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 Post subject: Quantity of carbonized particles before Arc Flash Initiation
PostPosted: Wed Feb 20, 2019 3:13 am 

Joined: Wed Feb 20, 2019 3:06 am
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In panels or switchgears where a tool dropped can initiate a major arc flash. I'd like to understand the behavior if carbonized particles (say from previous short) get into the conductors causing a short. What quantity before they can initiate and chain react into a major arc flash?

Imagine there is little amount of carbonized particles between the two phases or conductors. Once it conducts. It can vaporize immediately. Would this be enough to establish a major arc flash. What is the carbon deposit threshold before this can occur?


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Thu Feb 21, 2019 4:10 pm 
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Ommi wrote:
In panels or switchgears where a tool dropped can initiate a major arc flash. I'd like to understand the behavior if carbonized particles (say from previous short) get into the conductors causing a short. What quantity before they can initiate and chain react into a major arc flash?

Imagine there is little amount of carbonized particles between the two phases or conductors. Once it conducts. It can vaporize immediately. Would this be enough to establish a major arc flash. What is the carbon deposit threshold before this can occur?


I'm not sure what you are referring to. There are two different issues here. Contamination is sort of quantified in a very non-qualitative way in IEEE 516 where the evidence for this is documented. It's not very strong. Repeated incidents of tracking will eventually result in a fault but there's no definition of what is too bad. Eventually you get effects like dry banding, corona discharges, etc., which eventually result in a power arc. In NFPA 499 for instance referring to dust explosion problems such as coal they give a figure of 1/32 inch thickness and claim that this is when you can no longer see a white painted background clearly as a good indicator of when it is too thick.

As to vaporizing and such, what? That's not really what we're talking about here. It's conductive or semi-conductive materials that form surface partial discharges...basically bridging regions of high dielectric with low (or basically zero) ones causing increased voltage stress on the high dielectric regions resulting in insulation breakdown which causes the growth of what are called electrical trees which grow over time until a flashover occurs. Carbon vapor is not a necessity for this to occur...simple presence is all it takes. Google "partial discharge", "dry banding", "critical flashover voltage", "voltage stress", etc.

The concern with dropped tools is that it's a conductor. Simple as that.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Thu Feb 21, 2019 4:51 pm 

Joined: Wed Feb 20, 2019 3:06 am
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PaulEngr wrote:
Ommi wrote:
In panels or switchgears where a tool dropped can initiate a major arc flash. I'd like to understand the behavior if carbonized particles (say from previous short) get into the conductors causing a short. What quantity before they can initiate and chain react into a major arc flash?

Imagine there is little amount of carbonized particles between the two phases or conductors. Once it conducts. It can vaporize immediately. Would this be enough to establish a major arc flash. What is the carbon deposit threshold before this can occur?


I'm not sure what you are referring to. There are two different issues here. Contamination is sort of quantified in a very non-qualitative way in IEEE 516 where the evidence for this is documented. It's not very strong. Repeated incidents of tracking will eventually result in a fault but there's no definition of what is too bad. Eventually you get effects like dry banding, corona discharges, etc., which eventually result in a power arc. In NFPA 499 for instance referring to dust explosion problems such as coal they give a figure of 1/32 inch thickness and claim that this is when you can no longer see a white painted background clearly as a good indicator of when it is too thick.

As to vaporizing and such, what? That's not really what we're talking about here. It's conductive or semi-conductive materials that form surface partial discharges...basically bridging regions of high dielectric with low (or basically zero) ones causing increased voltage stress on the high dielectric regions resulting in insulation breakdown which causes the growth of what are called electrical trees which grow over time until a flashover occurs. Carbon vapor is not a necessity for this to occur...simple presence is all it takes. Google "partial discharge", "dry banding", "critical flashover voltage", "voltage stress", etc.

The concern with dropped tools is that it's a conductor. Simple as that.



What I was asking was supposed just for sake of discussion the drop thing is not a tool but a thin layer of carbon (again just for sake of discussion or better yet let's say the switch gear or whole facility is turn off and you put a thin layer of carbon between the conductors ). Since a thin layer of carbon is a conductor.. and this bridges the gaps between the two terminals or huge wires with large incident energy, would this initiate an arc flash or should the thing drop be a tool that can pass through huge current? Or can a thin layer of carbon enough to cause chain reaction (or negative resistance) effect that would build up the arc plasma from miniscule until the textbook arc flash occurs that throw the person across the room?

My


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Fri Feb 22, 2019 1:15 am 

Joined: Mon Jun 05, 2017 8:07 pm
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Hi ,

Since you mentioned for just discussion, It is interesting to know on how arc resistance may be a factor. Obviously we are dealing with some unkown parameters. I may not have the actual situation but however I do have a SIMILAR event as you mentioned. Watch this link below.

https://www.youtube.com/watch?v=g4ph-h7l_aM

May be we may expect something similar but inside an enclosed equipment that can cause an event with different physical reactions. Just a thought!!!

Regards
Raghu


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Fri Feb 22, 2019 2:34 am 

Joined: Wed Feb 20, 2019 3:06 am
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Ok. I'm investigating an arc flash accident. In the following image. The breakers encountered a first short which deposited carbon into the plastic enclosure and lugs. During a second event. The electrician just touch the right most live terminal into the right most lugs. It suddenly arc flashed causing the damage as well as hurting his arms.

What I want to know is. If there is merely carbon deposited in the breakers. Is it enough to cause arc flash between the two live terminals. It's like the carbon initiated the arc flash?

The transformers is an open delta 3 phase composing of two 75kVA transformers serving 240volts to an office building (the red and black line). It's connected directly by short wires to the service meter and directly the breakers. When the arc flash occurred, there was no overcurrent devices upstream of it. The transformer breaker didn't trip.

Just tell me if mere carbon deposit in the breakers can initiate arc flash. Notice the middle plastic is not damaged. It's as if the arc flash jumped in front of it between the terminals, is this possible? Note the smaller wire is not connected to the right most live wire. The smaller wire is just the ground hidden at the back of the live wire.

Image


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Fri Feb 22, 2019 10:05 am 
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While carbon is present, I believe the second event was caused by insulation failure from the heat of the 1st. Hope your electrician is doing well.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Fri Feb 22, 2019 11:11 am 
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Ommi wrote:
Ok. I'm investigating an arc flash accident. In the following image. The breakers encountered a first short which deposited carbon into the plastic enclosure and lugs. During a second event. The electrician just touch the right most live terminal into the right most lugs. It suddenly arc flashed causing the damage as well as hurting his arms.

What I want to know is. If there is merely carbon deposited in the breakers. Is it enough to cause arc flash between the two live terminals. It's like the carbon initiated the arc flash?

The transformers is an open delta 3 phase composing of two 75kVA transformers serving 240volts to an office building (the red and black line). It's connected directly by short wires to the service meter and directly the breakers. When the arc flash occurred, there was no overcurrent devices upstream of it. The transformer breaker didn't trip.

Just tell me if mere carbon deposit in the breakers can initiate arc flash. Notice the middle plastic is not damaged. It's as if the arc flash jumped in front of it between the terminals, is this possible? Note the smaller wire is not connected to the right most live wire. The smaller wire is just the ground hidden at the back of the live wire.

Image


Let's stop right there. First off this is a shock/electrocution case. Burning of flesh occurs at around 1 A. We don't need to go further than this. This is NOT arc flash. It's joule heating...cooking someone, plain and simple. There has been a bonafide arc flash case at 240/120 VAC in 2009 in Georgia documented by OSHA in which someone died from the injuries but what you are talking about doesn't sound anywhere close to that. In fact since the electrician touched he "right" terminal and was probably grounded that's only 120 VAC which has not been shown to be a significant arc flash hazard. Still something significant did happen here but you can't honestly attribute it to arc flash.

Second, you do NOT need any kind of previous damage of any kind in an actual arc flash. What typically happens is that we get an arc from line to ground which is what you had. Then this heats up the surrounding air. As air temperature goes up as with most substances (except carbon) the insulation properties go down. Thus within typically 1-2 cycles of an arc flash the surrounding air heats up to the point that it jumps from one terminal to the next and we get a full on 3 phase (or single phase in your case) arc flash regardless of how it started. Barriers, insulation blocks, etc., often have little to no effect as far as isolation goes. From a forensic point of view one of the tell tale signs is looking specifically for damage from a ground fault since once the arc flash gets going it usually has very little interaction with grounded surfaces after that except if you get A-enclosure-C arcing.

Carbon is not "deposited". If insulation is damaged the chemical results are water, CO2, etc., when hydrocarbons completely burned but you get soot and ash with incomplete burning and noncombustible materials such as fillers. The remaining material (the "carbon" as you called it) is a semiconductor. It has no electrical insulating properties whatsoever. Carbon "depositing" is something quite different. That's contamination. Just ask "smoke stack" industries (iron & steel, refineries, coal prep plants, mining, many chemical plants, wood plants, etc.) if it's a problem. The answer is a resounding yes. Routine cleaning of electrical gear is an absolute necessity for them in many cases. After damage NEMA standards among others are pretty clear: you remove and replace ALL damaged insulation, period. The previous repair was clearly improperly done based on the description.

To clear things up at low level voltages and currents we get streaming which is essentially nonvisible currents flowing across surfaces. As the surface becomes more contaminated or damaged, "glows", "sparks", St. Elmo's fire (corona discharges), etc. appear. Depending on the intensity these things can be mostly harmless. As the intensity increases we get progressively more energetic effects, and damage begins to ensue. It appears initially as a whitish powder residue and surfaces that are intended to be polished like insulators start to take on a dull appearance. Eventually the insulation turns black where it has burned completely leaving only a semiconductive ash and soot material behind. Whether white or black, the dielectric is compromised. Across the semi-conductive region whether it is due to contamination or insulation resistance, effectively the dielectric is ZERO and thus the voltage drop across this region is zero. The problem though is that we're dealing not with a 1 dimensional case like a wire but a 2D or maybe 3D problem. This sets up a situation where we have a much higher voltage flux (gradient) across the surrounding regions that are still intact and in parallel with the compromised region. Eventually this additional stress causes breakdown of the regions next to the compromised spot if the applied voltage across the insulation is high enough. This region is then damaged (oxidized/burns) which increases the size of the damaged area and puts yet more voltage stress on the remaining undamaged insulation. Physically the pattern is a Lichtenberg figure (frozen lightning) which is something that technicians look for with medium voltage which is a tell tale indication of an impending failure. The same thing still happens even at low voltages but the scales are drastically smaller.

Now going beyond this, there is no measurement of contamination or any sort of test to indication what is acceptable and what's not. At medium voltage (>1 kV) there is the "tin foil" test which is a performance test to verify whether or not the equipment clearances are sufficient, and there are hardly much in the way of UL standards for 600 V or less either. The equipment is wrapped in aluminum foil and then high potted to see if it leaks or arcs across the gaps. If it holds a high voltage (<1 mA leakage), it passes. However there is no NEC rule as to how much the minimum clearance distance should be. There is usually a good deal of "margin" built into terminal blocks though and as you are probably no doubt aware there is a lot of abuse out there. Electricians can do an absolutely horrendous job of routing and terminating cables at terminal blocks and still manage not to cause an arcing fault, particularly at 240 V. As you go up in voltage at 4160 respecting clearances is a much bigger deal. By the time you get to 7200 V if you even lay unshielded cables on a grounded surface you will get tracking and failures. At 15 kV and above, merely nicking some of the insulation when terminating the cable without smooth edges is all it takes to cause a failure. That's why you should be trained to do medium voltage terminations. At 240 VAC cleanliness is usually more of a matter of failures and overheating from being buried in dirt. But at medium voltage cleanliness is critical to safe operation so it is looked at very differently. But it is still instructive to understanding what happens at low voltages.

At low voltages equipment is rated and tested based on both the clearance (distance in air between two conductors) and creepage distance which is the linear distance from one conductor to another along a surface. Creepage distance is usually supposed to be at least twice the air distance as a rule of thumb. So with damage the air distance is probably hardly changed but the creep distance can quickly disappear.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Fri Feb 22, 2019 3:41 pm 

Joined: Wed Feb 20, 2019 3:06 am
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PaulEngr wrote:
Ommi wrote:
Ok. I'm investigating an arc flash accident. In the following image. The breakers encountered a first short which deposited carbon into the plastic enclosure and lugs. During a second event. The electrician just touch the right most live terminal into the right most lugs. It suddenly arc flashed causing the damage as well as hurting his arms.

What I want to know is. If there is merely carbon deposited in the breakers. Is it enough to cause arc flash between the two live terminals. It's like the carbon initiated the arc flash?

The transformers is an open delta 3 phase composing of two 75kVA transformers serving 240volts to an office building (the red and black line). It's connected directly by short wires to the service meter and directly the breakers. When the arc flash occurred, there was no overcurrent devices upstream of it. The transformer breaker didn't trip.

Just tell me if mere carbon deposit in the breakers can initiate arc flash. Notice the middle plastic is not damaged. It's as if the arc flash jumped in front of it between the terminals, is this possible? Note the smaller wire is not connected to the right most live wire. The smaller wire is just the ground hidden at the back of the live wire.


Let's stop right there. First off this is a shock/electrocution case. Burning of flesh occurs at around 1 A. We don't need to go further than this. This is NOT arc flash. It's joule heating...cooking someone, plain and simple. There has been a bonafide arc flash case at 240/120 VAC in 2009 in Georgia documented by OSHA in which someone died from the injuries but what you are talking about doesn't sound anywhere close to that. In fact since the electrician touched he "right" terminal and was probably grounded that's only 120 VAC which has not been shown to be a significant arc flash hazard. Still something significant did happen here but you can't honestly attribute it to arc flash.



When I mentioned the word "touched". I simply meant he connected the right live wire to the right terminal. I didn't mean he touched it with bare hands. "Touch" was the live wire and terminal lugs touching. Bad choice of words.


Quote:
Second, you do NOT need any kind of previous damage of any kind in an actual arc flash. What typically happens is that we get an arc from line to ground which is what you had. Then this heats up the surrounding air. As air temperature goes up as with most substances (except carbon) the insulation properties go down. Thus within typically 1-2 cycles of an arc flash the surrounding air heats up to the point that it jumps from one terminal to the next and we get a full on 3 phase (or single phase in your case) arc flash regardless of how it started. Barriers, insulation blocks, etc., often have little to no effect as far as isolation goes. From a forensic point of view one of the tell tale signs is looking specifically for damage from a ground fault since once the arc flash gets going it usually has very little interaction with grounded surfaces after that except if you get A-enclosure-C arcing.


What I was asking was whether the arc flash occured between the two terminals due to the carbon being deposited during the first short between live and chassis. The carbon not coming from the insulation but directly from the breaker black case. The heat carbonized it. Then during the second connection. The carbon initiate dthe arc flash between the two terminals or specifically:

Image

Can this occur? The flash going in between the two terminals higher up enough not to damage the plastic case between the terminals (the blue lines being the path of the arc flash)? So it was like the arc flash did all the damages to the chassis wall and the heat caused blisters in the electrican hand.

Again the electrician didn't touch the terminal with bare hands. He simply connected the live wire to the terminal which suddenly produced arc flash. The breakers was opened to examine if the inside were damaged or shorted. They were working perfectly and it was OFF during the incident and there was no load or wires connected at the load side yet.



Quote:
Carbon is not "deposited". If insulation is damaged the chemical results are water, CO2, etc., when hydrocarbons completely burned but you get soot and ash with incomplete burning and noncombustible materials such as fillers. The remaining material (the "carbon" as you called it) is a semiconductor. It has no electrical insulating properties whatsoever. Carbon "depositing" is something quite different. That's contamination. Just ask "smoke stack" industries (iron & steel, refineries, coal prep plants, mining, many chemical plants, wood plants, etc.) if it's a problem. The answer is a resounding yes. Routine cleaning of electrical gear is an absolute necessity for them in many cases. After damage NEMA standards among others are pretty clear: you remove and replace ALL damaged insulation, period. The previous repair was clearly improperly done based on the description.

To clear things up at low level voltages and currents we get streaming which is essentially nonvisible currents flowing across surfaces. As the surface becomes more contaminated or damaged, "glows", "sparks", St. Elmo's fire (corona discharges), etc. appear. Depending on the intensity these things can be mostly harmless. As the intensity increases we get progressively more energetic effects, and damage begins to ensue. It appears initially as a whitish powder residue and surfaces that are intended to be polished like insulators start to take on a dull appearance. Eventually the insulation turns black where it has burned completely leaving only a semiconductive ash and soot material behind. Whether white or black, the dielectric is compromised. Across the semi-conductive region whether it is due to contamination or insulation resistance, effectively the dielectric is ZERO and thus the voltage drop across this region is zero. The problem though is that we're dealing not with a 1 dimensional case like a wire but a 2D or maybe 3D problem. This sets up a situation where we have a much higher voltage flux (gradient) across the surrounding regions that are still intact and in parallel with the compromised region. Eventually this additional stress causes breakdown of the regions next to the compromised spot if the applied voltage across the insulation is high enough. This region is then damaged (oxidized/burns) which increases the size of the damaged area and puts yet more voltage stress on the remaining undamaged insulation. Physically the pattern is a Lichtenberg figure (frozen lightning) which is something that technicians look for with medium voltage which is a tell tale indication of an impending failure. The same thing still happens even at low voltages but the scales are drastically smaller.

Now going beyond this, there is no measurement of contamination or any sort of test to indication what is acceptable and what's not. At medium voltage (>1 kV) there is the "tin foil" test which is a performance test to verify whether or not the equipment clearances are sufficient, and there are hardly much in the way of UL standards for 600 V or less either. The equipment is wrapped in aluminum foil and then high potted to see if it leaks or arcs across the gaps. If it holds a high voltage (<1 mA leakage), it passes. However there is no NEC rule as to how much the minimum clearance distance should be. There is usually a good deal of "margin" built into terminal blocks though and as you are probably no doubt aware there is a lot of abuse out there. Electricians can do an absolutely horrendous job of routing and terminating cables at terminal blocks and still manage not to cause an arcing fault, particularly at 240 V. As you go up in voltage at 4160 respecting clearances is a much bigger deal. By the time you get to 7200 V if you even lay unshielded cables on a grounded surface you will get tracking and failures. At 15 kV and above, merely nicking some of the insulation when terminating the cable without smooth edges is all it takes to cause a failure. That's why you should be trained to do medium voltage terminations. At 240 VAC cleanliness is usually more of a matter of failures and overheating from being buried in dirt. But at medium voltage cleanliness is critical to safe operation so it is looked at very differently. But it is still instructive to understanding what happens at low voltages.

At low voltages equipment is rated and tested based on both the clearance (distance in air between two conductors) and creepage distance which is the linear distance from one conductor to another along a surface. Creepage distance is usually supposed to be at least twice the air distance as a rule of thumb. So with damage the air distance is probably hardly changed but the creep distance can quickly disappear.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Sun Feb 24, 2019 8:17 am 
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OK so if I understand this correctly to simplify it down effectively the breaker and the electrician had nothing to do with the arc flash incident except indirectly. It was de-energized at the time before the arcing fault occurred and wires were being landed live. So effectively we have at least one energized cable landing on a terminal which isn't connected to anything and at least one terminal not connected to anything either with a live cable being landed on it. So the question is how did it flash over.

First off someone is landing a cable in VERY CLOSE proximity to another cable onto a mechanical lug. We're talking less than an inch here. Even with situational awareness and the most careful pair of hands, mistakes still happen all the time. It is very easy to accidentally get the wire too close without any other extenuating circumstances. Add to this the fact that it is also very easy to have small frayed strands. It happens all the time. It is especially a big problem when attempting to replace a previously used wire without cutting off the ends. In a mechanical lug the strands are cold welded together....they are compressed out of shape and physically forced together in order to make the connection. Because they are so badly deformed the lug manufacturers tell you that you are supposed to cut off the end and land only a clean, freshly prepared wire every time. And no, none of us do that unless we have to because it takes extra time and because often especially when landing line side wiring we don't have that much slack to work with in the first place when replacing components or moving equipment around. All it takes is one very tiny strand or someone getting too close with the wire to cause a flashover. We don't need a damaged breaker to initiate an arcing fault this way. This is the very activity that NFPA 70E and OSHA are very adamant about never doing if at all possible and this is the very reason for not doing it. The vast majority of the connections I personally work with right now are larger sizes, mostly above #10 AWG sizes. Even when that's most of what I work with, I can't tell you how many times I've had a loose strand get away from me. So then I have to undo everything, pull the wire back out, try to retrain the strand back into the bundle where it belongs, or cut it all off and strip a clean section of cable. It's a pain. It happens a lot. And it can happen completely unnoticed even on de-energized cable particularly fine strand but even on coarse strand whether you think you have it all or not. It happens most frequently when the cable and lug maximum sizes are close but it can happen even on smaller cables too. Ask around about how often this happens with control power terminals. The answer will be that it happens all the time. It is a bigger problem with "open" style lugs without any kind of barrier in between. Attempting it energized seems like an accident waiting to happen. At a bare minimm if the insulation isn't stripped exactly according to the manufacturer's requirements which usually results in less than 1/8" of copper showing out of the lug in some cases and flush in others, if it sticks out at all, you are just asking for trouble.

Second, yes the burned surface of the plastic is a semiconductor. It is impossible to tell how much the surface is compromised now that it is completely destroyed so there is no way to tell what the conditions were before the incident as far as how compromised it is. Keep in mind that transients exist. A typical rule of thumb is that transients are up to around 150% of the line voltage so with a 240 V system, a 360 V transient wouldn't be all that unusual but in ungrounded systems for instance, transients can get up to 600-800% of line voltage. I doubt that breaker is 300 V class equipment so it is probably rated as 600 V class equipment which means that it is tested for transient protection purposes up to around 2,000 V. That's enough to withstand even a totally ungrounded worst case transient on the system. But that's with new, undamaged equipment. Your equipment was already compromised. The transient withstand would have been much lower. It is quite possible that a transient occurred at the time of landing the wire and that it was just bad luck that the electrician happened to be in the line of fire at the time. As far as transients go, I'm not even referring to lightning. Starting and stopping motors, or the utility performing switching somewhere on their system is all that it takes to create switching transients. So even if it appears to be sound right now, that situation can change in a fraction of a cycle minutes or even days later.

The various NEMA standards out there for flood and fire damage are very specific on how to properly repair equipment after it is damaged. You remove and replace everything that is damaged or contaminated from soot/smoke/liquids. Equipment must be properly repaired if it is reused and that means tested not just visually but electrically as well. A lot of times you can clean up and wipe things off and retest it just to be sure and that's all that needs to be done. BUT if the damage is more than just surface contamination as it was in your case, the equipment must be fully repaired or replaced, no exceptions. It is possible to repair and restore equipment that has been lightly damaged back to full operation. It has to be thoroughly cleaned first. We use dry ice blasting in our repair facility when we can't replace it. Then we use enamels and other coatings when necessary to restore the surface creep properties. Then we test it using the same tests the manufacturer would use (ohms then megger then corona or hi pot testing in this case) to verify that the equipment is back to the original factory tested condition. Companies that do this work on breakers follow PEARL standards for general electrical equipment which would apply in this case. From your description this was not done.

So if I'm following this scenario it sounds like something happened on top of the breaker prior to the actual incident. The breaker was cleaned up but not replaced or sent out to be repaired. Now at this point it is not clear if the breaker had never been put back into service until this incident or that this was the first time that it was placed back into service. If it's the first time then it's just a case of semiconductive charred material reduced the creepage distance to the point where it flashed over. But if this condition had existed for quite some time while in service, then either the circuit changes being made reduced the creepage or clearance distance even further such as burned insulation cracking or flaking off the power cable and/or the breaker, or the installation itself contributed or caused the issue...cable got too close or a frayed strand got too close.

Either way, recommend a couple things:
1. Review interpretations of what are acceptable repairs. NEMA AB-4 is the standard that every breaker manufacturer refers to. It is very clear, easy to read, and has full color pictures explaining what to look for when determining whether or not to put a breaker back into service. If you use that, this would not have happened. They also publish a standard for evaluating flood damaged equipment. It is GD-1. This is a good standard for really any kind of contamination such as fire. Again if you use that this would not have happened. These standards are useful not only to prevent future incidents and because when they are referenced by the manufacturers and so when OSHA and 70E refer to manufacturer recommendations, those are the recommendations. You can't really be following OSHA regulations or 70E without following these. It also provides cover for maintenance departments when they are pushed on decisions whether or not to repair or replace damaged equipment instead of just running it when questioned by management. The best part about these two standards by the way is that they are free...as in free to download and use. A lot of NEMA standards are expensive to purchase so take advantage of these two.
2. 70E (and OSHA) make energized circuit changes such as this one a management decision. Either management was not following the energized work permit procedure, or was misinterpreting it, or didn't recognize the importance, or is not enforcing it, or is unaware of it. There is an OSHA letter of interpretation when it comes to the phrase "continuous industrial processes" that you need to read, too. It is not an "out" for process plants contrary to popular belief. Review every aspect of the management side of things. Was an EEWP filled out and signed for this? Review especially the letters of interpretation around energized work permits that OSHA has put out. It should be very quickly obvious that when they say energized work should only be allowed when it is impossible to do it any other way, they mean just about impossible. This doesn't mean that we can't do a lot of testing such as testing voltages for instance because that obviously has to be done while energized. But physically landing energized wires is something that is almost never required. It's just that when we all get into butting heads and maintenance needs to shut down equipment or sometimes the entire plant to make what is essentially a minor repair, management starts looking at costs and loses focus on safety. Trust me, I've done a job where the only viable solution to the repair was to shut off all power to a large prison. This isn't something that anyone wanted to have happen, including me, and I'm just the contractor doing the work. The repair itself took about 30 minutes. It took over a year of planning and paperwork to make it happen. We did a lot more than just this one repair since we had the time available.
3. Review the distribution design. This is sometimes costly and sometimes relatively inexpensive to fix depending on the extent of the design issue. If live work is necessary, design for it. Utilities have extensive "extra" disconnecting means in their substations for this reason, and they use special saddles and clips, and hot line tools for doing energized work safely directly on power lines when they have to add or remove a "tap". The circuit that the breaker fed was obviously not energized so de-energizing is obviously possible. This points to a distribution design issue that needs to be addressed because cables feeding a breaker should be fed from another overcurrent protection device which should be opened to make repairs. Plants often make management decisions placing production needs over safety in this case, particularly if the breaker is not very large. The OSHA letter of interpretation on continuous industrial processes should make it very clear how OSHA views this practice: you de-energize even if doing so is costly. Energized work is a bandaid to a design problem at best and should be temporary. Common design issues are where the distribution system is designed around areas and not processes. Then shutting down a distribution panel that feeds an area shuts down multiple production processes instead of just one. The other design problem is cutting corners during construction, modifications, or repairs by using various tap rules to avoid installing additional overcurrent protection devices even hen they are necessary from an isolation point of view. It is very tempting for plant management to make decisions to do live work when this happens even if the equipment was never designed to support live work. In the prison scenario above there were multiple feeders and generators involved. But a couple design decisions made it impossible to utilize the redundancy built into the system. The generators were located on the same bus as the main breaker and the utility only had a single cutout feeding all feeders so isolation was impossible without taking everything down.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Sun Feb 24, 2019 4:43 pm 

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PaulEngr wrote:
OK so if I understand this correctly to simplify it down effectively the breaker and the electrician had nothing to do with the arc flash incident except indirectly. It was de-energized at the time before the arcing fault occurred and wires were being landed live. So effectively we have at least one energized cable landing on a terminal which isn't connected to anything and at least one terminal not connected to anything either with a live cable being landed on it. So the question is how did it flash over.

First off someone is landing a cable in VERY CLOSE proximity to another cable onto a mechanical lug. We're talking less than an inch here. Even with situational awareness and the most careful pair of hands, mistakes still happen all the time. It is very easy to accidentally get the wire too close without any other extenuating circumstances. Add to this the fact that it is also very easy to have small frayed strands. It happens all the time. It is especially a big problem when attempting to replace a previously used wire without cutting off the ends. In a mechanical lug the strands are cold welded together....they are compressed out of shape and physically forced together in order to make the connection. Because they are so badly deformed the lug manufacturers tell you that you are supposed to cut off the end and land only a clean, freshly prepared wire every time. And no, none of us do that unless we have to because it takes extra time and because often especially when landing line side wiring we don't have that much slack to work with in the first place when replacing components or moving equipment around. All it takes is one very tiny strand or someone getting too close with the wire to cause a flashover. We don't need a damaged breaker to initiate an arcing fault this way. This is the very activity that NFPA 70E and OSHA are very adamant about never doing if at all possible and this is the very reason for not doing it. The vast majority of the connections I personally work with right now are larger sizes, mostly above #10 AWG sizes. Even when that's most of what I work with, I can't tell you how many times I've had a loose strand get away from me. So then I have to undo everything, pull the wire back out, try to retrain the strand back into the bundle where it belongs, or cut it all off and strip a clean section of cable. It's a pain. It happens a lot. And it can happen completely unnoticed even on de-energized cable particularly fine strand but even on coarse strand whether you think you have it all or not. It happens most frequently when the cable and lug maximum sizes are close but it can happen even on smaller cables too. Ask around about how often this happens with control power terminals. The answer will be that it happens all the time. It is a bigger problem with "open" style lugs without any kind of barrier in between. Attempting it energized seems like an accident waiting to happen. At a bare minimm if the insulation isn't stripped exactly according to the manufacturer's requirements which usually results in less than 1/8" of copper showing out of the lug in some cases and flush in others, if it sticks out at all, you are just asking for trouble.

Second, yes the burned surface of the plastic is a semiconductor. It is impossible to tell how much the surface is compromised now that it is completely destroyed so there is no way to tell what the conditions were before the incident as far as how compromised it is. Keep in mind that transients exist. A typical rule of thumb is that transients are up to around 150% of the line voltage so with a 240 V system, a 360 V transient wouldn't be all that unusual but in ungrounded systems for instance, transients can get up to 600-800% of line voltage. I doubt that breaker is 300 V class equipment so it is probably rated as 600 V class equipment which means that it is tested for transient protection purposes up to around 2,000 V. That's enough to withstand even a totally ungrounded worst case transient on the system. But that's with new, undamaged equipment. Your equipment was already compromised. The transient withstand would have been much lower. It is quite possible that a transient occurred at the time of landing the wire and that it was just bad luck that the electrician happened to be in the line of fire at the time. As far as transients go, I'm not even referring to lightning. Starting and stopping motors, or the utility performing switching somewhere on their system is all that it takes to create switching transients. So even if it appears to be sound right now, that situation can change in a fraction of a cycle minutes or even days later.

The various NEMA standards out there for flood and fire damage are very specific on how to properly repair equipment after it is damaged. You remove and replace everything that is damaged or contaminated from soot/smoke/liquids. Equipment must be properly repaired if it is reused and that means tested not just visually but electrically as well. A lot of times you can clean up and wipe things off and retest it just to be sure and that's all that needs to be done. BUT if the damage is more than just surface contamination as it was in your case, the equipment must be fully repaired or replaced, no exceptions. It is possible to repair and restore equipment that has been lightly damaged back to full operation. It has to be thoroughly cleaned first. We use dry ice blasting in our repair facility when we can't replace it. Then we use enamels and other coatings when necessary to restore the surface creep properties. Then we test it using the same tests the manufacturer would use (ohms then megger then corona or hi pot testing in this case) to verify that the equipment is back to the original factory tested condition. Companies that do this work on breakers follow PEARL standards for general electrical equipment which would apply in this case. From your description this was not done.

So if I'm following this scenario it sounds like something happened on top of the breaker prior to the actual incident. The breaker was cleaned up but not replaced or sent out to be repaired. Now at this point it is not clear if the breaker had never been put back into service until this incident or that this was the first time that it was placed back into service. If it's the first time then it's just a case of semiconductive charred material reduced the creepage distance to the point where it flashed over. But if this condition had existed for quite some time while in service, then either the circuit changes being made reduced the creepage or clearance distance even further such as burned insulation cracking or flaking off the power cable and/or the breaker, or the installation itself contributed or caused the issue...cable got too close or a frayed strand got too close.





The first short circuit that created the semiconductive charred material was only a day earlier and the breaker was never used before and the panel was only opened during the second attempt to connect the live terminal with nothing at the load side of it. Now let's focus on the physics of how the flash occurred. When you mentioned "to the point where it flashed over". Did you mean "to the point where it arc flashed over"? My main question was supposed there was semiconductive charged material in the surface. Would the short circuit current just vaporize the charred material and then stop. Or would it initiate chain reaction meaning the charred material is enough to create a micro plasma arc that via negative resistance built up until there is a large arc flash between the two live terminals enough to damage the wall of the chassis and caused 2nd degree burn (rat size blisters) in the electrican arm? Kindly focus on this questions. And so many thanks for the detailed explanations above and below. But I'm interested in the exact physics of it. Let's assume the incident energy is big and there is really no overcurrent protection device upstream of it (there is really none). So given large source incident energy, whether the above can occur and how exactly (from the creation of micro flash caused by the charred material to chain reaction via negative resistance into much bigger arc flash whose plasma just explode, is this possible, this is the main question.. thanks).



Quote:
Either way, recommend a couple things:
1. Review interpretations of what are acceptable repairs. NEMA AB-4 is the standard that every breaker manufacturer refers to. It is very clear, easy to read, and has full color pictures explaining what to look for when determining whether or not to put a breaker back into service. If you use that, this would not have happened. They also publish a standard for evaluating flood damaged equipment. It is GD-1. This is a good standard for really any kind of contamination such as fire. Again if you use that this would not have happened. These standards are useful not only to prevent future incidents and because when they are referenced by the manufacturers and so when OSHA and 70E refer to manufacturer recommendations, those are the recommendations. You can't really be following OSHA regulations or 70E without following these. It also provides cover for maintenance departments when they are pushed on decisions whether or not to repair or replace damaged equipment instead of just running it when questioned by management. The best part about these two standards by the way is that they are free...as in free to download and use. A lot of NEMA standards are expensive to purchase so take advantage of these two.
2. 70E (and OSHA) make energized circuit changes such as this one a management decision. Either management was not following the energized work permit procedure, or was misinterpreting it, or didn't recognize the importance, or is not enforcing it, or is unaware of it. There is an OSHA letter of interpretation when it comes to the phrase "continuous industrial processes" that you need to read, too. It is not an "out" for process plants contrary to popular belief. Review every aspect of the management side of things. Was an EEWP filled out and signed for this? Review especially the letters of interpretation around energized work permits that OSHA has put out. It should be very quickly obvious that when they say energized work should only be allowed when it is impossible to do it any other way, they mean just about impossible. This doesn't mean that we can't do a lot of testing such as testing voltages for instance because that obviously has to be done while energized. But physically landing energized wires is something that is almost never required. It's just that when we all get into butting heads and maintenance needs to shut down equipment or sometimes the entire plant to make what is essentially a minor repair, management starts looking at costs and loses focus on safety. Trust me, I've done a job where the only viable solution to the repair was to shut off all power to a large prison. This isn't something that anyone wanted to have happen, including me, and I'm just the contractor doing the work. The repair itself took about 30 minutes. It took over a year of planning and paperwork to make it happen. We did a lot more than just this one repair since we had the time available.
3. Review the distribution design. This is sometimes costly and sometimes relatively inexpensive to fix depending on the extent of the design issue. If live work is necessary, design for it. Utilities have extensive "extra" disconnecting means in their substations for this reason, and they use special saddles and clips, and hot line tools for doing energized work safely directly on power lines when they have to add or remove a "tap". The circuit that the breaker fed was obviously not energized so de-energizing is obviously possible. This points to a distribution design issue that needs to be addressed because cables feeding a breaker should be fed from another overcurrent protection device which should be opened to make repairs. Plants often make management decisions placing production needs over safety in this case, particularly if the breaker is not very large. The OSHA letter of interpretation on continuous industrial processes should make it very clear how OSHA views this practice: you de-energize even if doing so is costly. Energized work is a bandaid to a design problem at best and should be temporary. Common design issues are where the distribution system is designed around areas and not processes. Then shutting down a distribution panel that feeds an area shuts down multiple production processes instead of just one. The other design problem is cutting corners during construction, modifications, or repairs by using various tap rules to avoid installing additional overcurrent protection devices even hen they are necessary from an isolation point of view. It is very tempting for plant management to make decisions to do live work when this happens even if the equipment was never designed to support live work. In the prison scenario above there were multiple feeders and generators involved. But a couple design decisions made it impossible to utilize the redundancy built into the system. The generators were located on the same bus as the main breaker and the utility only had a single cutout feeding all feeders so isolation was impossible without taking everything down.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Tue Feb 26, 2019 9:57 am 
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Ommi wrote:
The first short circuit that created the semiconductive charred material was only a day earlier and the breaker was never used before and the panel was only opened during the second attempt to connect the live terminal with nothing at the load side of it. Now let's focus on the physics of how the flash occurred. When you mentioned "to the point where it flashed over". Did you mean "to the point where it arc flashed over"?


No. Definitions first. I know what you mean but if we're going to nitpick on definitions...

See figure 3 here:
http://www.neiengineering.com/wp-conten ... ations.pdf

Basically we have a bunch of phenomena going on that have various names (glow discharge, streaming, corona, sparking) where more or less current draw is very low and resistances are high. These can lead to equipment failures eventually above around 2000 VAC or so. They might look scary since there is a lot of noise associated with them (stand in a utility substation on a very humid or wet day) but they are harmless from a safety (arc flash) point of view. Then at some point we cross the CFO or critical flashover voltage/resistance, defined by IEEE as:
"The amplitude of voltage of a given waveshape that, under specified conditions, causes flashover through the surrounding medium on 50% of the voltage applications."

Flashover means that we transition into a power arc at which point arc voltage is almost constant and current increases dramatically as shown in figure 3 above. Note that this is for DC...I'll get back to the AC part below. Suffice to say that under the right conditions an electric arc forms. When it does, it ignites and extinguishes in picoseconds or nanoseconds so for power and safety purposes it is effectively instantaneous.

NFPA 70E defines arc flash HAZARD as: "A dangerous condition associated with the possible release of energy caused by an electric arc. Informational Note No. 1: An arc flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc. Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard." Although there are multiple effects that are a hazard when it comes to arcing (heat, light, ejected materials), currently only the heat component is considered. Arcing is the phenomena. Arc flash is thermal safety effect of the arc.

Quote:
My main question was supposed there was semiconductive charged material in the surface. Would the short circuit current just vaporize the charred material and then stop. Or would it initiate chain reaction meaning the charred material is enough to create a micro plasma arc that via negative resistance built up until there is a large arc flash between the two live terminals enough to damage the wall of the chassis and caused 2nd degree burn (rat size blisters) in the electrican arm? Kindly focus on this questions.


OK, focussing on the phenomena of an arc itself. Once the voltage gets high enough and in this context we're talking about such a fast event that the fact that we're dealing with AC doesn't matter...this is a pure DC discussion that it exceeds the CFO (critical flashover voltage) of the system in it's current state, an arc starts. The core of the arc is a plasma which means that it is so hot that the electrons are so energized that they just float around in a cloud around the general vicinity of the nuclei. Chemical bonds don't really exist so instead of O2, N2, CO2, etc., we just have individual nuclei and a cloud of highly energized electrons. This cloud has very little electrical resistance for obvious reasons and it is also highly magnetic. It pulls in on itself so that effectively the arc diameter is almost a constant at around 1-2 mm. This is what the physicists have figured out about electric arcs. The properties of a plasma are temperature dependent. So physicists get really hung up on calculating plasma temperature.

Energy radiated from this arc core is 99%+ absorbed by the surrounding air though. As air molecules get hot they dump photons to cool down and we perceive photons in the visible spectrum as light. Then once a photon is emitted from one atom if it strikes the next (electron), it passes the energy on at which point it is absorbed and then emitted again in a cascade. Obviously there are losses along the way as the surrounding air absorbs a lot of the energy (heats up) and some passes through to solid objects nearby. That's what you are seeing...you don't actually "see" an arc, you see it's effects and you feel the infrared photons as heat. However nearly all of the energy emitted by the arc core is absorbed and re-emitted around it. So even though the arc core where plasma exists at some crazy high temperature that only a physicist cares about (the "35,000 degrees" claim you see in the arc flash scare literature) that temperature is NEVER seen outside the arc core. If it was, it would vaporize anything exposed to it which is obviously not the case. That's not to say that arcs don't emit a lot of thermal radiation, just trying to be realistic here. If you are at the receiving end of the arc (a shock) then you are being directly exposed to the effects of having all the electrons blown off the nuclei and some skin will be vaporized and it is going to do a lot more damage, but that's not where we're going here.

As far as the arc extinguishing at this point the resistance of the air within the plasma has essentially gone to almost zero. There is nothing to stop the current from flowing with a DC arc at all. There are really only three ways to extinguish a DC arc. First, disconnect the power so that the air cools back down (CFO goes back up) so that it can't reform. Second method is to increase the arc length to the point where the arc is unsustainable. The arc consumes a certain amount of power per unit length to sustain itself. If the arc is too long there isn't enough energy available to keep it going so it goes out.

Now switching over to AC at a DC perspective (milliseconds) if either the normal AC voltage or a transient gets above the CFO (critical flashover voltage) again the arc strikes. Once it strikes the resistance within the arc itself is obviously so low that there's nothing to extinguish the arc even if the voltage drops until the CURRENT goes through a zero which it does so 120 times per second because in this case it's an AC system. So the arc naturally extinguishes. But prior to this happening the air around the arc was also heated up quite a bit. As air temperature increases, resistance decreases. So the CFO also decreases. On the next voltage rise it takes a lot less voltage to restrike the arc. HOWEVER this is all something that you can see from laboratory measurements of arc tests. In practical calculations we ignore all this because the effect only happens during the first couple restrikes. After that the CFO and thus arc restriking are pretty much constant.

As to your question about how the surface contamination evolves, this is what I was explaining earlier. Even if the arc ablates away some material (vaporizes/burns off) it is also thermally damaging the undamaged layers next to it. It is not going to "burn clean" as you are suggesting. The charred area is going to grow in both surface area and depth, even if material is actually burned away, just as a conventional (non-electrical) fire first chars and then consumes material. Under all circumstances, CFO can only decrease even if the arc does self-extinguish. The only time that CFO ever goes back up is with what is known as "renewable" insulation which is a nice way of saying that if we stop the arc, air will cool back down and electrical resistance through air is restored back to normal.

Quote:
And so many thanks for the detailed explanations above and below. But I'm interested in the exact physics of it.


I can point you to a couple books. Both are very much out of print You won't care about the subject matter so much as the introductory sections that explain what's going on. First one and you really only need the introductory chapters is:
https://www.amazon.com/High-Voltage-Cir ... b_title_bk

The part that you need to read is only chapter 1 and part of chapter 2, and fortunately Amazon has that available for free. Circuit breaker theory is all about sustainability of arcs (and breaking them) which is part of what you are asking about, along with some basic material science concepts at high voltages and temperatures. Granted you are asking about very low voltage arcs. The theory still applies for the most part.

Second is this one:
https://www.cambridge.org/core/books/el ... 022EC27926

You can read this one entirely. It is freely available from several sources. I just linked the first one. Ayrton was interested in stable, sustainable arcs for lighting purposes (neon lighting) and did a lot of the first early physics experiments with arcs. She was NOT interested in power arcs since that usually results in destroying the equipment but instead working at the borderline before they form. From this point of view since increased voltage results in a decrease in resistance (see figure 3 above again) the concept of negative resistance applies. Once we have a power arc that concept is no longer valid...voltage is slightly increasing which is zero or positive resistance.

The third leg is arc flash. That is better covered at least from a basic theoretical point of view in the first article I referenced on DC arcs. If you really want to go down the AC rabbit hole this is one of the more useful articles that gets into the nuts and bolts of arc flash. Incidentally prior to the most recent update to IEEE 1584, it beats out IEEE 1584-2002 in terms of accuracy. And Wilkins way of explaining things is pretty straight forward:
https://www.google.com/url?sa=t&rct=j&q ... 7y5ndtQFB2

Quote:
Let's assume the incident energy is big and there is really no overcurrent protection device upstream of it (there is really none). So given large source incident energy, whether the above can occur and how exactly (from the creation of micro flash caused by the charred material to chain reaction via negative resistance into much bigger arc flash whose plasma just explode, is this possible, this is the main question.. thanks).


It is not "large source incident energy", it is available fault current that drives the intensity of the power arc. Incident energy is the thermal power generated by the arc, scaled for factors like distance from the arc and enclosure geometry, and multiplied by the arcing time in seconds. Aside from maybe a little longer arcing time after the first or second restrike which all happens in 1 cycle (16 milliseconds) before we get to essentially stable conditions, arc power isn't changing. Incident energy increases linearly because it is arc power integrated over time (multiply by arcing time in seconds). Everything else you're talking about doesn't apply and doesn't happen because the limiting factors on arc power are available fault current and arc gap. Arc gap might be increasing as material is burned away but this is only a slight effect and all arc flash models assume arc gap is a constant.

I'm not sure what "micro flash" is. That isn't a term used in electric arc literature. Plasma does not explode. An explosion is a rapid pressure rise caused by rapid expansion of gas. Plasmas are magnetically contained so they don't do that. There is a related effect to arc flash called arc blast that was thought to be a much more significant hazard with high currents than it turns out to be. Recent experimental evidence has dispelled most of the myths surrounding it. And negative resistance does not play a role except for maybe some macroscopic view of the reduction in the CFO. CFO is really only used conceptually to rate the insulation properties of an electrical system to ensure that an arc does not start. Once it starts, CFO isn't useful anymore. We know it decreases but we don't measure this and we don't care since the equipment has already faulted.

The one thing missing from this discussion is the assumption that an AC arc is self-sustaining. At DC it should be pretty obvious that this is almost always the case. As we get close to instability even DC arcs aren't guaranteed to be self-sustaining. But with AC since it must restrike 120 times per second, this is not guaranteed. As the system voltage decreases below around 300 VAC we are getting close to the minimum possible CFO. The arc just doesn't stay lit long enough to maintain an arc. Arcs tend to be more unstable and often do not restrike. For instance at 208 VAC only a single test was successful in the IEEE testing done for the 2002 edition of 1584. All the other tests failed because the arcs extinguished in the middle of the test. And as another example the ABB HK type circuit breaker which is a 5000 V, 1200 A rated breaker has a mechanical bellows that "blows the arc out" by blowing the hot air away from the electrodes, preventing restriking. In fact it was believed that conditions were not even possible to cause a serious injury or fatality below around 250-300 VAC from an arc flash until about 10 years ago. Then in 2009 a fatality occurred and subsequent laboratory test results proved that there is an arc flash hazard at these voltages, so the stance at low voltages in the 2018 edition of IEEE 1584 has been revised to include voltages down to 208 VAC except for very low fault current (under 2,000 A). So suffice to say that although it is rare and unlikely because stable arcing conditions are also rare and unlikely, serious injuries and fatalities down to around 200 VAC at least it can happen.

So far predicting arc stability hasn't been consistent in any of the experiments so there is no scientific way to prove whether or not a stable arc exists. We upper bound it...we can conservatively estimate the CFO and thus calculate whether or not conditions might be right to allow an arc to form. We have ways to analyze stable arcs (DC and AC arc flash models). We know enough to know how to intentionally and reliably extinguish an arc (breaker and fuse technology) but we cannot predict conditions at the edges...where we are marginally stable or marginally unstable to the point where an arc can go out on its own.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Tue Feb 26, 2019 12:33 pm 

Joined: Wed Feb 20, 2019 3:06 am
Posts: 45
PaulEngr wrote:
Ommi wrote:
The first short circuit that created the semiconductive charred material was only a day earlier and the breaker was never used before and the panel was only opened during the second attempt to connect the live terminal with nothing at the load side of it. Now let's focus on the physics of how the flash occurred. When you mentioned "to the point where it flashed over". Did you mean "to the point where it arc flashed over"?


No. Definitions first. I know what you mean but if we're going to nitpick on definitions...

See figure 3 here:
http://www.neiengineering.com/wp-conten ... ations.pdf

Basically we have a bunch of phenomena going on that have various names (glow discharge, streaming, corona, sparking) where more or less current draw is very low and resistances are high. These can lead to equipment failures eventually above around 2000 VAC or so. They might look scary since there is a lot of noise associated with them (stand in a utility substation on a very humid or wet day) but they are harmless from a safety (arc flash) point of view. Then at some point we cross the CFO or critical flashover voltage/resistance, defined by IEEE as:
"The amplitude of voltage of a given waveshape that, under specified conditions, causes flashover through the surrounding medium on 50% of the voltage applications."

Flashover means that we transition into a power arc at which point arc voltage is almost constant and current increases dramatically as shown in figure 3 above. Note that this is for DC...I'll get back to the AC part below. Suffice to say that under the right conditions an electric arc forms. When it does, it ignites and extinguishes in picoseconds or nanoseconds so for power and safety purposes it is effectively instantaneous.

NFPA 70E defines arc flash HAZARD as: "A dangerous condition associated with the possible release of energy caused by an electric arc. Informational Note No. 1: An arc flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc. Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard." Although there are multiple effects that are a hazard when it comes to arcing (heat, light, ejected materials), currently only the heat component is considered. Arcing is the phenomena. Arc flash is thermal safety effect of the arc.

Quote:
My main question was supposed there was semiconductive charged material in the surface. Would the short circuit current just vaporize the charred material and then stop. Or would it initiate chain reaction meaning the charred material is enough to create a micro plasma arc that via negative resistance built up until there is a large arc flash between the two live terminals enough to damage the wall of the chassis and caused 2nd degree burn (rat size blisters) in the electrican arm? Kindly focus on this questions.


OK, focussing on the phenomena of an arc itself. Once the voltage gets high enough and in this context we're talking about such a fast event that the fact that we're dealing with AC doesn't matter...this is a pure DC discussion that it exceeds the CFO (critical flashover voltage) of the system in it's current state, an arc starts. The core of the arc is a plasma which means that it is so hot that the electrons are so energized that they just float around in a cloud around the general vicinity of the nuclei. Chemical bonds don't really exist so instead of O2, N2, CO2, etc., we just have individual nuclei and a cloud of highly energized electrons. This cloud has very little electrical resistance for obvious reasons and it is also highly magnetic. It pulls in on itself so that effectively the arc diameter is almost a constant at around 1-2 mm. This is what the physicists have figured out about electric arcs. The properties of a plasma are temperature dependent. So physicists get really hung up on calculating plasma temperature.

Energy radiated from this arc core is 99%+ absorbed by the surrounding air though. As air molecules get hot they dump photons to cool down and we perceive photons in the visible spectrum as light. Then once a photon is emitted from one atom if it strikes the next (electron), it passes the energy on at which point it is absorbed and then emitted again in a cascade. Obviously there are losses along the way as the surrounding air absorbs a lot of the energy (heats up) and some passes through to solid objects nearby. That's what you are seeing...you don't actually "see" an arc, you see it's effects and you feel the infrared photons as heat. However nearly all of the energy emitted by the arc core is absorbed and re-emitted around it. So even though the arc core where plasma exists at some crazy high temperature that only a physicist cares about (the "35,000 degrees" claim you see in the arc flash scare literature) that temperature is NEVER seen outside the arc core. If it was, it would vaporize anything exposed to it which is obviously not the case. That's not to say that arcs don't emit a lot of thermal radiation, just trying to be realistic here. If you are at the receiving end of the arc (a shock) then you are being directly exposed to the effects of having all the electrons blown off the nuclei and some skin will be vaporized and it is going to do a lot more damage, but that's not where we're going here.

As far as the arc extinguishing at this point the resistance of the air within the plasma has essentially gone to almost zero. There is nothing to stop the current from flowing with a DC arc at all. There are really only three ways to extinguish a DC arc. First, disconnect the power so that the air cools back down (CFO goes back up) so that it can't reform. Second method is to increase the arc length to the point where the arc is unsustainable. The arc consumes a certain amount of power per unit length to sustain itself. If the arc is too long there isn't enough energy available to keep it going so it goes out.

Now switching over to AC at a DC perspective (milliseconds) if either the normal AC voltage or a transient gets above the CFO (critical flashover voltage) again the arc strikes. Once it strikes the resistance within the arc itself is obviously so low that there's nothing to extinguish the arc even if the voltage drops until the CURRENT goes through a zero which it does so 120 times per second because in this case it's an AC system. So the arc naturally extinguishes. But prior to this happening the air around the arc was also heated up quite a bit. As air temperature increases, resistance decreases. So the CFO also decreases. On the next voltage rise it takes a lot less voltage to restrike the arc. HOWEVER this is all something that you can see from laboratory measurements of arc tests. In practical calculations we ignore all this because the effect only happens during the first couple restrikes. After that the CFO and thus arc restriking are pretty much constant.

As to your question about how the surface contamination evolves, this is what I was explaining earlier. Even if the arc ablates away some material (vaporizes/burns off) it is also thermally damaging the undamaged layers next to it. It is not going to "burn clean" as you are suggesting. The charred area is going to grow in both surface area and depth, even if material is actually burned away, just as a conventional (non-electrical) fire first chars and then consumes material. Under all circumstances, CFO can only decrease even if the arc does self-extinguish. The only time that CFO ever goes back up is with what is known as "renewable" insulation which is a nice way of saying that if we stop the arc, air will cool back down and electrical resistance through air is restored back to normal.

Quote:
And so many thanks for the detailed explanations above and below. But I'm interested in the exact physics of it.


I can point you to a couple books. Both are very much out of print You won't care about the subject matter so much as the introductory sections that explain what's going on. First one and you really only need the introductory chapters is:
https://www.amazon.com/High-Voltage-Cir ... b_title_bk

The part that you need to read is only chapter 1 and part of chapter 2, and fortunately Amazon has that available for free. Circuit breaker theory is all about sustainability of arcs (and breaking them) which is part of what you are asking about, along with some basic material science concepts at high voltages and temperatures. Granted you are asking about very low voltage arcs. The theory still applies for the most part.

Second is this one:
https://www.cambridge.org/core/books/el ... 022EC27926

You can read this one entirely. It is freely available from several sources. I just linked the first one. Ayrton was interested in stable, sustainable arcs for lighting purposes (neon lighting) and did a lot of the first early physics experiments with arcs. She was NOT interested in power arcs since that usually results in destroying the equipment but instead working at the borderline before they form. From this point of view since increased voltage results in a decrease in resistance (see figure 3 above again) the concept of negative resistance applies. Once we have a power arc that concept is no longer valid...voltage is slightly increasing which is zero or positive resistance.

The third leg is arc flash. That is better covered at least from a basic theoretical point of view in the first article I referenced on DC arcs. If you really want to go down the AC rabbit hole this is one of the more useful articles that gets into the nuts and bolts of arc flash. Incidentally prior to the most recent update to IEEE 1584, it beats out IEEE 1584-2002 in terms of accuracy. And Wilkins way of explaining things is pretty straight forward:
https://www.google.com/url?sa=t&rct=j&q ... 7y5ndtQFB2

Quote:
Let's assume the incident energy is big and there is really no overcurrent protection device upstream of it (there is really none). So given large source incident energy, whether the above can occur and how exactly (from the creation of micro flash caused by the charred material to chain reaction via negative resistance into much bigger arc flash whose plasma just explode, is this possible, this is the main question.. thanks).


It is not "large source incident energy", it is available fault current that drives the intensity of the power arc. Incident energy is the thermal power generated by the arc, scaled for factors like distance from the arc and enclosure geometry, and multiplied by the arcing time in seconds. Aside from maybe a little longer arcing time after the first or second restrike which all happens in 1 cycle (16 milliseconds) before we get to essentially stable conditions, arc power isn't changing. Incident energy increases linearly because it is arc power integrated over time (multiply by arcing time in seconds). Everything else you're talking about doesn't apply and doesn't happen because the limiting factors on arc power are available fault current and arc gap. Arc gap might be increasing as material is burned away but this is only a slight effect and all arc flash models assume arc gap is a constant.

I'm not sure what "micro flash" is. That isn't a term used in electric arc literature. Plasma does not explode. An explosion is a rapid pressure rise caused by rapid expansion of gas. Plasmas are magnetically contained so they don't do that. There is a related effect to arc flash called arc blast that was thought to be a much more significant hazard with high currents than it turns out to be. Recent experimental evidence has dispelled most of the myths surrounding it. And negative resistance does not play a role except for maybe some macroscopic view of the reduction in the CFO. CFO is really only used conceptually to rate the insulation properties of an electrical system to ensure that an arc does not start. Once it starts, CFO isn't useful anymore. We know it decreases but we don't measure this and we don't care since the equipment has already faulted.

The one thing missing from this discussion is the assumption that an AC arc is self-sustaining. At DC it should be pretty obvious that this is almost always the case. As we get close to instability even DC arcs aren't guaranteed to be self-sustaining. But with AC since it must restrike 120 times per second, this is not guaranteed. As the system voltage decreases below around 300 VAC we are getting close to the minimum possible CFO. The arc just doesn't stay lit long enough to maintain an arc. Arcs tend to be more unstable and often do not restrike. For instance at 208 VAC only a single test was successful in the IEEE testing done for the 2002 edition of 1584. All the other tests failed because the arcs extinguished in the middle of the test. And as another example the ABB HK type circuit breaker which is a 5000 V, 1200 A rated breaker has a mechanical bellows that "blows the arc out" by blowing the hot air away from the electrodes, preventing restriking. In fact it was believed that conditions were not even possible to cause a serious injury or fatality below around 250-300 VAC from an arc flash until about 10 years ago. Then in 2009 a fatality occurred and subsequent laboratory test results proved that there is an arc flash hazard at these voltages, so the stance at low voltages in the 2018 edition of IEEE 1584 has been revised to include voltages down to 208 VAC except for very low fault current (under 2,000 A). So suffice to say that although it is rare and unlikely because stable arcing conditions are also rare and unlikely, serious injuries and fatalities down to around 200 VAC at least it can happen.

So far predicting arc stability hasn't been consistent in any of the experiments so there is no scientific way to prove whether or not a stable arc exists. We upper bound it...we can conservatively estimate the CFO and thus calculate whether or not conditions might be right to allow an arc to form. We have ways to analyze stable arcs (DC and AC arc flash models). We know enough to know how to intentionally and reliably extinguish an arc (breaker and fuse technology) but we cannot predict conditions at the edges...where we are marginally stable or marginally unstable to the point where an arc can go out on its own.


Thanks so much for the incredible details. You must be a good instructor or professor. Hope you can write articles about arc flash like in Scientific American.

Just a quick question or clarification. When you say arc being self-extinguishing, stable arcs, number of cycles, etc. What are the mininum cycles or duration before injury can occur like causing a rat size bubble or blister in the electrician hands. Can 1 or 2 cycles caused this?

Lastly. In essence and summary. If an industrial switchgear is say turned off, and a saboteur put a thin layer of carbon between the terminals. When it is opened, can it cause an arc flash enough to create the textbook damage to the vicinity from the arc flash? Has this test ever been done? In most youtube video of arc flash. I think the experimenter just put a dead short in the panels. But has anyone done any carbon layer testing to see how it would progress?

Thanks a whole lot and appreciated zillions for the help. Everyday one dies from arc flash so we need to spread the knowledge far and wide.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Tue Feb 26, 2019 5:13 pm 

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In addition to the above clarification. I still have the breaker that allegedly suffered arc flash.

Image

Image


This is what I want to understand. It is possible for the arc to take the middle screw path then go up by curving outside enough not to damage the plastic enclosure between them then go up and hit the right terminal and melt one third of it? Can arc flash do this? Remember what happened was the electrician simply lowered the live wire to the right terminal without touching the middle and it suddenly flashed damaging the terminals and his hands. It's not dead short. Is the above consistent with arc flash jumping between the two terminals with only carbon as the triggering factor?


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Tue Feb 26, 2019 8:39 pm 

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In addition to the above. I just wanted to know how the arc flash damaged the breakers. I just think now there may be two separate arc flashes in the terminals. See:

Image

The first arc flash occured between the terminal screws where it curved outside enough not to damage the plastic enclosure between them. Then the second arc flashed occured between the right terminal and live wire (from above (not shown).. We can say the separate arc flashes occured at same time.. just separate areas. Was this possible?

Before today I was thinking there was just one big arc flash that took the path of 2nd terminal screws to the right lugs. But why does the right terminal suffered so much damage versus the middle terminal? A two stage or separate arc flash can explain them right?

Image

Which of the above scenario is more likely and matched experimental setup? I was sure the electrician didn't connect the two live wires together so it wasn't just a normal dead short.

My concern about all this is what if the panel full of dust would be opened just to turn off the breakers (in case the rooms need service). Would it just spontaneously arc flashed if the dusts happen to have a lot of carbon content. So I was thinking what is the best PPE for Arc Flash Category 2 Hazard I can buy and be ready to give to electrician.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Wed Feb 27, 2019 4:39 am 
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Ommi wrote:
Thanks so much for the incredible details. You must be a good instructor or professor. Hope you can write articles about arc flash like in Scientific American.


I'm an engineer for an electrical equipment service shop. Arc flash is something I deal with on an up front close and personal basis.

Quote:
Just a quick question or clarification. When you say arc being self-extinguishing, stable arcs, number of cycles, etc. What are the mininum cycles or duration before injury can occur like causing a rat size bubble or blister in the electrician hands. Can 1 or 2 cycles caused this?


What you describe is what Alicia Stoll's experiment was testing for and is known as the Stoll curve. It is equal to 1.2 cal/cm2 at 1 second and extends out to about 2.0 cal/cm2 at 2 seconds. This is the onset of a second degree burn. However your question is also talking about the onset of a second degree burn to a hand. IEEE 1584 uses the onset of a second degree burn to the face/chest area so the arc flash boundary would be defined at 18" away. In open air the incident energy quadruples if you half the distance. So 1.2 cal/cm2 at the face/chest would be 4.8 halfway down the arms. Let's just say that other than avoiding burns in the first place, we are never going to protect the hands.

This is one of the key things to remember about IEEE 1584 and most current safety codes. We would like to reduce the chance of a fatality down to less than 1 in a million. Other standards such as EPA environmental standards use the same threshold. So that also means we would like to reduce serious injuries down to a threshold of less than 1 in 100,000 and injuries requiring doctor's visits such as the one you describe to about 1 in 10,000 or less. IEEE 1584 does this in two ways. First and foremost it promotes turning off power whenever practically possible to do so. Second using equipment, PPE, and so forth to reduce the chance of a second degree burn to the face and chest area to such a rare or unlikely event that a trip to the burn center is minimized or eliminated (down to 1 in 100,000 or less).

Quote:
Lastly. In essence and summary. If an industrial switchgear is say turned off, and a saboteur put a thin layer of carbon between the terminals. When it is opened, can it cause an arc flash enough to create the textbook damage to the vicinity from the arc flash? Has this test ever been done? In most youtube video of arc flash. I think the experimenter just put a dead short in the panels. But has anyone done any carbon layer testing to see how it would progress?

[/quote]

The dead short is a thin piece of copper wire that vaporizes during the test that is called a "fuse" for good reason. It is intended to give a consistent and stable arc. Some of the problems with the earlier 208 V tests were that they used a #12 wire for every test but it turns out that this is too big for low voltage tests. Now they vary the wire size with low voltage tests.

Second instead of carbon they use salt water in insulation contamination tests so yes they've done this at least indirectly. If you have a contamination problem you are supposed to relate it back to brine conductivity.

Third, we have some of the largest tire plants in the world as customers in the Carolinas. The Banberry mixer areas where they make the rubber compounds are always covered in a thin layer of carbon black and they have to almost constantly clean all the equipment to keep the arcing down to an acceptable level. They might have tested it compared to the salt and brine conductivity tests but mostly they just know that everything gets totally cleaned about once a week. If they try to skip a week, they are risking flashovers at one large plant where I know most of the guys there. At the largest plant they push it a little beyond that point based on what I've observed during service calls. The 1/32" dust layer definition seems to hold true for them but this is loose dust, not burned plastic.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Wed Feb 27, 2019 4:06 pm 

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PaulEngr wrote:
Ommi wrote:
Thanks so much for the incredible details. You must be a good instructor or professor. Hope you can write articles about arc flash like in Scientific American.


I'm an engineer for an electrical equipment service shop. Arc flash is something I deal with on an up front close and personal basis.

Quote:
Just a quick question or clarification. When you say arc being self-extinguishing, stable arcs, number of cycles, etc. What are the mininum cycles or duration before injury can occur like causing a rat size bubble or blister in the electrician hands. Can 1 or 2 cycles caused this?


What you describe is what Alicia Stoll's experiment was testing for and is known as the Stoll curve. It is equal to 1.2 cal/cm2 at 1 second and extends out to about 2.0 cal/cm2 at 2 seconds. This is the onset of a second degree burn. However your question is also talking about the onset of a second degree burn to a hand. IEEE 1584 uses the onset of a second degree burn to the face/chest area so the arc flash boundary would be defined at 18" away. In open air the incident energy quadruples if you half the distance. So 1.2 cal/cm2 at the face/chest would be 4.8 halfway down the arms. Let's just say that other than avoiding burns in the first place, we are never going to protect the hands.

This is one of the key things to remember about IEEE 1584 and most current safety codes. We would like to reduce the chance of a fatality down to less than 1 in a million. Other standards such as EPA environmental standards use the same threshold. So that also means we would like to reduce serious injuries down to a threshold of less than 1 in 100,000 and injuries requiring doctor's visits such as the one you describe to about 1 in 10,000 or less. IEEE 1584 does this in two ways. First and foremost it promotes turning off power whenever practically possible to do so. Second using equipment, PPE, and so forth to reduce the chance of a second degree burn to the face and chest area to such a rare or unlikely event that a trip to the burn center is minimized or eliminated (down to 1 in 100,000 or less).

Quote:
Lastly. In essence and summary. If an industrial switchgear is say turned off, and a saboteur put a thin layer of carbon between the terminals. When it is opened, can it cause an arc flash enough to create the textbook damage to the vicinity from the arc flash? Has this test ever been done? In most youtube video of arc flash. I think the experimenter just put a dead short in the panels. But has anyone done any carbon layer testing to see how it would progress?



Quote:
The dead short is a thin piece of copper wire that vaporizes during the test that is called a "fuse" for good reason. It is intended to give a consistent and stable arc. Some of the problems with the earlier 208 V tests were that they used a #12 wire for every test but it turns out that this is too big for low voltage tests. Now they vary the wire size with low voltage tests.

Second instead of carbon they use salt water in insulation contamination tests so yes they've done this at least indirectly. If you have a contamination problem you are supposed to relate it back to brine conductivity.

Third, we have some of the largest tire plants in the world as customers in the Carolinas. The Banberry mixer areas where they make the rubber compounds are always covered in a thin layer of carbon black and they have to almost constantly clean all the equipment to keep the arcing down to an acceptable level. They might have tested it compared to the salt and brine conductivity tests but mostly they just know that everything gets totally cleaned about once a week. If they try to skip a week, they are risking flashovers at one large plant where I know most of the guys there. At the largest plant they push it a little beyond that point based on what I've observed during service calls. The 1/32" dust layer definition seems to hold true for them but this is loose dust, not burned plastic.


Many thanks. I have read all your messages many times and again and again analyzing each sense. I'd like to focus on this "The core of the arc is a plasma which means that it is so hot that the electrons are so energized that they just float around in a cloud around the general vicinity of the nuclei."
This is specifically as it relates to the following:

Image

Is it possible for there to be separate arcs? The first arc flash occured between the terminal screws where it curved outside enough not to damage the plastic enclosure between them. Then the second arc flashed occured between the right terminal and live wire (from above (not shown).. We can say the separate arc flashes occured at same time.. just separate areas. Was this possible?

Lets focus on the one between the screws. If an arc forms there like in the violet curve line. Can the arc diameter enclosed the heat within the plasma such that the plastic enclosure between them is not scorched and it is only the screws at the ends of the arc that are scorched because the arc releases the heat along the ends of the arc because it originated there? Or do you think it's odd the plastic in between it is not scorched? Based on your experiencs and knowledge. What kind of behavior of the arc where it only releases heat at the ends with the middle one sparing the immediate vicinity (plastic), or maybe the arc forms further outside at middle (more curving and farther from the plastic in between) that its heat didn't melt it?

Thanks a whole lot.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Thu Feb 28, 2019 4:35 am 
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Ommi wrote:
Many thanks. I have read all your messages many times and again and again analyzing each sense. I'd like to focus on this "The core of the arc is a plasma which means that it is so hot that the electrons are so energized that they just float around in a cloud around the general vicinity of the nuclei."
This is specifically as it relates to the following:

Image

Is it possible for there to be separate arcs? The first arc flash occured between the terminal screws where it curved outside enough not to damage the plastic enclosure between them. Then the second arc flashed occured between the right terminal and live wire (from above (not shown).. We can say the separate arc flashes occured at same time.. just separate areas. Was this possible?


Yes. Usually several arcs. Look at the high speed photos out there. With 3 phase systems especially between A and C phase often you get line-enclosure-line arcs since they are physically so far apart.

Quote:
Lets focus on the one between the screws. If an arc forms there like in the violet curve line. Can the arc diameter enclosed the heat within the plasma such that the plastic enclosure between them is not scorched and it is only the screws at the ends of the arc that are scorched because the arc releases the heat along the ends of the arc because it originated there? Or do you think it's odd the plastic in between it is not scorched? Based on your experiencs and knowledge. What kind of behavior of the arc where it only releases heat at the ends with the middle one sparing the immediate vicinity (plastic), or maybe the arc forms further outside at middle (more curving and farther from the plastic in between) that its heat didn't melt it?

Thanks a whole lot.



Pretty normal to see undamaged material right next to badly damaged material. There are some odd things that go on at the ends of the arcs and then the rest of it is pretty consistent. Arc flash models treat arcs as so short that it's a point source of radiation which isn't quite true. Either way don't forget shadows. This is radiation after all. It travels in straight lines until it is absorbed or reflected. It takes a lot to burn through material and the arc may only last tenths of a second. Not enough time really for conduction to occur to any great amount. A lot of the time the smoke and burning damage happens after an arcing fault where it ignites non fire retardant materials which continue to burn. Most electrical materials are reasonably fire retardant. They burn but do not sustain flame. And char may be conductive but it is still reasonably thermally insulating. Don't get hung up on the big luminous fireballs you see in arc flash videos. Steel starts to visibly glow at around 800-1000 F. Air is more like 3000+ F but in low pressures these temperatures go down like near a very hot arc so you get bubbles of gas. See the YouTube videos on grapes in microwaves creating bubbles of glowing air. Lots of research people see glowing stuff and instantly call it plasma. Talk to a welder. They will give you a very different perspective about heat, glowing objects, and plasma since they work with it all the time.

Conduction heat transfer is proportional to the temperature difference and thermal resistance. Radiation is proportional to emissivity and the fourth power of the temperature difference. So it should be obvious why we don't see much of any conduction. Conduction is when you grab something hot and it burns you. Radiation is why a camp fire feels warm even when everything around you is cold. Arc damage other than after effects is mostly radiation except when it's a stable controlled arc like an arc furnace.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Thu Feb 28, 2019 5:21 am 

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PaulEngr wrote:
Ommi wrote:
Many thanks. I have read all your messages many times and again and again analyzing each sense. I'd like to focus on this "The core of the arc is a plasma which means that it is so hot that the electrons are so energized that they just float around in a cloud around the general vicinity of the nuclei."
This is specifically as it relates to the following:

Image

Is it possible for there to be separate arcs? The first arc flash occured between the terminal screws where it curved outside enough not to damage the plastic enclosure between them. Then the second arc flashed occured between the right terminal and live wire (from above (not shown).. We can say the separate arc flashes occured at same time.. just separate areas. Was this possible?


Yes. Usually several arcs. Look at the high speed photos out there. With 3 phase systems especially between A and C phase often you get line-enclosure-line arcs since they are physically so far apart.

Quote:
Lets focus on the one between the screws. If an arc forms there like in the violet curve line. Can the arc diameter enclosed the heat within the plasma such that the plastic enclosure between them is not scorched and it is only the screws at the ends of the arc that are scorched because the arc releases the heat along the ends of the arc because it originated there? Or do you think it's odd the plastic in between it is not scorched? Based on your experiencs and knowledge. What kind of behavior of the arc where it only releases heat at the ends with the middle one sparing the immediate vicinity (plastic), or maybe the arc forms further outside at middle (more curving and farther from the plastic in between) that its heat didn't melt it?

Thanks a whole lot.



Pretty normal to see undamaged material right next to badly damaged material. There are some odd things that go on at the ends of the arcs and then the rest of it is pretty consistent. Arc flash models treat arcs as so short that it's a point source of radiation which isn't quite true. Either way don't forget shadows. This is radiation after all. It travels in straight lines until it is absorbed or reflected. It takes a lot to burn through material and the arc may only last tenths of a second. Not enough time really for conduction to occur to any great amount. A lot of the time the smoke and burning damage happens after an arcing fault where it ignites non fire retardant materials which continue to burn. Most electrical materials are reasonably fire retardant. They burn but do not sustain flame. And char may be conductive but it is still reasonably thermally insulating. Don't get hung up on the big luminous fireballs you see in arc flash videos. Steel starts to visibly glow at around 800-1000 F. Air is more like 3000+ F but in low pressures these temperatures go down like near a very hot arc so you get bubbles of gas. See the YouTube videos on grapes in microwaves creating bubbles of glowing air. Lots of research people see glowing stuff and instantly call it plasma. Talk to a welder. They will give you a very different perspective about heat, glowing objects, and plasma since they work with it all the time.

Conduction heat transfer is proportional to the temperature difference and thermal resistance. Radiation is proportional to emissivity and the fourth power of the temperature difference. So it should be obvious why we don't see much of any conduction. Conduction is when you grab something hot and it burns you. Radiation is why a camp fire feels warm even when everything around you is cold. Arc damage other than after effects is mostly radiation except when it's a stable controlled arc like an arc furnace.



Ok. Thanks. About this "What you describe is what Alicia Stoll's experiment was testing for and is known as the Stoll curve. It is equal to 1.2 cal/cm2 at 1 second and extends out to about 2.0 cal/cm2 at 2 seconds."

What would happen if the 1.2 cal/cm2 lasted only a few cycles. Can it cause 2nd degree burn if it's only 10 cycles? Also if there is an overcurrent protective device or breaker upstream of it. How many equivalent cycles do breakers trip? (10 cycles or so for example?) In the accident above. There is no breakers upstream of it. If there is. Then it can prevent this 1.2 cal/cm2 at 1 second damage? Or if the damage can occur for 5 cycles at 1.2 cal/cm2 then the breakers upstream won't help very much. isn't it?


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Thu Feb 28, 2019 4:35 pm 
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Ommi wrote:
PaulEngr wrote:
Ommi wrote:
Many thanks. I have read all your messages many times and again and again analyzing each sense. I'd like to focus on this "The core of the arc is a plasma which means that it is so hot that the electrons are so energized that they just float around in a cloud around the general vicinity of the nuclei."
This is specifically as it relates to the following:

Image

Is it possible for there to be separate arcs? The first arc flash occured between the terminal screws where it curved outside enough not to damage the plastic enclosure between them. Then the second arc flashed occured between the right terminal and live wire (from above (not shown).. We can say the separate arc flashes occured at same time.. just separate areas. Was this possible?


Yes. Usually several arcs. Look at the high speed photos out there. With 3 phase systems especially between A and C phase often you get line-enclosure-line arcs since they are physically so far apart.

Quote:
Lets focus on the one between the screws. If an arc forms there like in the violet curve line. Can the arc diameter enclosed the heat within the plasma such that the plastic enclosure between them is not scorched and it is only the screws at the ends of the arc that are scorched because the arc releases the heat along the ends of the arc because it originated there? Or do you think it's odd the plastic in between it is not scorched? Based on your experiencs and knowledge. What kind of behavior of the arc where it only releases heat at the ends with the middle one sparing the immediate vicinity (plastic), or maybe the arc forms further outside at middle (more curving and farther from the plastic in between) that its heat didn't melt it?

Thanks a whole lot.



Pretty normal to see undamaged material right next to badly damaged material. There are some odd things that go on at the ends of the arcs and then the rest of it is pretty consistent. Arc flash models treat arcs as so short that it's a point source of radiation which isn't quite true. Either way don't forget shadows. This is radiation after all. It travels in straight lines until it is absorbed or reflected. It takes a lot to burn through material and the arc may only last tenths of a second. Not enough time really for conduction to occur to any great amount. A lot of the time the smoke and burning damage happens after an arcing fault where it ignites non fire retardant materials which continue to burn. Most electrical materials are reasonably fire retardant. They burn but do not sustain flame. And char may be conductive but it is still reasonably thermally insulating. Don't get hung up on the big luminous fireballs you see in arc flash videos. Steel starts to visibly glow at around 800-1000 F. Air is more like 3000+ F but in low pressures these temperatures go down like near a very hot arc so you get bubbles of gas. See the YouTube videos on grapes in microwaves creating bubbles of glowing air. Lots of research people see glowing stuff and instantly call it plasma. Talk to a welder. They will give you a very different perspective about heat, glowing objects, and plasma since they work with it all the time.

Conduction heat transfer is proportional to the temperature difference and thermal resistance. Radiation is proportional to emissivity and the fourth power of the temperature difference. So it should be obvious why we don't see much of any conduction. Conduction is when you grab something hot and it burns you. Radiation is why a camp fire feels warm even when everything around you is cold. Arc damage other than after effects is mostly radiation except when it's a stable controlled arc like an arc furnace.



Ok. Thanks. About this "What you describe is what Alicia Stoll's experiment was testing for and is known as the Stoll curve. It is equal to 1.2 cal/cm2 at 1 second and extends out to about 2.0 cal/cm2 at 2 seconds."

What would happen if the 1.2 cal/cm2 lasted only a few cycles. Can it cause 2nd degree burn if it's only 10 cycles? Also if there is an overcurrent protective device or breaker upstream of it. How many equivalent cycles do breakers trip? (10 cycles or so for example?) In the accident above. There is no breakers upstream of it. If there is. Then it can prevent this 1.2 cal/cm2 at 1 second damage? Or if the damage can occur for 5 cycles at 1.2 cal/cm2 then the breakers upstream won't help very much. isn't it?


There are academic debates about the right “limit” but 70E uses a flat 1.2 cal/cm2 irrespective of time. That means the heat flux (power per unit area) multiplied by time so it becomes heat energy per unit area exceeds 1.2 cal/cm2. Since we already factored in time, distance, gap, and voltage, how we got there does not matter. If the arc heat flux is twice as much it takes half as long to get to 1.2 cal/cm2. If the effect is burns to the hands and arms only then obviously you didn’t get to 1.2 cal/cm2 at the face/chest. So this would be an unfortunate but acceptable injury under 70E standards. It is a survivable injury. Since all standards treat arc energy as a point source and heat flux goes exponential towards zero distance, no existing standard addresses exposure for the hands and arms.


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 Post subject: Re: Quantity of carbonized particles before Arc Flash Initia
PostPosted: Thu Feb 28, 2019 7:15 pm 

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Quote:
There are academic debates about the right “limit” but 70E uses a flat 1.2 cal/cm2 irrespective of time. That means the heat flux (power per unit area) multiplied by time so it becomes heat energy per unit area exceeds 1.2 cal/cm2. Since we already factored in time, distance, gap, and voltage, how we got there does not matter. If the arc heat flux is twice as much it takes half as long to get to 1.2 cal/cm2. If the effect is burns to the hands and arms only then obviously you didn’t get to 1.2 cal/cm2 at the face/chest. So this would be an unfortunate but acceptable injury under 70E standards. It is a survivable injury. Since all standards treat arc energy as a point source and heat flux goes exponential towards zero distance, no existing standard addresses exposure for the hands and arms.


You are saying if the incident energy is large, main breaker (or breakers upstream of it) won't even help. But if the incident energy is small. Breakers upstream would help a lot, right?
In actual arc flash accidents. How much did the breakers upstream help in decreasing the incident energy?


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