What are the Arc Flash requirements with respect to dwelling units?

This question is geared more towards high rise apartments and condominiums, with higher fault currents and potential for self sustaining arcs than single family homes. These are also likely to be run by a management company.

• NFPA 70 110.16 Exempts dwelling units from labelling requirements.
  
• NFPA 70E doesn't appear to make a distinction between dwelling and commercial.
  
• IEEE 584 doesn't cover single phase systems, but I see no reason not to apply conservative results from 3 phase systems.
  
• OSHA Doesn't exempt residential as far as I can tell. A management company may have subs or in house qualified people tasked with maintenance on residential load centers. While there's no justification for working hot in an apartment, they still have to suit up to check it dead.

How is the Arc flash community handling this type of installation? Should we recommend studies be performed in these occupancies? Is there any downside to using the 3 phase approximation for 240V or 208V single phase systems?

Thanks for your responses

For now there is no Arc Flash label requirement for dwelling units, but I don't see why we cannot recommend it as good safety engineering practice, what happens if an electrician is to replace a fuse with HRC above 4? it won't matter if it's residential or commercial, danger is danger, and the right thing and good engineering practice would be to do everything necessary to protect electrical workers, hopefully this will be addressed in the new standards to come, as NFPA70E and CSA Z462 are still in their infancy stages

[quote="Ex twidget"]
Is there any downside to using the 3 phase approximation for 240V or 208V single phase systems?

What is the 3 phase approximation for 240v or 208v single phase systems?

ok. There is no "3 phase approximation". Let me rephrase it...

IEEE 1584 equations are derived from 3 phase arc events. They say single phase to phase, phase to ground arcs rapidly escalate into three phase arcs.

I've read that energy from a single phase arc is substatially lower than from 3 phase arcs. If this is true, then applying IEEE 1584 equations to single phase systems would yield conservative results.

On the other hand, I don't know if this is true. What if the single phase arc presented a higher impedence and persisted longer than a three phase arc? This would lead to longer trip times and possibly higher energy.

It's really a 2 part question:

1. Can the IEEE 1584 equations be conservatively applied to single phase, residential type systems?
   
2. Does OSHA and NFPA 70E apply to these residential systems even though the 2008 NEC exempts them from labelling requirements?

Thanks for sharing information

Yes.



OSHA applies to employers and employees, so no, not unless there is construction or remodelling going on by a contractor.

NFPA 70E is a voluntary standard. It is referenced by NEC so it is not Code required but Code recommended. It is also recognized by OSHA but not required by OSHA. Similarly the companion to both of them, NFPA 70B, is not Code required but is referenced by NFPA 70E. You are free to substitute an alternative so long as you meet Code (NEC) requirements.

The IEEE 1584 committee has a cutoff in the current standard below 240 V so for residential cases the assumption is that an arc cannot be sustained long enough to create an arc flash hazard. The assumption is that unqualified personnel (homeowners) are never put in a situation of having exposed conductors and NEC requirements are designed to meet this. Maintenance is of course on the homeowner.

So a contractor who performs electrical repairs on homes/apartment complexes and works in these panels...knowing that the short circuit currents can sometimes be found to be higher than previously assumed, how does the contracting company ensure OSHA compliance? No scc levels posted, no arc flash labels required, thus no idea of the actual danger levels, etc., yet OSHA required the employer to ensure a safe working environment from known hazards...hmmmm....Feedback?

Have to be careful here. OSHA holds employers responsible for RECOGNIZED hazards and taking industry recognized steps for responding to them. Although there is some experimental data out there that suggests that there are arc flash hazards <240 VAC, it is not a hazard which is generally recognized by industry...ie, no standard documenting the size and scope of the hazard so it is not practical to do anything yet other than perhaps take some reasonable precautions such as standing away from the breaker while energizing/de-energizing it. If we turn the clock back 20 years ago although you could hardly say that arc flash was not a recognized hazard, the industry consensus standard for safety was just that...stand to one side when operating the breaker.

Stepping away from your situation, I have a 22.9 kV distribution system. There are only three available methods for addressing the hazard/risk. First is the tables in 70E. The second is to use ArcPro software. The third is to use the calculation in Annex D.1 or D.2 which results in grossly overly conservative values...3 times the values actually measured at 15 kV and it gets increasingly distorted as the voltage and/or current increase and is only valid for equipment configurations which don't address my major concern (indoor switchgear lineups). Given that only one of the three methods (70E tables) has anything close to an industry consensus behind it, that's what I use.

In the case of residential work, the industry consensus that is currently published in IEEE 1584 is that there is no appreciable arc flash hazard with transformers smaller than 125 kVA and <240 VAC. Even using the tables in 70E, you'd be looking at H/RC 0 for almost all cases. There is a caveat here...the size of the transformer for IEEE 1584, and the available fault current and interrupting time for 70E. You don't have enough information to even begin to verify either one. You may consider at best, looking at the BLS data to see if a similar injury has ever occurred to what you are considering.

So similar to the world above 15 kV, you have to make an evaluation of the available standards themselves and go with that. Hence the reason for adopting the tables in 70E as the best you can do given limited information. It's better than "stand to one side". If you try to do something like recommending something beyond a recognized industry consensus safety standard "just in case", you also put yourself out there defining the standard with no defensible standard to work from...a legally precarious position to be in because if something ever did happen, your judgement gets called directly into question as to whether or not you went far enough as opposed to leaving the lawyers to argue with the NPFA and a panel of industry experts which carry far more weight than a single engineer or safety manager or maintenance department manager.