2-Core or 3-Core

[JohnW2]

any length of any cable with a non-metallic sheath (i.e. virtually any multicore cable) has 'adequate mechanical protection' for the cable to be unprotected by a RCD in a TT system. That doesn't really seem to be fully 'within the spirit', so I wonder if I'm misinterpreting?

No, that's exactly what it is saying, or how else would you get your electricity from the meter into the RCD enclosure?]

Yes, there's obviously always going to be a need for an exception to avoid that 'Catch 22', but I guess that can be largely served by a requirement to have an RCD 'close to the origin' of the installation, hence at least limiting the length of that unprotected cable. I was commenting more on the fact that this would seemingly allow an unlimited length of,say, T&E to exist without any RCD protection in a TT installation.

Indeed. I find it difficult to be sure whether 412.2.2.4 theoretically allows a CPC to enter a pre-RCD Class II enclosure in order to connect to the armour (and CPC, if appropriate) an outgoing armoured cable - but, as others have said, I'm not at all sure that one should be doing this, whatever 412.2.4 may say. If the enclosure is the one which actually contains an RCD (so that the outgoing cable was RCD-protected, I suppose it could still be said that there was an issue with having a CPC within it, but that would perhaps be getting rather pedantic.

Note much thinking required there - the reg is for CPCs of other circuits, not the one connected to the RCD.

Are you saying that this reg theoretically does not allow, say, an outgoing SWA cable to have its armour (and maybe CPC) earthed (or even 'exposed') in an enclosure which contains non-RCD protected conductors (e.g. the enclosure containing the most upstream RCD) in a TT installation? If that's the case, I guess one has to earth it only at the other end, and take steps to 'isolate' the upstream SWA gland; in practice, I suspect it would usually be simpler to run unarmoured cable (with CPC not connected at one end) to a second enclosure for the SWA termination?

Kind Regards, John

 

[mfarrow]

I was commenting more on the fact that this would seemingly allow an unlimited length of,say, T&E to exist without any RCD protection in a TT installation

You wouldn't use T&E. The cable is not unprotected. It is protected against overload by the fuse, and you are protected against electric shock by double or reinforced insulation.

Remember that ADS isn't the only way to achieve protection against electric shock.

Are you saying that this reg theoretically does not allow, say, an outgoing SWA cable to have its armour (and maybe CPC) earthed (or even 'exposed') in an enclosure which contains non-RCD protected conductors (e.g. the enclosure containing the most upstream RCD) in a TT installation? If that's the case, I guess one has to earth it only at the other end, and take steps to 'isolate' the upstream SWA gland; in practice, I suspect it would usually be simpler to run unarmoured cable (with CPC not connected at one end) to a second enclosure for the SWA termination?

It only talks about other circuits passing through the enclosure, so I can't answer the first part. But it is practicable and safe, so I wouldn't have thought it to be a problem. Remember that 531.4.1 only relates to the installation between the origin and the RCD, so if you've got good separation of circuits inside the enclosure, then no doubt you'll be alright when viewed pragmatically, though an external gland plate may be advisable.

 

[JohnW2]

I was commenting more on the fact that this would seemingly allow an unlimited length of,say, T&E to exist without any RCD protection in a TT installation

You wouldn't use T&E.

Probably not, given that one wouldn't want to use the CPC. The point I was trying to make was that the regs would appear to allow 'any old PVC insulated/sleeved cable' (without armour or any other protection), and without too much thought threw in "say, T&E" as an example of such a cable.

The cable is not unprotected. It is protected against overload by the fuse, and you are protected against electric shock by double or reinforced insulation. Remember that ADS isn't the only way to achieve protection against electric shock.

Agreed. Provided that there is no 'earthed' CPC in the cable, there is minimal risk of an L-E fault arising in association with it. If there were an 'earthed' CPC within a pre-RCD cable, the risk would presumably be that an L-E fault within the cable would (with TT) raise all the metalwork etc. in the premises to a high voltage without any protective device operating. Admittedly, proper bonding should prevent that representing a hazard to life, but it's not a desirable situation.

It only talks about other circuits passing through the enclosure, so I can't answer the first part. But it is practicable and safe, so I wouldn't have thought it to be a problem. Remember that 531.4.1 only relates to the installation between the origin and the RCD, so if you've got good separation of circuits inside the enclosure, then no doubt you'll be alright when viewed pragmatically, though an external gland plate may be advisable.

Yes, again, I agree with all that. As you say, the common sense of the situation is fairly clear, and I suppose this is one of the many situations in which 'the words of the regs' do not answer all questions.

Kind Regards, John

 

[securespark]

Seem to remember when I did the 15th, it was possible to use metallic units on TT as long as you used special insulating grommets/ glands.

Were some of them made of wood, or have I imagined that??

 

[JohnW2]

Seem to remember when I did the 15th, it was possible to use metallic units on TT as long as you used special insulating grommets/ glands.

Yes, I've seen mention of such things (even 'special kits') quite recently, almost suggesting that such things might even still be around. However, quite apart from the current regs, acceptability of that approach would rely on the assumption that the only risk of a L-to-case fault was at the point of cable entry; although that's undoubtedly the most likely fault, it's not the only possibility (e.g. L cable pops out of loose terminal and flops onto case!). Do I take it that the cases of such metal units would have been earthed in those days?

Kind Regards, John.

 

[Darkness]

Just to clear things up, at the meter, the tails go into a CU which has a main switch 'Hager 80A 30mA RCCB. Also there is a 40A MCB which the run of 16mm² is fed from to the garage.

The CU in this garage has a normal 100A switch.

Going to price some cable this evening or tomorrow at the latest so do you guys still think 2-core is fine with a new rod installed at the new building?

Cheers.