Ring final earth leakage fault-running out of ideas!

[FlatMax]

Hi everyone.

My query involves a ring final circuit in our lower ground floor.

 

The house was built in 2003 so is 16th Edition regs. The CU is a Hager split unit with six 32A MCBs with a 30mA RCD, and six 6A MCBs with the isolator switch to the far right of the box. The room has six twin sockets and as far as I am aware none are spurs; they all run around the perimeter of the room and I can identify where the ring enters/leaves the room. There have been no alterations to any sockets in the room since new.

Last week the RCD tripped the whole house and by elimination I isolated the lower ring final as the issue.The fault occurred after heavy rain here, and after exposing the socket tops I observed that box 6 was very damp with badly corroded conductors and switch connectors. I replaced the socket with new after treating the corroded cables, but still the RCD trips when the phase is energised.

From the CU I have continuity and resistence as follows... L=0.43 N=0.43 CPC=0.7; the ring length I estimate at 45 meters in 2.5/1.5mm twin and earth. On my clamp meter I show 4mA leaking through the main earth bonding leaving the board. I did not do a figure of eight test as I only wanted to establish there was no break in the cable, as we do get mice here quite regularly.
 
I am totally perplexed that I have continuity with correct resistence, but still the RCD trips.

 

Here in South Devon we can't get electricians out for love nor money, so I'm hoping someone on the forum can point me in the right direction. Once this issue is resolved I plan to upgrade the CU to RCBOs for all the power MCBs. In case anyone is wondering, I am not an electrician but do have a background in automotive electronics so feel competent enough to tackle this stuff. 

Many thanks in advance.

[FuerteStu]

Could you explain what you mean by 'treated the corroded cables'? 

 

Have you double checked the terminations in the socket you replaced? They are not always in the same places for different brands. 

 

The next step would be to split the circuit at other sockets (isolating different legs of the cables) and see if it holds. 

[Mattg4321]

The first thing to do with a fault like this is always always always insulation resistance testing. It solves 90% of these faults or more. 
 

If you haven’t got a tester or don’t know how to use it properly then your going to be flailing around in the dark… possibly literally!

[mistake_not]

You can do the very slow and boring resistance testing.... Disconnect at CU completely, then test resistance between cables pairs at every socket and see where resistance is lowest. Might tell you when you are closet to problem.

[andyscotland]

2 hours ago, FlatMax said:

I only wanted to establish there was no break in the cable, as we do get mice here quite regularly.

 
I am totally perplexed that I have continuity with correct resistence, but still the RCD trips.

 

As @Mattg4321 says you really need to test the resistance between cores.

 

Your continuity tests have proved that the copper is not broken/damaged.

 

However that doesn't tell you anything about whether there's a section of insulation either damaged (e.g. by rodents) or bridged (e.g. by water).

 

If you're only testing continuity of each core separately then the other cores are not connected to the test circuit, so even if there was a dead short somewhere the test current will flow entirely within the core you're testing and give you the continuity result you expect.

[ProDave]

As above, insulation resistance testing is needed Live to earth and neutral to earth.  If you don't have one, buy one, or get an electrician.

 

Clearly there is damp where the wet socket is. It sounds likely there is a lot of damp in the cable as well as just in the socket.

[Mattg4321]

9 minutes ago, mistake_not said:

You can do the very slow and boring resistance testing.... Disconnect at CU completely, then test resistance between cables pairs at every socket and see where resistance is lowest. Might tell you when you are closet to problem.

That works if you have a low resistance fault. A lot, maybe even most, are not. It might be outside the scope of a low reading ohm meter/continuity meter and require an insulation resistance tester. No need to disconnect all the legs at this stage yet either. 

[Alan Ambrose]

>>> On my clamp meter I show 4mA leaking through the main earth bonding leaving the board.

 

That’s a good test, but is that with all the other MCBs off? Check that reading on each circuit by switching the MCBs on one-by-one to double check which circuit(s) the leakage is on?

 

Similarly, take your suspect box out of circuit temporarily by taking it out of circuit? That is, binary search until you prove to yourself exactly what circuit / box / connection / device is causing the majority of the leakage.

 

Edited 6 hours ago by Alan Ambrose

[andyscotland]

7 minutes ago, Alan Ambrose said:

>>> On my clamp meter I show 4mA leaking through the main earth bonding leaving the board.

 

That’s a good test, but is that with all the other MCBs off? Check that reading on each circuit by switching the MCBs on one-by-one to double check which circuit(s) the leakage is on?

 

Personally I wouldn't spend too long chasing that - unless you confirm that it's entirely from the suspect circuit and there are no appliances of any kind connected (including sockets with built-in usb chargers etc).

 

That sort of leakage would be quite normal once you have a few electronic circuits connected as they often intentionally leak to earth as part of electromagnetic interference protection circuits etc. It's nowhere near the 30mA that would be required to trip the RCD and well within the 30%-of-tripping-current safe limit whhich would be 9mA.

[Nickfromwales]

If one link wire is dead in the ring you can omit it and reduce the 32a breaker to 20a and run it as a radial. 

[ProDave]

22 minutes ago, Nickfromwales said:

If one link wire is dead in the ring you can omit it and reduce the 32a breaker to 20a and run it as a radial. 

But you are not going to find which one without an insulation tester.  Unless you have the patience of a saint and try the "disconnect one and see if it still trips" method.

[Nickfromwales]

1 hour ago, ProDave said:

But you are not going to find which one without an insulation tester.  Unless you have the patience of a saint and try the "disconnect one and see if it still trips" method.

That one is DIY friendly and at a price even you’d like   

[FlatMax]

Thanks ever so much everyone; really helpful response.

• I cleaned the corroded cable ends (that is the end outside the plastic) back to copper using dilute hydrochloric acid and rinsing/drying. It looks like corrosion has crept up the CPC right into the flat sheathing. Assuming that's the same for the phase and neutral (and that it's very damp as well), will that be enough to allow current to leak past the plastic insulation surrounding the copper core? I have blown through the conduit between box 5 and the suspect box 6, and there is no water pooling inside.
• The earth leakage clamp test was done with all the board energised; I thought that would indicate if there was any significant loss elsewhere.

The maximum load likely to be taken from any appliance in this particular room will never exceed 7 amps, so a 20A RCBO/MCB will be fine if I decide to convert to a radial as suggested by #FuerteStu. I'll start by isolating the run between the suspect socket 6 and socket 5; if it holds then I guess I'll be ordering a 20A MCB.

 

(Just as an aside, I did put 12v DC down the ring and got no voltage drop whatsoever, but I'm guessing this would not be enough voltage to expose any fault. I think a Megger uses 500v DC or something?)

 

Anyway, thanks again for the very positive and constructive suggestions; I'll post the progress (or not, as the case might be).

[andyscotland]

8 minutes ago, FlatMax said:

Thanks ever so much everyone; really helpful response.

• I cleaned the corroded cable ends (that is the end outside the plastic) back to copper using dilute hydrochloric acid and rinsing/drying. It looks like corrosion has crept up the CPC right into the flat sheathing. Assuming that's the same for the phase and neutral (and that it's very damp as well), will that be enough to allow current to leak past the plastic insulation surrounding the copper core? I have blown through the conduit between box 5 and the suspect box 6, and there is no water pooling inside.

 

I wouldn't necessarily expect corrosion/damp within the cable sheath to cause an issue of itself - the sleeving on the phase and neutral cores should in theory be enough to insulate them from the CPC and any contamination around it. However it's possible in practice that the inner insulation is degraded or cracked (especially if the cable has tight bends and/or is old) which has allowed liquid or salts to migrate into the gap and reduce the resistance. It's also possible the hydrochloric acid has damaged it - in theory I think that's compatible with pvc but I think it depends on the exact makeup of the plastic.

 

I'd certainly agree that I'd start by suspecting this cable if I was fault-finding, unless I could prove it was fine.

 

8 minutes ago, FlatMax said:

• The earth leakage clamp test was done with all the board energised; I thought that would indicate if there was any significant loss elsewhere.

 

Yeah in that case I'd ignore that, it's a very acceptable result for a complete energised installation.

 

8 minutes ago, FlatMax said:

The maximum load likely to be taken from any appliance in this particular room will never exceed 7 amps, so a 20A RCBO/MCB will be fine if I decide to convert to a radial as suggested by #FuerteStu. I'll start by isolating the run between the suspect socket 6 and socket 5; if it holds then I guess I'll be ordering a 20A MCB.

 

(Just as an aside, I did put 12v DC down the ring and got no voltage drop whatsoever, but I'm guessing this would not be enough voltage to expose any fault. I think a Megger uses 500v DC or something?)

 

Yep - faults like this will generally require a higher voltage to overcome the initial resistance of the fault. An insulation resistance test needs to be done at 250/500/1000V to give meaningful results (voltage depending on the characteristics of the circuit and whether any sensitive devices are connected).

 

8 minutes ago, FlatMax said:

Anyway, thanks again for the very positive and constructive suggestions; I'll post the progress (or not, as the case might be).

 

[Nickfromwales]

The elephant in the room is that this is terminally ill though? 
 

If water / moisture are a constant then this’ll keep recurring. The only cure would be to replace the twin and earth with HO5 or 7, for a long term solution, and swap the metal boxes for PVC ones. 

[ProDave]

1 hour ago, Nickfromwales said:

That one is DIY friendly and at a price even you’d like   

You can buy an old style, not calibrated insulation tester second hand on the likes of ebay for not much money that would solve this quickly if intent on doing it DIY.

[andyscotland]

1 hour ago, Nickfromwales said:

The elephant in the room is that this is terminally ill though? 
 

If water / moisture are a constant then this’ll keep recurring. The only cure would be to replace the twin and earth with HO5 or 7, for a long term solution, and swap the metal boxes for PVC ones. 

 

This is a very good point. I'd assumed as this is indoors then the source of the leak has been identified & fixed but you're right OP doesn't specifically say so.

[ProDave]

Go and buy something like this https://www.ebay.co.uk/itm/397148477114?

 

That will enable you to test each individual leg of the circuit and identify where the fault is.