S-type RCBOs

[peterinreading]

Does anyone make S-type RCBOs?

For discrimination you should use an S-type RCD to protect a sub-main feeding a CU with a normal 30mA RCD or RCBOs. This can be handled with a 100mA 100mSec S-type RCD. It would be convenient if this were combined with a type B or C 63 amp MCB.

This is in a domestic context with a main DB feeding a number of CUs.

 

[davesparks]

What do you need the RCD protection for?

 

[Leesparkykent]

If the installation method requires additional protection then having an S type RCD is going to be no good.

 

[peterinreading]

RCD required on submain consisting of 16mm2 T&E (not SWA) which is <50mm from surface of stud wall. Really you should use 30mA but then you don't get discrimination.

 

[Andy78]

RCD required on submain consisting of 16mm2 T&E (not SWA) which is <50mm from surface of stud wall. Really you should use 30mA but then you don't get discrimination.

There's no "should really use" about it. If you are complying with the regs it has to be 30mA.

I'd be redesigning the distribution circuit installation myself.

 

[Richard Burns]

I doubt you will find delayed 100mA RCBOs, certainly not in a domestic capacity.
You should have designed the submain to not require RCD protection.
Doing your method of delayed 100mA RCD protection will not meet the regulations.

EDIT: beaten to it by Andy78

 

[davesparks]

RCD required on submain consisting of 16mm2 T&E (not SWA) which is <50mm from surface of stud wall. Really you should use 30mA but then you don't get discrimination.

Only 30mA non time delayed RCD protection is allowed for this application.

Presumably this domestic installation is split across multiple CUs? If so the having a single RCD protecting each CU is not really a problem as far as the regs are concerned.
For example, if you have an installation spread across two CUs each with an RCD main switch is it any different to having the whole lot on one split CU with 2x RCD?

 

[peterinreading]

Thanks for the comments. The submains already existed probably since pre-17th edition days protected by 63 amp MCBs. A risk analysis might suggest that the S-type RCDs would be better than none at all. It may be possible to change some of the more accessible cables for SWA which would solve the problem for some of them

But anyway thanks for the confirmation that S-type RCBOs are probably unavailable.

 

[Andy78]

Thanks for the comments. The submains already existed probably since pre-17th edition days protected by 63 amp MCBs. A risk analysis might suggest that the S-type RCDs would be better than none at all. It may be possible to change some of the more accessible cables for SWA which would solve the problem for some of them

But anyway thanks for the confirmation that S-type RCBOs are probably unavailable.

As said, this will not comply with the requirements of BS7671 for alterations to existing circuits. You can either provide 30mA protection for the distribution circuits, change the installation of the distribution circuits so they do not require additional protection, or leave it as it is.

 

[telectrix]

but is it necessary to RCD protect the sub-main feeds if they complied with previous edition/s of regs.?

 

[Andy78]

but is it necessary to RCD protect the sub-main feeds if they complied with previous edition/s of regs.?

I would say yes if he is to set about altering the nature of OCPD protection of the distribution circuits.

 

[davesparks]

A risk analysis might suggest that the S-type RCDs would be better than none at all. .

Would it? If you take the purely mathematical view of the situation then it does appear that way. But what if you actually think about the purpose of the RCD?
30mA RCDs are designed to operate before death occurs in a person making contact with a live conductor, any delay in the operation and they will not disconnect before death has occurred.
Any RCD which operates after the death of the person might as well not be there at all from the point of view of that person.

 

[IzzyS]

The 30mA RCD requirement is to (hopefully) prevent a fatal shock. A time delayed 100mA RCD doesn't do that, so you're wasting your time and money.

Add mechanical protection, re-route the submain, or change it for a cable type which will comply.

Protecting a sub-main with a 63A MCB is unlikely to offer discrimination for a downstream MCB anyway.

That's why switch-fuses take fuses, not circuit breakers.

I type too slowly!

 

[Octopus]

Back to the OP - if the distribution circuit is existing why are you looking to add RCD protection?

 

[peterinreading]

Thanks for all your comments. Looks like a combination of tactics may be required as IssyS says: add mechanical protection where feasible, re-route the submains where feasible, or otherwise change the cables for SWA. Whatever tactic is used, the S-type RCDs don't add any useful additional protection and therefore the question I asked of whether S-type RCBOs are available is irrelevant.

 

[Octopus]

Thanks for all your comments. Looks like a combination of tactics may be required as IssyS says: add mechanical protection where feasible, re-route the submains where feasible, or otherwise change the cables for SWA. Whatever tactic is used, the S-type RCDs don't add any useful additional protection and therefore the question I asked of whether S-type RCBOs are available is irrelevant.

Try answering my question!

If the distribution circuit is existing why are you looking to add RCD protection?

 

[davesparks]

Why are you dead set against adding 30mA protection to the submains?
If the installation is divided across multiple boards then it's no different to dividing it across a twin RCD split board

 

[Dave OCD]

It's equally likely that each sub main is feeding a complete flat or dwelling so in that situation having a remote 30ma RCD feeding complete CUs would at best be hugely inconvenient in the event of tripping and at worst dangerous.