On the site I'm working at, one phase has blown its fuse twice in the last few days.
Its a 400Amp supply from the transformer (I havemt confirmed this, I was told it was that), which is fused down to 200A at the 3 ph cut out.
From the cut-out there are 2x 40+ way Lucy DB with individual 100A fuses for the 70+ flats and communal services.
I have noted here before about the poor design of the installation, but now it seems even dafter, firstly the flats will never pull anything like 100 amos (50A would be plenty), and why give a 400A supply, when it is fused down to 200A at the incoming supply?
Anyway, one of the phases has broken its fuse in the cut-out twice in 3 days. The Private DNO have been out, and replaced the fuse, but, strangely, have done no testing to see why it has broken. There is a minimal load on the full installation now, as it isnt occupied, just water pumps, some lighting, general safety services, and around 10 computers etc, and a few panel heaters in the site office in the building.I would estimate a peak of 30 amps per phase. There has been no breaking/blown individual fuses to the many DB's after the Lucy DB.
The DNO Employee wasnt saying much at all, I asked if he'd done any clamp testing on that phase, but he said he was only there to change the fuse!
I've been asked to have a look at it on Monday, firstly to check the consumption on each phase, and then inspect to see where the fault coudl lie. We arent going to be fixing it, we'll just be reporting to the Main Contractor,who will contact the DNO.
The main thing I think could be causing it will be a high resistance joint on the LV supply between the Cut-out and the Lucy board. If the outgoing side of the Lucy board was faulty, surely the 100A fuse upstream of that will break, rather than the cut-out 200A fuse. All of the testing after the Lucy DB has been done by our Guys, so I'm pretty confident it isnt that cabing that is at fault.
Any other ideas as to what to look for?
