Ring Circuit on its Last 'Legs'?

[acvc]

Scenario: Landlord has put in a new c/u in a house, then called me to test and certify it! The wiring and c/u were actually ok - off the shelf unit complete with mcbs, dual rcds, and main switch, all BS numbers good. It's a 3bed terrace, old colours and definitely ageing, looks like lots of diy spurs.

Problemo: testing a ring final, 2.5mm 6242Y. Readings are all over the place.

End2Ends are: Line 0.04, Neutral 0.04, Earth 0.09 (ohms). This would suggest around 5m of 2.5mm and 7.5m of 1.5mm cpc! Clearly not the case.

Crossovers at the c/u are: R1Nn 0.03, R1R2 0.04, RnR2 0.04 (ohms), and the socket readings are all consistently higher and variable. R1-Rn varies from 0.12 - 0.98. R1-R2 varies from 0.15-1.08, and Rn-R2 varies from 0.21-1.12 . They should be more or less the same.

Question 1: How can there be such a difference between the c/u reading and the sockets in general?

Question 2: Is this just a case of ageing and deteriorating wiring on it's last 'legs' (excuse the pun)?

 

[electricAl]

I`d suggest the Ring is bridged close`ish to the CU. When you do your measures there its just looping around the shortest ring, but when testing at s/o`s its path back to CU is varying...

Not pretty. Best of luck with it. I`ve gotta a Periodic next week on a rented house - i know the owners mate (a `Property Developer` has been installing new s/o`s & lights (inc in kitchen) Should be fun! Again, they`ll be looking for a `get out of jail free` cert. They might be disappointed.

 

[acvc]

Yep the final straw for me was getting above maximum Zs on both rings. At that point i stopped and called the landlord to say 'you're not getting any certificate from me mate'.

On the positive side, I think I've persuaded him to rewire the lot. I just hope I get the job!

 

[NoSparks]

0.04 does seem awfully low, but I have come across variations like these many times. How are you taking your measurements, from the wires at the back of the socket or plugging into the socket? In my opinion much of the prolem is contact resistance. I have tested the two sockets of a twin socket and got .2 on one and .8 on the other (very repeatably) thus proving the fault was not in the wireing. Oh and by the way this was a brand new, flashy, shiny socket!

I now normally take the maximum and just ignore the differences unless they take me out of range, then the first option is to try a new socket.

The next most likely fault I find is loose or corroded connections. Check, clean and re-make where possible.

If the wireing is in PVC then the chances of it failing are minimal unless attacked by vermin.

 

[acvc]

Thanks NoSparks.

I'm using a plug-in socket adaptor into which i put my Megger 1552 (unfused) leads. I will definitely have a go at comparing the readings I get from the wiring inside the socket to outside the socket (plug-in figure). Should be interesting.

At the c/u, i'm putting the ends into a simple connector block, screw it tight for a good connection and then test across the screws on either side of the block.

If I get these crazy low figures again, I'll cut back the end to behind the pvc and try again.

ElectricAl. Do you mean that low figures like this could indicate an interconnected ring (is this what you mean by "bridge") close to the c/u?