125A consumer unit powered by 63A Switched BS88 Fused Unit

[JR2687]

Hi, Ive got an upcoming job to determine the cause of a BS88 fuse tripping in a switched fuse unit that powers a 125A consumer unit yet none of the MCB's trip, Just the one cartridge fuse.

What is a concern about this site is that the old fused switch unit is 63A, yet it is feeding a 125A consumer unit. This is apparently a recent install too. The consumer unit is used for 4 ring circuits rated at 32A.

Surely this shouldnt have been signed off on the installation certificate as the switched unit is under rated?

Any help is greatly appreciated, Thank you

 

[Lucien Nunes]

So you need to discover whether the 63A fuse is blowing due to fault or overload. It sounds like overload, but you might have to clamp the circuit to prove that, if it is not obvious from the connected load.

You can't say that the switch-fuse is under-rated without knowing what the load is. If there are ten 200W computer loads plugged into each ring, the four rings might be completely justified and well utilised, with a total load of only 35A and the fuse is perfectly adequate. The 125A rating of the consumer unit is neither here nor there.

 

[JR2687]

Oh ok, thank you.
So the wiring regs wouldn't say that bad practice?

 

[pc1966]

It would be bad practice if the original installation exceeded the 63A fuse rating, but before laying the blame at the original sparky who signed off, it might be worth checking to see what has changed (if anything) since then in terms of how it is used.

Having 4 RFC at 32A does not imply they were planned to be used to that load, it might have been for fault tolerance or some other reason.

But equally it might be the design expected close to 100A but for some reason or another the supply was not uprated to suit. Is this a domestic set-up? Has someone added a 10kW shower or EV charger (in addition to several electric heaters in normal use)?