cable behind 50mm insulated plasterboard

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nicademus

Hi, doing new build first fix ,chased wall for 25mm conduit to run ring mains on 32 amp rcbo (TT installation) behind 50mm insulated platerboad, is OK to run in 2.5 t&e. Without factoring in thermals, would be very expensive to beef up to 4.0mm.
 
nicademus said:
Hi, doing new build first fix ,chased wall for 25mm conduit to run ring mains on 32 amp rcbo (TT installation) behind 50mm insulated platerboad, is OK to run in 2.5 t&e. Without factoring in thermals, would be very expensive to beef up to 4.0mm.
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Leave a small gap between the wall & back of switched socket to allow air to enter the back box. This air will be drawn into and up the conduit due to Thermal Convection and keep the cable cool.
 
nicademus said:
Hi, doing new build first fix ,chased wall for 25mm conduit to run ring mains on 32 amp rcbo (TT installation) behind 50mm insulated platerboad, is OK to run in 2.5 t&e. Without factoring in thermals, would be very expensive to beef up to 4.0mm.
Click to expand...

Time to get your note pad, pen and calculator out.
 
nicademus said:
Hi, doing new build first fix ,chased wall for 25mm conduit to run ring mains on 32 amp rcbo (TT installation) behind 50mm insulated platerboad, is OK to run in 2.5 t&e. Without factoring in thermals, would be very expensive to beef up to 4.0mm.
Click to expand...
I don't understand why you've chased walls then buried conduit then buried it under 50mm boards. It seems like an awful lot of work for no benefit.
What do your derating factors tell you about the 2.5?
Why are you running T & E in conduit?
 
trev said:
I don't understand why you've chased walls then buried conduit then buried it under 50mm boards. It seems like an awful lot of work for no benefit.
What do your derating factors tell you about the 2.5?
Why are you running T & E in conduit?
Click to expand...

Steady on Son ..... you're asking two questions at once there.

Break it down a bit - keep it simple. ;)
 
D Skelton said:
Why are you using RCBOs on a TT?
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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).
 
happyhippydad said:
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).
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If it had an upfront two pole RCD, in the event of a N-E fault the upfront RCD will operate whether it is 100mA or S type. The reason is that a single pole RCBO will not disconnect a N-E fault.

Therefore, an RCBO is pointless in this scenario.
 
Last edited:
no. if he's got an up front 100mA S type, he still nees a 30mA RCD/RCBO for the rfc.
 
telectrix said:
no. if he's got an up front 100mA S type, he still nees a 30mA RCD/RCBO for the rfc.
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Correct, but if it's a single pole RCBO, the 100A S type will still operate on a N-E fault as a SP RCBO won't disconnect it. This would disconnect the whole installation causing unnecessary disruption.

So both RCBO's and a 100mA S type are pointless in this instance - a 30mA two pole RCD is required to provide adequate protection to the final circuit or a group of circuits in a split load board.
 
happyhippydad said:
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).
Click to expand...

Yes it does need 30mA protection but the commonly available single pole RCBO is unsuitable, just read the manufacturers instructions.
The use of SPSN or DP RCBOs would be acceptable.
 
davesparks said:
Yes it does need 30mA protection but the commonly available single pole RCBO is unsuitable, just read the manufacturers instructions.
The use of SPSN or DP RCBOs would be acceptable.
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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?
 
happyhippydad said:
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).
Click to expand...

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.
 
HandySparks said:
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?
Click to expand...

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 said:
Where does it say that an RCD or RCBO requires an additional RCD in series as a backup?
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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.
 
D Skelton said:
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.
Click to expand...

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)?
 
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!
 
Simlec said:
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!
Click to expand...

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 said:
OK, but why?
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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.
 
davesparks said:
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.
Click to expand...

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)
 
SimpleSimon said:
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)
Click to expand...

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

Cheers
 
GEOFF S said:
Unfortunately on a TT the dno fuse is unlike to operate due to the high impedance.

Cheers
Click to expand...
So the trick is to make sure the impedance is low enough that any fuse will operate.
It's not difficult, I do it all the time. My own home is on a TT and has a sub 1 ohm impedance. If memory serves me well it's around the 0.5 - 0.6 mark (can't be bothered to go and get the cert) my record is 0.29.
 
trev said:
So the trick is to make sure the impedance is low enough that any fuse will operate.
It's not difficult, I do it all the time. My own home is on a TT and has a sub 1 ohm impedance. If memory serves me well it's around the 0.5 - 0.6 mark (can't be bothered to go and get the cert) my record is 0.29.
Click to expand...

Impressive, are we talking Ra or Zs?

Hot or cold site?

Cheers
 
Well if we want to be pedantic it's actually a Ze although the terms are interchanged all the time when talking about TTs. Ra is the resistance of the electrode itself measured with the spike system. The 0.29 was a domestic
 
trev said:
Well if we want to be pedantic it's actually a Ze although the terms are interchanged all the time when talking about TTs. Ra is the resistance of the electrode itself measured with the spike system. The 0.29 was a domestic
Click to expand...

Well to be pedantic, where RA isnt Known it may be replaced by Zs, 411.5.3. So the RA was 0.29 ohms?
 

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