Arc Flash Studies

[Qckrun]

All,

I am currently working on modifying an old RFP at my work in regards to having an Arc Flash Study done. One item in the RFP states:

"Lumped low voltage motor groups for MCC's shall be modeled per IEEE standards using groups less than 50HP. Low voltage motors rated more than 50-HP shall be modeled individually. Where motor list data is not available, single lumped groups may be modeled per IEEE-141 “Red Book.” "

Is this standard still applicable?

Also, is it a TOTAL number of motors equaling 50HP or just all motors under 50HPs? For instance if I have (20) 10HP motors on an MCC, would it be modeled as (4) 50HP Loads?

Thank you,

 

[jim dungar]

Simplistic answer.

The limitation of <50HP has to do with the the ability for the motor to contribute substantial short circuit current as its magnetic field collapses.
One school of thought is that if a motor is not worth consideration, effectively Isc=0, then multiple such motors will still not be worth considering.

If you lump the motors together, your example (4) @ 50HP, you may be overestimating your arc flash short circuit contribution and therefore may underestimate device operating times.

I have been known to run scenarios with and without lumped motor contribution and then report the worse AF incident energy.

 

[Qckrun]

Simplistic answer.

The limitation of <50HP has to do with the the ability for the motor to contribute substantial short circuit current as its magnetic field collapses.
One school of thought is that if a motor is not worth consideration, effectively Isc=0, then multiple such motors will still not be worth considering.

If you lump the motors together, your example (4) @ 50HP, you may be overestimating your arc flash short circuit contribution and therefore may underestimate device operating times.

I have been known to run scenarios with and without lumped motor contribution and then report the worse AF incident energy.

Jim, Thank you for the reply. Would it may be beneficial to include in my RFP that the scenarios be ran both ways and report the worse AF Incident Energy level? Based on your experiences, has it ever been a higher AF if you ran the motors without lumped contribution? I'd imagine this would be a small increase in man hours or would it possibly double the total work load? I have over 380 breakers / motors that would be analyzed total (not broken apart under 50HP).