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.

 

[davesparks]

OK, but why?

I'm struggling to put the answer in to written words that will make sense at the moment, but as I understand it, it's basically along the lines of this.
The neutral and the installation's earth are not directly connected to each other so can be at different potentials in a TT supply under normal conditions, in a TN supply they are directly connected together and to the same physical earth connection.

 

[nicademus]

Thanks Dave for the reply, your thread seems to go along with what my understanding was. Many thanks.

 

[SimpleSimon]

I'm struggling to put the answer in to written words that will make sense at the moment, but as I understand it, it's basically along the lines of this.
The neutral and the installation's earth are not directly connected to each other so can be at different potentials in a TT supply under normal conditions, in a TN supply they are directly connected together and to the same physical earth connection.

Think that is a pretty easy to follow explanation.
Imagine there is a N-E fault on your lighting circuit on a TT system. If you have on the lighting circuit only SP RCBO this is going to go undetected. But further upstream you have a double pole RCD main switch which is going to trip. Because of this scenario, you can't re-energise your system. If you had SPSN RCBOs installed then this would trip first narrowing down your fault to a lighting circuit (remembering that our main switch is 100mA)

 

[D Skelton]

Think that is a pretty easy to follow explanation.
Imagine there is a N-E fault on your lighting circuit on a TT system. If you have on the lighting circuit only SP RCBO this is going to go undetected. But further upstream you have a double pole RCD main switch which is going to trip. Because of this scenario, you can't re-energise your system. If you had SPSN RCBOs installed then this would trip first narrowing down your fault to a lighting circuit (remembering that our main switch is 100mA)

A SP RCBO will still trip, it just won't disconnect the fault, thus leaving it to the downstream DP RCD to perform that task.

 

[SimpleSimon]

Course it will!!

Sorry, I've edited my post now, cheers for the correction

 

[Engineer54]

A SP RCBO will still trip, it just won't disconnect the fault, thus leaving it to the downstream DP RCD to perform that task.

Think you meant to say ''Upstream'' here!! lol!!

 

[D Skelton]

Think you meant to say ''Upstream'' here!! lol!!

Upstream/downstream, who cares lol

Everyone knows I mean the one BEFORE it

 

[GEOFF S]

Imagine two propertys, both TT, one of the houses has no rcd protection and has developed a Line to earth fault. Both propertys have a common lead water pipe.

If i was working in the other house what risk would there be?

Cheers

 

[trev]

If it had no RCD would there not be another type of protection?
What about what the DNO put in their cut out thingys, what are they called?

 

[GEOFF S]

If it had no RCD would there not be another type of protection?
What about what the DNO put in their cut out thingys, what are they called?

Unfortunately on a TT the dno fuse is unlike to operate due to the high impedance earth fault path.

Cheers