Ring main continuity test

[Deleted member 112675]

I'm not sure if this is a reasonable question or so stupid it shouldn't be asked. If the latter than I apologise and hope I don't get too much negativity back.

So I want to perform the test Continuity of ring final conductors on an existing ring main. Because I'm lazy and its dark outside, could I do this whilst the rest of the CU is live?

My thoughts are to pull the MCB for that circuit and pull the earth cables from the bar out too. Then for neutral, first check the neutral bar in the CU is at the same potential as earth by using a cheap multi meter to check for continuity to earth. If good, then remove the neutrals for that circuit.

Then with both cables removed, carry out the testing as per normal.

 

[Deleted member 112675]

Since it was so quick to do, I decided to try Wilko's kind suggestion. I've only done the easy first part partly because I know it is a ring (its in my house and I've seen it) and partly because I just wanted a sanity check before doing a load of other work and then a final test.

Anyway, the numbers around the ring in ohms were:
L 0.18
N 0.17
E 0.41

This is 2.5mm twin and earth.

That last reading was higher than I would expect since I think it should be around 0.3 but not sure if its anything to worry about. Perhaps my earthing terminals are not as tight as the others.

 

[MJPD29]

Your results sound like they are I'm the right ball park. Out of interest why are you doing this?

 

[Sparky5473]

looking at those results is the cable 2.5mm with 1.0mm cpc?

 

[Highresdis]

Since it was so quick to do, I decided to try Wilko's kind suggestion. I've only done the easy first part partly because I know it is a ring (its in my house and I've seen it) and partly because I just wanted a sanity check before doing a load of other work and then a final test.

Anyway, the numbers around the ring in ohms were:
L 0.18
N 0.17
E 0.41

This is 2.5mm twin and earth.

That last reading was higher than I would expect since I think it should be around 0.3 but not sure if its anything to worry about. Perhaps my earthing terminals are not as tight as the others.

The 2 most likely reasons for the discrepancy from the x1.666 ratio are:

• The cable is over 40 years old, and still had a 1mm earth with the 2.5mm live cores.
• There is at least one case of a poor connection. Quite often you will find the result improves incrementally after reterminating the earths in each socket.

BTW, if this is the main ring circuit for the house, it appears to be quite small, and may indicate that a number of sockets (usually the downstairs ones) have been wired as spurs from a small ring. This is not a major problem, but isn't ideal for any modifications, and usually indicates an old installation.

 

[7029 dave]

Since it was so quick to do, I decided to try Wilko's kind suggestion. I've only done the easy first part partly because I know it is a ring (its in my house and I've seen it) and partly because I just wanted a sanity check before doing a load of other work and then a final test.

Anyway, the numbers around the ring in ohms were:
L 0.18
N 0.17
E 0.41

This is 2.5mm twin and earth.

That last reading was higher than I would expect since I think it should be around 0.3 but not sure if its anything to worry about. Perhaps my earthing terminals are not as tight as the others.

Your cpc will have higher resistance because the conductor is smaller.

 

[westward10]

As above in #19 you either have a poor connection or the cpcs are 1.0 in which case you use a ratio of ×2.5

 

[telectrix]

i see no need to isolate a CU in the OP's case. switch off MCB. diss all 6 conductors, do dead teats on that RFC, reconnect and reset MCB, assuming dead tests are within spec.

 

[Deleted member 112675]

Thanks very much to the majority who have given me good advice on this thread. I have nevertheless learned my lesson that talking about working live esp for a DIYer is going to annoy one or two so I shan't do that again. I do deliberately use "DIY" terms such as earth instead of CPC etc so that it is very clear I am a DIYer just in case you miss the DIY badge of shame above all my posts.

I have a lot of respect for those in many trades including electricians. DIY "bodgers" have had a terrible press of recent years in many cases with justification. The problem people face is that when you call someone out to do some work, how do you know they've done a good job? Most people are honest and will do good work even when the work is out of site but some will not. If Mrs Miggins sees her shiny new sockets & switches and they all seem to work ok how is she going to know if the installation is dangerous ? That is my motivation to DIY.

Your cpc will have higher resistance because the conductor is smaller.

Yes but not that high. It should be 2.5/1.5=1.6667 times higher but is much closer to 2.5/1 which as some have kindly suggested might indicate a 1mm conductor somewhere in the ring.

That did worry me for a short while because much of that ring was installed perhaps 20-30 years ago but I did compare the earth with a known 1mm earth and a known 1.5mm earth and it is definitely 1.5mm.

I finally resolved the issue though. Whilst the raw numbers pointed to a 1mm somewhere, I have less faith in my conductor termination skills than some of you do. So I removed all the newly added sockets one by one and connected the earths together using a wago connector (which even I can get right). As I did so I took fresh readings and sure enough the earth resistance around the ring started to come down. When I'd change all 6 I was down to 0.32 Ohms which in conjunction with the other numbers is much more consistent with a 1.5mm earth. THe problem was I simply wasn't terminating well so I went back to the drawing board, watched a couple of John Ward videos, and I plan to start looping the ends of the conductors back on themselves to fill the connector.

Thanks once again to most all of you who helped me with this.

 

[magnoliafan89]

Was I the only one who assumed he meant he wanted to work in the CU whilst everything else bar the socket circuit was still live?