NFPA 70E TABLE MAX FAULT CLEARING TIME

[wbdvt]

I just don't see where it indicates in NFPA 70E_2015 that the actual bolted fault current and clearing time is required to use Table 130.7(C)(15)(A)(b). It does says, "The estimated maximum available short-circuit current, maximum fault-clearing times, and minimum working distances for various ac equipment types or classifications are listed in Table 130.7(C)(15)(A)(b)." It then lists the scenarios where you cannot use the table. I can tell you that when the actual bolted fault current from the utility is unknown, an infinite bus is often used. As I said before, when you get down to the lower voltage buses (ie 240 & 120) from an medium voltage service, the actual bolted fault current is not much different than an infinite bus.

Ok, I am traveling so I don't have my NFPA 70E copy with me but consider this:

Your actual bolted fault current is less than the maximum bolted fault current as shown in the tables but the clearing time is much higher due to the actual fault current being less than the maximum bolted fault current you decide to use based on an infinite bus. This puts you outside the parameters of the table. You have an employee working in PPE you said was acceptable based on your interpretation of the table parameters. There is an accident and the employee has less than the necessary PPE for the actual incident energy. He/She is severely injured. You are now on the witness stand with a lawyer (and they have been attending seminars on Arc Flash) who is questioning you on your decision. Will you be able to defend your decision on use of maximum available fault current from an infinite bus versus maximum available fault current able to be delivered by the utility?

Your call.

 

[Bugman1400]

Are you saying that the table should be ignored and a formal study must always be done because of liability risk? Also, are you assuming that the tables have no margin built in? IOW, the table may call for PPE Level 3 but, you think a higher PPE level (4 or 5) may be needed because there is a chance that a lower fault current would lead to increased clearing times and higher incident energy. I assumed the Table took this into account and the PPE Level 3 would be highest level needed and a higher fault current with a faster clearing time and lower incident energy perhaps would call for a PPE Level 1 or 2.

I will test this assumption on a few models and see what I get. Thanks for sticking through this......it sometimes takes a few whacks with a 2x4 before I get the message.

 

[wbdvt]

Nobody really likes the tables but they had to give people something. To adequately use the tables the fault current and associated clearing time must be known. This would require a short circuit study. So once that is done it is not much more effort to do incident energy. In some cases the tables may be conservative, in others not so.

When protecting people you want to utilize the best tools.

 

[jim dungar]

I will test this assumption on a few models and see what I get.

Remember the task tables also contain an element of probability.

Effectively these tables say:
If the fault current is below this maximum value
And
If the protective device opens at least this fast,
Then this PPE, with this ATPV level, is likely to protect you 95% of the time you perform this task.