Prospective Fault Current

[LearnerDave]

Hi Guys,

We measured PSCC on a three phase dist board in a commercial establishment. This board is fed from a main switch board with a main 400Amp switchfuse with BS88-2 fuses rated at 88kA. Feeding a second 160Amp switchfuse with BS88-2 fuses protecting the cabling to the board we were looking at.

The board has Dorman Smith series 15 MCBs rated at a PSCC of M4.5 which I believe = 4.5kA. We measured the highest PSCC/PEFC as 2.9kA. I believe the rule of thumb is to double it, which = 5.8kA. This meant the value is larger than the rated kA of the MCB .

In GN3 I’ve read that - Reg. 434.5.1 states that the breaking capacity rating of each protective device shall be not less than the prospective fault current at its point of installation. From this I would say that the MCB would have to be higher rated than the measured kA and therefore in this case need to be changed. One of the other chaps said because the board was supplied by a BS88-2 88kA fuse, there wasn’t an issue as this has the capability to blow its fuse safely. I can’t find anything in the Reg’s to back this up.

Is it acceptable to rely on the up stream BS88-2 fuse? If not, how serious/necessary is it to up-rate the MCB?
Out of interest, what PIR code would you give it, if any?

Thanks a lot Dave

 

[telectrix]

if you are measuring the pscc before the dorman MCBs. i.e on the incoming side of the board , it is the fuses feeding this board that have to withstand the pscc.

 

[LearnerDave]

Hi Telectrix, It was measured at the incomer of the Dorman Smith board.

If It was measured on the load terminal of the MCB's and got a similar reading, am I correct in thinking that a dead short in the cable just after the MCB could theoretically allow the measured PFC to flow through the MCB and if larger than the MCB rating could destroy it, and may not work/open at all?

 

[telectrix]

theoretically, yes. in practice unlikely. better minds than mine may disagree.