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 Post subject: Equipment greater than 600V
PostPosted: Mon Jan 12, 2015 8:01 am 
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NFPA 70E 130.7(A) Information Note 2 states:
It is the collective experience of the Technical Committee on Electrical Safety in the Workplace that normal operation of enclosed electrical equipment, operating at 600 volts or less, that has been properly installed and maintained by qualified persons is not likely to expose the employee to an electrical hazard.

This implies, but doesn't state, that MV equipment does expose the employee to an electrical hazard, and therefore PPE (shock and arc flash) would have to be worn even to operate a disconnect switch with the doors closed.

What are the thoughts on this one? Our practice currently is to let anyone open or close MV starter breakers, and operating MV switchgear is not mandated to be done with rated PPE on.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Tue Jan 13, 2015 2:44 pm 
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I think the 70E Committee is being reasonable in cutting off their experience at 600 V but to suggest that medium voltage is less safe than 600 V class equipment, as they have done in the new task tables, is utterly foolish and shows a lack of knowledge on the committee's part. We've already been through this over the last 30 years in the mining industry and even the federal agency responsible for safety in U.S. mining (MSHA) went back on their own rule banning use of medium voltage equipment after finding it as safe as low voltage equipment.

Roberts who is one of the sources of the "normal work" concept that showed up in 70E-2015 keeps inserting similar language with no justification at all. There is no evidence of any sort that medium voltage equipment is inherently more dangerous than 600 V equipment. Quite the contrary in fact for several reasons. First, try this. Start with say a 2500 kVA transformer with ANSI standard impedance (5.5% for 2400 V and 5.75% for 480 V). The short circuit current with an infinite source is then 6.3 kA for 4160 V and 52 kA for 480 V. Using IEEE 1584 empirical equation with switchgear and assuming a dead short on the transformer leads, the incident energy for 4160 V is 2.8 cal/cm^2 with a 0.5 second arc. The incident energy with 480 V with a 0.5 second arc is 28 cal/cm^2 with the same arc gap. That's a 10 fold increase. So before even proceeding further it is clear that higher voltages have less arc flash to worry about in the first place.

Second reason. Medium voltage starters (E2 equipment), metal clad switchgear, and even most metal enclosed switchgear has the medium voltage power bus and the low voltage controls physically isolated by barriers to where the arc flash potential while working on the control circuits is effectively nonexistent. Most of the time this is a requirement because otherwise (by NEC) the insulation would need to be rated at the highest voltage in the compartment. Isolation is much less expensive. In 600 V equipment they are all mixed together in most cases such as MCC buckets and panelboards so no such isolation exists. Since most of the time control circuits are where the work is at if for no other reason than more connections, the likelihood of an arc flash is less. The cost difference between 150, 300, and 600 V insulation is not even worth it for most cable companies these ddays to even offer it.

Third reason. Medium voltage gear is subject to corona induced failures, known popularly today as partial discharge. No such effect exists at low voltage. The up shot is that low voltage equipment can be pretty much buried in nearly any material and will continue to operate as long as it doesn't overheat. So it gets no respect in terms of conamination. Medium voltage equipment in the same conditions flsahes over and trips out. It is inherently "less reliable" and thus is maintained in better condition because it has to be.

Forth reason. Wellman (OSHA Arc Flash Injury Data Analysis, IEEE paper #ESW-2012-28) looked at arc flash injuries vs. voltage. There were no fatalities below 480 V. The number of fatalities for 480 V was 11%. It was 12% for the entire range of 1 kV-35 kV voltages. 1% given the nature of the statistics is statistically insignificant. Thus there is no difference in fatality rates between medium voltage and low voltage equipment. It would not be valid to compare the raw scores (480 V vs. medium voltage) because there is a lot more of one than the other.

Fifth reason. MSHA has already looked a tthis extensively. Until very recently medium voltage equipment (over 600 V) was banned in coal mines as a general rule. MSHA's view can be best described by a report, "History or Coal Mine Electrical Fatalities since 1970" which examined fatalities from 1970 to 1983. It found that the overwhelming majority of injuries on surface equipment (75%) involved medium voltage equipment. However the vast majority involved contact with overhead power lines by trucks, cranes, and drill rigs. In every case it was reported that the operator was preoccupied with some other task and simply forgot that the power line was there. Another 40% involved working with medium voltage equipment that was energized. Over time since 1983 as motor sizes grew, MSHA granted specific use cases where medium voltage was allowed. Finally in 2010, they published an update to the standard and made it acceptable as a general rule. MSHA had determined that the equipment and procedures used with medium voltage equipment provided the same level of safety as with low voltage.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Wed Jan 14, 2015 5:54 am 
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So, the conclusion I draw from what you wrote is that MV is just as safe to work on as LV. My concern right now isn't with reality so much as being code compliant. And the conclusion I get from the code is that we need to treat MV as more dangerous and take extra precautions because of that.

I have some brand new MV starters. I can conclude that to work in the control circuit section of the starter the workers need to wear arc flash gear rated to the incident energy of the power portion of the starter. Which, thankfully, does tend to be lower than that I find on most of my LV MCC's, but still above 1.2, so requiring face shield and balaclava.

PaulEngr wrote:
I think the 70E Committee is being reasonable in cutting off their experience at 600 V but to suggest that medium voltage is less safe than 600 V class equipment, as they have done in the new task tables, is utterly foolish and shows a lack of knowledge on the committee's part. We've already been through this over the last 30 years in the mining industry and even the federal agency responsible for safety in U.S. mining (MSHA) went back on their own rule banning use of medium voltage equipment after finding it as safe as low voltage equipment.

Roberts who is one of the sources of the "normal work" concept that showed up in 70E-2015 keeps inserting similar language with no justification at all. There is no evidence of any sort that medium voltage equipment is inherently more dangerous than 600 V equipment. Quite the contrary in fact for several reasons. First, try this. Start with say a 2500 kVA transformer with ANSI standard impedance (5.5% for 2400 V and 5.75% for 480 V). The short circuit current with an infinite source is then 6.3 kA for 4160 V and 52 kA for 480 V. Using IEEE 1584 empirical equation with switchgear and assuming a dead short on the transformer leads, the incident energy for 4160 V is 2.8 cal/cm^2 with a 0.5 second arc. The incident energy with 480 V with a 0.5 second arc is 28 cal/cm^2 with the same arc gap. That's a 10 fold increase. So before even proceeding further it is clear that higher voltages have less arc flash to worry about in the first place.

Second reason. Medium voltage starters (E2 equipment), metal clad switchgear, and even most metal enclosed switchgear has the medium voltage power bus and the low voltage controls physically isolated by barriers to where the arc flash potential while working on the control circuits is effectively nonexistent. Most of the time this is a requirement because otherwise (by NEC) the insulation would need to be rated at the highest voltage in the compartment. Isolation is much less expensive. In 600 V equipment they are all mixed together in most cases such as MCC buckets and panelboards so no such isolation exists. Since most of the time control circuits are where the work is at if for no other reason than more connections, the likelihood of an arc flash is less. The cost difference between 150, 300, and 600 V insulation is not even worth it for most cable companies these ddays to even offer it.

Third reason. Medium voltage gear is subject to corona induced failures, known popularly today as partial discharge. No such effect exists at low voltage. The up shot is that low voltage equipment can be pretty much buried in nearly any material and will continue to operate as long as it doesn't overheat. So it gets no respect in terms of conamination. Medium voltage equipment in the same conditions flsahes over and trips out. It is inherently "less reliable" and thus is maintained in better condition because it has to be.

Forth reason. Wellman (OSHA Arc Flash Injury Data Analysis, IEEE paper #ESW-2012-28) looked at arc flash injuries vs. voltage. There were no fatalities below 480 V. The number of fatalities for 480 V was 11%. It was 12% for the entire range of 1 kV-35 kV voltages. 1% given the nature of the statistics is statistically insignificant. Thus there is no difference in fatality rates between medium voltage and low voltage equipment. It would not be valid to compare the raw scores (480 V vs. medium voltage) because there is a lot more of one than the other.

Fifth reason. MSHA has already looked a tthis extensively. Until very recently medium voltage equipment (over 600 V) was banned in coal mines as a general rule. MSHA's view can be best described by a report, "History or Coal Mine Electrical Fatalities since 1970" which examined fatalities from 1970 to 1983. It found that the overwhelming majority of injuries on surface equipment (75%) involved medium voltage equipment. However the vast majority involved contact with overhead power lines by trucks, cranes, and drill rigs. In every case it was reported that the operator was preoccupied with some other task and simply forgot that the power line was there. Another 40% involved working with medium voltage equipment that was energized. Over time since 1983 as motor sizes grew, MSHA granted specific use cases where medium voltage was allowed. Finally in 2010, they published an update to the standard and made it acceptable as a general rule. MSHA had determined that the equipment and procedures used with medium voltage equipment provided the same level of safety as with low voltage.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Wed Jan 14, 2015 11:05 am 
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Based on your description and following 70E not as a safety standard but as a regulatory standard, nope, can't do that.

When it comes to arc flash, you need to make one very critical decision before proceeding further: whether to use tables or not.

If you use the tables, then you have to use all 3 tables (whether arc flash PPE is required, the "level" based on the equipment type, and the PPE as specified according to the level). You can't have one without the other, sort of (will get back to this). For practical reasons this is a very important decision. Doing nothing has been proven to be very ineffective. Following the tables according to a study performed by Doan has a 50% success rate when it comes to protecting against arc flash injuries. Using an engineering study approach so far has a 100% success rate. Furthermore at least with the current tables you have to do the same amount of leg work to comply with the notes in the equipment table (verifying the opening times) so it's just as much work to use the engineered approach and is a good deal safer. The only groups that are unlikely to be able to do this are contract workers that don't have control over the areas they are working in and are at the mercy of the arc flash studies done (or not done) by the host employer.

If you choose the engineering study route, you can't use the tables. You MUST do your own risk assessment. The hazard assessment, commonly done with either IEEE 1584, actual testing, or ArcPro (as recommended by OSHA 1910.269 for >10-15 kV), is only ONE component. This still means that you have to perform the risk assessment part which means determining whether or not PPE is required in the first place. In prior editions, this second step was not required. I'm saying this specifically because according to the rest of your explanation you have clearly done a hazard analysis outside of 70E and are now attempting to apply the task tables without applying the equipment and PPE tables. You can't have one without the other. You have to do all 4 levels as per the table.

Now for the "mixing" part. That does not in fact mean that you can't review at least the first table as part of your risk assessment and adopt (or alter as needed) it to suit your situation. In this case your table may look something like this:
Task: Operating breakers as normal work. No arc flash PPE required as per NFPA 70E-2015.
Task: Landing a wire onto an energized terminal. Arc flash PPE required as per NFPA 70E-2015.
Task: Performing noncontact inspections where not crossing the restricted approach boundary with tools or equipment. No arc flash PPE required as per OSHA 1910.269 Annex F...


You can't do what has been done in the past...using IEEE 1584, declaring everything hazardous beyond belief, dressing everyone in 40 cal suits and hoping for the best, and stopping right there. Not doing a risk assessment is simply not optional under 1910.269 for distribution equipment, and no longer optional under 70E for everyone else at least in the U.S. Risk assessments are already required for every other task and as of 2015, this also applies to electrical tasks. The basic understanding that "just walking by" is not a hazard MUST be analyzed, as the 70E Committee themeselves has done countless timesin numerous responses to proposals for changes, even if going by the "old" (2012) approach that most sites took.

And by the way, there i still a PPE table. It's in Annex H. If you use this table, you stop using "H/RC" or just "PPE levels" and just use cal/cm^2. Annex H allows higher cal/cm^2 ratings for the "mid grade" (8 cal/cm^2) level up to 12 cal/cm^2 as long as your PPE can do this so it is more forgiving in the first place.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Wed Jan 14, 2015 11:33 am 
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I have done an assessment on the entire facility. I have a MV MCC with an incident energy of 2.2 cal/cm2. What I am debating is whether or not I have to wear PPE rated to 2.2 cal/cm2 when I operate the disconnect switch with the doors closed, or whether I have to wear PPE rated to 2.2 cal/cm2 when I have the control compartment open.

PaulEngr wrote:
Based on your description and following 70E not as a safety standard but as a regulatory standard, nope, can't do that.

When it comes to arc flash, you need to make one very critical decision before proceeding further: whether to use tables or not.

If you use the tables, then you have to use all 3 tables (whether arc flash PPE is required, the "level" based on the equipment type, and the PPE as specified according to the level). You can't have one without the other, sort of (will get back to this). For practical reasons this is a very important decision. Doing nothing has been proven to be very ineffective. Following the tables according to a study performed by Doan has a 50% success rate when it comes to protecting against arc flash injuries. Using an engineering study approach so far has a 100% success rate. Furthermore at least with the current tables you have to do the same amount of leg work to comply with the notes in the equipment table (verifying the opening times) so it's just as much work to use the engineered approach and is a good deal safer. The only groups that are unlikely to be able to do this are contract workers that don't have control over the areas they are working in and are at the mercy of the arc flash studies done (or not done) by the host employer.

If you choose the engineering study route, you can't use the tables. You MUST do your own risk assessment. The hazard assessment, commonly done with either IEEE 1584, actual testing, or ArcPro (as recommended by OSHA 1910.269 for >10-15 kV), is only ONE component. This still means that you have to perform the risk assessment part which means determining whether or not PPE is required in the first place. In prior editions, this second step was not required. I'm saying this specifically because according to the rest of your explanation you have clearly done a hazard analysis outside of 70E and are now attempting to apply the task tables without applying the equipment and PPE tables. You can't have one without the other. You have to do all 4 levels as per the table.

Now for the "mixing" part. That does not in fact mean that you can't review at least the first table as part of your risk assessment and adopt (or alter as needed) it to suit your situation. In this case your table may look something like this:
Task: Operating breakers as normal work. No arc flash PPE required as per NFPA 70E-2015.
Task: Landing a wire onto an energized terminal. Arc flash PPE required as per NFPA 70E-2015.
Task: Performing noncontact inspections where not crossing the restricted approach boundary with tools or equipment. No arc flash PPE required as per OSHA 1910.269 Annex F...


You can't do what has been done in the past...using IEEE 1584, declaring everything hazardous beyond belief, dressing everyone in 40 cal suits and hoping for the best, and stopping right there. Not doing a risk assessment is simply not optional under 1910.269 for distribution equipment, and no longer optional under 70E for everyone else at least in the U.S. Risk assessments are already required for every other task and as of 2015, this also applies to electrical tasks. The basic understanding that "just walking by" is not a hazard MUST be analyzed, as the 70E Committee themeselves has done countless timesin numerous responses to proposals for changes, even if going by the "old" (2012) approach that most sites took.

And by the way, there i still a PPE table. It's in Annex H. If you use this table, you stop using "H/RC" or just "PPE levels" and just use cal/cm^2. Annex H allows higher cal/cm^2 ratings for the "mid grade" (8 cal/cm^2) level up to 12 cal/cm^2 as long as your PPE can do this so it is more forgiving in the first place.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Fri Jan 16, 2015 3:14 pm 
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When you do a risk assessment, there are several standards to use. Annex F in 70E is there but it is nonfunctional. To see what I mean, follow the procedure and attempt to do a risk assessment. You can't do it. It looks like they took ANSI B11.TR3 and tried to convert it to use for electrical safety. This is fundamentally flawed for several reasons:
1. Probability of occurrence of the hazard is given as "very high, likely, possible, rare, negligible". No definitions are given such as say "1 in a million" so you can't possibly gauge what "likely" even means. Part of this is because ANSI B11.TR3 states that risk is "industry and company specific". But then does not define it any further or give any guidance. There are real, acceptable standards out there. For instance the federal government in many places gives "1 in a million" per year as acceptable risk to the public. Most industry and government standards place fatalities at between 1 in a million and 1 in 100,000 based essentially on average fatality rates. The idea is to set the standard to be no more and no less likely than current rates (neither inflate nor deflate risk).
2. Second problem is with injuries. Again, it's simply not defined.
3. Third area is with frequency of exposure and time frames. Here for once Annex F gives some numbers but the fact of the matter is that in most cases frequency of exposure for arc flash in electrical equipment is measured in fractions of years. This is a holdover from a standard that is intended to address moving mechanical equipment on an assembly line and has no place at all in an electrical standard.

Instead I suggest using Layers Of Protection Analysis from the CCPS Handbooks on the subject. The intended time frame are chemical plant accidents that occur very rarely but involve one or more serious injuries or fatalities. Thus it suits arc flash very well. It gives concrete data on probability of occurrence and injury estimates from a variety of industries. And it is very easy to apply it directly to electrical work. If you do this, we end up with a very simple criteria of maintaining the probability of an arc flash injury occurring at a rate of around 1 in 10,000 to 1 in 100,000 per year as acceptable. Average arcing fault rates from IEEE 493 are well below those rates for equipment that is in good shape and receives "average" maintenance, whatever "average" means. Unfortunately I can't ind anyone with a definition of acceptable maintenance for safety purposes anywhere.

Either way, I am not aware of a case and I can't even see a good way that anyone could cause an arcing fault from the control enclosure short of fishing wires through holes or some similar activity which is blatantly and obviously dangerous even if it were done from outside the enclosure with no doors open. Doors open/closed came from Roberts (ESW2011-22) and essentially eliminates shock hazards by ensuring that the equipment is not exposed. There are a number of places within the new tables where 70E then defines where doors can be open, especially when working around batteries, which often don't have doors. I have not uncovered any evidence of a significant arc flash hazard with 120 VAC equipment. The one exception is that there was an OSHA investigation of an IT worker who was burned on his hands when a metal cover on a power strip came loose and caused the plug to explode in his hands. 70E makes no attempt to protect against arc flash at the hands but instead focusses on the chest and face so this case is immaterial. OSHA 1910.269 gives a risk assessment table of tasks that looks at whether or not the worker is within the restricted approach boundary as the criteria for arc flash hazards rather than whether the doors are open or closed. This makes far more sense than the doors. In general I like OSHA's table better because it is far less confusing than the mess than the 70E Committee finally came up with and is much closer to the first and second draft tables they started with. I'm not sure what happened when they got to the final draft because they reject changes to the drafts that are beyond the original proposals for changes.

So in summary, I would suggest that 2.2 cal/cm^2 or less PPE would be not be required in the control compartment (only nonmelting clothing), and when operating the equipment under normal conditions or performing tasks that are outside the restricted apprroach boundary as long as the equipment is properly designed, installed, and not showing evidence of impending failure, and not faulted.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Mon Jan 19, 2015 10:05 am 
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"This implies, but doesn't state, that MV equipment does expose the employee to an electrical hazard, and therefore PPE (shock and arc flash) would have to be worn even to operate a disconnect switch with the doors closed."

I would disagree that it implies this at all.

As an (absurd) example if you found the statement to say
" Information Note 2 states: It is the collective experience of the Technical Committee that normal operation of motorcycles with windscreens is likely to keep bugs out of your teeth." would not imply that "driving a sedan" IS likely to result in bugs in your teeth.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Tue Jan 20, 2015 7:15 am 
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JKlessig wrote:
"This implies, but doesn't state, that MV equipment does expose the employee to an electrical hazard, and therefore PPE (shock and arc flash) would have to be worn even to operate a disconnect switch with the doors closed."

I would disagree that it implies this at all.

As an (absurd) example if you found the statement to say
" Information Note 2 states: It is the collective experience of the Technical Committee that normal operation of motorcycles with windscreens is likely to keep bugs out of your teeth." would not imply that "driving a sedan" IS likely to result in bugs in your teeth.


No, it would imply that driving motorcycles without windscreens would be more likely to result with bugs in your teeth.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Tue Jan 20, 2015 7:18 am 
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The reply I got back from NFPA is:

There are no voltage limitations associated with 130.2(A)(4). If a worker performs a normal operation (turning a circuit breaker on or off) and meets the conditions specified that worker does not need to wear PPE.

Now I need to figure out if "Normal Operation" can include opening and closing of distribution breakers, as well as racking of same.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Tue Jan 20, 2015 1:42 pm 
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Opening/closing breakers: Yes.

Racking them on and off the bus: No. There is a specific task in the list for this.

It's a bit of a grey area. With MCC's, the end user is entirely responsible for alignment and there are plenty of cases where attempting to insert/remove buckets while energized is a really bad idea.

With switchgear, we have a mechanism that SHOULD maintain alignment and keep the contacts in the open position. The end user is still responsible for providing the motive energy but hopefully everything stays nice and aligned. The trouble is that this doesn't always happen. ABB at least reports that 80% of arcing faults involve the racking mechanism, not the breaker. I've seen that number pop up here and there but I can't give a more definitive source.

That's not to say that adding a true "disconnect" is a solution either. GE Limitamp starters have a manual disconnect handle that makes them lift/lower to connect to the bus and opens/closes the shutters. The starters themselves are on wheels. Sometimes the handle breaks and there is no way to tell externally that the stabs haven't opened. This has resulted in at least one fatality.

http://work.alberta.ca/documents/WHS-PU ... -12-12.pdf


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 Post subject: Re: Equipment greater than 600V
PostPosted: Thu Jan 22, 2015 6:40 pm 
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PaulEngr wrote:

That's not to say that adding a true "disconnect" is a solution either. GE Limitamp starters have a manual disconnect handle that makes them lift/lower to connect to the bus and opens/closes the shutters. The starters themselves are on wheels. Sometimes the handle breaks and there is no way to tell externally that the stabs haven't opened. This has resulted in at least one fatality.

http://work.alberta.ca/documents/WHS-PU ... -12-12.pdf


There are numerous roll out MV starter designs with the same potential problem. We had enough alignment issues with the roll out starters that we switched to fixed mount starters years ago. I am aware of one major heavy industrial company that still uses roll out starters, but most folks have switched to fixed mount contactors now since the disconnect switches have fewer degrees of freedom than the roll out mechanism, meaning fewer potential failure modes. FMEA or other formalized assesment tools confirm this, and the use of fixed mount MV contactors has been mentinoed several times in safety by design literature. In some fixed mount starter designs the disconnect switch blades can be visually inspected with the outer doors closed.

I find this somewhat interesting that most folks have switched to fixed mount contactors but still use drawout MV breakers. Maybe in time this will shift as well for the same reasons.


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 Post subject: Re: Equipment greater than 600V
PostPosted: Thu Jan 22, 2015 8:27 pm 
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I have become convinced that drawout gear adds failure modes and decreases reliability. The number of times where repairs can be made while energized is small. There is a third potentially better option used by Duke (largest US utility) in their underground gear. It is a sealed vacuum breaker that uses elbow connectors for line and load sides. There is no “enclosure“. Each breaker is in its own housing and elbow connectors up to 200 A can be load break. So swapping breakers is done with disconnects (elbows).


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 Post subject: Re: Equipment greater than 600V
PostPosted: Thu Apr 07, 2016 7:50 am 
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I appreciate all the details in this thread. This topic has recently emerged with several of our plants.

My first response is “Why risk it?” .
My second response is “Do I really know this disconnect has been properly maintained?”

The pushback I’ve received is that if a worker is going to do mechanical work instead of electrical work, he shouldn’t have to wear PPE. I argue that there is arc flash and arc blast risk regardless of the work intention. Right?

I’ve certainly heard maintenance workers tell me (even in 30A 480V disconnects) that an arc flash happened while the door was closed and they were glad they had their glove on. Again, why risk it?

Are there any regulatory requirements here? This sounds so subjective…or are we to rely on our own electrical safety program to govern the procedures here.

Thank you for this discussion!


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 Post subject: Re: Equipment greater than 600V
PostPosted: Thu Apr 07, 2016 6:31 pm 
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uriah1 wrote:
I appreciate all the details in this thread. This topic has recently emerged with several of our plants.

My first response is “Why risk it?” .
My second response is “Do I really know this disconnect has been properly maintained?”

The pushback I’ve received is that if a worker is going to do mechanical work instead of electrical work, he shouldn’t have to wear PPE. I argue that there is arc flash and arc blast risk regardless of the work intention. Right?

I’ve certainly heard maintenance workers tell me (even in 30A 480V disconnects) that an arc flash happened while the door was closed and they were glad they had their glove on. Again, why risk it?

Are there any regulatory requirements here? This sounds so subjective…or are we to rely on our own electrical safety program to govern the procedures here.

Thank you for this discussion!


Actually arc blast may not be the major hazard that it is purported to be. Out of the published data the "open air" cases show that the pressure is relatively small, only a few PSI at worst. It is true that arc temperatures can reach "35,000 F" (20,000 K) or more but this temperature is within the core of the arc itself which even with high power arcs is only a couple millimeters across. The vast majority of the emitted energy is immediately absorbed by the air surrounding the arc so only a small amount of energy actually reaches beyond the core to cause arc flash/blast. In the only open air tests that have been published which was produced by two electrodes facing each other and measured with a microphone, the pressure seems to be almost inverse linear with distance although intuitively it would seem like it should be related to the inverse of the square of the distance. A better curve fit was achieved with a 0.9 exponent by Lee.

In an enclosure what happens is that it builds up pressure and then the enclosure ruptures releasing the pressure all at once. The pressure wave dissipates rapidly though so the biggest danger is from the door that comes flying off. Again most of the energy is absorbed in the air surrounding the arc. Thus the arc blast is not related to characteristics of the arc at all for sealed enclosures but only to the mechanical strength of the enclosure. If the equipment leaks out pressure then the peak pressure will be less and the onset is extended. The highest reported pressure I've seen is 30 PSI inside the enclosure but since the enclosure was heavily vented (modelling MV switchgear with two huge openings) and all the MCC-style enclosure measurements never get above about 10 PSI, this one is probably invalid. These pressures are inside the enclosures. Outside the enclosure pressure is going to fall off at a cubic rate (expansion of gas...so it's volumetric) until it dissipates down to again where we are at the open air case again. I'm slowly pulling together a paper on this that I'll be releasing soon. In comparison to published data on the effects of pressure, it is pretty likely that ear drums will be blown and there is a small outside chance of potential minor lung injury, and that workers can be knocked around by the pressure especially if they are close by. There is simply not enough pressure/force to launch shrapnel other than the door itself. It is more likely that the "shrapnel" reported is being launched by either parts of conductors or resting on conductors and being magnetically propelled. A fatality or severe direct injuries from an arc blast have been vastly overblown.

Second getting back to the arc flash argument, keep in mind that for pretty much any disconnect regardless of the design, even if it's installing jumpers on open air overhead lines, there are basically three types of equipment. One type which is typical of either powered equipment (circuit breakers) or the types that use some kind of spring loaded or overhead cam mechanism are characterized by rapid opening of the contacts which is driven by the mechanism, no matter what the arc quenching mechanism is. The second case is where there is some kind of alignment mechanism such as a drawout mechanism or cheap non-load-break "air switches" used in outdoor switchgear (Igor, throw the switch!) These devices rely entirely on the operator for motive force. In some cases it may be possible to rapidly open them but in others this is not possible. Finally we have equipment in which both alignment and motive force are entirely under control of the operator. This would be the case for various types of jumpers used in overhead power lines as well as buckets in MCC's.

Arcing failure rates according to IEEE 493 are around 10^12 for disconnects and drop down to around 10^6 for bolted breakers and contactors and then drop down to around 10^5 for draw out breakers. From the data it appears that failures in mechanisms that are self-aligning but rely on human motive power are somewhere around an order of magnitude less reliable than the other types. In "high risk" industries (chemicals, nuclear) human error rates vary between 1% and 40% depending on the situation. Different people have analyzed different data sets but suffice to say that the ratio of serious injuries to fatalities is between 10:1 and 20:1. Industry standards vary but in general fatalities from most sources are around 10^6 when we look at say fatal car crashes, fires, etc. Thus on average with the exception of the various mechanisms like "draw out" switches, non-load break switches, and mechanisms that work in a similar fashion with fatality rates about an order less than other types of equipment it would seem prudent to treat it differently (ie, require arc flash PPE) while for equipment that is in good shape and not exhibiting any obvious outward signs of a potential failure, the likelihood of a fatality or serious injury is no different from other sources and thus PPE would not normally be required. Requiring PPE for this case is akin to requiring clerks to wear FR jump suits, head-neck injury restraints, and 5 point harnesses to drive a sedan over to the post office to drop off mail as compared to requiring NASCAR racing drivers to use this type of PPE when driving inches from each other at 200 MPH on a super speedway.

The regulatory requirements fall of the general duty clause. Employers have a duty to protect workers from recognized hazards and arc flash and shock are certainly recognized hazards. In doing so OSHA has long recognized that employers should do a risk assessment in doing so. Risk assessments are quite easy for risks such as requiring electrically insulated power tools or whether or not to provide protection from falling meteors around parking lots...in both cases the risk is very clear. Electrical injuries are tough to analyze because humans simply have a hard time comprehending the statistics. Electrical injuries are the 7th leading cause of electrical fatalities, and shocks account for about 2/3rds of those statistics with the rest being arc flash. But in terms of total injuries electrical injuries in general fall into the "miscellaneous" category since they account for less than 1% of all industrial injuries. So electrical risks fall into a difficult to analyze category where the hazards are significant and thus we need to pay attention but that they are so rare that even in very large companies with thousands of employees, arc flash and shock injuries may only happen every few years. Thus also confounds analysis because especially small plants may never experience a major arc flash injury and there simply aren't enough of them to do any meaningful statistics except over a large population such as looking at national statistics from OSHA. And I can tell you from analyzing OSHA statistics myself that there are hardly any "minor arc flash injuries". Almost all require hospitalization, but conversely fatalities are also pretty rare.

So I would suggest three things in terms of protecting non-electrical workers from electrical hazards:
1. When operating disconnects, position your body clear of the doors if possible.
2. If the equipment has any signs of problems such as obvious damage, water coming out of the panel, burn/scorch marks or discoloration, or if it recently tripped, don't mess with it. Get professional help (electrician) to check it out and make it safe.
3. If you see downed electrical lines or see or suspect energized conductors or feel a "tickle" (shock), again get professional help and avoid the area.
4. Especially when it comes to cranes, stay clear of electrical wires. OSHA has recently (in the last 6 years) updated the standards for cranes and derricks but the rules should be applied to ANY equipment (raised truck beds, ladders, carrying pipes and conduits, working with heavy equipment around power lines).

But making mechanics and operators or other unqualified personnel dress up in arc flash PPE is silly unless the plant has done such a poor job of maintaining equipment that they should be shut down.


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