cable behind 50mm insulated plasterboard

[D Skelton]

I'm confused with that one Mr DS? It may have an upfront RCD of 100mA or greater so would need 30mA protection on this new work. Come on.. put me on the right track (looking for smiley with a deep, accepting sigh).

I can't see the reason for confusion?

No self respecting spark would rely on 30mA RCBOs alone to provide earth fault protection, therefore that's the reason behind fitting an s-type up front for back up protection. The only thing is, with a SP RCBO, it won't disconnect a N-E fault, meaning that such a fault would take out the up front s-type.

Two options here, fit DP or SPSN RCBOs, or, fit a dual RCD board.

The former of those two options is limited to either those who can afford to pay for really expensive gear, or, those who are prepared to have what in my personal opinion appears to be cheap and not very well made gear fitted.

99 times out of 100, fitting a dual RCD board with up front protection is the only realistic option on a domestic TT system.

 

[Plonker 3]

Help me out here. I understand that for most TT supplies, we're relying on a residual current device of some sort for earth fault protection.

What is it about a TT supply that makes an all SP RCBO board unsuitable where it would have been OK for a TN supply? What is the particular additional risk with TT in using single pole devices?

2 RCDs/RCBOs one n-e fault and not way of isolating the neutral will lead to both RCDs/RCBOs trippping and not able to reset the upfront device.

 

[HandySparks]

2 RCDs/RCBOs one n-e fault and not way of isolating the neutral will lead to both RCDs/RCBOs trippping and not able to reset the upfront device.

I understand that.

Are you saying that you would never install on a TT supply without a 'backup' RCD in the supply?

 

[Plonker 3]

No, you need to install as others have said either spsn RCBOs or DP RCDs

 

[HandySparks]

No, you need to install as others have said either spsn RCBOs or DP RCDs

In which case, do you think that the current regs are inadequate in not mandating backup RCD protection on TT supplies?

 

[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.