More electrical Qs: FCU fuse rating

[Grumps]

I have some B10 MCBs feeding single electric heaters which are connected via FCUs with 13A fuses. Max heater rating is 2kW. As discrimination isn't an issue (only one load per MCB), should the fuses be changed for smaller values? Surely I'd need a 10A fuse for the FCU feeding the 2kW heater, but the MCB is also 10A.

[Robin]

I don't see an issue.

The MCB is there to protect the cable to the FCU. At 10A it does that.

The fuse is there to protect what's attached to the FCU. At 13A it does that.

I suppose that leaves the theoretical possibility that the manufacturer requires something /less/ than 10A protection. But I wouldn't worry about. I don't think BS1362 provided a value between 5A and 10A.

For completeness there's also discrimination in the sense of "Which goes first when there's a fault, the MCB or the fuse?" You don't need worry about that with only one thing - the heater - on the circuit . It doesn't matter if a fault with the heater causes the MCB to open, the fuse to blow, or both. Any which way the result is the same: the fault is isolated; and nothing else is affected.

[Robin]

Please ignore the last para of my earlier post.

Off now to do another 999 lines of

"I must always read the question twice, carefully."

[John Rumm]

A 13A fuse in the FCU will be fine, its main purpose in life (unless directed otherwise by the heater manufacturer) is to protect the flex connecting FCU to heater. The heater should have its own protection for anything else.

There may be cases where a manufacturer will direct that its appliance be protected at a specific level (e.g. an extractor fan may need protection at 3A, and hence will need a 3A fuse even when powered from a

6A lighting circuit.

In this case, even if the manufacturer dictated the appliance be protected at 10A (unlikely), that would be achieved by the MCB anyway, so the fuse could still be 13A.

[ARW]

Probably both.

Now it was two fuses it would of course be the one you did not have a spare for.