Can a RCD be used as an Isolating Switch

[THRIPSTER]

537.1.4 A main linked switch or linked circuit breaker shall be provided as near as practicable to the origin of every installation as a means of switching the supply on load and as a means of isolation.

For households this would need to be a double pole switch.


The point being that there should be one switch which is capable of de-energising the installation.


Regards

 

[JohnW2]

537.1.4 A main linked switch or linked circuit breaker shall be provided as near as practicable to the origin of every installation as a means of switching the supply on load and as a means of isolation. For households this would need to be a double pole switch. The point being that there should be one switch which is capable of de-energising the installation.

That is, indeed, true, but it seems to be commonly violated, and I can't recall having noticed people complaining about that. 'Secondary' CUs or switchfuses for submains to outhouses etc. seem to be commonly fed from post-meter Henleys without any upstream isolator which covers the whole of the "installation" (as you and I would probably regard it - but see below). I wonder what people think about that?

Perhaps one issue is the BS7671 definition of "an (electrical) installation" could be taken to indicate that it does not necessarily mean 'what we would think'. If there are, say, multiple CUs or submains, one could argue that, although they shared a cutout and meter, they could be regarded for the purpose of BS7671 as being separate 'installations', each of which had its isolator as required by 537.1.4?

Kind Regards, John

 

[EFLImpudence]

I would agree with that but that does, I think, depend on the size of the premises.

In the case of a very large building with 'separate installations' a single main-main isolator would likely be very rarely used as it would not be necessary, each having a switch fuse where required.

Fuses are considered adequate for isolation so - cut-out ?


In the case of the OP where the consumer units are adjacent it may be advisable although, with absolutely no disrespect to the OP whatsoever, it may not even be realised what the joint isolator was and anyone who knows what they are doing would immediately realise the situation.
Second CUs for storage heaters 'never' have joint isolators.

 

[JohnW2]

I would agree with that but that does, I think, depend on the size of the premises. In the case of a very large building with 'separate installations' a single main-main isolator would likely be very rarely used as it would not be necessary, each having a switch fuse where required.

Indeed - but what about a small house with a switchfuse for, say, an outbuilding supply taken from Henley's without an upstream 'overall' isolator. Do you feel that the submain could, for the purpose of 537.1.4, be regarded as a separate 'installation'?

Fuses are considered adequate for isolation so - cut-out ?

True. However, 537.1.3 seems to deal with that, but has a Note "A cut-out fuse may only be withdrawn by a person authorized to do so by the distributor" and 537.1.4 seem to require that isolation 'by ordinary persons' has to be DP.

In the case of the OP where the consumer units are adjacent it may be advisable although, with absolutely no disrespect to the OP whatsoever, it may not even be realised what the joint isolator was and anyone who knows what they are doing would immediately realise the situation.

Quite so - and then we would be back to reliance, for 'ordinary people, on(fallable) 'labelling' again.

Second CUs for storage heaters 'never' have joint isolators.

Indeed - and, similarly, 'dual tariff CUs'.

Kind Regards, John

 

[EFLImpudence]

Indeed - but what about a small house with a switchfuse for, say, an outbuilding supply taken from Henley's without an upstream 'overall' isolator. Do you feel that the submain could, for the purpose of 537.1.4, be regarded as a separate 'installation'?

I suppose, yes if a large house can be.
See below.

Fuses are considered adequate for isolation so - cut-out ?

True. However, 537.1.3 seems to deal with that, but has a Note "A cut-out fuse may only be withdrawn by a person authorized to do so by the distributor" and 537.1.4 seem to require that isolation 'by ordinary persons' has to be DP.

A lot of non-compliant installations around, then.

The OP's second CU could be supplied from the load side of the first CU's main switch.
However, this approach is frequently dismissed as poor practice - not sure why - probably due to the 'fashion' of using over-sized conductors.

 

[JohnW2]

Indeed - but what about a small house with a switchfuse for, say, an outbuilding supply taken from Henley's without an upstream 'overall' isolator. Do you feel that the submain could, for the purpose of 537.1.4, be regarded as a separate 'installation'?

I suppose, yes if a large house can be. See below.

If you're prepared to accept such things as separate 'installations' that presumably gets rid of most of the potential non-compliance issues.

A lot of non-compliant installations around, then.

That was my initial comment above - but if one accepts that each CU or switchfuse (all of which would have DP isolators) can create a separate 'installation', then there would be no non-compiance, would there?

The OP's second CU could be supplied from the load side of the first CU's main switch. However, this approach is frequently dismissed as poor practice - not sure why - probably due to the 'fashion' of using over-sized conductors.

Indeed - and, yes, I suspect that's the most likely reason why some people don't like it. I must say that I didn't used to be very the keen on sticking conductors into a main switch along with the end of a busbar. However, it's now so common (with multiple RCD CUs etc.) that I suppose I've come to accept it!

Kind Regards, John

 

[PrenticeBoyofDerry]

You would NOT expect the isolator of one distribution board to isolate the circuit of another distribution board.

Yes I did mean that, thanks BAS for pointing this out, I have corrected the post, to prevent misleading information.
Thanks again.