fluctuations in earth loop result

[Sparky7573]

hi guys , im just wondering if anyone has had this problem before, were testing a commercial winery premises, main control gear in one room upstairs (old metal clad MEM stuff) but there has been a shop alteration downstairs, the main installation is around 50 year old, but the shop was done around 8year ago, its all new colours, and it has its own 3 phase board, this 3 phase board is fed from 80amp 3phase circuit breaker (8year old) from old main MEM cupboard upstairs, the submain is a 16mm 4core, armourings as the earth , all earth rings are fitted and linked via 16mm earths with crimps lugs , looks fine, outgoing earth loop is 0.10ohms , the submain is about 30meters long, but at the 3phase board we got a reading of 5.3ohms at first, then we tested again about 10 mins later and got a reading of 15 ohms, then 10 mins later we tested again and it dropped to 4ohms, went back upstairs and tested the out going armoured again , it still read 0.10 ohms, went back downstairs and tested again at the 3 phase board and got a reading of 28ohms this time, it has me beat, ive inspected the cable run which is 70% clipped direct , but disappears into a flat roof space , any ideas guys , any help would be appreciated, i have not meggered the cable yet as the shop is in use, which i suspect im going to have to do.

 

[ericmark]

I have had that before, found a loose earth wire, bolt had worked loose I assume with vibration.