RCD Discrimination

[WhydidIstart]

Hi, looking for some advice on RCD discrimination please.

Incoming mains goes via the meter and splits to feed 1 consumer unit for one section of the house and another feed goes via a switch that holds what looks like a single BS1361 fuse (60A – too high I think as 10mmsq t&e not clipped direct for the whole run). This feeds a second consumer unit in a different part of the house.

The second consumer unit currently has no RCD protection. Some new circuits need to be added to the second consumer unit and it makes sense to change this CU to a 17th edition dual RCD type plus use RCBOs in the high integrity positions for circuits that would benefit from being separately protected such as shed / workshop / utility room feeds.

Changing the 2nd CU gives the required results – capacity for extra circuits and RCD protection arranged to minimise safety & inconvenience issues.

The issue is that the feed to the 2nd CU is not RCD protected and I assume this would need to be updated at the same time? Mechanical protection of the feed cable isn’t practical, but protecting it with a standard 30mA RCD would introduce all the safety and inconvenience problems present back in the days of a single RCD protecting the whole installation.

Replacing the existing 60A BS1361 fuse with a type S RCD and 45A MCB would seem to be the best overall solution, but doesn’t seem to be compliant from what I can see – any ideas on options greatly appreciated please??

 

[EFLImpudence]

The issue is that the feed to the 2nd CU is not RCD protected and I assume this would need to be updated at the same time?

As it is already there you do not, strictly speaking, have to 'update' it.
If it were fitted new and had cables installed in a way that requires RCD protection then you would have to fit a 30mA one so there would be no discrimination.
Armoured cable not requiring protection would be chosen.

I suppose if you just want to, for the sake of it (I wouldn't), then you could fit an 'S' type 100mA.

Mechanical protection of the feed cable isn’t practical, but protecting it with a standard 30mA RCD would introduce all the safety and inconvenience problems present back in the days of a single RCD protecting the whole installation.

As you say. Is it likely that anyone may drill into it?

Replacing the existing 60A BS1361 fuse with a type S RCD and 45A MCB would seem to be the best overall solution, but doesn’t seem to be compliant from what I can see – any ideas on options greatly appreciated please??

No, it technically is not - but as you don't have to do it, I suppose you may fit what you want.

 

[WhydidIstart]

Thanks for your comments, yes the original install is pre-16th edition, possibly even pre-15th so before an RCD requirement at the time of install.

Armoured cable is an interesting thought, the size and bend radius would prevent it being easily retrofitted though.

The feed is unlikely to be drilled into or damaged, so yes leaving it alone could be an option, perhaps just fitting a 45A fuse in place of the 60A fuse.

No, it technically is not - but as you don't have to do it, I suppose you may fit what you want.

Clearly a modern type S RCD plus MCB gives better protection than the existing 60A BS1361 fuse and is a sensible option, but just becuase it doesn't have to be updated, doesn't updating it mean that whatever goes in should be compliant with the current specs? So a 'like for like' swap would be ok - meeting the spec of the original install, swap for a compliant 30mA RCD would technically be ok but has the issues mentioned above, but a 'like for similar non-complaint' swap seems a bit more of a grey area?

 

[EFLImpudence]

Clearly a modern type S RCD plus MCB gives better protection than the existing 60A BS1361 fuse and is a sensible option,

It does.

but just becuase it doesn't have to be updated, doesn't updating it mean that whatever goes in should be compliant with the current specs?

Yes, but that's not the case. It is already there.

So a 'like for like' swap would be ok - meeting the spec of the original install,

There is no mention of 'like for like' in any regulations. The term used is 'replacements'.

swap for a compliant 30mA RCD would technically be ok but has the issues mentioned above,

Well, it wouldn't; you would be making the situation worse.

but a 'like for similar non-complaint' swap seems a bit more of a grey area?

Nope. It is a replacement.

Most people look for reasons NOT to comply with the regulations because they don't want to do it.
You are looking for reasons TO DO something you don't have to do. If you want to do it, then do so.


Incidentally, what is the reason you think the cable requires a lower rated protective device?
You say it is not clipped all the way; what are the other methods?

 

[eveares]

Well, it wouldn't; you would be making the situation worse.

If not of because of discrimination against the first RCD, then how will a 30mA RCD along with a 45A MCB be worse over a 60A BS1361 fuse?

I am confused.

 

[EFLImpudence]

The whole installation will be switched of with one little leakage fault.

 

[eveares]

The whole installation will be switched of with one little leakage fault.

But your not going to eliminate that issue without doing something about everything being downstream of the first and main RCD. Same issue as in the days where the whole house was protected by just one RCD.

 

[Iggifer]

But your not going to eliminate that issue without doing something about everything being downstream of the first and main RCD. Same issue as in the days where the whole house was protected by just one RCD.

Yes, but the point is, that isn't happening now.

No, the concealed cable isn't protected by an RCD, is it a major issue? No.

If it's something you don't want, then you either install the cable in a method that does not require an RCD, or you use a cable that means you don't need an RCD.

Protecting submains via an RCD is stooopid

 

[EFLImpudence]

But your not going to eliminate that issue without doing something about everything being downstream of the first and main RCD.

Don't understand.

Same issue as in the days where the whole house was protected by just one RCD.

Yes, and not supposed to be done now.

He doesn't have to fit one so he's making it worse.

 

[eveares]

Thank's get where your coming from now.

 

[JohnW2]

The issue is that the feed to the 2nd CU is not RCD protected and I assume this would need to be updated at the same time? Mechanical protection of the feed cable isn’t practical, but protecting it with a standard 30mA RCD would introduce all the safety and inconvenience problems present back in the days of a single RCD protecting the whole installation.

Whilst I would not advocate RCD-protecting that distribution circuit ('feed to second CU') the 'downside' you cite is not really, in itself, as bad as you suggest. Having a house's installation split into two groups of circuits each with its own RCD is precisely what we usually do these days, albeit usually within one CU rather than two. Admittedly, the usual practice is to split the circuits in such a manner that one part of the house does not lose all its power (i.e. both lights and sockets) in the event of a single fault, but that is perhaps not such a crucial consideration.

However, having said that, unless you have a TT earthing system, I agree with EFLI that adding RCD protection to the existing distribution circuit is probably not desirable (as well as not being 'required').

Kind Regards, John