use of 10ka mcbs

hodgey1972 · Apr 13, 2015

Gents, Im not the most technically minded electrician in the world but could someone explain why on commercial installations its necessary to fit 10ka mcbs not 6ka throughout the installation?
or mores the point, im being asked to fit a main panel and off that a few single phase domestic high integrity dbs, but use cheaper 6ka mcbs, which ive said no to.
ive read bs7671 434
Any help would be great.

LeeH · Apr 13, 2015

You fit what is required, measure or calculate the pfc and bob's your uncle.

Darkwood · Apr 13, 2015

Domestic usually single phase... commercial and industrial usually 3ph so a short circuit could be between 2 phases and thus approx twice that of a L - N or E fault, there is no specific requirement that says use 10Ka its more to be the result of the manufacturers designing them higher in their 3ph boards and in reality you only need to fit 10ka if your PFC could be larger than 6Ka.... and even then you still don't need a downstream device to meet this requirement if it is covered by an upstream device that meets the PFC requirements but its always good practice to fit the correct rated mcb's at any point in the circuit reqardless of upstream protection.

hodgey1972 · Apr 13, 2015

Lee, how can you measure or calc pfc, if the supply isn't installed yet?

shanky887614 · Apr 13, 2015

hodgey1972 said:
Lee, how can you measure or calc pfc, if the supply isn't installed yet?

it sounds like a 3pn in commercial so he could test at the head, i would be surprised if the ka warranted 10ka in a lot of commercial supplies but i could be wrong, im surprised there asking for three single phase boards though

is the cost much more thouh, i was under the impression there wasnt a lot in it

Engineer54 · Apr 13, 2015

We use 10KA MCB's as a matter of course for all our general MCB DB's whether they are single or 3 phase DB's. For our substation lighting and small power DB's our MCB's are 25KA...