High(ish Zs and extending ring final question.

[Marcus Vaughan]

Hi all. Love the forum by the way.

I do have a question though...

I have just visited a house (on a Sunday!). Customer is asking for additional sockets onto 32amp ring final. Circuit is 30mA RCD protected. Wiring is 2.5mm/1.5mm tinned copper.

Thought I’d better do a live Zs test.

Zs = 1.29, down to 1.22 back of socket. Checked a nearby socket - same results.

So it doesn’t comply with Max Zs values for ADS and I am not one of those who likes to use the ‘RCD get out of jail card’.

So by extending it I will be knowingly making the circuit less compliant.

Impractical to split circuits (no room in the CU). Impractical to rewire in larger cable.

Feels like a no-go.

I was wondering if, with those values, it would make enough difference if I conducted the dead test for more accurate results?

Clutching at straws.

Flooring going in Wednesday apparently....

Irony is that I know they will just use extension leads across the floor instead.

I wonder what you would do?

 

[Marcus Vaughan]

Perhaps I need not worry too much about mix and match manufacturers in the consumer unit. Just noticed on my photo there is a Hager to the right and a Wyler to the left.

Yuk.

 

[Marcus Vaughan]

Wyler. I meant Wylex!

 

[KEV 1 N]

Sometimes you do get a “high” Zs reading when testing a circuit with an RCD. I’ve had this happen before and out of curiosity have bypassed the RCD or put the lives into the non RCD side, and the results have been a lot lower.
As the others have mentioned, do the ring dead tests and that will highlight any connection issues.

 

[Bob Geldoff1234]

I agree that it could be the rcd causing the high readings.To find out attach your test earth lead to the MET and test on the incoming of the rcd.Note the reading and then test on the output side of the rcd.They should be the same but i have come across up to 1 ohm variation on some makes of rcd.

 

[Octopus]

Perhaps I need not worry too much about mix and match manufacturers in the consumer unit. Just noticed on my photo there is a Hager to the right and a Wyler to the left.

Yuk.

View attachment 40263

Id rather fit a used MCB to match the cu that start fitting another make. Just make sure your quote and invoice state a used part will be fitted ........

 

[Marcus Vaughan]

Sometimes you do get a “high” Zs reading when testing a circuit with an RCD. I’ve had this happen before and out of curiosity have bypassed the RCD or put the lives into the non RCD side, and the results have been a lot lower.
As the others have mentioned, do the ring dead tests and that will highlight any connection issues.

Thank you - seems to be the consensus.

 

[Marcus Vaughan]

Id rather fit a used MCB to match the cu that start fitting another make. Just make sure your quote and invoice state a used part will be fitted ........

Good advice that. I’m wondering if bus bars have been hacked about or not secured particularly well. Something to look forward to!

 

[Marcus Vaughan]

Well - that's the end if that thread. We'll never know. Customer called me at 5pm the night before to cancel the job. 'Cost outweighs the benefits'. I'd kept the day free for it as well - so ended up grovelling around in the phone in the evening to bring jobs forward.

I did let the customer know that 'Further Investigation' was still recommended (an extra sockets and 5 or 6 metres of cabling would only have made the Zs slightly higher).

Grrrrrrrr.

Thanks for posts everyone though - I have learnt something ready for next time.