Two similar problems occurred recently both were resolved but would like learned persons on the forum just to run an eye over the MCB tripping problem scrutiny in other words, if any deviation from the none statutory BS7671has taken place.
A garage with a paint shop compressor motor failed, very old 3phase motor tested and condemned by me. He got a new motor and connected it up but it was a 5HP instead of 3.5HP, the difference in size of the new motor (smaller) than the old motors is remarkable .However it ran for a while then two 32 amp fuses blown in a three- phase supply main -switch . I did a temporary fix of placing a 30amp melt able link (fuse wire) across the fuses that I still carry from the old days saying I will be back with three dual element fuses in a week or two , did replace the temporary fix 3 HRC 30 amp motor fuses £12 + vat. Goes to show the old fuse wire like the dual element HRC has an over current limit and a time delay before activation.
The second problem, due to ill health a garage mechanic gave up his rented garage, but still wanted to do small jobs in his home garage, he transferred his single phase motor compressor to the home garage which had two 13amp double sockets, he sent for me because the garage MCB kept tripping 20 amp type B, I changed it to a 32amp type C, problem solved. My question is there any reference in BS7671 as to fitting a C type MCB in a domestic ring main. The only danger I can think on is to check for a robust mains supply RCD is there a sequence of past events leading to problems.
jcm
