NFPA-70E Table 130.7.c.a question

[Eleceng01]

Ok, I am fairly new to the arc flash issue so I am hoping to get some assistance from the more experienced engineers out there.

In the table notes, it mentioned clearing times of 0.03sec (2-cycles). What devices clear in 2-cycles (other than fuses)? Does this include breaker operating time?

I am just trying to fully understand this 2-cycle note. Thank you in advance for the advice.

 

[dpc]

Molded case circuit breakers generally clear in less than 2 cycles on the instantaneous trip portion of their protective curve. Some are much faster than that. You can look at a TCC for any molded case circuit breaker. There may be some exceptions for very old breakers, but modern breakers is generally faster than 2 cycles. You may be thinking of medium-voltage and high-voltage breakers that can take 3, 5 or even 8 cycles to clear. But they have a much harder job than a 480 V breaker.

"Clearing Time" includes all time from initiation of the fault until the arc is extinguished (and stays extinguised).

 

[Eleceng01]

dpc, thanks. I do come from the MV/HV world and I usually think of breakers opening in 3,5, or 8 cycles (plus the time for the protective device to detect the fault).

What about if the bolted fault current is not in the instantaneous region of the breaker? I would think you need an arc flash analysis to know if this is the case or not.

 

[dpc]

If not in the instantaneous band, you're in trouble. Clearing times will jump from 2 cycles to 2 seconds. In the 480 V world, more fault current is usually better when it comes to arc-flash.

 

[Eleceng01]

dpc, I agree - wouldn't you need to perform an arc flash analysis instead of using the table to know where on the TCC curve you are? My belief is you do, but I am hoping others will give their view. Thanks.

 

[Zogzog]

You basically have to do an analysis to use the tables. You need to know your fault current and clearing times.

 

[dpc]

Doing an analysis would be the best approach and the only way to find the areas where long clearing times may be a problem.

As a rule of thumb, the smaller the transformer, the more problems you will have.

 

[julesm]

Hang On- I was following this thread in full agreement until that last statement about smaller transformers causing more arc flash problems. Can you explain? SKM has the option to not include smaller transformers based on actuals. That is for 480-208/120V. Maybe you are talking about higher voltage??

 

[dpc]

Sorry, I was referring to 480 V systems. It's usually the smaller 480 V transformers that can create arc-flash problems because the reduced fault current can lead to some very long clearing times.

For 208 V systems, there is a statement in IEEE 1584 that only transformers 125 kVA and larger need to be evaluated. That's probably the origin of the option in SKM.