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?

 

[Deleted member 72666]

Have you recently used a wander lead and still have your meter nulled for that?

 

[Octopus]

If you were using a plug in tester for the figure of 8 then dirty contacts can do exactly what you describe.

Have you allowed for spurs?

What were the actual Zs readings like?

 

[Marcus Vaughan]

Hi Stantheman. I had just used the wander lead. Pretty sure I 're-nulled' though.

Dirty contacts - possible. Some of the sockets are past their best.

Live Zs was coming in at just over 1.1 ohms on a few 'DIY' spurs.

Thanks for prompt responses - I'll be back there on Thursdayto try and resolve what's going on. It's good to be armed with ideas....

 

[Octopus]

When you get unusually high Zs readings on sockets, you'll find that plugging in and unplugging the tester several times will, in 90% of cases lower the actual Zs readings

 

[Deleted member 9648]

As long as Zs readings are acceptable I wouldn't lose any sleep over it. As long as you are sure your continuity readings are correct just note it on your MW.
There's a site I work at wired about 10 years ago where all the power circuits are 4.0 32a radials on RCBO's, Zs is invariably higher than that f0r the OCPD on the longer runs.....I just note that the RCD element of the device meets the disconnection time. It's poor initial design, but as long as there are no issues with the use of the circuit or fault protection just accept it and crack on.

 

[robo83]

I had similar situation, after going round and checking the termination of all the sockets and removing every Fascia. I realised my lead for my megger was damaged and wasn't making the correct termination with the meter

 

[Deleted member 72666]

Usually best to take readings from the terminals themselves as to avoid poor contact issues.

 

[Risteard]

As long as Zs readings are acceptable I wouldn't lose any sleep over it.

I'm not sure I'd agree. I think it would definitely need to be established why such readings were being obtained. It doesn't seem entirely credible that the circuit is indeed that length (but who knows?).

 

[davesparks]

Did you test both sockets of the double sockets? I've often found that one side is different to the other.

 

[Marcus Vaughan]

Hi All. Thanks for responses. I have spent the morning changing a few accessories - including 3 sockets and a FCU. The readings are coming down - it an still getting various Ze of between 0.56 and 1.26 ohms. In fact there is a group that is compliant then a step change to a group that are not. I'm starting to think a kitchen extension is to blame? Interconnections? Now to retest properly from the board.

 

[davesparks]

If Ze is varying then that is another matter entirely, that's the DNO's problem.

 

[Marcus Vaughan]

If Ze is varying then that is another matter entirely, that's the DNO's problem.

Sorry - I didn't mean Ze, no idea why I wrote that. I meant R1+R2.

 

[Marcus Vaughan]

Well - I returned. Found even more sockets on the ring. Mapped it out. Used IR test to confirm everything disconnected. r1, rn, r2 all still high (the same as before). Tested back and front of sockets and swapped a few out there were a little 'resistive'.
All new R1+R2 readings - very uniform but at 0.89 ohms give or take 0.05 for those on the ring (couple of spurs a teeny but more). Add a 0.3 Ze on a 32 amp type B.......

It's non-compliant. I think the house was at close to maximum upon design. Add an extension, add some electric velox windows, add this, add that...

Now trying to come up with a solution that won't destroy the place...

 

[Deleted member 9648]

I'm still not sure whether the effort is justified, unless I missed it what's the Zs? Are disconnection times met?
It's an existing circuit that may not be compliant but is unlikely to cause a problem based on information given. I think you are making a mountain out of a mole hill

Edit...just noted you've stated 1.1 ohms on spurs? Within required max for the OCPD. I cant see the point in it.