cable behind 50mm insulated plasterboard

[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.

 

[Geordie Spark]

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.

 

[Plonker 3]

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.

 

[trev]

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?

 

[D Skelton]

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.

Why are you using RCBOs on a TT?

 

[Bobster]

Why are you using RCBOs on a TT?

Beat me to it.

 

[Geordie Spark]

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.

 

[HappyHippyDad]

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).

 

[ImpededLoop]

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.

 

[telectrix]

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

 

[Bobster]

Some manufacturers literature forbid the use of RCBOs on TT/IT systems.

 

[ImpededLoop]

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.

 

[davesparks]

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.

 

[Geordie Spark]

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).

From Rommel????? :laugh:

 

[HandySparks]

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?