OK, now I understand where you are coming from.

If we can't prevent an arc flash then we really have three choices:
1. Increase working distance.
2. Decrease the current (voltage has little effect).
3. Decrease arcing time.

For #1 there is only so much you can do from a practical point of view.
For #2 current limiting breakers and fuses HELP as well as increasing transformer impedance somewhat but usually the amount of current limiting that you get in reality isn't very much. One of the big problems with this approach is that while you are reducing the current which reduces incident energy, the reduced current also causes protective device operating time to increase which offsets the reduction in incident energy.
For #3, trip faster. This is usually the least expensive solution with the largest reduction. Limitations on this solution are that we are limited on how low we can set trip settings before we either run into a problem with downstream device coordination, or we run into limitations on the minimum current needed to operate the equipment. For example using your scenario if I try to control arc flash by reducing the size of the fuses or breaker on the primary side of the transformer, I will usually run into a problem where the transformer magnetizing inrush prevents me from providing much of any protection on the secondary side of the transformer. And when it comes to the first breaker/fuses on the secondary side, incident energy is usually very high because of this. On larger switchgear type setups I can often put a set of CT's directly on the transformer bushings and then use an overcurrent relay to shunt trip an upstream primary side breaker, or else use trip logic in the breaker to use a high curve during breaker closing then immediately switch to a lower curve once the transformer inrush passes. But with low voltage scenarios like yours 99% of the time the best, least expensive solution is to insert a fused disconnect or even just a junction box with low peak fuses in it between the transformer and the distribution panel. Arc flash at this panel is high and you normally shut the transformer off from the primary side so there is really no reason to access this panel in the first place. For that reason usually I just use a fuse block in a panel to discourage operating a disconnect.

There are also exotic devices. There are arc flash relays that detect the light flash from an arc and trip much faster than overcurrent protection (and have no limitations on current) which are relatively inexpensive. But from years of experience I've seen these demonstrated but I have never installed one.

Why did you mention fuses and not breakers?

Originally there was supposed to be this main breaker to be installed (upstream of the arc flashed breaker/disconnect and 3 other disconnects downstream of the main breaker). The main breaker is supposed to be a GE General Electric TQD32225 225A Circuit Breaker 3 Pole 10 kA @ 240 VAC 225 Amp. However the contractor didn't install it to save money because he argued the budget was used up elsewhere in the building. What would be wrong if I'd go ahead and and use this breaker instead as main breaker? The contractor is not used to install fuses. Is 10kA interrupting current not enough to reduce the incident energy? What values must it be before it can significantly reduce it? Won't this breaker significantly reduce the incident energy? Many thanks.

There is not enough information. You need to know the short circuit current fairly accurately (IEC method as opposed to ANSI). If you have the short circuit current everything else is fairly easily calculated.

However the ANSI part is what you need to watch out for. There are a lot of ways of calculating short circuit current and today there are two reasons for doing it. The first (and original) is because the magnetic force on an electrical component during a fault is proportional to the square of the current. A 10% increase in current increases the magnetic force by about 25% for instance. If the short circuit current exceeds the rating of a component, it can fly apart. The breaker you mentioned for instance has an AIC of 10 kA. With that in mind we don't care if we miss the short circuit current to the high side when doing these calculations. For instance knowing just transformer impedance, size (kVA), and secondary voltage I can calculate short circuit by assuming that the bus on the primary side of the transformer is so big that the transformer can draw an infinite amount of current and that the impedance of all of the bus and cables on the load side is zero. Now that NEC requires short circuit posted on equipment as of 2017, this is how most electricians are doing it. Unless there is a very large motor as one of the loads this number is reasonable. Going further we can use a set of tables and estimates for large motors for instance to calculate a much more accurate answer on a notepad in a few minutes. This is the ANSI (simplified) method that was developed back before calculators were widely available. There is a similar simplified IEC method as well. This calculation has been around longer than most electrical equipment and that's how we've done it for years. The calculated short circuit is also called "bolted fault" current because you can literally bolt the ends of all 3 phases together and energize the system to test it.

Arc flash calculations are relatively new. The first ones were only available starting around the 1990's. The arc has resistance (not dead shorted) so the arcing current is less than the bolted fault short circuit current, usually about 65-85% of the bolted fault current. The calculation even starts by using the short circuit current. HOWEVER there is a catch. If for instance we cut the short circuit current in half, the obvious assumption is that arc flash should be cut in half (Arc power cut in half). This is close to accurate as far as the arc thermal power goes. BUT the breaker timing is also increased but since breakers operate on an inverse time curve most of the time, the increase in opening time more than doubles. So the resulting incident energy often actually goes up rather than down. This is important because when it comes to calculating short circuit current, "napkin math" doesn't work. An estimate that works fine for checking AIC on breakers is utterly useless for calculating incident energy because the number that we calculate will be too low. So we need to do a full calculation without simple short cuts like the one I previously described.

So for all these reasons I can confidently say that inserting a breaker as you are suggesting will probably decrease incident energy but I can't tell you how much it will decrease by, nor can I tell you what the incident energy is based on the information you have. Incident energy is not something that a manufacturer can simply add to the tag on the equipment. To calculate it we have to know realistically what the utility's short circuit current is. Then we need to know the sizes of the transformers, cables, and buswork leading up to the location in question along with the sizes of all of the motors that are 25 HP and larger in the plant. Since breakers and fuses may not be coordinated (upstream device trips before downstream) we also need full data on all the fuses and circuit breakers. This is the part where arc flash safety gets very costly simply because of the man hours involved in collecting and entering data to do the calculation. The calculations themselves are not very hard. The above reference to the simplified method gives you a taste of the calculations. The full most recent IEEE 1584 (2018) model is to date the most complicated model out there but it only took m about 2 hours to put all the calculations into a spreadsheet for modelling purposes for 600 V or less. Doing impedance calculations for short circuit purposes is not hard either. But there is a LOT of data both from the name plates and from the reference data (trip curves, cable impedance, conduit magnetic impedance, etc.). Realistically you can do it by hand for maybe a couple locations with a spreadsheet but it becomes prohibitively time consuming for a whole plant. There is power system analysis software that incorporates all the calculations and libraries of data that cuts the time down to reasonable for a plant-wide study that costs a few thousand for a license. But it also requires training (several days) to know how to use the software. And training for the calculations themselves is also several days. The guy that hosts and runs this web site, Jim Phillips, is one of the best in regards to the knowledge part of how to do it.

So the short answer is that I can't answer your question. We can talk about theory here and roughly what happens and why but when it comes to the answer you really want...what does it require to knock the incident energy down to 1.2 cal/cm2, that's a question that can't be answered on this forum. If you know how to do power system analysis which is typically a senior level engineering class or you take Jim's week long course on doing it, doing arc flash calculations isn't hard but you'll also know that doing them by hand even with spreadsheet software isn't very practical either.

Thanks for the info.

Well. The Poco told me the voltage of the primary is 34.5kV for the 3-phase open delta composed of two 75kVA transformers. I'm computing for the exact short circuit current
from source. Are you supposed to use the primary or secondary voltage in the calculation assuming 2% impedance? For the 34.5kV primary, should the short circuit current be:

75,000/34,500V/ 0.02 = 108.69A

or is it

75,000/240v/0.02 = 15,625A?

If it's the latter, then the main breaker with 10k interrupting current not enough?

the open delta schematic is:



What arc flashed was when the 208v high leg was lowered to the right terminal of the breaker. Beside it was the 120v wire. Can this increase the voltage to 328v?

That is the infinite bus assumption. So for breaker sizing purposes if you do not consider any other impedance, yes the breaker is too small. This calculation is not valid for arc flash purposes. It will calculate too small of a value.



No. This is basically a version of a high leg delta. The voltage from corner to corner is 240 V. High leg to neutral is 208 V. So a short from the high leg to a 120 V "hit" is 240 V and to ground is 208 V. To get additive voltages you'd have to use a buck-boost arrangement which is essentially using a second small transformer as an autotransformer arrangement on the with the same phases.

Also the primary (utility) fuses though very much inadequate are what is protecting your low side. The contractor should lose his license and the utility should have never passed inspection. If the fuses don't trip which they probably won't we use 2 seconds maximum arcing time. Knowing that the injury was nothing like the calculations would indicate though I'm suspecting the arc self extinguished.

Thanks for all the details. I have read your messages from the beginning over and over again. I also read a lot of references and have understood already how the carbon
surfaces can cause flashover (or avalanche of electrons that starts the flash over process itself). Now I'd like to analyze the exact flashover sequence of the following:






When the live wire was lowered to the right terminal. There was no dead bolt between the live middle terminal/wire and right live wire. Was it possible that when the live wire
was lowered and it contacted the terminal. The flashover occurred between the screws in front enough to conduct current that can create the separate arc flash on top of
the right terminal? This seems to be one of few ways to describe the sequence of events for the damages.

Let's assume before the live wire was lowered, the surfaces of the breakers had semiconductive charred material as result of the first short between live wire and chassis.
Cardboard was put all over it to prevent another one like it. Then all our eyes were focused at the live wire as it was lowered to the right terminal. I (and the electrician) was certain
there was no strands that contacted the middle terminal. You can see in the picture the top of the middle terminal was not damaged. The flash over seemed to be between the
screws (to cause conduction between the two terminals). My question is. Can arc current flow from middle screw to right screw enough for the short between top live wire
and right terminal top to create bigger arc flash than the one between the screws? What do you think? Let's focus on the arc flash dynamics of the damage. Thank you!

Once the conductive path is formed during the first half cycle (up to the first current zero crossing) a lot of heat envelopes the area. Typically in a 3 phase system within the first 1 to 2 full cycles you will get a full 3 phase arcing fault where all 3 terminals arc between each other. During this time arcs typically extinguish and establish themselves in new locations other than the original ones. Arcs can even move significantly far away from the original arcing location, even hundreds of feet, although in this case that didn't happen. Looking at the forensic evidence (melted spots) after the fact tells you where the arc finally settled and burned but tells you nothing about how it initially formed. Sometimes the initial formation is obvious, such as when you can see an obvious phase-to-ground arcing path and all the rest are clearly phase-to-phase. But often you can't really tell anything about how it first started.

Second a longer arc gap actually creates a longer arc and thus more heat, not less. The current is going to be limited by the power circuit feeding it, not the path the arc takes. But as a general rule once the air heats up everything in the area is conductive so it's just a matter of what path is the path of least resistance establishing the relatively stable arc path along with convection of the air among other things (long arcs tend to arch up and lift off the terminals from air flow) but for arc flash purposes you get about the same results regardless of the path.

So it's possible the arcs formed between the middle screw going slant to top of the right terminal as heat enveloped the whole area? Have you seen it elsewhere?
Can't you guess the arc formations from the image above?

Anyway I plan to buy a PPE suit with protection up to 12cal/cm2 for the above office building only (loads is only typical office loads):



https://www.amazon.com/gp/product/B013J ... X0DER&th=1

Specs:
"Meets and exceeds NFPA 70E PPE Category 2 standards and has an ATPV rating of 12 cal/cm2.
Arc flash suit set comes equipped with a True Color Grey hard cap and shield, balaclava, coveralls, and storage bag. The balaclava and coveralls
are made from an inherently flame resistant fabric. Color Navy Blue "


But you mentioned the arms can't protect the molten vapor from the cooper/lugs? Then what use is the coveralls? And what can protect the arms? A metal shield
of some kind? Thanks.

While some PPE may be a good idea, the root cause of this accident appears to the decision to work live on damaged equipment. What was the justification for performing this work live? Are the work rules adequate, and were they followed?

Everyone seems to be assuming the arc went line to line initially, jumping quite a distance. Don't forget the breaker is mounted on grounded metal. I suspect damaged insulation allowed the breaker lug to move toward ground and the arc was initiated by contact. The second lug became involved eventually also.

If you follow 70E as written, you will survive. It does not guarantee no injury, only to minimize injury to the face and chest area. You can survive with no legs or arms. But the chance of survival goes down quickly with severe burns to the face or chest area and that's what 70E is designed to target in terms of PPE. So let's talk about what the PPE is and what it does and what it doesn't do. First, it has an ATPV rating. This means that you will not receive more than a second degree burn in the chest area. That's what the ATPV rating represents. It is a thermal rating. You could achieve the exact same thing with a set of winter insulated and non-arc rated coveralls up until the point where the coveralls melt or ignite. The second key component is that the PPE is completely fire retardant. So for instance if a piece of slag (molten debris) hits the coveralls it will burn a hole right through them and burn the skin beneath. BUT if the coveralls were not fire retardant, they can melt or ignite from the piece of slag subjecting the entire body to fire and causing a much more severe burn including in the key face/chest area. So the PPE performs as required under any circumstance. Victims of arc flash will survive, but won't necessarily go injury free.

So it's not that the coveralls aren't performing as expected, just that there is a false belief that arc flash PPE is designed to eliminate injury when it face it is never intended to do so. And in your particular case based on the description of the injury...first and second degree burns to the hands and arms, arc flash PPE is not and will not prevent that. It might help but it is not going to prevent it.

As far as how to stop it entirely, that's an entirely different matter. I think if you look to industries that deal directly with thermal processes you will find your answer. Welders switch from simple FR overalls to heavy chromated leather PPE when they are welding in an overhead position where slag and molten metal drop down on them or where there is a danger of splash and flying slag. Iron and steel workers use aluminized fire suits when they are doing tasks such as clearing a tap hole in a ladle where the molten metal is under pressure and has a tendency to spray them while performing the task. I can give these two examples from personal experience with both situations.

Moreover going back to my original point....the problem here wasn't even the fact that an injury occurred. The injury could have been completely avoided. As a counter example I have a customer that had a 3000 A draw out type circuit breaker that fused itself to the bus due to some other (unknown) issues. So we needed to break the fingers of the draw out mechanism loose to remove the breaker and repair it. It is possible to reach under the breaker with rubber gloves and sleeves on with a screwdriver and pry the fingers loose. So if you have no idea what I'm talking about basically you would end up with your body wrapped around a 3000 A breaker forcing spring loaded clips loose with a screwdriver in an area that you can't even see. Danger is hardly the word for it...stupid is more like it. To make this worse the breaker was at a large relatively high security prison and killing power to this particular breaker also means killing power to the entire security system. Add to this the fact that it's a government agency so difficult to get something like this done. And we had to coordinate with the utility to cut power at the pole. All that being said, we did exactly that. And it all worked. And we got it all fixed. And everyone was happy with the result. And no chance of anyone eating an arc flash at all.

I googled "chromated leather PPE". I couldn't find the exact hits. What are other words for these? Any pictures of them? It's good I could still cancel the previous order at amazon. So I must look for PPE that
can prevent entry of any molten slags. The panel opening vertical is less than 1 feet only so what must be protected is the arm and chest area. About aluminized fire suits. Couldn't
molten slugs enter it? How come arc flash PPE suits don't have equivalent molten slags resistant rating too ?


Btw. I realized you were right. The initial short or arc flash was between the right terminal and the chassis screw strip holder. Then it enveloped the area with so much heat the arc jumped
to the middle terminal. That's it. This is the only explanation that can explain the damages. Thank you!

You are correct above! Thank you.

Looking at the old photos taken in 2015. I found the smoking gun.



The above photo was after the cardboard was removed and the scorched chassis painted. But there was a hole at the back of chassis where the breaker metal strip was attached to the chassis. You can see scorched marks in the metal strip holding the breakers. So what happened obviously was the short and arc flash was between the 208v high leg to the ground (because the chassis was grounded). Then when the area was enveloped with so much heat. There was a phase to phase arc or another arc from screw of third terminal to the second terminal explaining the screws scorched marks.

I don't want to focus how third live wire was shorted to the chassis. The trigger could be strands or carbon or whatever (in case the electrician would read this in future, you could have just shorted it with a strand since it was half an inch to the chassis metal strip). But the short (or whatever) triggered arc flash between phase to ground that created another minor phase to phase arc flash too.

The following picture showed the top of the breakers in closeup showing the damage extended to the chassis strip holding the breaker:



I need to know something. I heard that in US 120/240v residential ac power, it's arc flash category 0. Meaning no arc flash risk. Does it mean when even if there was accidental contact between phase and chassis like the above. You don't have arc flash that sends molten snag to the arms or wall of chassis. Or does it also occur? If so, then why do you refer to your 120/240v as Arc Flash Category 0? If there is even 1.2cal/cm2 incident energy at the arms length. It's already Arc Flash Category 1. What do you think? This is the last thing I wanna know before I move on from all this. Thank you.

PhilEngr. Do you have any videos how a 120v to chassis short look like. I mean. If in experimental lab, the 120v live wire would be continuously contacted to the chassis. Would
it be like a welding spot where you can see continuously short but no arc flash found that can injure a hand?

I'd like to see videos how a short without arc flash and a short with arc flash look like for 120v US residential house. This would demonstrate how 120v can't sustain
arc flash and whether it means no arc flash would form at all. I couldn't find a video of short circuit done on purpose continuously as experiment. I guess the
208v high leg shorted to ground can produce a arc flash that 120v to chassis in US residential house can't? Thank you.

You are going down the wrong road here. First off, we have OSHA 1910.333(a)(1): "'Deenergized parts.' Live parts to which an employee may be exposed shall be deenergized before the employee works on or near them, unless the employer can demonstrate that deenergizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations. Live parts that operate at less than 50 volts to ground need not be deenergized if there will be no increased exposure to electrical burns or to explosion due to electric arcs." As far as how OSHA views infeasability, they've explained that as well:
https://www.osha.gov/laws-regs/standard ... 2006-12-19
"...to qualify for the exception found in Note 2 of §1910.333(a)(1), the employer must, on a case-by-case basis, determine if the orderly shutdown of the related equipment (including the panel) and processes would introduce additional or increased hazards. If so, then the employer may perform the work using the electrical safe work practices found in §§1910.331-1910.335, including, but not limited to, insulated tools, shields, barrier, and personal protective equipment. If the orderly shutdown of the related equipment and processes would not introduce additional or increased hazards, but merely alter or interrupt production, then the de-energization of the equipment would be considered feasible, and the exception found in Note 2 of §1910.333(a)(1) would not apply."

You can call the utility and have the transformer nnd thus the top of the breaker de-energized. It's a pain in the rear because of the coordination involved but OSHA doesn't care about that. I've managed to get it done with even the most bureacratic utilities in the country. They are only interested in whether it is physically possible to de-energize or whether doing so creates additional or increased hazards. You haven't met that standard so there is no reason to do energized work in the first place. In Canada substantially similar wording is used in CSA Z462. In Europe they mandate ALARA (As Low As Reasonably Achieveable) which is the exact same concept.

So answering the questions:

1. You need to protect the whole body (head-to-toe). You could theoretically pick and choose parts to protect but here's the whole problem with the direction you are trying to take. We have established peer reviewed safety standards (70E) for electrical hazards. If something happens in my shop and I'm following 70E, then when OSHA or the victim's lawyers start knocking on my door, I can shut them down with the fact that I'm following a consensus safety standard. BUT if I do my own thing and I am faced with people knocking on my door, I'm faced with the problem of proving the science behind what I'm doing as well as whether or not I did it. Arc flash is not cut-and-dry simple science. There are lots of legitimate, published scientific arguments poking holes in 70E that apply equally to any other approach. At this point the strongest argument for the approach supported by 70E (and IEEE 1584) is that both have withstood the test of time. In other words the major argument for doing it the 70E/1584 way is because everyone is doing it that way. If I was hired by a lawyer, I'd use every one of the anti-1584 and anti-70E arguments against you and you don't stand a chance. Your ideas do not have the credibility of being a consensus safety standard, nor have withstood the test of time. So you don't stand a chance in court.
2. Leather welder's jackets are so common you can find them at the local welding supply store, Tractor Supply, Northern Tool, etc. You are making this too hard. The tanning process makes them fire retardant. It used to be a chrome solution but since certain forms of chrome are highly toxic they don't use chrome anymore but old school welders call it that. I'm just old school.
3. Slag can enter aluminized fire suits as well as welding leathers. The various pieces are overlapping. The surface properties of the liquid metal cause it to just bead up and roll off harmlessly with most metals. The few that actually splatter and stick also tend to melt at a much lower temperature such as lead. This is not a perfect solution. Have you ever looked at a welder's arms? It happens if you work in the metal business even if you aren't working in a foundry.
4. Arc flash PPE doesn't have a slag rating or a "plasma rating" because as of right now the standards do not address this issue. The whole reason that arc flash as a concept took off is because back in the 1990's we started to develop ways to measure it and to calculate the hazard. Prior to that arc flash was a known hazard but it was one of those rare situations that since we lacked the scientific understanding, there were few standards or work practices addressing it. Things like stand to one side and turn your head when closing a breaker. It is currently based around protection from the thermal effects only, other than the tinting of the face shields and goggles also protects against the light. Oberon at one time made a "ballistic" arc flash PPE but there was very little interest in it. Remember...this is a rare event. We don't wear cut resistant gloves while doing office work even though paper cuts are a very common injury because the injury severity is so low. The focus today is on eliminating exposure first and foremost and then using PPE only when we have no other choice. The PPE isn't perfect but it's better than nothing. You will walk away. That's all it promises.

Have there been tests of the maximum velocity of these slags? Are they always in liquid metal state? At least it's not like bullets that can pass through metal. I was imaging them as like bullet fragments. But if they are liquid metal, then they are lighter and have less penetrating powers.

These are separate from the vapors, isn't it? Usually vapor only expand for Arc Flash categority 3 or higher?

PPE is just mostly cotton coverall. You have mentioned "arc shield". is there small version of this that one can hold and put against the opening of the panel? Theoretically in the industry. What materials are as strong as iron but not conductive and don't melt like plastic. Something that is stronger than leather. Perhaps some kind of fiberglass or kelvar? i just need something to insert in the panel like hand shield for purposes of turning on and off and breakers only for services in the units or room they power. Not for live work. It should be insulator and not a conductor. Hence iron shield and titanium doesn't count. Note this will be used in addition to the normal PPE outfit with long sleeved cotton and head shield, etc. The panel
was so full of dusts so just taking precautions in case there would be spontaneous arc flash.

Also in my last message I was asking about the difference between short circuit with and without arc flash. I couldn't find video in youtube where there is continuous short between 120v wire and the chassis. How does it look like? Would the appearance be like localized welding or does even this simple short can send slug to the hands? Then why is it called Arc Flash Categority 0 (for US residental) when the arms can be injured too. Or this doesn't occur? This is also to tell if what happened in the panel
was just a simple short without any involvement of arc flash. That's why I need to see video what's it like to short circuit 120v to panel in US home setup and doing it continuously. But couldn't find even one video like this. So kindly share one if you have seen one. Thank you.

The above is very important points.. where 70E ATPV rating means that you will not receive more
than a second degree burn in the chest area. The arms can still be injured. I have seen it with my
own eyes. The injured electrician was even running after me. Therefore lessons learn is do not just
use 70E required PPE but use *additional* PPE with slag rating if not plasma rating. Hence I have to
create arm shield made of material as strong as metal but not conductive and don't melt like plastic.
What is it? Something better than leather for higher velocity liquid slags. So if you guys can think of
one. I need to fabricate extra protection in addition to the full face and head shield with PPE coverall
and other standards. Remember the fabricated shield would not replace them but complement them
to avoid arms injury.

(btw.. if one shares any pictures, the format of the paragraph would become longer than the screen
in the whole page.. so hope this defect can be fixed. I have to press 'enter' every line above to make
it fit the screen).

Don't worry about categories unless you are using the table method for determining PPE in 70E. The key metric is incident energy for electrical calculations or ATPV for PPE ratings.

A piece of textile 12 oz/yard thickness cannot get above ATPV 12. Above that point requires a multilayer approach. PPE Level 3 is 25 cal/cm2 so it has to be multilayer. That's the only difference. Nothing to do with the thermal energy that it is exposed to.



Not true. It's just that treated cotton is about 1/3 of the cost of the next best option so that's mostly what you see.

Kevlar is a brand name of an aramid fiber. Others are Spectra and Nomex. It is optimized for strength and often used for the thread in the cotton PPE (no seam busting). Nomex is a relatively well known brand optimized for flame resistance and that's the main competitor to treated cotton. In recent years there have been some others. For instance Glenguard is a patented modacrylic blend that holds color (hi viz and arc resistant is possible) as well as more chemical resistant.



Arc flash blankets but this is a totally different purpose. These are for containing an arc flash enough to be used in conjunction with 8 cal/cm2 PPE, balaclava and face shield. These are intended for use when escape is impossible (2 second rule doesn't apply) such as down a manhole in a vault. This does not apply to your case.



70E does not apply if you do not maintain the equipment properly. In a coal prep plant for instance we routinely have to go in and clean everything top to bottom annually.

But on top of that, read the definition of an arc flash hazard carefully. Arc flash protection is not an ABSOLUTE standard. Spontaneous flash overs do not require PPE which means normal operation of the equipment is not included such as switching tasks. Just walking by isn't included either. As an example did you know that if you are struck by a meteor, you are pretty much dead on the spot? But since this is such a rare event we do not erect meteor shields over parking lots to protect employees as they walk to the buildings. Similarly electrical equipment is generally safe for normal use. It's only when there is something wrong with it or we go to do a task which is inherently dangerous like attempting to land a wire live that we get into a situation where PPE becomes necessary.



That's because short circuits are boring. You will see a jerk of the conductors as they get hit by the high magnetic forces but that's it. Cooper Bussmann's fuseology handbook (free PDF's on their web site) has several videos showing the effects of overduty (exceeding AIC rating) and short circuits. I like using the one with about a 50 foot long cable laying loosely on the floor hit with a very high short current as it goes whipping across the floor.



Uhh, it jerks in a fraction of a second then you usually hear transformer buzzing. That's about it.



If you expose some popular synthetic fibers to even the heat of an incandescent light bulb, they will melt onto your skin. We all learned that in the 1970's disco days when this type of material first became popular. Or at least those of us over 40 did. The heat source (light bulb, cigarette lighter, or minor electrical arcing or sparking) itself is relatively harmless...maybe a 1st degree burn, that's it. BUT when the synthetic fibers melt into your skin you will receive a much more severe burn. Natural fibers char instead of melting. Also PPE is designed to keep your skin from exceeding the Stoll curve (at the face/chest area) which is generally considered around 1.2 cal/cm2. The underlayers will see that amount of heat. So what we want to do is to avoid the "work-dry" shirts, hi viz shirts, polypropylene long underwear, and other clothing underneath the PPE that can melt into the skin and do a lot more damage even if we are dressed in the correct PPE.

As an extreme example Mattisse who is a coworker and all around nice guy I worked with in Philadelphia was wearing a full aluminized fire suit including the gloves. ALSO this was against company rules about meltable materials but since the iron melting area was very dirty, Mattisse wore nitrile gloves underneath the aluminized gloves. One day unknown to him there was a leak in an oxygen line which blew pure oxygen down inside his aluminized fire glove. This lowered the flash point of the nitrile glove until it caught fire essentially at room temperature severely burning his hand inside the aluminized glove. Similar incidents have happened where sleeves catch fire and travel up inside PPE (not wearing head-to-toe) when guys do things like cut the sleeves out in summer time or because they wear their underarmor stuff to stay cool and then get burned inside the PPE.

Level 0 has been deleted as of 2015. As Al Havens who put in the public input that changed it put it, clothing is not PPE.

I nearly ordered the cotton treated PPE today but learnt about "inherently flame resistant" PPE last minute so bought the following
professional suit instead.
Do you know what the Oberon "inherently flame resistant" PPE fabric are made of? See:

https://www.oberoncompany.com/wp-conten ... Series.pdf

https://www.oberoncompany.com/faq/is-ob ... ed-cotton/

I forgot to ask about the above. I was asking what if you or a machine holds the live wire to the chassis in US residential
*continuously* without any upstream overcurrent protective device turning off. How long before it melts the wires or chassis
resulting in a 1.2cal/cm2 energy and molten metal from the melting at arms length away (without any arc flash or Hazard
category 0)?

And to repeat my question in last message:

I nearly ordered the cotton treated PPE today but learnt about "inherently flame resistant" PPE last minute so bought the following
professional suit instead.



Do you know what the Oberon "inherently flame resistant" PPE fabric are made of? See:

https://www.oberoncompany.com/wp-conten ... Series.pdf

https://www.oberoncompany.com/faq/is-ob ... ed-cotton/

There isn't a good formula for predicting time to melt. With fuses they basically test it. An uncontrolled experiment with vaporizing or melting wire is not nearly as predictable as a controlled environment like a fuse. And obviously current plays a role in this. Then we get to a question of arcing time. At 120 VAC it's going to self extinguish. I can't put an exact number on it but most likely it will never get there. As far as molten metal goes, try it yourself. It's not very exciting.



You can call and ask on that specific one. Obern makes both the treated cotton and the Nomex grade (considerably more expensive) stuff. The fact that it says "inherently flame resistant" is important because welding greens were first used for arc flash PPE. They lose their retardant properties after about 50 washings. At the same foundry I mentioned we had a mechanic get a severe burn because the greens he was wearing lost their fire retardancy and a piece of slag hit him and lit him up like a candle. The ammonia (Westex ammonia) process that is in use with all arc flash PPE that have been available in several years is chemically bonded to the cotton. There is no way to wash it out unlike welding greens.