FCU Fuse Size

stl · 25 Jan 2011

Hi,

I have a query regarding the size of fuses to go in a FCU.

1) - I have an FCU high up on the wall outside the bathroom that isolates the supply to the main bathroom light (and pull coard switch) on the upstairs lighting circuit which itself is protected by a 5A fuse on the CU. What size fuse should be in the FCU? - at the moment it is 13A which seems pointless!

2) - Also, I have a similar situation for the en-suite. This has a light / switch for light and also a fan with its own one/off switch built into the fan - pull coard. Again what size fuse should be in the FCU? as at the moment it is 13A.

The upstairs light circuit is not on the RCD side of the board, and we have just moved in and noted the 13A fuse!!

Many thanks!

1john · 25 Jan 2011

You need to provide discrimination so it needs to be a smaller fuse that the one feeding the circuit, it also has to be greater than the load being subjected to it, I would think a 3A fuse would do, all that being said why not replace the FCU with DP switches as the MCB will trip quicker than the cartridge fuse will blow anyways

riveralt · 25 Jan 2011

stl said:
Hi,

I have a query regarding the size of fuses to go in a FCU.

1) - I have an FCU high up on the wall outside the bathroom that isolates the supply to the main bathroom light (and pull coard switch) on the upstairs lighting circuit which itself is protected by a 5A fuse on the CU. What size fuse should be in the FCU? - at the moment it is 13A which seems pointless!

Not sure why there is an FCU there but if it is only protecting the light in the bathroom 3Amp

2) - Also, I have a similar situation for the en-suite. This has a light / switch for light and also a fan with its own one/off switch built into the fan - pull coard. Again what size fuse should be in the FCU? as at the moment it is 13A.

whatever the extractor fan manufacturer says normally 3/5Amp.

Steve · 25 Jan 2011

They are just bog standard FCU's? No RCD in them?

I'd stick 3 amp fuses in them, but theres not much discrimination between a 3 amp fuse and a 5 amp fuse or MCB, for most faults.

Sometimes an FCU is used before a bathroom light, to isolate a fan instead of a 3 pole switch, the disadvantage being that it also isolates the light.

Chri5 · 25 Jan 2011

Amps = watts divided by 230v

So add up the lighting load via counting the lamp(s) watt rating, add 100w for a fan and a nominal 50w for a shaver point. Add in anything else (mirror lighting ?)

Normally 3 amp will do.

xr4x4 · 25 Jan 2011

a 3 amp fuse..

you could replace it with a RCD protected FCU if you wanted to protect the light with a RCD.

Risteard · 25 Jan 2011

1john said:
You need to provide discrimination so it needs to be a smaller fuse that the one feeding the circuit, it also has to be greater than the load being subjected to it, I would think a 3A fuse would do, all that being said why not replace the FCU with DP switches as the MCB will trip quicker than the cartridge fuse will blow anyways

A 3A BS 1362 fuse will not provide discrimination with a 5A BS 1361 fuse.

The fuse is to protect the cable and not the accessory.

Aside from the fact that fan manufacturers tend to stipulate that a 3A fuse is to be used (and the Regulations require compliance with manufacturers' instructions) then there isn't really any other reason to use one.

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FCU Fuse Size

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