Application of 2007 NESC Arc-Flash Requirements 2

[dpc]

I'm curious how others are dealing with the 2007 NESC arc-flash protection requirements and the apparent contradictions with existing NFPA 70E?

In particular, I'm wondering about the provisions for dealing with systems less than 1000 V. The NESC says that in lieu of doing an actual arc-flash study, a hazard of 4 cal/cm2 can be assumed under 1000 V. For 480 V systems, this can be absurdly low when compared with the requirements of NFPA 70E and IEEE 1584. This is especially true when dealing with the 480V portion of a pad-mounted transformer that probably has minimal primary protection.

Does anyone know the basis or origin of the 4 cal/cm2??

Also, for substations using metal-clad switchgear, the IEEE 1584 calculations would seem to be much more relevant than the NESC tables when dealing with switchgear. Anyone made any decisions on this?

 

[Breakerboy]

When talking about the hazard of a 480v load center, ours range from a low of 16.1cal/cm2 to a high of 25.1 cal/cm2. So using 4 cal/cm2 as a base for your hazards would be a bad assumption. For 480v MCC busses we have a low of 10.6 cal/cm2 and a high of 38.1 cal/cm2. Your calculations need to include the actual clearing times for the components that you have in your circuts. The clearing time for the total of the fault detecting relay, the trip relay and the breaker clearing speed must be added and the calculated with the maximum current. Remember you must clear the bus not the breaker with the fault. The breaker with the fault may not be able to clear since it was involved in the arc flash blast. I cannot emphasis enough the importance of doing your own calculations with the equipment installed in your plant, not some imaginary gear and you must include all of the equipment necessary to clear the fault.

 

[Grollor33]

I have subsequently performed the arc flash calcs for the 480 volt system I was working on.

Per the calcs I agree with Breakerboy, 4 cal/cm^2 is certainly not a good number to use for a rule of thumb...

that is if the standards have a correct approximation method...


would be interested to see any test data of the LV side of life...