ARC FLASH LABELING FOR 120V CONTROL PANELS

[PE (always learning)]

Hey Everyone,

I am being hired to do an arc flash study for an industrial facility that contains hundreds of control panels. The owner is wanting to provide arc flash labels out to almost every panel in the facility and he even included 208V single phase and 120V control panels in his list. I know that table 130.7(C)(15)(A)(b) in the NFPA-70E shows that single phase panels, 240V and below should be labeled as PPE category 1, but I'm curious if it's still valid to use this table if I'm using the incident energy analysis for every other panel 240V and greater in this facility. Is it standard to just provide a general arc flash warning label for these small panels? Please let me know your thoughts.

Best Regards

 

[tom baker]

I am sure you are familiar with the requirements in 110.16 (A). It does not mention voltage. There is an IN to 70E. I would put a label on the 240V and below, it could be a generic label that shows the voltage and PPE.

 

[Jraef]

If it is under 1.2 cal./cm^2, it is HRC/PPE zero, which I think has been more recently changed to "no special PPE necessary". Long sleeve shirt and pants of non-melting fabric (like cotton), safety glasses/goggles, ear plugs, voltage rated gloves. Basic stuff.

Technically, you COULD have enough arc flash energy in a "120V" panel if the current were high enough, but if it's a control panel fed from a 1P 20A breaker in a panelboard somewhere, you be somewhere south of 0.4cal.cm^2, meaning not even close.

 

[paulengr]

You cannot sustain arcs under around 200 V. This is the reason IEEE 1584 is only valid starting at 208 V. At 208 or 240 an arc is possible. There has been one documented case. Nobody believed it was possible before about 2010. Then there was an incident and some lab tests have corroborated it.

You can’t get there under around 300 V in open air or in the VCB or HCB scenarios. You need a box-barrier scenario which unfortunately is what you get where cables terminate into the top of an MCCB.

The data in the following is old (before the new 1584) and the guys at Etap basically plagiarized me and added a little to it. There is further testing data privately held in the 1584 database I can’t release but since the results are public I just used published papers. But the principles remain. This is the reason 1584 was extended down to 208 V.

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The problem with 1584 and really ALL models below about 300 V is that arcs self extinguish but we don’t have a good model to predict this. 1584 just ignores the issue and assumes arcs are sustainable indefinitely...well up to 2 seconds anyway. Thus the best and most correct way to model this is by using actual test data, which you can get from EPRI, or using the NESC or 70E tables. The trouble is neither contains a lower cutoff.

As referenced in the paper you can’t get an arc to sustain long enough to get to level 1 even with DC which is naturally self sustaining because it has no zero crossings at 130 VDC even up to 20 kA. But if you have a typical panel (240 or 208) the top of the panel will be an issue. But still in IEEE testing they had to drop the arc gaps to 1/4” and use box barrier to get anything. So depending on the equipment design you can usually safely ignore it anyways.

This is where a monkey plugging numbers into a computer program is just that...a trained monkey. You must use engineering judgement when doing the analysis or you get garbage in, garbage out. Or in Trumps words, You’re Fired!

 

[GeorgeB]

You cannot sustain arcs under around 200 V.

Umm, when I was a wee one, we took the carbon positive rods from D cell batteries, made a saline resistor, and struck and maintained arcs for many minutes from "110V". Later I was given "carbons" by a friend who owned a movie theater and we went a lot longer.

How many arc welders do you think have over 100V at the electrode?

I think there is more to your reason than what you have given us.

 

[paulengr]

Umm, when I was a wee one, we took the carbon positive rods from D cell batteries, made a saline resistor, and struck and maintained arcs for many minutes from "110V". Later I was given "carbons" by a friend who owned a movie theater and we went a lot longer.

How many arc welders do you think have over 100V at the electrode?

I think there is more to your reason than what you have given us.

In both cases though how far are you away? Maybe a foot or two? So did you receive a second degree burn in under 2 seconds? No? Did you trip the breaker? That’s not an arc flash or power arc.

Those two examples use a conductive or semi conductive vapor, not air. Carbon arc lamps arc through a carbon vapor.

Arc lamp - Wikipedia

en.wikipedia.org

Similarly arc welders arc through a metal vapor.

In an arc flash the vapor is just air. Arcs are not linear. In one region things stay pretty much under control and we can do useful things like HID lighting and welding. In another region the arc voltage drop becomes almost constant irrespective of current and so we lose control and arc current becomes limited almost entirely only by system available short circuit current.

That’s IF the arcs self sustain. That’s easy in a conductive gas but not in air. Hence your examples show that low voltage arcs in conductive gas can self sustain quite easily but a generally nonconductive gas is another matter. Air can only sustain restriking if it gets hot enough so that it doesn’t cool so far between restrikes (every half cycle) that it is self sustaining. At DC this is possible only at extreme conditions (20 kA available current, 1/4” gap) up to 80 milliseconds at 130 VDC. At lower currents it would not sustain at all. This just crosses the 1.2 cal/cm2 threshold at 18”. Some people have seriously misrepresented this data (lied) by using the experimentally measured arc thermal power, changing the working distance to 12” and just recalculating it at 2 seconds given the arc power at 80 milliseconds. Which is clearly fraudulent science at best. At 120 VAC there is no way it can possibly approach the same conditions given the limited time the voltage exceeds even 130 VDC.

If this seems confusing let’s make it more clear. Here is what happens at DC.

https://d3i71xaburhd42.cloudfront.net/1da9b310465bcf1039f0ffe5d3772ef1e4d6f408/2-Figure2-1.png

Notice the arc is not linear. To the left we get low currents (neon lamps, welders, HID). To the right we get power arcs where current is limited only by the system short circuit current. So in AC once we ignite an arc it continues to burn until the current goes through zero...technically arc voltage reaches a few Volts where it drops out of the power arc region. This happens every half cycle. Then air starts cooling down. As voltage rises again if it reaches a pint where it can restrike the arc reignites and remains lit again fir the rest of the half cycle. If the air cools down too far it just self extinguishes and stays that way. This gives us the classic “square wave” arcing fault voltage. It is far more pronounced at low voltages than at medium voltage where the line voltage gets to the flashover voltage very quickly so it’s only really obvious in AFCI test data. I have some nicer charts but they are from IEEE papers (pay wall).

https://www.researchgate.net/profile/Zoran_Radojevic/publication/3275462/figure/fig2/AS:394655339433999@1471104573916/Real-arc-voltage-and-current-waveforms_Q320.jpg

This is not to say that AFCIs aren’t a solution looking for a problem. They are intended to address sparks (left side of the above), not arcs. In 120 V circuits power arcs will easily trip the instantaneous protection but not before wood can catch fire.

 

[PE (always learning)]

This is where a monkey plugging numbers into a computer program is just that...a trained monkey. You must use engineering judgement when doing the analysis or you get garbage in, garbage out. Or in Trumps words, You’re Fired!

 

[PE (always learning)]

Ignore the previous pose, this is what I was trying to say:

This is where a monkey plugging numbers into a computer program is just that...a trained monkey. You must use engineering judgement when doing the analysis or you get garbage in, garbage out. Or in Trumps words, You’re Fired!

I'm a highly trained monkey! But, yes I see your point. Engineering judgment must be used on this. All of the transformers ahead of these 120V control panels are 75 KVA and lower. Because I can't measure the single phase fault currents with my program, I'm going to double check the available fault currents on the secondaries of these transformers and see if they are at an acceptably low level. If they are low enough, then I will provide a generic label. The label will give no arc flash PPE level, but it will give the shock boundary limitations.