Discuss Need to extend a RFC - Zs not good enough in the Commercial Electrical Advice area at ElectriciansForums.net

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Just after some thoughts on this. I need to extend a RFC, but the Zs isn't good enough to comply (it doesn't comply currently, but it becomes my problem when I touch it). It's currently about 0.70 ohms - what I need to extend it by will be minimal, maybe taking it up to 0.75 ohms. It's protected by a C32 MEM1 RCBO, top of my head that's 0.55 ohms to meet Zs requirements. It could go on a B32 no problem - only feeding a few computers - but, they didn't make B32 RBCOs for MEM1, or if they did I can't find any in the used market.

So, where do I go from here - I can't run a new circuit, a new board is out of the question really at the minute. Does the RCD mean I only need to be looking for 1667 ohms Zs or is that all a bit naughty?

Appreciate any thoughts.
 
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Ian1981

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The regulations allow an rcd to be used for fault protection in a TN arrangement in compliance with table 41.5
 
RCBO means you only need to comply with the requirements for RCD Zs however I’d check that earth leakage isn’t going to be an issue if the extended circuit will be used for PC’s
 
T

Toneyz

why not fit a C20 RCBO?
The MEMSHEILD 1 I think was only in 16 and 32A
 
T

Toneyz

What is the existing Zdb and R1+R2 how does it compare with past results could be a loose/poorly tightened in one of the existing.
Since MEM 1 don't forget that we have adjusted for 230v and Cmin so once could have complied.
Whilst a stand-alone RCD can be used to overcome the high loop value I try not to rely on them in this scenario as many a time they are not tested with the test button regularly and left to stick.
How is the existing circuit wired as can additional earth be installed?
 
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The cpc had an r2 of around 0.2something. It was the line that had a value of around 1.7. This is because there's a tonne of conduit and the likes on the run. I've no doubt it once complied, but I've also no doubt this RFC has been previously extended without testing by the installing company. Thing is, 0.75 ohms is a perfectly acceptable reading on a type b.
 
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James

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I have a few of the pods you are referring to.
make me an offer I cant refuse.
anything over £300 will be instantly accepted and I will even clean the dust off.
they are all second hand and have come out of various boards i have changed.
 
What about putting a small DIN enclosure adjacent to board with a B32 breaker inside, fed from the MEM C32? A bit heath robinson but compliant?
 

Dave OCD

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I was just going to suggest that, but as you're altering the circuit why not a B32 30ma RCBO in the small enclosure ?
 

stevethesparks

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RCBO means you only need to comply with the requirements for RCD Zs however I’d check that earth leakage isn’t going to be an issue if the extended circuit will be used for PC’s
Although not a requirement, I would also conduct a line test to make sure L&N also return a low enough measurement to satisfy disconnection times. An RCD would not detect such a fault.
 
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  • #17
Thanks for the responses all. Greatly appreciated again, apologies for my delay in getting back to you all, it's been a manic 2 weeks years.

The regulations allow an rcd to be used for fault protection in a TN arrangement in compliance with table 41.5
This is what I've gone with - documented on the certificate that I am relying on the RCD for fault protection as is my wont per chapter 41.

RCBO means you only need to comply with the requirements for RCD Zs however I’d check that earth leakage isn’t going to be an issue if the extended circuit will be used for PC’s
Thanks, only 5 PCs on this RCD and I've got circuits that have never grumbled with 10 PCs on the same model RCD.

I have a few of the pods you are referring to.
make me an offer I cant refuse.
anything over £300 will be instantly accepted and I will even clean the dust off.
they are all second hand and have come out of various boards i have changed.
Not a pod for MEM1, that's MEM2.

What about putting a small DIN enclosure adjacent to board with a B32 breaker inside, fed from the MEM C32? A bit heath robinson but compliant?
I thought and thought on this design, and it just seems a lot of a bodge. The truth is, these MEM1 are old hat now - we need new boards that have room for future growth too. Thing is, we're bidding for grants for a full rewire so they won't give me the cash to update the boards because it would then be money lost should the rewire funding be approved.

Again, thank you for all your help all.
 
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