I am completing an AF study at a facility that has a battery bank supplying DC loads (125 vdc). Input voltage to the rectifier is 480 vac 3 phase. There is the exclusion in NFPA 70E (pg 114) that AF analysis is not required for circuits below 240 volts if fed from a single transformer below 125 kVA. However, the closest transformer upstream to the rectifier is 300 kVA. Are there any regs or standards excluding the 125 vdc portions of the circuit downstream from the rectifier from analysis? Thank you

The exception to 'skip' an analysis does not provide an exception to skip labeling and advisement of proper PPE.

Guidance from ETA

Below are some guidelines from ETA which was generated following an Arc Flash incident on a UPS 600 volt battery supply.

PPE Selection Chart
Types of equipment
DC Voltages
Arc Flash Boundary
Minimum PPE Hazard/Risk Category
DC based control systems such as switchgear controls with DC voltages as indicated:
50 volts or more but less than 150 volts
Minimum of 4 ft.
Category 1
Small portable UPS systems 20KVA or less in size with DC voltages as indicated:
50 volts or more but less than 300 volts
Minimum of 4 ft.
Category 1
DC based power distribution and / or battery power distribution systems such as UPS systems, rectifiers, inverters, and similar telecommunications power support equipment operating with DC voltages as indicated:
50 volts or more but less than 600 volts
Minimum of 4 ft.
Category 3
DC based power distribution and / or battery power distribution systems such as UPS systems, rectifiers, inverters, and similar telecommunications power support equipment operating with DC voltages as indicated:
600 volts or more but less than 1000 volts
Minimum of 6 ft.
Category 4

I would use extreme caution using the above.Some of these battery cell units can have very high fault currents.(10,000-14,000 amps).There does not seem to be any provision for these high fault currents in this listing.

DC Arc Flash Analysis

I find that the amount of energy released by dc arc would greatly depend on arc gap, circuit time constant, system voltage and time current characteristic of circuit protective device clearing the fault.

Eta

Who is ETA? Couldn't find anything in the search engine on Google.

Who is ETA

Eta is Engineering Consultants....

I just received an email from the folks at NFPA. The email discusses several new proposals that may be added to the 2012 version of NPFA 70E. Below is one of these proposals that deal with DC systems.


NFPA 70E — Moving Forward with the 2012 Edition
Jeffrey S. Sargent

Addition of Specific Requirements and Annex Material on Direct Current (dc) Systems. Users frequently ask whether NFPA 70E applies to direct-current systems. Nowhere in the standard does it specify that it applies only to alternating-current systems. OSHA obviously does not exclude dc hazards from workplace safety requirements. While it is quite obvious that many of the 70E requirements are founded based on ac systems (because ac is predominantly used), the purpose of the standard does not indicate that it is to only provide a safe working area for employees exposed to alternating-current hazards. Electricity includes ac and dc systems, and there are hazards associated with both types of systems. It is interesting that in many circles the same belief exists relative to the NEC®; that it is only an installation requirement for ac systems. This is equally untrue. In fact, there are many NEC requirements that are specific to dc systems. Based on committee actions related to proposals specific to dc systems, the 2012 edition of NFPA 70E will clearly address the hazards associated with direct-current systems. Included in the committee actions on dc system safety requirements are actions to add a dc approach boundary table similar to current Table 130.2(C) and a dc task table similar to current Table 130.7(C)(9).
The complete article can be found at
http://www.necplus.org/Features/Pages/NFPA70E%E2%80%94MovingForwardwiththe2012Edition.aspx?sso=0

Summary of article

I read the NFPA article referenced in the prior post, here is a quick summary, 5 topics.

Quick summary:

1) Inspections tasks will be added, in section 90.2, the "Scope" section at the beginning of NFPA 70E.
2) Hazard/Risk Category Table will be made easier to read; notes at end of table moved to body of table.
3) Equipment labeling changes, method of label information development must be on label and other changes too numerous to list here.
4) DC systems covered, this is new.
5) Elimination of Hazard/Risk category "2*"; it was not easy to see what the "*" stood for, in some handbooks. It is/was a distinction on type of headwear.

I've been following the 2012 edition closely and the one thing that jumped out at me is labeling. - Many of you have been following the debate that I have been having with a few NPFA members)

Now the label requires AT LEAST ONE of the following:

Incident Energy
Minimum arc rating of clothing
Level of PPE
Highest HRC

This no longer says one OR the other any more. In fact, it looks like they had to add a grandfather clause for labels that only had one OR the other. - The postion that some strongly held.

My congratulations and thanks to the NFPA committee for sorting out what was a very confusing issue and making it more practical.