Lost cpc at socket

[derbyspark]

Hi i have been working at a 70's prefab bungalow that has no earth reading at 1 socket, it's part of a ring circuit and all readings good end to end from cu r1 0.44 rn o.43 but no end to end on cpc. readings at socket end to end r1 0.44 rn 0.43 but no cpc end to end. have crossed r1 + rn and have proved it to be a ring and no alterations on electrics since built. have checked all cpcs at every socket and all others ok. There's only a 100mm cavity under a ply floor for access. Apparently the houses were bolted together in 2 parts and all electrics were done in the factory, So i am thinking something must be joining them when erected? just seems very strange that readings are so good on L-N but not the earth if there's some sort of loose connection.

 

[mogga]

have you taken the socket off and measured the Zs direct at terminals? Iv had a couple of old sockets where tvs have been plugged in and due to the time the earth pin doesnt make contact with the socket part