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).

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.