2009 70E 130.3 Exception 1

Our facility uses 25kVA transformers (480 to 120/240v) for control power for discrete I/O controls. This is an example of meeting Exception 1 in section 130.3.

The tables suggest that performing a voltage test requires Category 1 PPE.
Is there a way to justify using Category 0 PPE - not requiring the use of a face shield?

Are you doing voltage tests at the I/O mod or at the transformer? If at the I/O, maybe fuse the secondary side of transformer to lower the AF after the transformer?

The exception says that transformers under 240V and 125KVA are exempt from analysis. That is because it is unlikely to have a sustained arced at this low a voltage. You just need to be in non-melting PPE, or level -1 to service.

There is no longer a HRC -1.

To clarify my previous post.
We have a 25kVA transformer (480 to 120/240) feeding a lighting panel. A 20A single pole breaker will then feed a junction box where the 120v is distributed to multiple I/O. Now each individual I/O has a 2A or 1A fuse that will feed the field device (such as solenoid valves, limit switches etc.)

I feel comfortable considering the load side of the fuses as a Category 0, but am not sure how to regard the line side of the fuses. The table in 130.7(C)(9) for panelboards rated 240V and below would seem to indicate a Category 1 PPE for troubleshooting. But...we have done an arc flash study at our facility and do not know how to deal with incident energy calcs with single phase.

My ultimate goal is to justify calling the junction box (that is fed by a 20A single pole breaker) a Hazard Category 0 so that we do not need to wear a face shield to troubleshoot.

I think you can safely make that assumption.

Zog,
I agree with your opinion. But I was hoping for some fact(s) that would back up the assumption. Or do I ask for too much?
I am new to all of this but it does seem that calculations for single phase is non-existant. The only info I have for this case is the table.

If you have the study available, put the load in as a 208V three phase behind the transformer, with the breakers in question. You will probably be pleasantly surprised by the results.
The 3 phase case should be worse case, so your actual energy levels should be even lower.

That sounds like a reasonable approach to assume 3 phase with a 3 phase bolted fault.

Just for kicks, I input a 25kVA 208V Xfmr with typical impedence into my software (SKM) and connected it directly to my main 161kV bus.
After clearing out the exception for <240V (which defaults to Cat 0 if less than 10KA bolted fault or < 125 kVA Xfmr), and giving it a 2 second clearing time, I ended up with 5.1 cal/cm.

Since you will also have other impedences and shorter clearing times, your values should be much less. I think Cat 0 would be reasonable, judging from this test case.

Actually, this raises the point that assuming Cat 0 for this case is endorsed by the IEEE 1584 and NFPA 70E if less than 240V, less than 125kVA Xfmr fed, or less than 10kV bolted fault current.

Interesting, thanks. The point is for these low energy system is that the IEEE 1584 group assumes that an arc will not be self sustaining (Like unplugging a hair dryer while running) and the clearing time is irrevelant. However, I have discussed this with people that have done a lot of arc flash testing that disagree with IEEE's assumption. Until we get some good single phase calculations (Is being developed now) we will have to go with the HRC 0 for the exception being discussed here.

Is there still a less than 10kA fault current exclusion? I thought the exclusion was just 240V or lower tnf less than 125kVA?

You are correct.

EasyPower under 240V <125kVA Options?

Does anyone know of such a option for this exception (circuits under 240V fed from <125kVA xfmr) with EasyPower software?

EasyPower: under 240V <125kVA exception option?

Does anyone know of an option for this exception (under 240V, <125kVA xfmr) in EasyPower software?

I don't know the internal of EasyPower but I have had some conversations with Chet Davis the owner, and he is always willing to help. Just email him and ask:
chet@easypower.com

Limit Arc Duration

I use EasyPower and overcome this issue by limiting the maximum arc duration for circuits <240 volts to .167 seconds. This .167 seconds comes from PG&E's arc flash studies whereby the longest sustained arc they produced at 240 volts was .167 seconds. The longest duration at 480 volts was 1.67 seconds.

Does 70E explicitly say you can assume Cat 0 under 130.3 Exception No. 1? I see that it says an arc flash hazard analysis is not required, but I can't find where Cat 0 is implied. The alternative is that you have to use table 130.7(C)(9) which does show examples of Cat 1 Hazard when performing tasks such as voltage testing.