Fault finding advice

[dg2908]

I got asked to find an occasional fault by a regular customer of mine. I'm now totally stumped.

This is how the relevant part of the installation stands today...

[ Main House ]  5m T&E  [ Conservatory ]     30m SWA    [ Garden ]  20m T&E in conduit [ Greenhouse    ]
[ CU - Non   ]----->----[ CU           ]-------->-------[ IP56   ]----------->---------[ 2 way CU with ]
[ RCD side   ]          [ all RCBOs    ]                [ Socket ]                     [ RCD main sw   ]

Split load main board, half RCD half not. Fault causing RCD to trip. Narrowed it down to Greenhouse circuit on Conservatory CU. When I get there problem has gone - IR and Zs on the greenhouse circuit fine.

To reduce the impact of this occasional fault, I offered the option to replace the conservatory CU (old BS3036 fuse wire) with a board of separate RCBOs, and change the feed to the Non-RCD side of the main CU as the submain is run on the surface. Customer took me up on that. On the day when I installed the new conservatory CU, the IR, Zs, and RCD tripping times on the new CU were fine.

Few weeks later he calls up, has realised that at some point the greenhouse RCBO has tripped. By the time I get there, the RCBO will reset and the IR, Zs, and RCD tripping times on the greenhouse circuit were fine. I swapped out the RCBO for his peace of mind.

Few weeks later he tells me it's tripped out and I get there after a period of wet weather, hoping this will help find something. I test the Garden Socket-Greenhouse cable. IR fine. I test the Conservatory CU-Garden Socket cable which has low IR. We agree to replace this cable (was buried 2.5 T&E under a patio etc) with 4mm SWA round the edge of the garden. All tests out fine.

Three weeks later I go to do another job there and I'm told it's tripped out again. RCBO won't reset. I disconnect the supply into the Greenhouse CU and put it into terminal blocks. RCBO Still won't reset so the problem is between the Conservatory CU and the greenhouse. I disconnect the Garden socket-Greenhouse cable at the garden socket, so the circuit is just the new SWA and the garden socket. RCBO resets no problem. I then test the IR of the Garden socket-Greenhouse cable and it's fine. I admit I'm stumped.

Long story short, RCBO trips with the Garden socket-Greenhouse cable connected even if it's IR is fine and there's no load connected to it. I have triple checked all connections! Anybody able to provide pointers as to what I can do next?

 

[telectrix]

damp. IR at 1000V. use a hair dryer on external accessories etc.

 

[Guest111]

As above,IR at 1000v,in fact when fault finding I never test at anything less.

 

[NickD]

As above,IR at 1000v,in fact when fault finding I never test at anything less.

Cos if you don't find the fault you blow something up and call that the fault?

 

[telectrix]

yeah. 1000V IR tester is a man's toy. if boys ain't got the balls...........

 

[MDJ]

As above,IR at 1000v,in fact when fault finding I never test at anything less.

Another electrician here, they are appearing daily, good point phil, I do tend to whack a 1000 volts down myself as well.

 

[Des 56]

RCBO trips with the Garden socket-Greenhouse cable connected

What was the result of the test at the consumer unit for the offending Rcbo,that you carried out with the load circuit disconnected ?

 

[dg2908]

My IR tests were done at 500V so I will probably go back and test at 1000V now.

Des, which test are you referring to? I didn't record any results that seemed obviously fine during my faultfinding sessions