CSA Z462 Protective Device Clearing Time

[mahan4]

I have encountered the following issues during my Arc-Flash studies and would like to share and discuss:

1) Protective Device Clearing Time: Several sections of the code (e.g. hazard/risk category classifications table, under the "Notes") state that these categories will be valid if maximum 25kA short circuit current, and maximum of 0.02 cycles fault clearing time for example. My question is the following: by maximum fault clearing time, it means the time required to clear which fault? the maximum arcing fault? or the maximum bolted fault? I am assuming the maximum arcing fault, but in order to know the arcing fault, shouldn't there be an analysis done? or we could use the 38% bolted fault rule? Also, how are the electricians going to access the time current curves to determine this?

2) The CEC requires equipment that are likely to be services, examined, adjusted or maintained while energized to be field marked. So I am assuming that arc-flash labels are required for splitters, panelboards, switchboards, MCC's, industrial CP's, disconnects, etc... that are likely to be examined WHILE ENERGIZED. But then, voltage testing for lockout/tagout is also considered energized work and they need PPE for that, so how do we tackle this? should we ask the electricians to use tables when testing for voltage and locking out for any other equipment that is not likely for work to be done on it while energized? or should they just use a category 4 suit, test it, and then remove the suit since it's energized, the code is unclear on this

3) Another side note on DC systems, any news on what to do?

[brainfiller]

I'll try to address these one at a time.

The tables are meant for use when a full calculations study has not been performed. In my opinion, they have a few short comings but are better than nothing. The current is bolted and what happens to the clearing time if it is arcing? That is one of the problem. The tables are made to appear simple, and there are footnotes to help address the details but when you dig deep, there are a few holes. In addition, at the same location you can have several types of PPE even though if an arc flash occurs, it would be the same event - they factor in "risk".

A full arc flash calculation study just gives the hazard in absolute terms, i.e. how much incident energy - period, you then dress for that energy level.

Determining the clearing time does ususally requires the use of time current curves. Not always availabe or understood by everyone that uses the "simple" tables.

Ideally, everything that can be serviced, etc. should be labeled but some may interpret this differently due to a few exceptions in 70E regarding 208V / 125 kVA transformers. My concern about labeling some but not all equipment is when you do not see a label, does that mean it is catagory 0?, was there an exception? or did someone just miss it.

In the absence of a full arc flash study, calculations, etc. the best option is to use the tables doing the best job with the footnotes that you can.

As for DC arc flash, there are quite a few technical papers out there. None of them seem to answer the whole question but I took what I felt was the best of the best, filled in a few holes and added it to my arc flash class in the absence of any stanard yet. NFPA is going to add approach distances for DC as well as additions to the "tables" for DC but that is it for now.

Good luck! Stop back if you have any more questions.

[mahan4]

I agree, but I'm also a big fan of using the tables in lieu of the calculations for the following reason:
Equipment that get labelled, per NEC and CSA, are equipment that are likely to be serviced while energized. What about those equipment that will not be energized, but still require a final voltage test before doing work on them to make sure they are de-energized? It would not be feasible to produce labels for everything within the power system, such as cables, disconnects, etc..., they'll be de-energized, locked out and tagged out before work, but in order to check the voltage to make sure they're de-energized, the electricians can use the tables, and also the short circuits from the analysis (fuse curves and fault levels), to perform those tasks. In my opinion, the tables and the analysis should be used together and one is not complete without the other.