High r1, rn, r2 etc on ring final.

[Marcus Vaughan]

Hi All.

I had a job which was going to be a 'quicky' fault finding and repairing a couple of sockets, then it came to the testing.....

The consumer unit gave me no cause for alarm but was all labelled up wrong, so it turned out there were far more sockets and square meterage on this ring final than I first thought.

So I get, on end to end continuity (2.5/1.5 T&E)
r1=1.41 ohms, rn=1.43 ohms, r2=2.38 ohms. Then from that fig 8 test a range of R1+R2 from 1.53 to 2.61 ohms.

Way too high. Nulled the leads (again). Checked again...

If it was just one of those end to end readings that was too high, I would set about looking for a high resistance connection, but as the r1 is so similar to rn and the r2 is about right too - it seems unlikely that there is a high resistance joint in all three?

If I reverse input those readings to get a estimated cable length it comes out at around 190 metres!

The house is not particularly old - about 10 years I think. The floor area of the circuit is round about the 100 square metre mark, maybe more, and includes ground floor and first floor rooms - so the cable length could well be pretty high. On the limits at guess - but not way over as I have found.

Splitting this all down would be no easy task, and I've been told the wiring was 'inspected' three years ago during a change of ownership. So I'm looking to avoid the disruption, if possible. If anyone has any advice or ideas I'd certainly appreciate it - big time.

Are there any factors which might have given me misleading high readings?

 

[Marcus Vaughan]

Max Zs is 1.29 ohms on a B32. Correct for temperature I usually work to 1.10. So no - disconnection times not met.

Mountain out of a mole hill? I thought disconnection times were there for a reason - I mean how do I write it up. Is it not in -unsafe condition?

 

[Deleted member 9648]

I assume it's on an RCD? Bs 7671 permits an RCD to take care of earth fault protection on a TN system, in which case your max Zs becomes 1667 ohms. I'm not saying that would be usual practice, but this is a circuit only marginally over required criteria. I just cant see the point in major disruption and expense for a circuit like this. Alternatively loadings permitting, stick it on a 20a OCPD

 

[Marcus Vaughan]

Max Zs is 1.29 ohms on a B32. Correct for temperature I usually work to 1.10. So no - disconnection times not met.

Mountain out of a mole hill? I thought disconnection times were there for a reason - I mean how do I write it up. Is it not in -unsafe condition?

I assume it's on an RCD? Bs 7671 permits an RCD to take care of earth fault protection on a TN system, in which case your max Zs becomes 1667 ohms. I'm not saying that would be usual practice, but this is a circuit only marginally over required criteria. I just cant see the point in major disruption and expense for a circuit like this. Alternatively loadings permitting, stick it on a 20a OCPD[/QUOTE

 

[Marcus Vaughan]

Ring Finals have to be on a 30amp or 32amp MCB. - 433-1-204.

Yes. RCD protected. What about overload and disconnection times?

The customer wants to know 'is it safe'?

 

[Deleted member 9648]

It's up to you to determine whether it's safe, your name on the form, I'm just saying what I'd do. Regarding the 30/32a device for a ring final, if you feel the load is OK for a 20a then put it on the form as a departure and justify it with the reason you've given that it is a poorly designed circuit and reducing the OCPD rating is making it safer.
Personally I'd list the high circuit readings on the form with explanation, leave it on a 32a and state that the RCD is meeting disconnection times and that I consider the circuit poorly designed but safe for continued use.

 

[Marcus Vaughan]

Thanks Wirepuller. Seems like sound advice. It's obviously not a new build or initial verification - I'll talk it through with the customer - who will probably want to go down the changing the MCB route.