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