Fault on Ring final circuit

[Timbo]

Did you split the ring at the furthest point into 2 sections, or just separate at the CU?

 

[AJB Temple]

Doing this in breaks from work so a bit slow.
No - so far I have just split it at the CU. I don't know where the furthest part is really as the circuit seems to run through part of the upstairs. However, I will now remove a socket some way away from the CU and do the tests again. I presume I just leave the wires disconnected in free air at the socket point.

 

[pc1966]

Right side
L-N 0.00
L-CPC 0.17
N-CPC 0.15
L+N -CPC 0.15

This suggests to me that I have a dead short on the live side somewhere? Is that correct? Even though L-L continuity seems to be OK as testing at o.81 ohms today.

The L-N of near-zero on IR could be a fault, but it could be some load you have not found to disconnect (e.g. fridge with socket behind it).

The IR of below 1M on either L or N to E is very suspicious. With it is your fault, or you have some device with SPD included (such as a protected extension block), or there is an old RCD socket that has some internal path to trip it on L-N reversal.

First step is to search again and unplug / switch off everything on sockets or FCU.

Second step is to repeat IR at 250V, if now high you have SPD, if still low a fault (or odd RCD socket)

Third step is to find somewhere on the RFC and break L and N (you can leave CPC connected. Repeat the left/right test and see if only one "half" is now faulty. Rinse and repeat the socket break position until you pin it down. Probably in the region of recent work!

 

[AJB Temple]

Thank you. That is really helpful.

There is no chance of a fridge or similar being connected as all that stuff runs off a totally new RCBO CU with totally different feed from meter. But, as I don't know exactly where this circuit runs, it is possible that there is something connected that is hidden that I don't know about.

 

[AJB Temple]

Ah. Brainwave. Thanks for the prompt, I have just realised that the circuit must feed an outside socket that has a rainwater sewer over flow pump connected to it. Float operated industrial grade motor. Also neons in the switch. Will unplug that and test again.

 

[AJB Temple]

New results on full circuit

Left side
L-N 0.01 M ohms
L-CPC 42.0
N-CPC 15.1
L+N-CPC 29.0

Right side
L-N 0.01
L-CPC 13.7
N-CPC 29.8
L+N -CPC 29.1

Bemused. Will break the circuit at a socket now.

 

[westward10]

No need for line to neutral IR tests. As it stands those readings are above minimum limits.

 

[AJB Temple]

OK, so what am I looking for?

 

[westward10]

Connect the cpcs back to the earth bar then IR test with all four line and neutral conductors connected together to earth.

 

[timhoward]

r1 Live to live 0.96 ohms
rn Neutral to neutral 0.75 ohms
r2 CPC to CPC 1.9 ohms

This post showed you had acceptable end-to-end readings.
So actually there isn't very much to be gained by doing the same test at both ends of the same wires that have been proven to be acceptably continuous!

L+N-CPC 29.0 (presumably MOhms)

As already explained above, the L+N to E results are adequate right now and would not cause an RCD to trip.
(In general <1Mohm is inadequate, <2 Mohm should be investigated. )

So splitting the circuit apart at a socket isn't required and won't help you. (If you had got 0.xx then breaking the circuit apart and trying each half would be the start of tracking it down)

The reason the next step suggested above is to connect the CPC's to the CPC bar is that it might be a fault to a water/gas pipe or something else that is bonded or has a low resistance to earth.
So connect the CPC's back up, and test L+N (either end!) to the CPC bar and see what you get.

If that is also a reasonable reading then you have the joy of an intermittent fault, where it depends on weather, timing, movement, or simply some equipment not being connected right now.

Report back and let us know!

 

[pc1966]

L+N -CPC 29.1

Bemused. Will break the circuit at a socket now.

It looks like the pump you just disconnected. Test it separately and see if low IR to E, most likely water in somewhere!

 

[pc1966]

While it is possible to get a L-N fault that does not involve the CPC, it is hard to do with T&E cable due to its physical layout (CPC in the middle) and at accessories it is usually a fault to the back-box (cable nipped by screw sort of thing, missing grommet allowing cable-box cut, etc)

Do a continuity test L-N and see if it is low enough to trip the MCB side (so under 5-ish ohms) or it is tens of ohms or more like a small-medium motor waiting to start.

 

[AJB Temple]

This is so confusing! You guys are so helpful.

Right, I have connected CPCs back to the earth bar in CU. Put both live and both neutral from the ring into a 5 way wago, with a new wire out from it to test. Put green lead from Megger on the CU earth bar and red onto the lead from the wago connecting ring together. Insulation resistance test on that gives 35.8 M Ohms.

 

[pc1966]

Right, I have connected CPCs back to the earth bar in CU. Put both live and both neutral from the ring into a 5 way wago, with a new wire out from it to test. Put green lead from Megger on the CU earth bar and red onto the lead from the wago connecting ring together. Insulation resistance test on that gives 35.8 M Ohms.

That is good.

As a general rule, you always do IR testing with the CPCs connected to the MET (earth bar) so a fault to external metal / true Earth (e.g. screw through cable) is seen even if it is not touching the CPC conductors.

 

[pc1966]

Now check the pump!

 

[AJB Temple]

Right. Checking the pump is not so easy as the inspection hatch is a two man lift, as it is an inspection cover filled in with stone. However, I have reconnected the ring main back into the CU temporarily using Wagos and it has not tripped so far.

 

[pc1966]

Right. Checking the pump is not so easy as the inspection hatch is a two man lift, as it is an inspection cover filled in with stone.

Can't you IR test at its plug?

However, I have reconnected the ring main back into the CU temporarily using Wagos and it has not tripped so far.

That is encouraging!

 

[timhoward]

Right. Checking the pump is not so easy as the inspection hatch is a two man lift, as it is an inspection cover filled in with stone.

Do a quick L-E and N-E test using the clips on plug pins.

 

[davesparks]

No need for line to neutral IR tests. As it stands those readings are above minimum limits.

They aren't, post 26 states line to neutral is 0.01Mohms

 

[pc1966]

They aren't, post 26 states line to neutral is 0.01Mohms

I think we all suspect that is not really the cable alone, and something is still connected. But the L/N to E are now good with the pump unplugged, so the culprit would now appear to be cornered awaiting the electron police...