FR clothing in Electrical Equipment

[Mike McDonald]

What is the risk category and therefore the FR clothing , arc flash PPE, and shock hazard protection required for working in electrical equipment not associated with switchboards, panels or MCC's, such as terminations and contactors located inside a chiller control cabinet? I can say it would be prohibitively cumbersome to troubleshoot and perform repairs to a ground level installed package unit operating at 480v between phases as an example. If anyone knows i sure would appreciate it. Thanks co-forum dudes!!

 

[zog]

It is not that simple, you need to conduct an arc flash study to determine the hazard. And wearing this stuff is not cumbersome, burn units and skin grafts are cumbersome.

 

[raider1]

Mike,

Take a look at NFPA 70E table 130.7(C)(9)(a) under heading "Other 600V Class (277 V through 600V, Nominal) Equipment. I would say that a chiller control cabinet that is 480 volts would fall under this section.

There is a line under that section titled "Working on energized parts including voltage testing." This section requires catagory 2* type PPE. It also requires the use of voltage rated gloves and tools.

Table 130.7(C)(10) will give you all the required PPE for catagory 2*

Hope this helps, Chris

 

[raider1]

you need to conduct an arc flash study to determine the hazard.

You can use a tabulated method as long as you know the avaliable short circuit ampacity, This doesn't require a full arc-flash analysis.

I agree that you should always wear the proper PPE when ever you work on energized equipment.

Chris

 

[raider1]

Oh, and as an added thought Mike, if you don't have a copy of NFPA 70E you can view it on-line Here.

Follow the instructions, you will have to agree to a disclaimer to view it. You also cann't do a search or cut and paste.

Chris

 

[zog]

Chris,

You also need to know the clearing time of the protective device to ensure you are within the limitation of the tables, so why make assumptions by using the tables? If you know the S/C current and the clearing times, you have the info you need for a analysis.

You can use a free calculator like the one found here

http://www.epowerplus.com/Training.htm/download.shtml

 

[raider1]

zog,

Yes, I forgot to mention that you need to know the clearing times of the overcurrent devices as well to use the tables, sorry

The Tables in NFPA 70E are very conserative, and if used properly assure appropriate protection of personnel.

Chris

P.S. thanks for the link

 

[zog]

Well according to the tables he would need a double layered switching hood, which is most likely too much protection.

What I mean by too much protection is that the 2* requirement for the hood may impair his vision and increace the risk of causing an arc flash. There are so many flaws in those tables, the 70E group got tired of answering questions at the NETA conference in Memphis this year and finally admitted the tables are full of flaws and need to be revisited for the next revision.

However, I agree they are conservative.

 

[raider1]

Well according to the tables he would need a double layered switching hood, which is most likely too much protection.

You are probably right about too much protection.

I agree that the best way to select PPE is with an arc-flash analysis, as you are able to eliminate excessive requirements that are established in the tables.

All I was pointing out was that you can use the table method instead of a full arc-flash analysis, and not arguing against an arc-flash analysis.

Chris

 

[dlhoule]

If an arc flash study to determine the hazard, you can frequently get down to a "0" level of protection. Most of our machines are now down to a class "0" level. I strongly urge everyone not to do any troubleshooting or work on energized circuits without proper PPE.

 

[Shockedby277v]

Nice links guys

 

[Mike McDonald]

flash hood

flash hood

thanks a lot guys for your time and positive and insightful comments and suggestions
my apologies for my lack of typing proficiency...
but the double layer switching hood is exactly what i meant by cumbersome and creating a visual hazard...made so by the construction of the unit...ie..on-ground, low control panel, and packed with anything from 480 3ph, 120v, and 24 volt control power...but hey who am i to nitpick...after all the mfgr obtained a UL label for it...

 

[don_resqcapt19]

Be careful with thinking that a lower available fault current always means a lower incident energy. I just got an arc flash hazard "slip stick" from Littelfuse. It says it is for 600 volts and less, but I expect that is applies mostly to 480 volt systems as the arc tends to self extinguish at each zero crossing for lower voltage systems.
They show that for a 101 to 200 amp RK1 or Class L fuse, you would need protection Level 0 for fault currents above 4kA, Level 2 for a fault current of 2kA and Level X (incident energy exceeds 40 cal/cm^2 and energized work is not permitted) for a fault current of 1kA. For breakers in the same size range they show Level 1 for fault currents of 16 to 60kA, Level 0 for 4 to 14kA and Level X for 2kA and under.
Don

 

[zog]

Exactly, the clearing time of the protective device is key, lower fault currents usually mean higher Ei due to increased clearing times.

 

[paul32]

Be careful with thinking that a lower available fault current always means a lower incident energy. I just got an arc flash hazard "slip stick" from Littelfuse. It says it is for 600 volts and less, but I expect that is applies mostly to 480 volt systems as the arc tends to self extinguish at each zero crossing for lower voltage systems.

Does this mean lower voltage systems don't have an issue? Or at least can we say a 120/240V service needs to be a certain size before it is?

They show that for a 101 to 200 amp RK1 or Class L fuse, you would need protection Level 0 for fault currents above 4kA, Level 2 for a fault current of 2kA and Level X (incident energy exceeds 40 cal/cm^2 and energized work is not permitted) for a fault current of 1kA. For breakers in the same size range they show Level 1 for fault currents of 16 to 60kA, Level 0 for 4 to 14kA and Level X for 2kA and under.
Don

So how can you ever be safe? How can you guarantee you have at least 4kA or whatever of fault current? I can see how you can find a maximum fault current, limited by the transformer, wire, etc, but not how you can find a minimum.

 

[don_resqcapt19]

Paul,

Does this mean lower voltage systems don't have an issue? Or at least can we say a 120/240V service needs to be a certain size before it is?

I think there is still an issue at the lower voltages, but in some cases the arc will self extinguish and not restrike at the first zero crossing thereby limiting the duration of the arc flash to less than 1/2 cycle. The incident energy is based on both the fault current and the amount of time before the fault is cleared.

So how can you ever be safe? How can you guarantee you have at least 4kA or whatever of fault current? I can see how you can find a maximum fault current, limited by the transformer, wire, etc, but not how you can find a minimum.

That is the big problem with this issue...you can't. It is also the problem with doing the calculations as the utility system is so dynamic that you can have big changes in the available fault current caused by their normal day to day switching. There are very few times when you are really permitted to work on energized equipment. This is the main point of 70E and the OSHA rules. Yes, you do need PPE to verify the lockout and that is still a problem, but the risk when you are just verifing the lockout is much less than when doing other work on energized systems.
Don

 

[dlhoule]

Quote:
Originally Posted by don_resqcapt19
Be careful with thinking that a lower available fault current always means a lower incident energy. I just got an arc flash hazard "slip stick" from Littelfuse. It says it is for 600 volts and less, but I expect that is applies mostly to 480 volt systems as the arc tends to self extinguish at each zero crossing for lower voltage systems.

Don, aren't you reaching that same zero crossing regardless of voltage? Isn't it going to depend on the resistance and the distance arc has to travel? In my mind the higher the voltage the more space you need to prevent the start of an arc as opposed to lower voltage. When you use a carbon arc type machine to cut metal, I believe what you say is true, the arc will self extinguish a the zero crossing. However, that 1/60 of a second is not real noticeable regardless of the voltage.

Does this mean lower voltage systems don't have an issue? Or at least can we say a 120/240V service needs to be a certain size before it is?

IMO a 30A service has the same issues as a 600A service if they are both fed off same xfmr.


Quote:
They show that for a 101 to 200 amp RK1 or Class L fuse, you would need protection Level 0 for fault currents above 4kA, Level 2 for a fault current of 2kA and Level X (incident energy exceeds 40 cal/cm^2 and energized work is not permitted) for a fault current of 1kA. For breakers in the same size range they show Level 1 for fault currents of 16 to 60kA, Level 0 for 4 to 14kA and Level X for 2kA and under.
Don


So how can you ever be safe? How can you guarantee you have at least 4kA or whatever of fault current? I can see how you can find a maximum fault current, limited by the transformer, wire, etc, but not how you can find a minimum.

 

[don_resqcapt19]

Larry,

Don, aren't you reaching that same zero crossing regardless of voltage? Isn't it going to depend on the resistance and the distance arc has to travel?

It is much more likely that the arc will "restrike" as the voltage climes after the zero crossing in higher voltage systems. This fact is the basis for the GFP protection that is required on 277/480 volt wye systems and not required on lower voltage systems.

IMO a 30A service has the same issues as a 600A service if they are both fed off same xfmr.

That is true only if you have the same impedance in the service conductors between the transformer and the service. Given that the 600 amp service will require much larger conductors the availbable fault current will be much greater at the line side of the 600 amp service disconnect.
If you have a transformer with 25,000 amps of fault current available at the bushings, you will have ~11,450 at the end of a 50' #4 feeder and ~22,450 at the end of a 50' set of parallel 350kcmil.
Don

 

[paul32]

That is the big problem with this issue...you can't. It is also the problem with doing the calculations as the utility system is so dynamic that you can have big changes in the available fault current caused by their normal day to day switching.

My point was even if the utility didn't change and you knew the AVAILABLE fault current, you still wouldn't know the actual fault current since it would depend on the impedance of the circuit.

Getting back to your "level X for 2kA and under", there must be some lower limit that would be no problem. A fault of 1A? Maybe because I don't know anything about how it is calculated.

Can we say a transformer needs to be above a certain size before PPE would be needed for 120/240V? I think I have seen comments on the forum before that there isn't an arc flash hazzard in a typical load center in a house. But talk of faults in the single digit kA range makes me wonder. If the statement is assuming a small transformer, then I wonder how small. My house is served by a 37.5 kVA transformer.

 

[zog]

Regarding conducting an arc flash study the IEEE 1584 states "Equipment below 240V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply"