DIY Doctor UK DIY Forums

Hello,

Apologies if this question has been asked before... I've tried looking for the info on lots of forums, but it I think it might be too basic for it to have been asked before!

Anyway, the question...

FCU's provide an extra layer of protection in the form of a fuse, I understand that, but can any type of FCU be connected to any circuit?

For example, can a 3A FCU be connected to a 32A circuit without fear of it blowing? I read a topic saying the wiring would be too big and unwieldy for this particular example, but in theory is that ok?

I just like to understand things, and in my mind there must be a limit to the size of a circuit going in to a FCU - in other words how does it cope with reducing the power of the current? Are they designed to deal with anything that they'd encounter in a home? So up to 45A or even 60A? Or am I looking at it in completely the wrong way?

Just to be clear, I'm not talking about the appliance going into the FCU, or the size of the wiring. I'm assuming using the correct FCU for that appliance, and the correct size of wire. I just mean the theory behind connecting the FCU to a circuit.

Cheers,

triggerfish

Hi triggerfish,
A FCU is used to discriminate down steam of a circuit, so say you wanted to connect a wall light and the only means of a supply was a ring final circuit Protected by a 32 amp breaker.
You would then install a FCU rated at 3amps as the wall light would work fine on this ampage you'd break in to the ring final circuit and connect the FCU up.
The light would only need to have a cable size of 1mm from the FCU to the lamp you could use a larger size but that would cost more money!
It's the resistance in the fuse that cause the reduced ampage.
If ever there was an overcurrent the fuse would break protecting the cable and the equipment (wall light) from damage.
Has this answered your question
Remember most household installation at supply are protected by 80-100amp breakers but you will see 6amp breakers for lighting circuts in your CU. Discrimination has been applied to protect cable and equipment.
Hope this is what you was after?
KB
This is a very basic description. fuses can operate for a period of time with an overcurrent that may not be a fault without breaking.

Thanks kbrownie... really useful. That was exactly what I was after... very clear! :)

Cheers,

triggerfish