Strange/Unusual Fault

[Lucien Nunes]

It's a valid point, and my hunch in post #8 was that there might be leakage contributions from the capacitance (mainly causing the difficulty resetting) and damp (causing the tripping during rain as mentioned by the customer).

Though I would find it surprising for cable with insulation damage underground to recover from the tens of kilohms needed to cause a trip, all the way to 70MΩ L-E as measured by the OP. Plus the L-E and N-E readings are identical and the L-N double the resistance, which is fairly convincing. The IR of one of the gate systems was much lower at 12MΩ.

The factor that suggests to me that capacitance is at least a significant player here is that both the original and replacement RCBOs tested fine on their own, and the cable tested fine at 70 - 140MΩ, but as soon as the cable was connected to the RCBO it became difficult to reset. I.e. the effect was apparent at the same time as the cable was showing good insulation, regardless of whether the insulation always remains good.

History is increasingly of interest. How old is it? When did it start tripping?

 

[mattg4321]

It was extremely wet here today, so I doubt rainfall will take the IR any lower than that found.

Customer has just moved in a few months ago. It’s tripping intermittently since they moved in. Consumer unit was changed early this year. Very plausible there would’ve been no rcd on the gate circuit beforehand.

I really don’t think there is any other realistic explanation than lucien’s. I was on those lines myself, but wasn’t sure if that was realistic. To be honest I’ve not come across this before. But I’ve also not come across a final circuit 400m plus in length that I can remember!

 

[marconi]

Can one buy a single length of 6mm2 swa 400m long?

 

[telectrix]

might be an idea to use an earthleagage clamp meter ( a decent one). once or twice i've had tripping issues and IR tests have been in the 100s of Megs. sometimes a IR test just will not show up a fault, esp. if it's a slight water/damp ingress. if the SWA does not need RCD protection (regs. wise), feed from a non-RCD way.

 

[James]

Can one buy a single length of 6mm2 swa 400m long?

Yes, most wholesalers will order it in for you.

in general I would not expect any issues until about 1km
if I remember right, you can have about 4 tonne on a large reel, after that it would be more difficult to obtain.

 

[baldelectrician]

I would be inclined to move the RCD to the gate location, even install an IP rated board with 2-3 RCBO's, then each socket would have an individual RCD

I know that they are phasing out RCD sockets but that is also an option at the gate end

 

[DPG]

I would be inclined to move the RCD to the gate location, even install an IP rated board with 2-3 RCBO's, then each socket would have an individual RCD

I know that they are phasing out RCD sockets but that is also an option at the gate end

Are they phasing out RCD sockets?

 

[timhoward]

if the SWA does not need RCD protection (regs. wise), feed from a non-RCD way.

Agreed. I think the issue's going to be ADS for the cable, with Zs of 4 ohms, unless the load is light enough for a B6 breaker, or even 10 amp BS88 fuse, it looks to me like it's going to need an RCD at source for fault protection.

 

[newfutile]

It’s bad practise to add RCD to comply with ADS ,this would not work on a Line-Neutral fault . This would be a departure from the regulations which does not improve safety , its better to run a bigger cable or parallel cables or even an extra R2 (cpc)

 

[timhoward]

It’s bad practise to add RCD to comply with ADS ,this would not work on a Line-Neutral fault . This would be a departure from the regulations which does not improve safety , its better to run a bigger cable or parallel cables or even an extra R2 (cpc)

I agree in ideal circumstances but we are not talking about adding one, we are talking about the effects of removing one. The original design which the OP has inherited is not ideal, and far from easy to change. It’s likely questionable with regard to volt drop too.
In any case I’m fairly sure it is in fact permitted to have an RCD for fault protection and separate overload protection as per reg 411.4.5

 

[Pretty Mouth]

It’s bad practise to add RCD to comply with ADS ,this would not work on a Line-Neutral fault . This would be a departure from the regulations which does not improve safety , its better to run a bigger cable or parallel cables or even an extra R2 (cpc)

RCD for fault protection is perfectly compliant. L to N faults do not need to disconnect within stated disconnection times, only L - E. 411.3.2.1

 

[Mikegh]

When you're IR testing the connected loads at the outlet

How can you be sure you're actually getting everything

Some stuff may not be in-circuit until aftehe supply is energised


Ime SWA faults show on 500v or 1000v , could be wrong though but usually depend on the IR tester to find an issue on fixed wiring

 

[Pete999]

When you're IR testing the connected loads at the outlet

How can you be sure you're actually getting everything

Some stuff may not be in-circuit until aftehe supply is energised


Ime SWA faults show on 500v or 1000v , could be wrong though but usually depend on the IR tester to find an issue on fixed wiring

Are you testing just the installation, or the power consuming equipment? two different testing procedures ime

 

[Mikegh]

Are you testing just the installation, or the power consuming equipment? two different testing procedures ime

I was respkndiu to the first post where he said he had IR tested the SWA.eun and then IR tested.the equipment with L+N connected

I'm saying you may not be effectively checking all the wiring on the connected load as some may not be in-circuit when supply is disconnected

 

[Mikegh]

I was responding to the first post where he said he had IR tested the SWA run and then IR tested.the equipment with L+N connected

I'm saying you may not be effectively checking all the wiring on the connected load as some may not be in-circuit when supply is disconnected

Don't seem to be able to edit anything

 

[Lucien Nunes]

I agree, and it woukd be helpful to confirm the gate operator leakage current when energised. But one of the OP's key points was that it is difficult to get the RCBO to hold with just the cable connected, and the loads completely unplugged. And the cable IR tests very good. I think we're going round in circles now, there's no mystery here IMO.

 

[mattg4321]

Exactly. The rcbo was tripping at a current too low to be registered by my tester with just a cable and a socket on the end of it with nothing plugged in. Sometimes it wouldn’t even reset.

Remove cable and all ok.

 

[Pete999]

I was respkndiu to the first post where he said he had IR tested the SWA.eun and then IR tested.the equipment with L+N connected

I'm saying you may not be effectively checking all the wiring on the connected load as some may not be in-circuit when supply is disconnected

Then if it's that strange carry out two seoerate tests , the wiring and the power consuming equipment \, to try and eliminate one from the other, no good just testing it all in one procedure, any good detective will tell you to eliminate one area at a time just saying, others may call me an idiot, but been called worse. and have a thick skin.

 

[Mikegh]

I agree, and it woukd be helpful to confirm the gate operator leakage current when energised. But one of the OP's key points was that it is difficult to get the RCBO to hold with just the cable connected, and the loads completely unplugged. And the cable IR tests very good. I think we're going round in circles now, there's no mystery here IMO.

Didn't spot that , I usually shoot from the hip anyway

 

[SparkySy]

Sorry I can't remember if you said you had disconnected the socket from the swa as well, it's not moisture in that causing it? I had a gate supply that kept tripping even when it hadn't been raining. Turned out to be the socket, the enclosure it was in was ip rated but was really damp inside, it was sweating and running off the top internally down into the socket causing a N-E short occasionally.
Sy