# Ring final test results

Hello,

I've added a couple of sockets to a ring final circuit, and have taken the opportunity to test the circuit. I've followed the instructions here. My results are:
R11: 1.6
Rnn: 1.6
R22: 4.1

At stage 2 in the instructions, the values between live & neutral for each socket are 1.4 or 1.5 & then between live & earth they're all 1.8 or 1.9 except... one socket gives 2.2.

So I've got two questions:
1. Is there any explanation for why I seem to be getting double the value I'd expect for the socket readings? E.g. I was expecting to get (R11+Rnn)/4=0.8 for the first set of socket readings.
2. What's with the weird high reading for one of the sockets. I've re-tested and checked the connections etc.
Thanks for any help,

Your r1 and rn (note lower case 'r' for end to end measurements) are very high and as you say the R1+R2 is even higher.

They (r1 & rn) would indicate the circuit is 216 metres long (2.5mm² conductor has a resistance of 7.4 mΩ/m).

What are you using to test?

R11: 1.6 ... Rnn: 1.6 ... R22: 4.1
Something's surely wrong there, for a start? Not only are the figures very high (unless it's a ludicrously long circuit), but (for 2.5/1.5mm² cable), one would expect the end-to-end CPC resistance to be about 1.6 times that of L & N, not ~2.6 times.

Kind Regards, John

Sounds like you're using a cheapy multimeter, which is no good reading low values.

I am using an old & cheap multimeter :/

If I get a new one, is there any way to tell if it'll actually be accurate enough before I buy it?

Does anyone have any ideas on my second question? The socket is on the ring, not a spur, so I'm confused about how it can be giving a higher reading than all the others! I tested from the back of the socket.

You need a pretty specialist meter for measuring low ohm values. Not yet seen a cheap one that will do it even from our friends in china. Happy to be proved wrong as I'd like another one. If you are happy to tinker and can do maths then you make a low ohm meter from your existing meter. Works surprisingly well and pretty accurate. Obviously this isn't a calibrated meter but it should give a reasonably good idea if something is wrong: http://www.robotroom.com/Measuring-Low-Resistances.html

Not only are the figures very high (unless it's a ludicrously long circuit), but (for 2.5/1.5mm² cable), one would expect the end-to-end CPC resistance to be about 1.6 times that of L & N, not ~2.6 times.

Putting aside just a moment that (as you say) it is unlikely that the circuit is as long as those values would suggest. It is perhaps worth considering that it is not 2.5/1.5.... as 2.5/1 would have the cpc end to end equal to 2.5 times the live end to end. Which is awfully close to the 2.6x reported.

Of course all we can do is speculate unless readings which can be trusted can be obtained!

Some low cost multi-meters use a very low current for measuring resistance and their measurements are confused by small voltages that are created in the loop by junctions of dis-similar metals. ( the thermo couple effect ).

A meter that drives a constant current ( 100 mA ) through the circuit under test and measures the voltage needed to drive that current round the circuit and calculates R from R = V / I is more accurate than a multi-meter with a test current less than 5 mA

Putting aside just a moment that (as you say) it is unlikely that the circuit is as long as those values would suggest. It is perhaps worth considering that it is not 2.5/1.5.... as 2.5/1 would have the cpc end to end equal to 2.5 times the live end to end. Which is awfully close to the 2.6x reported.

Kind Regards, John

Not only are the figures very high (unless it's a ludicrously long circuit), but (for 2.5/1.5mm² cable), one would expect the end-to-end CPC resistance to be about 1.6 times that of L & N, not ~2.6 times.

Putting aside just a moment that (as you say) it is unlikely that the circuit is as long as those values would suggest. It is perhaps worth considering that it is not 2.5/1.5.... as 2.5/1 would have the cpc end to end equal to 2.5 times the live end to end. Which is awfully close to the 2.6x reported.
My ballpark estimate for the total length is 50m, (give or take 10m!). I'd seen about the possibility of it being old 2.5/1 in another thread but I know that almost all of the cabling is brown & blue (which I assume means 2.5/1.5 There are just a couple of places (awkward spurs particularly) where I've seen black & red.

Anyway, thanks for all the comments. I'm a bit stuck about the meter as I don't really want to spend a fortune buying a fancy one that I'll use maybe twice & don't have the bits for the home-made option. I guess I can at least be confident that I've got continuous connections for the three wires.

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