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drypulse
Just spent a frustrating day fault finding and still left scratching my head, was wondering if anyone else had experienced something similar!
The kitchen we had been working in had been a proper DIY job (diagonal cable chases, no earth sleeving etc etc etc) so we were testing a ring circuit to ensure it was indeed a ring before adding new sockets to it, when we found that the neutral and cpc had continuity between them. (0.16 ohms, e-e was 0.76 ohms, l-l 0.46 ohms) We then proceeded to remove all the other sockets, take up floor boards and check all the junction boxes, visually fine, yet neutral and cpc were still connected. We traced this all the way back to the fuse board (old re-wireables) where it became more interesting...
By switching off the main switch, the neutral-earth fault would be cleared on the sockets. Turn it back on, and the fault would come back. We then checked the cooker circuit, same problem...
Now, the earthing for the fuse board is not via an earth cable back to the incoming mains, it is clamped to a metal box which houses the meter, fuse board and incoming tails. There is another clamp that connect from the box to the incoming earth cable, such as in a TNS system.
I thought perhaps the incoming neutral cable had broken when being pulled through steel conduit between the fuse board and the meter, resulting in an insulation resistance fault between neutral and earth that way. replaced the cable, fault still there.
Worse still, even when the main neutral and main earth had been disconnected, i could still get a continuity reading between the earthing clamp on the incoming earth and the neutral bar in the fuse board. The result was over 600 ohms however.
This leads me to believe that somehow the incoming neutral cable is touching the steel wrapping of the incoming cable, under the ground somewhere? It must have been like this for many a year, and no fuse had tripped and no signs of damage, nicked cables, screws hitting cables when fitting the board/meter etc.
No idea how to proceed from here? Really stumped I don't mind saying! Has anyone had a similar experience or has any suggestions (other then run and hide!) that I could try?
I will try and get a photo of it all tomorrow if need be, I hope I have been able to give as much information as I can, if you need any more I will try my best to help!
Any advice appreciated, Joe
The kitchen we had been working in had been a proper DIY job (diagonal cable chases, no earth sleeving etc etc etc) so we were testing a ring circuit to ensure it was indeed a ring before adding new sockets to it, when we found that the neutral and cpc had continuity between them. (0.16 ohms, e-e was 0.76 ohms, l-l 0.46 ohms) We then proceeded to remove all the other sockets, take up floor boards and check all the junction boxes, visually fine, yet neutral and cpc were still connected. We traced this all the way back to the fuse board (old re-wireables) where it became more interesting...
By switching off the main switch, the neutral-earth fault would be cleared on the sockets. Turn it back on, and the fault would come back. We then checked the cooker circuit, same problem...
Now, the earthing for the fuse board is not via an earth cable back to the incoming mains, it is clamped to a metal box which houses the meter, fuse board and incoming tails. There is another clamp that connect from the box to the incoming earth cable, such as in a TNS system.
I thought perhaps the incoming neutral cable had broken when being pulled through steel conduit between the fuse board and the meter, resulting in an insulation resistance fault between neutral and earth that way. replaced the cable, fault still there.
Worse still, even when the main neutral and main earth had been disconnected, i could still get a continuity reading between the earthing clamp on the incoming earth and the neutral bar in the fuse board. The result was over 600 ohms however.
This leads me to believe that somehow the incoming neutral cable is touching the steel wrapping of the incoming cable, under the ground somewhere? It must have been like this for many a year, and no fuse had tripped and no signs of damage, nicked cables, screws hitting cables when fitting the board/meter etc.
No idea how to proceed from here? Really stumped I don't mind saying! Has anyone had a similar experience or has any suggestions (other then run and hide!) that I could try?
I will try and get a photo of it all tomorrow if need be, I hope I have been able to give as much information as I can, if you need any more I will try my best to help!
Any advice appreciated, Joe
