Transformers smaller than 125KVA, 208V

Should an arc flash study be performed on 208V systems fed by transformers smaller than 125KVA? IEEE 1584 indicates an arc-flash hazard should not be a concern. NFPA 70E specifically does not address this issue.

I think the idea is that the fault current would be low enough on the secondary of small transformers that it is not considered a severe threat. We don't review the system beyond the small transformers for that reason. I am sure some will review everything but many stop at the small transformers. You are correct that this is not covered in 70E. 70E requires a flash hazard analysis that could involve using the hazard risk tables and simple flash boundary formulas or it could involve detailed calculations but they don't say how to conduct the calculations so you rely in 1584.

208 circuits

IAW the IEEE 1584, the cutoff is 208VAC systems fed by transformers that are below 112.5KVA. They aren't required to be calculated, because a fault on their secondary would be so small as to be classified as a "non-event". The do however, have to be labeled, if you follow the NEC 110.16 edict regarding labeling of components.

This can come down to the fault current level. If the max fault current is under a few hundred amps, this is no more dangerous than a typical industrial welder.

208 system, less 125kVA transformer

For 208 system, less than 125 kVA transformer, you can use NFPA, table 130.7(c)(9)(a) because you will have less than 10kA short circuit current available=> can reduce hazard/ risk category by one number.


This means you will end up with category 0 for 208V system.

Raffle

We have been trying to sort this out in a 'least liable' way. Performing calculations in such instances has resulted in risk category 1 or lower, for our given parameters. We have not been able to get the results above that level.

(at this particular site, they have decided to label all installations as requiring RC 1 PPE as a base level of protective PPE to enter the plant, so that has been the threshold we seek)