new CU and garage wiring questions

[sherlockholmes]

I don't know if the planned isolator provides overcurrent protection or not, it's just labelled 100A double pole isolator on the diagram.

Ah, then maybe it is just as isolator then (a manually-operated 'switch' in common parlance, although unlike switches, an isolator is not intended to be used to switch off a circuit 'under load'). It could well be that the electrician has specified an isolator there to faciliate his subsequent work (and that of other electricians in the future). What made me think that you knew it afforded overcurrent protection was that you wrote:

...the upstream isolator (one is being put in before the Henley block), which has no fuse but as I understand it does the same job of disconnecting the current in the event of disaster

My intended question would perhaps have been more clearly put as why a switch fuse for overcurrent protection instead of a cheaper, smaller modern circuit breaker. Either could be rated at a level to protect the cable downstream. I think BAS's answer covers it with discrimination.

Indeed. I thought you were primarily asking why the switch fuse (or some alternative) was there at all (given the presence of the upstream 'isolator'), with the fuse vs. MCB question as a 'secondary' one.

I hope the work goes well.

Kind Regards, John.

Thank you. I have a rather limited knowledge of electrical equipment, and regularly get the terminology/function wrong, sorry for the confusion. I was originally under the impression that an isolator rated to a certain current operated as an MCB as well.

 

[JohnW2]

Thank you. I have a rather limited knowledge of electrical equipment, and regularly get the terminology/function wrong, sorry for the confusion. I was originally under the impression that an isolator rated to a certain current operated as an MCB as well.

It's not really you - some of this terminology is ambiguous and/or confusing and/or sometimes used rather loosely. An 'isolator' simply has to be able to do what it says - isolate a circuit or appliance etc. from the electrical supply (another word for an isolator is a 'disconnector'). It could just be a 'switch', as I described in my previous post, but many/most modern double pole protection devices (MCBs, RCDs etc.) can also be used to isolate - hence the possible variation in what is meant when someone talks of an 'isolator'.

Kind Regards, John

 

[ban-all-sheds]

I was originally under the impression that an isolator rated to a certain current operated as an MCB as well.

No - it's just a switch. The main switch in your CU is probably labelled 80A or 100A - that doesn't do any overcurrent protection either.

Nor does an RCD saying 63A, or 80A etc.