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 Post subject: ARC Flash rating
PostPosted: Mon Dec 09, 2019 9:51 am 
We have a tie breaker rated at 48 kal. Currently needing to switch one main to the other side. If all your doing is switching with doors all bolted shut wouldn't the kal rating be under the 40 kal limit?

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 Post subject: Re: ARC Flash rating
PostPosted: Sun Dec 15, 2019 11:42 am 
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Location: Rutland, VT
Unless the equipment is the newer arc flash resistant switchgear, the doors provide no reduction in incident energy values. I assume by 48 kal, you mean 48 cal/cm^2.

Tie breakers are interesting and it would good to check the report. Usually the tie breaker is closed to connect two energized buses together prior to disconnect the source from one of the buses. This means at one point the arc flash would be generated by two sources so it may be greater than the 48 cal/cm^2. You should check the report to see if the tie breaker incident energy is from both sources.

There are certain conditions (see NFPA 70E-2018) that if met, allow you to operate the breaker with no arc rated PPE. You could also look at remote racking. Another item to consider is the working distance used in the calculation, which if 480V, is probably 18 inches. In reality, for operating the tie breaker, you are probably farther away than 18 inches so you could see what the incident value is at say 36 inches.

Barry Donovan, P.E.

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 Post subject: Re: ARC Flash rating
PostPosted: Thu Dec 26, 2019 6:31 am 

Joined: Tue Oct 29, 2019 6:23 am
Posts: 19
If I may add. I will make some assumptions.
1) When you state the rating is 48 calories at the time, I am assuming that is from one "bus" with the time open.
2) I will also assume that this represents the energy level at 18" (typical LV working distance) based on available fault current and the protection provided by one of the mains.
3) One also assume all doors and panels are properly secured and the equipment is in good condition and well maintained with an up-to-date AF study which is where the 48 cal/cm^2 came from.

As the previous response indicated, assuming all appropriate conditions exists this "may" be considered normal operations and may not, technically, need PPE. However, that does not mean PPE would not be a be a good idea as things do not always go as they are ideally expected to occur. If that is taken as a possibility (I.e. a risk to be addressed), some questions... will the tie parallel the sources for a moment? If so, does the 48 cals represent 1 source or the 2 in parallel? Because if it represents only one and now you have paralleled the sources the energy has probably doubled! A mechanical failure of the switching device provides a particularly nasty situation should that happen while paralleling two sources.

If what you are doing is pushing a button on the front of the gear to close the device you are probably more than 18" away, not to mention you have a door, though the impact of the door on incident energy is unknown hence it is not normally considered at all. Remote operation, however it is safely and reliably achieved, is the best risk control that can be applied!

In short, I suggest PPE is recommended, considering the "high" energy level the PPE should be appropriate, and if you are paralleling the sources, for however short a time then even more energy is possible. If a high energy fault happens inside equipment that is "closed" the thermal energy "may" be limited a bit by the equipment (unknown), however because the equipment is not AR rated the panels pose additional risk of becoming a projectile or source of other trauma... again distance is a good thing here!

Another thought would be... why is the energy so high? 48 calories for a 480V, or even 600V bus seems very high! Would consider what can be done from the perspective of improved protection to get energy lower, at least below 12 calories.

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 Post subject: Re: ARC Flash rating
PostPosted: Thu Dec 26, 2019 11:15 am 

Joined: Tue May 29, 2018 8:19 am
Posts: 35
What Marcelo said...

A few other thoughts:

Yes, arc-rated PPE may not be required per the NFPA 70E-2018. Generally the rule is that normal operation of electrical equipment in good condition, within it's ratings, with all covers in place and properly secured, etc., does not require arc-rated PPE. Until the 2018 edition, this was not clearly stated. Is PPE still a good idea? Of course!

As for the tie breaker, the incident energy exposure depends on how it is operated (open-transition or closed-transition). Make sure the incident energy calculation is based on the proposed mode of operation.

As Marcelo suggested, perhaps it would be possible to adjust the trip characteristics of upstream protection to reduce the IE.

Alan Lehman

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