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 Post subject: 70E: Table method for Selection of PPE
PostPosted: Thu Jan 08, 2015 5:00 pm 

Joined: Tue Dec 02, 2014 7:46 am
Posts: 14
Do I need to apply this table method to select PPE to work on a 120V panel board ? According to this table panel boards and other equipment rated 240V or below shall have Category1 PPE. We have simplyfied the PPE categories to two levels. Leve l2 PPE for Category 1&2 and Level 4 PPE for Category 3&4. So in this case I guess our employees need to use Categry 2 PPE for working on a 120 volts panel board. Please let me know if I am correct in my assumptions.

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 Post subject: Re: 70E: Table method for Selection of PPE
PostPosted: Tue Jan 13, 2015 2:51 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
Tricky subject. IEEE 1584 has an exception that is much talked about that says that circuits fed by 3 phase transformers rated 208 V or less and that are under 125 kVA need not be rated. Data from Duke Power for DC (not AC) circuits shows that the incident energy for 130 VDC circuits is less than 1.3 cal/cm^2 when corrected for distance and the fact that the arc only sustained for 0.8 seconds. Unlike DC, single phase AC equipment extinguishes and must restrike 120 times a second so it is even less likely to sustain an arc. Similar data abounds. I'm finding very little evidence to support any arc flash PPE requirement for 120 VAC equipment. At 240 V and above though there is much more of a case for arc flash PPE of some kind. NESC published a table recommending 4 cal/cm^2 for all equipment rated 250 VAC or less based on actual test data but the lower cutoff for utilities is 4 cal/cm^2 PPE so this isn't really necessarily a lower limit in reality and you'd have to track down their actual test data to determine how it really applies.

The problem with low voltages is that arcs tend to self extinguish and that the arcs aren't very stable so the actual incident energy is lower than predicted by IEEE 1584 calculations. Best thing to do is to take a tabular approach. That's what we did using the IEEE 1584 exception as a "1.2 cal/cm^2" cutoff and everything outside of that as a 4 cal/cm^2 cutoff (using NESC as a reference).

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