NFPA 70E Battery Requirements 1

[EEbyChoice]

We operate a fleet of diesel generators. Most of these generators utilize two 12 Volt batteries in series to provide 24 VDC starting power at approximately 400 Amps continuous cranking current. The batteries are rated at 190 Amp Hours (20 hour rating) with 1300 cold cranking amps.

We recently completed an NFPA 70E arc flash hazard analysis and were told by the engineer conducting the survey that our batteries meet the requirements of NFPA 70E article 320 (Batteries and Battery Rooms). It seems to me that the intent of article 320 is to cover larger battery installations, such as large commercial UPS installations. (It states that any battery with more than 1 KWH of storage is covered)

The hazard associated with starting batteries certainly warrants caution, but according to the analysis done, we have to wear a 20 cal/cm2 suit just to work near the batteries (due of course to electrical arc flash hazard, not chemical hazard).

Something about this just doesn't sound quite right. If this is truly the requirement, we will certainly abide by it, but I want to make sure that we are not mis-applying the intent of the NFPA 70E article 320 standard and creating an undue burden on those working on this equipment.

If anyone else has been down this road, your assistance will be greatly appreciated.

 

[stevenal]

The current is low, but the time of exposure is long; providing you are just standing there like a test dummy. Did your engineer factor in some human response? At high currents damage can occur before any response, but lower currents would allow a person time to remove himself from the hazzard. I think the chemical protection would be enough in most cases as long as the exit route is unobstructed.

 

[dpc]

I don't think the requirement for 20 cal/cm2 suit for working on a 24 V battery system can be found in NFPA-70E. It might be a good idea, but I don't believe it's required by 70E.

The arc-flash section of 70E is limited to 50V and above. The battery section (320) is limited to larger battery installations and says nothing about arc-flash. As you mention, the scope of Article 320 would definitely not include the type of installation you are describing.

 

[EEbyChoice]

dpc -

I agree with your train of thought, however, the 20 cal/cm2 requirement wasn't taken from article 320. Our engineer used the incident energy calculation taken from Annex D, paragraph D.8.3. This calculation does not specify a minimum application voltage. If you plug in the available short circuit current and use 12 volts, he came up with 18.5 cal/cm2 incident energy.

It is interesting to note that in Article 320.8, PPE Requirements, there is no requirement for arc flash/electrical hazard related PPE, only PPE to prevent chemical burns is required.

Why is this when in Article 320.7(f) a sign indicating arc hazard due to short circuit current is required? Also, the SCOPE of article 320 states that it applies to batteries with a stored capacity of 1 KWH or greater, which these batteries appear to have, i.e. 12 volts with a 190 amp hour rating (20 hour rating) yields 9.5 amps @ 12 volts for 20 hours or approximately 2.28 KWH.

Please let me know if anyone thinks that I am missing the boat, but I am trying to find out exactly why the incident energy calculations performed by our contract engineer should not apply.

 

[dpc]

All of the information in Annex D pertains only to ac systems, as far as I know. The equations in Section D8.3 in Annex D are taken directly from the IEEE-1584 standard and cannot be used for dc systems. This is stated explictly in IEEE-1584. So if the arc-flash calculations for your 24V dc systems are based on equation D8.3, the results are pretty much meaningless, in my opinion. Your engineer needs to read IEEE-1584.

Besides, Annex D is not even part of the official NFPA 70E.

Also, no PPE requirements are given in Article 130 for systems below 50 V.

DC systems remain poorly defined in the standards in terms of arc-flash hazard. No doubt there are hazards, but we do not have any accepted method for calculating the potential arc-energy.

 

[alehman]

12 or 24V is not sufficient to sustain an arc. If there is a fault there will no doubt be some flying sparks which may cause minor burns or injur eyes, but I don't think there's any arc flash risk. I agree that this would be a misapplication of 70E. I'm curious how the engineer came up with the 20cal number.

For higher voltage batteries, yes there is a significant hazard, but 1584 doesn't apply.

A certain amount of common sense needs to be used in applying codes. As another exmple, I recently had a fire marshall in a large U.S. city insist that we install exhaust fans with hoods over the engine starting batteries. How far should we take this? By that logic, we would have needed exhaust fans for battery powered egress lights also. I didn't ask.

 

[EEbyChoice]

Thank you for the many responses to my original question. One last issue that you may be able to help with. Can anyone direct me to a copy of the IEEE 1584 standard, or at least the section that shows that it applies only to AC. Not trying to circumvent copyright laws, but I don't need the whole thing, just the one part.

Any help would be appreciated.

Thanks,

 

[stevenal]

1.2 Purpose
This guide presents methods for the calculation of arc-flash incident energy and arc-flash boundaries in
three-phase ac systems to which workers may be exposed.....

Single-phase ac systems and dc systems are not included in this guide.