Hi, Went to customers house following A N Other who had extended socket circuit for a conservatory, They said to customer that RCD wasn't tripping properly, interestingly the put NA against trip values on test cert for sockets that could be used outside... but thats another issue. So I repeated tests and couldn't get the RCD to trip on any setting, ZS at test socket was 0.70. Its a TT system but I was getting a ZE of 0.55 so probably parallel earths on water and or oil pipe. Overhead supply and not converted to TNCS. Changed the CU RCD for a new one still not tripping. There was also a much older RCD unit in series with the RCD on the CU, took that out and put tails into 100a DP as it could fail in some way, made no difference to RCD CU test. Oddly I found another mains RCD in the cupboard identical to the original in the CU, so it looks like someone tried changing it already,,,? Tried testing the RCD on the load side mains connection with and without MCBs on, still fails. Test button works fine. So to rule it out I changed for another new RCD, makes no difference. Tried disconnecting one neutral at a time and retesting but wouldn't trip either at mains or sockets. I haven't done any IR tests as I was running out of light and time but I recall somewhere that a N-E fault can cause this even though normally it should trip. Any ideas please?!