Arc Flash PPE Requirements - Covers On

[mayanees]

I've concluded from months of working on this topic, that the appropriate interpretation of Arc Flash PPE requirements around energized equipment is that the same PPE is required whether equipment covers are on or off.
The Arc Flash Boundary is a diameter around the equipment developed using the IEEE 1584 Arc-Flash Hazards calculations. (not using 70E tables - I'm using the calcs)

Given this interpretation, how can this practice be enforced? Is it being enforced in the field?

Using the example where there's a Power Distribution Unit (PDU) in a given floorspace. This PDU has a 300 kVA 480:208V transformer. The calculated incident energy at the secondary of the transformer is 40.1 cal/sq cm, necessitating a Dangerous Hazard Risk Category (HRC) - no safe PPE exists to approach the PDU in a 13 foot Arc Flash Boundary (AFB). (no main sec'y breaker on the PDU xfmr)

The "letter of the law" states that this equipment can't be approached while energized within that 13 foot boundary. Is anyone actually practicing this level of caution in the real world?

I want to issue Arc Flash labels, but I'm reluctant to advise a client that per NFPA 70E, they must stay out of the AFB around the equipment. The equipment is safe from Shock Protection with covers on, but it could explode from Arc Flash based on my calcs. Real world says the chances are very low that an Arc will develop, but since I've calculated the Incident Energy, I can't go back to the 70E tables for a relaxed PPE requirement.

What is a real-world approach to this situation?? Spray insulatory material on all busses and terminals to minimize the chance of an Arc?? But then how would that be tested...

Thanks for any comments.

John M

 

[zog]

I've concluded from months of working on this topic, that the appropriate interpretation of Arc Flash PPE requirements around energized equipment is that the same PPE is required whether equipment covers are on or off.
The Arc Flash Boundary is a diameter around the equipment developed using the IEEE 1584 Arc-Flash Hazards calculations. (not using 70E tables - I'm using the calcs)

Given this interpretation, how can this practice be enforced? Is it being enforced in the field?

Using the example where there's a Power Distribution Unit (PDU) in a given floorspace. This PDU has a 300 kVA 480:208V transformer. The calculated incident energy at the secondary of the transformer is 40.1 cal/sq cm, necessitating a Dangerous Hazard Risk Category (HRC) - no safe PPE exists to approach the PDU in a 13 foot Arc Flash Boundary (AFB). (no main sec'y breaker on the PDU xfmr)

The "letter of the law" states that this equipment can't be approached while energized within that 13 foot boundary. Is anyone actually practicing this level of caution in the real world?

I want to issue Arc Flash labels, but I'm reluctant to advise a client that per NFPA 70E, they must stay out of the AFB around the equipment. The equipment is safe from Shock Protection with covers on, but it could explode from Arc Flash based on my calcs. Real world says the chances are very low that an Arc will develop, but since I've calculated the Incident Energy, I can't go back to the 70E tables for a relaxed PPE requirement.

What is a real-world approach to this situation?? Spray insulatory material on all busses and terminals to minimize the chance of an Arc?? But then how would that be tested...

Thanks for any comments.

John M

Increase your working distance, thats the easy answer. The best answer is to mitigate the hazard, hard to do on a system like this but possible, not cheap.

 

[don_resqcapt19]

300kVA sounds like a small system. Is it normal to have over 40cal/sq cm in this small of a system?

 

[jim dungar]

The "letter of the law" states that this equipment can't be approached while energized within that 13 foot boundary.

Where do you see it written that the equipment cannot be approached?

I ask only as a point of discussion.
Many people try to implement NFPA 70E as if it were an exact step-by-step procedure and not one that requires/allows accommodations. NFPA70E 110.7(F) requires your 'electrical safety program' to identify potential hazard/risks that are appropriate for the 'voltage, energy level, and circuit conditions'. 130.3(B) talks about arc flash PPE that is required when "work will be performed" within the arc flash boundary. While not part of NPFA70E, annex J shows a typical flow chart for deciding if an energized work permit is required, it does not list a requirement for PPE unless the energized parts are exposed. Annex F provides a possible flow chart for determining a hazard/risk associated with a task, it contains a possible decision path that may allow approaching equipment if not interacting with it. Annex E suggests using a "logical approach to determine potential hazard of task".

 

[rbalex]

What you are describing is not an arc-flash, although it may be the result of one; rather it is an arc-blast and is the one out of the three identified types of electrical hazards that 70E does very little about. Beyond noting that Arc-blast can exist and cautioning awareness of it, 70E does virtually nothing, in fact.

Annex K summarizes all three identified types and describes how dangerous arc-blast is. But the rest of 70E is basically silent until it is mentioned in an FPN in Chapter 3. The definition of Electrical Hazards in Article 100 only gives it passing mention.

However the 2009 definition of Arc-Flash Hazard hints at and its FPN No 1 amplifies that a direct exposure to the arc is what is under consideration and enclosed equipment is not the intended application.

Why is arc-blast acknowledged but detailed discussion avoided? - Because there is NO effective PPE, Work Practice or product manufacturing standard that can be used reliably beyond good maintenance or staying away from the equipment altogether. This is why some companies use remote racking of their switchgear.

During the 2002 cycle there was posited the idea that C100 PPE was readily available. The response to that was it would make the difference between an open or closed casket funeral; any arc that released that much energy would also have a blast force that would crush a worker.

 

[mayanees]

Good points..

Good points..

Thanks for the discussion items.

Don,
I'm confident in the calculation at the secondary of the 300 kVA transformer, using the IEEE 1584 Standard. Although there's some ambiguity in the method used in this situation, it's the IEEE 1584 method. In this case, there's no secondary main, and the fault current is not high enough to trip the main breaker in its instantaneous region. In fact for this breaker, the upstream breaker doesn't trip for several seconds, but the IEEE 1584 method times out the calculated energy buildup at 2 seconds, expecting that's how long it takes you to get out of the way. At that point it's up to 41 cal/sq cm.

Jim,
I can not point to where it is written that energized equipment with doors on can only be approached in the AFB with appropriate PPE. It's just been hammered into me that once you've calculated the Incident Energy, you can't go back to the tables to perform listed tasks. So if the table says you could operate a meter on the switchgear with no PPE, and then I calculate that it's actually a level 4 PPE requirement for the exposed bus, then there's the argument that you must stick to the calcs. Even though...

Bob,
I totally understand that it's arc-blast that's in question. And you make the point that there are no no calcs for that. If it were Arc-Resistant switchgear, I'd feel better about approaching it with lessened PPE.

I've been laboring over this for months, and I appreciate the quick enlightenment. Jim, you gave me the answer. It's been staring at me all along - The required Electrical Safety Program (ESP). We need to emphasize owner-management of the ESP as a means to minimize exposure.

It will also effect future designs in terms of placement of equipment, and system design parameters. But there are allot of systems out there with extremely high energy, and the ESP needs to focus on awareness of that potential hazard in their ESP.

The answer from a design standpoint, is Medium Voltage. We recently did a comparison of two (2) 3000 kVA systems with different secondary voltages: 480 and 4160. We modeled them with 75 feet of cable to switchgear. At their respective switchgear, the 480V system had a Dangerous HRC level, and the 4160V system was category ZERO! It was cat 1 at the secondary of the 4160V xfmr, but after 75 feet it was down to cat 0.

Thanks again for the comments.

John M

 

[zog]

, and the fault current is not high enough to trip the main breaker in its instantaneous region.

Can you lower the INST settings? Have you considered an arc flash reduction switch, like the quick trip system?

I can not point to where it is written that energized equipment with doors on can only be approached in the AFB with appropriate PPE.

Does not say that anywhere, it does say that when you "Interact" with the equipment inside the AFB PPE must be worn. The key here is interaction, not approach.

The answer from a design standpoint, is Medium Voltage. We recently did a comparison of two (2) 3000 kVA systems with different secondary voltages: 480 and 4160. We modeled them with 75 feet of cable to switchgear. At their respective switchgear, the 480V system had a Dangerous HRC level, and the 4160V system was category ZERO!

That is very typical, what scares me is the unqualified electrical workers that think arc flash hazards increase with voltage and dont see 480V systems being all that hazardous. Shudder.......

 

[jim dungar]

Can you lower the INST settings? Have you considered an arc flash reduction switch, like the quick trip system?

We have been recommending adding CT's and a protective relay on the secondary of transformers like this, The relay is used to trip the primary breaker, thus acting like a virtual main.

 

[jghrist]

The calculation of incident energy using IEEE 1584 is based on tests that were done with direct exposure to the arc. There is no accepted method to calculate the incident energy where the arc is behind a metal barrier that I know of.

The fact that the incident energy is 41 cal/cm? does not mean that there is a high arc blast. You can get 41 cal/cm? with an arc flux of 2050 cal/cm?/s for a duration of 20 ms or with an arc flux of 20.5 cal/cm?/s for a duration of 2 seconds. The former would have a much higher arc blast.

 

[zog]

We have been recommending adding CT's and a protective relay on the secondary of transformers like this, The relay is used to trip the primary breaker, thus acting like a virtual main.

Thats what we do too. We have designed a mini VCB that can fit in the fuse section of load break switches, good solution for the MV/LV transformer problem.

 

[don_resqcapt19]

Thanks for the discussion items.

Don,
I'm confident in the calculation at the secondary of the 300 kVA transformer, using the IEEE 1584 Standard. Although there's some ambiguity in the method used in this situation, it's the IEEE 1584 method. In this case, there's no secondary main, and the fault current is not high enough to trip the main breaker in its instantaneous region. In fact for this breaker, the upstream breaker doesn't trip for several seconds, but the IEEE 1584 method times out the calculated energy buildup at 2 seconds, expecting that's how long it takes you to get out of the way. At that point it's up to 41 cal/sq cm. ...
John M

Thanks John, I forgot about the issue where lower available current does not quickly trip the OCPD.

 

[ron]

John,

but the IEEE 1584 method times out the calculated energy buildup at 2 seconds, expecting that's how long it takes you to get out of the way.

Being that this recommendation shows up in an annex, I've not been using the 2 second limitation, unless it is obvious that the worker would be blasted clear of the equipment, which it usually is not.

 

[mayanees]

John,

Being that this recommendation shows up in an annex, I've not been using the 2 second limitation, unless it is obvious that the worker would be blasted clear of the equipment, which it usually is not.

Ron,

That 2-second "fall or get out of the way" time certainly illustrates the arbitrary nature of the 1584 method, but I think you need to cut it off somewhere.

If I wait for the breaker to trip in this application, it takes it 52 seconds and builds up 888 calories/square cm!

John M

 

[mayanees]

Can you lower the INST settings? Have you considered an arc flash reduction switch, like the quick trip system?

Does not say that anywhere, it does say that when you "Interact" with the equipment inside the AFB PPE must be worn. The key here is interaction, not approach.

Instantaneous is set at LOW. THe breaker is a Square D MH Thermal mag so I don't think there are options for lessening the Inst any more than the settings offer.

I appreciate your "interaction" comment. That will let me rationalize AFB approach.

You and Jim mentioned secondary CTs tripping a primary device - do you offer a compact system to do that? Can you recommend somehting?


Thanks
JM

 

[ron]

John,
I don't want to be the guy that gets blasted against the wall or gets my clothes hung up on the equipment and can't get away. I don't use the 2 second limitation because of liability and being really conservative. I often get calories/cm^2 >40 and tell the client they can't work on it energized.

 

[mayanees]

John,
I don't want to be the guy that gets blasted against the wall or gets my clothes hung up on the equipment and can't get away. I don't use the 2 second limitation because of liability and being really conservative. I often get calories/cm^2 >40 and tell the client they can't work on it energized.

... makes good sense Ron.

 

[Doug S.]

mayanees,

Have you contacted the manufacturer regarding the energy suppression of their enclosures? I don't remember were but I thought I read some devices (panels boards / MCCs ?) must retain there components within there rated A.I.R. In other words they must have some sort of energy "suppression" inherently built in to them... and I bet if it's some one like SQ-D they've got numbers on it...

Obviously some devices like a dry NEMA 1 x-former enclosure will do very little to suppress an arc flash/blast. But I bet a panel board or MCC with it's doors closed and latched suppresses quite a bit?

Some one wanna help me out? ( Or chew my head off...)

Doug S.

 

[jim dungar]

But I bet a panel board or MCC with it's doors closed and latched suppresses quite a bit?

Unless the equipment is specifically designed, constructed, and listed as "Arc-Resistant" it is normally not considered to reduce the arc flash hazard.

When a OCPD has a UL AIC it means that the device will open and clear a fault within its rating. However a fault on the 'busbars' of the panel is not addressed in the UL listing procedures. So an enclosure would not be expected to contain or reduce an arc flash caused by an internal fault.

 

[Doug S.]

Well now ya'll went and turned over a rock...

The breaker shown is one of two mains, load side of my buildings sub's...

The warning label reads....

356 inches Flash Protection Boundary
160 cal/cm^2 Incident Energy at 18" Working Distance
Dangerous! Exceeds NFPA 70E-2009 PPE Categories

480VAC Shock hazard when cover is open
42 inches Limited Approach
12 inches Restricted Approach
1 inch Prohibited Approach


The fun begins...
For these OCPD on/off, do I need a 1" stick and a bomb suit, or a 30 foot stick and I can switch them in my skivvies... ?

I've got some reading to do...

Thanks,
Doug S.

 

[ron]

Doug,
You need to see what the PPE requirements are for the upstream equipment to see if you can turn it off from there.
You cannot manually operate that breaker @ a working distance of 18" as the IE is >>>40.
You can ask for the IE to be recalculated at the working distance of a hot stick and see if you can get it so a Cat 4 suit (<40 cal/cm^2) would work.