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Topic Title: RCD or RCBO ?
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Created On: 20 March 2012 12:39 PM
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 20 March 2012 12:39 PM
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jacks888

Posts: 3
Joined: 20 March 2012

Hi, I have just had a quote from local spark for installation of new 9kW shower unit. He insists that he needs to supply / fit an additional "Shower CU" alongside my existing 12way consumer unit as I require an RCBO which cannot be fitted into my Contactum CU. My issue is that my CU has an 80A RCD covering the installation and plenty of spare capacity. Surely all I need is a 40A MCB to run the circuit. I have a rod (TT) earth which I'm told has 18ohm "loop".Also a 10mm T&E cable upgrade(which I understand the need for). Am I missing something here?
 20 March 2012 12:51 PM
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tattyinengland

Posts: 792
Joined: 23 November 2006

As long as it is a 30mA RCD at the front end - I think you're fine to use the existing DB.
 20 March 2012 12:55 PM
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AJJewsbury

Posts: 11786
Joined: 13 August 2003

I suspect the reasoning might be that the 80A RCCB (RCD) probably isn't up to carrying the additional 40A on top of the existing load (what's your incoming supply rated at?), or that the RCD isn't of the 30mA variety required for bathroom circuits these days (100mA units were common as incomers on TT installations).
- Andy.
 20 March 2012 12:58 PM
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spinlondon

Posts: 4580
Joined: 10 December 2004

Shower circuits, and in fact any circuit of a special location require additional protection by means of a 30mA RCD.
As such your 80mA upfront RCD does not satisfy the requirement for additional protection.
You could swop the 80mA RCD for a 30mA RCD, but that would then fall foul of other requirements, and would possibly suffer nuisance tripping, due to cumalative earth leakage.
The 80mA RCD should be an 'S' type (time delayed), otherwise there will be no discrimination between the 80mA and the 30mA devices.
It sounds as if the electrician you have employed is advocating the simplest and cheapest solution to comply with all of the requirements of the Regulations.
Another option would be to replace the existing Consumer Unit, and install perhaps a dual RCD Consumer Unit, or even one with individual RCBOs.
 20 March 2012 01:06 PM
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AJJewsbury

Posts: 11786
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I've never heard of an RCD with Idn (residual rating) of 80mA spin - I suspect it's In (thermal rating) = 80A as the OP said.
- Andy.
 20 March 2012 02:01 PM
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spinlondon

Posts: 4580
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Yes Andy, I'm still having trouble with this reading malarkey.
 20 March 2012 03:46 PM
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OMS

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Originally posted by: jacks888

Hi, I have just had a quote from local spark for installation of new 9kW shower unit. He insists that he needs to supply / fit an additional "Shower CU" alongside my existing 12way consumer unit as I require an RCBO which cannot be fitted into my Contactum CU.

My issue is that my CU has an 80A RCD covering the installation and plenty of spare capacity.

If it's a 30mA unit then yes, that's all you need.

Surely all I need is a 40A MCB to run the circuit.

See above

I have a rod (TT) earth which I'm told has 18ohm "loop".Also a 10mm T&E cable upgrade(which I understand the need for).

Are you saying you already have a 6.0mm2 shower circuit - if so, 6.0mm2 will be fine for a 9kW shower


Am I missing something here?

Maybe - a competent electrician explaining the reasoning would help you



regards

OMS

-------------------------
Failure is always an option
 21 March 2012 12:27 AM
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jacks888

Posts: 3
Joined: 20 March 2012

Thank you everyone. I'm clear now as to the requirement. My CU has a 30mA RCD covering the busbar. The only issue is that it isn't a split-load CU, thus any tripping will take out the whole house. How often is that going to happen? As I see it , the only way that a 40A RCBO is going to discriminate from the main RCD is in an overload situation.
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