Discuss cable behind 50mm insulated plasterboard in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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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.
 
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.

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.
 
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.

Time to get your note pad, pen and calculator out.
 
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.
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?
 
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?

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

Break it down a bit - keep it simple. ;)
 
Why are you using RCBOs on a TT?

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'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).

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.
 
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no. if he's got an up front 100mA S type, he still nees a 30mA RCD/RCBO for the rfc.
 
no. if he's got an up front 100mA S type, he still nees a 30mA RCD/RCBO for the rfc.

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.
 
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).

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.
 
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.

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?
 
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.
 
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.
 
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?
 

Reply to cable behind 50mm insulated plasterboard in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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