cable behind 50mm insulated plasterboard

[Plonker 3]

It does say that, you just need to apply 314.1 to 411.5.2 from the BGB.

 

[HandySparks]

It does say that, you just need to apply 314.1 to 411.5.2 from the BGB.

Where does it say that an RCD or RCBO requires an additional RCD in series as a backup?

 

[D Skelton]

Where does it say that an RCD or RCBO requires an additional RCD in series as a backup?

In short, it doesn't, and I don't think anyone here is saying that it does, but given the number of faulty RCDs I've come across, in my opinion, you'd be an idiot not to fit an s-type up front as back up on a TT system where the only means of providing fault protection is by way of an RCD.

 

[HandySparks]

In short, it doesn't, and I don't think anyone here is saying that it does, but given the number of faulty RCDs I've come across, in my opinion, you'd be an idiot not to fit an s-type up front as back up on a TT system where the only means of providing fault protection is by way of an RCD.

I don't disagree that it's a 'good idea' to provide a backup device in the event of a faulty RCD or RCBO where these are the only means of earth fault protection.

I was just trying to get to the bottom of the assertion 'you can't use SP RCBOs on a TT supply'. So is it just the lack of discrimination with the backup RCD in the event of a neutral to earth fault or is there something more fundamental about a TT system that makes SP RCBOs unsuitable (assuming that the particular manufacturer doesn't advise against their use)?

 

[davesparks]

Manufacturers don't 'advise' against their use, they state that they are not to be used. There is a big difference!

 

[Simlec]

Easiest way to look at it is page 45 OSG (green) 5.1.1 (ii)
All live conductors in a TT system and line conductors in a TN, remembering what your neutral is classed as!

 

[D Skelton]

Easiest way to look at it is page 45 OSG (green) 5.1.1 (ii)
All live conductors in a TT system and line conductors in a TN, remembering what your neutral is classed as!

The regulation that refers to is applicable to the entire installation. It isn't talking about individual circuits.

More often than not, the only effect this regulation has is forbidding the use of three pole main switches for TP TT systems.

 

[HandySparks]

Easiest way to look at it is page 45 OSG (green) 5.1.1 (ii)
All live conductors in a TT system and line conductors in a TN, remembering what your neutral is classed as!

Thanks. So the answer is for purposes of isolation.
BGB / BYB 537.2.1.1

Anyone know of any other reason?

 

[HandySparks]

Manufacturers don't 'advise' against their use, they state that they are not to be used. There is a big difference!

OK, but why?

 

[nicademus]

Hi , original poster of question, thanks to all for helpful comments.