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# MCB Short circuit level sizing for Control circuit

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## MCB Short circuit level sizing for Control circuit

(OP)
Hello Experts,

I am about to select the proper SC level for a 2A,2P (L & N) MCB which protect a control circuit of motor feeder inside low voltage switchgear.

I am feeding this circuit directly from main bus bar which has rated short circuit level 65KA

I need to know that is it mandatory to size SC of MCB to be as main bus bar =65KA?

Best Regards,

### RE: MCB Short circuit level sizing for Control circuit

Dear Mr. Mahmoud-Abdullah (Electrical)(OP)18 Dec 22 13:28
".....I am about to select the proper SC level for a 2A,2P (L & N) MCB which protect a control circuit of motor feeder inside low voltage switchgear. I am feeding this circuit directly from main bus bar which has rated short circuit level 65KA. I need to know that is it mandatory to size SC of MCB to be as main bus bar =65KA? "
.
1. I assumed that the conductors (L + N) from the BB to the MCB are say < 1.5 m and the size say 2.5 mm2. The prospective SC at the MCB would be close to 65kA !

2. MCBs are rated not >10kA. To circumvent the issue, add (one 6A fuse) [on the line only] as close as possible to the BB. As 6A fuse is > 2 x 2A MCB; the series combination will achieve total discrimination.
2.1. Alternatively, replace the MCB by 1-pole MCCB rated 70 or 85kA at 250Vac. Note: MCCB min. current is 6A , which would be ok for SC protection only; (NOT over-load).
Che Kuan Yau (Singapore)

### RE: MCB Short circuit level sizing for Control circuit

Dear Mr. Mahmoud-Abdullah (Electrical)
Omission submission
I refer to my earlier post dated 19 Dec 22 10:57 and wish to submit the omission for your kind consideration.
1. In practice, very often it is impractical/inaccessible/inconvenient to place the short-circuit protection device (SCPD) (be it a fuse or CB) [ directly on the BB]. Therefore, there will always be a (short) length of (small size) conductor between the BB up the SCPD. This short length but small conductor is NOT protected !
2. FYI: In the IEC world, IEC 61439-x has look into it and covered extensively; with respect to the limitation on the length, wiring to be within the enclosure, the improvise of additional insulation/higher voltage insulation, installation methods ....etc. ; on how to prevent it from short-circuit.
Che Kuan Yau (Singapore)

### RE: MCB Short circuit level sizing for Control circuit

(OP)
Dear Mr. Che Kuan Yau,

Thanks a lot for your comment.

I will search IEC documents & get back to you if you can guide me.

Best Regards;

Keep Moving Forward ;)

### RE: MCB Short circuit level sizing for Control circuit

Yes, the protection should be rated for the fault current of the bus.

In North America, MCBs can't be connected as the sole power circuit protection. There are MCBs with ratings of 65kA, but the rating is dependent on it being series protected by another MCCB or fuse as specified.

UL and CSA also don't allow you to randomly put a fuse in front of a 10kA MCB hooked to a 65kA capable bus. There are exceptions in UL508A that do allow it for certain fault levels and fuse sizes.

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