Arc Resistant Switchgear 2

[whycliffrussell]

What does arc resistant switchgear do with respect to mitigating arc flash hazards? I know, qualitatively it reduces the risk an an arc flash incident, but how may this be quantified with respect to calculating arc flash hazards?

For instance, if I have a switgear lineup that has an arc flash hazard rating of 10cal/cm^2 (level 3), and I replace the line up with arc resistance switchgear, what is the NEW arc flash hazard at the switchgear lineup in cal/cm^2? Surely, there must be a limit to how much energry it may divert from a worker in the event of an arc flash hazard?

Additionally, if someone were to say perform some live work on the exposed bus in the arc resistant switchgear how is there any arc flash mitigation?

Who certifies arc resistant switchgear to say that this switchgear is arc resistant? For instance I could install insulated bus in a switchgear lineup and call it arc resistant (but no arc proof) since there will always be some exposed area of buswork inside the switchgear that my present some arc flash hazards, granted i understand that arc resistant switchgear is designed to contain and expel the hot gasses associated with the explosion in a controller manner through a chimney or duct, but i would have still reduced the liklihood or an arc flash incident occuring my insulating the exposed live parts.

Am I correct in saying that arc resistant switchgear generally only provides protection for non-intrusive work (ie: work such as switching circuit breakers) where an individual is not removing the covers to do work? Addtinoally, does this mean, for tasks such as switching circuit breakers that for arc resistant switchgrear that the arc flash hazard has been reduced from, say 40 cal/cm^2 to 0 cal/cm^2?, even though the same may not hold true for such intrusive tasks as opening up the switchgear live to perform work?

 

[Laplacian]

It is not arc-resistant with the doors open. Here is Powell's tech brief on the subject:

http://www.powellind.com/main/Uploa...ant switchgear tested to ansi and nfpa70e.pdf

 

[Zogzog]

Your assumptions are correct, the switchgear only protects with covers on. The 2009 70E tables will take arc resistant switchgear in to account for PPE requirements (HRC).

Here is a good link for arc rated switchgear, watch the movies on the bottom right of the page.

http://www.abb.com/product/seitp328/c1256ccb004e3abbc1256841004c460a.aspx

 

[dpc]

Regarding certification of arc-resistant switchgear, ANSI has an arc-resistant switchgear standard (as does Canada)that includes testing requirements. There are different levels of protection that can ordered. Lowest category is for protection from the front of the gear only. Another level covers front, back, and sides. Highest level requires protection between adjacent sections.

All these require some means of getting the high blast pressure out of the enclosure - normally a big duct or exhaust hood that runs outside.

 

[Eleceng01]

dpc is correct (as usual) but also remember that the standard (IEEE Std C37.20.7) requires that the fault be cleared within 0.5 seconds.

We recently ran across a situation where this could not be met and the switchgear manufacturer sent us a nice letter stating that they tested in accordance with the standard and that after 0.5 seconds "all bets were off" (after talking to their legal department).

 

[whycliffrussell]

Eleceng01:

Am I understanding you correctly in saying that in order for the switchgear to be arc resistant the fault in it must be cleared w/in 0.5 seconds? (I've seen several applications where this is not the case and sometimes there was not simple resoution to managing the FCT).

Switchgear Mfgs. test their switchgear to IEEE Std C37.20.7 OR ANSI OR BOTH (is this IEEE std. and the ANSI std. the same thing?).

One last thing:

How much protection can the switchgear provide? ie: If i have a situation, say where the incident energy available is 100cal/cm^2 with "regular" switchgear and i replace it with arc resistant switchgear, all other parameters consider the same (bus gap, working distance, and FCT, etc...)(and for arguments sake lets say that the FCT is < 0.5 seconds), does this mean that with the covers on the incident energy available around the equipment (say someone is doing something non-intrusive like throwing a CB) is going to be 0 cal/cm^2 (level 0?)

Surely there is a limit how how much incident energy the switchgear can "shunt" away from an individual and exhaust in a controlled manner.

There does not appear to be any quantative merit to this arc resistant (though not arc proof) switchgear.

Cheers.

 

[dpc]

You have to look at the parameters used in the ANSI requirements and tests. It is quantitative, but there are limits to what the switchgear can withstand.

If the fault current and clearing time is within the specified parameters and the doors are closed and latched, then the incident energy is low enough that cotton cloth adjacent to the switchgear will not ignite.

So I'm not sure what you mean when you say there is no quantitative merit.

By the way, a NFPA Hazard/Risk Category of Level #0 does NOT correspond to 0 cal/cm2.

 

[JJHorak]

I can see what Cliff is getting at. Suppose arc flash is calculated to be level 4, given standard switchgear. Now suppose you have some arc resistant switchgear but the same relaying and fault duty and theoretical arcing time. Maybe it is safer, but what does it mean to the guys that need to work on or near the gear?
--It is still level 4 once they open the doors to do their work, so the arc resistance has no value when you work on the gear, and there is no quantifiable benefit with the doors open.
--It seams having arc resistant gear means you can work near the gear more safely, as long as the doors are closed. However, on the other hand, if I knew that there was level 4 arc flash, I would steer clear of suggesting standing in front of the door in level 0 clothing as you rack breakers in and out. Is that the quantifiable benefit? "Sure, you are safer now, but we still recommend you treat this like the door was going to blow off, just in case."

Given the above, I am not sure the gear gained you that much.

 

[dpc]

Well, I know which one I'd rather be standing in front of, regardless of what level PPE I was wearing.

 

[Zogzog]

I would rather be standing 30 feet away using remote operators/racking myself.

 

[dpc]

Yes, I think we all agree about that. But someone still has to set up the remote racking device, and this is not the only work done around switchgear that has some risk.

 

[Eleceng01]

whycliffrussell,

I wasn't trying to be the expert on this, but I was only trying to share the information that we were given recently. As dpc says above, they test in accordance with IEEE and/or ANSI standards. My understanding is after 0.5 seconds, it doesn't matter if you are standing in front of arc-resistant switchgear or not - they both will act the same.

Also, if the protection isn't clearing the fault in 0.5 seconds, then you aren't getting any benefit by spending the additional money on the arc resistant switchgear.

Just MHO.

 

[whycliffrussell]

guys:

thanks for the discussion/brainstorming on this.

dpc/Eleceng01 you points are noted; however, the point that I was actually trying to make was caught by Timesabroom. thanks guys, i appreciate all the constructive commentary. i just wanted to see what other peoples views/opinions are on this, and i'm of the same frame of mind as Timesabroom.