Yet another arc flash question

[grumpyoldgeek]

I'm a little-wire electrical engineer and I own my own business. I'm looking at having a new facility built and I was wondering about all this arc-flash stuff I've been seeing. The new facility will likely have a 600 ampere 208 3 phase service entrance. Is there any concern about arc-flash with this sort of installation? Should I specify a study to be provided with the installation? Thanks in advance.

 

[dpc]

IEEE-1584 (recommended calculation methods for arc-flash) suggests that 208 V systems 125 kVA and larger be evaluated for arc-flash hazard using their calculation method. Unless the utility really undersizes your transformer, your service will probably be larger than 125 kVA.

But if you're trying to meet NFPA-70E a study is not required. NFPA-70E has (fairly) simple task-based tables that can be used in lieu of doing an arc-flash hazard evaluation, provided your system meets the requirements in the fine print.

 

[grumpyoldgeek]

Thanks. Can you tell me when, as a building owner, I might need to have an arc-flash study available?

 

[wbd]

Bear in mind that the tables in NFPA 70E are conservative and by having a study done, you may find that your PPE requirements are different than what the tables in 70E indicate. Typically, you usually end up with less PPE than the tables call for. That can save dollars and make it easier for the electricians to do their work.

 

[dpc]

What OSHA would want to see first is your safety plan or procedures. They would also look for appropriate signage for any workplace hazards.

You can just get a copy of NFPA-70E and make that the basis of your safety plan. That will make OSHA happy, especially if you actually follow it.

 

[Electic]

You can meet the intent of NFPA 70E by posting generic arc flash warning labels without a study having been completed.

The study is performed to determine Personal Protective Equipment if the electrical system needs to worked on while energized, with workers exposed to energized conductors.

If you institute a policy, that no work will be done on energized systems, I don't see a need for a study.

If your system is simple a study could be minimal.

 

[wbd]

Just remember that work on energized systems includes voltage and current measurements using hand held test equipment. Making a blanket no work on energized systems eliminates the possibility of troubleshooting using multimeters.

 

[peebee]

Or even an infrared scan, or just removing the cover to see what wire or breaker sizes you have in your panels....

 

[Electic]

There is a lot of fear being traded under the guise of arc flash protection. Some of that is justified on large industrial installations, but this question pertained to a 208V system serving an office.

1) Most offices can afford an outage for maintenance (usually adding a new circuit)

2) How often does an "office" panel require troubleshooting while energized; most I've seen either work or something obvious is wrong. These are not complex control systems apt to require trouble-shooting. Not unlike your house.

3) Under 240V, NFPA 70E prescriptive tables (no calculation required) allows hinged covers to be removed with exposed bare energized conductors with Hazard risk category "0" PPE; that is an untreated long sleeve cotton shirt, pants and safety glasses. That would allow thermal scan and current measurements (clamp on with no contact to energized conductors).

4) Under 240V NFPA 70E prescriptive tables (again, no calculation required) will allow work on energized parts with risk category "1" PPE; that includes flame resistant long sleeve shirt, pants or coverall (basically one layer of nomex or flame resistant cotton), hard hat safety glasses etc. That is a base level of protection some electrical employers are requiring.

5) Once again as an office facility, it is unlikely the owner of this building will have his own maintenance electrician or be in some way responsible to determine PPE requirements from NFPA 70E. Maintenance work at office facilities is generally contracted out, and I would expect the journeyman be aware of arc flash hazard and provide the above basic level of PPE, determined without calculations from the prescriptive tables within NFPA 70E.

6) The legal obligation is to warn of potential hazards. I don't see that as having to 'quantify' potential hazards and in fact, trying to quantify could constitute a greater legal liabilty (I.E.:are you also going to qualify a contractor's PPE?).

The generic labels have been installed in other locations and accepted by all interested parties. If there is some aspect of this office that renders greater than normal fault current or the need to maintain electrical systems while energized, then probably a full blown analysis is needed, as part of an electrical safety plan COMPLETE WITH EMPLOYEE TRAINING. That is a big deal, and I do not read that into the question.

The above answers are obviously simplified and do not constitute a full application of NFPA 70E or NEC 110-16.

 

[Electic]

Corrections (Oops!) In the previous response, I assumed from the original post that this was an office facility (i.e.: "engineer"), whereas if it constitutes some sort of continuous process plan needing electrical work performed while energized, that would constitute reason for a study.

Also, I am not sure where the 125kVA requirement came from in the first reply (I might have overlooked it). Within NFPA70E Table 130.7(C)(9)(a) notes there is a requiremnt that these rules be applied to no greater than 25kA fault current. Our local power company specifies transformers such that a 150kVA transformer has approximately that fault current available at the secondary bushings. It would be substantially less with a typical 40' service entrance conductor. Also, the local power company would typically size an general commercial building transformer at approximately 50% of calculated (NEC) demand so it is plausible that locally you would have less than 25kA available fault current.

 

[wbd]

The 125 kVA requirement comes from IEEE 1584, Section 4.2:

When the basic electrical system scheme is complete on the diagrams, add the data needed for the shortcircuit
study. The study must take into account all sources, including utilities, standby and power generators,
and large motors—those 37 kW and larger that contribute energy to short circuits.
10
The diagrams must
show all transformers, transmission lines, distribution circuits, electrical system grounding, current limiting
reactors and other current limiting devices, voltage correction or stabilization capacitors, disconnect
switches, switchgear, motor control centers (MCCs), panelboards/switchboards including protective devices,
fused load interrupter switches including fuse types and sizes, feeders and branch circuits, as well as motors
down to the 600 V or 400 V level, and transformers supplying instrument power and protective devices.
Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger lowimpedance
transformer in its immediate power supply.

 

[dpc]

One interesting aspect of IEEE-1584 is that is does not really specify the impedance network to be used for the short circuit calculations. Many people do not include small motor contributions when doing short circuit calcs for arc-flash. I believe that it just says to calculate the bolted three-phase fault current.

 

[wbd]

IEEE 1584 does state that motors greater than 37 kW (50 hp) must be taken into account to consider the energy added to the short circuit.

 

[dpc]

True.

Of course, it's all an approximation anyway. For a 15 cycle fault, a 50 hp motor fault contribution will only contribute for perhaps a cycle or two. Including all the motor fault contributions could be non-conservative if the higher fault current leads to an overly optimistic clearing time.