Arc Resistant Rated MCC

[mls1]

I'm working on a study for a client interested in replacing existing MCCs with arc resistant MCCs. All of the duration rated MCCs I've found have a duration rating of only 100 ms. For this particular system, the available fault current is less than 20 kA. With the short time trip delay on the upstream feeder breaker they do have levels of about 10 cal/cm2. However, to meet the manufacturer arc resistant rating they would have to use an instantaneous trip to keep the arc duration less than 100 ms. At that point they are already at a very low energy level. Common sense says the C37.20.7 rating requires the manufacturer to run the test at MCC rated fault current so the gear would be resistant at lower fault levels for a longer period but there is nothing that documents that. It seems to me that if they have to adjust delay settings or use a device rating to get an arc resistant rating there's really no point in using arc resistant gear. Just lower the energy level and call it good. Am I missing something here?

 

[Wfg42438]

Based on what i have read in IEEE C37.20.7-2017 you are bounded to the ratings of the Arc Rated gear.

In other words for you to rely on the gear to work as designed you must not allow the internal arc to last longer than the max arc duration tested by the manufacturer.

For example clause 2.4.3 says the following
"It is not possible to calculate the permissible arcing duration for a current which differs from that used in the test. The maximum pressure during the arcing event will generally not decrease with a shorter arcing time, and there is generally no agreed-upon rule according to which the permissible arcing duration may be increased (by calculation) with a lower current."

So in turn I would say that yes if you introduce protective device settings within the range of the arc rated gear you will have already mitigated the IE without the need of the arc rated gear. However keep in mind the additional benefit of the arc rated gear would also bring the benefit such as routing the harmful gases and reducing the pressure build up using the built in arc chambers if a internal arc takes place for which the standard MCC would not be able to. But then again this arc rated equipment is expensive so that may not be sufficient justification for the upgrade.