UK Dead short neutral to earth

[Steviea]

Hi.
I am after a bit of advice if anyone can help. I got called to a job due to RCD tripping. Ir circuits and fault was on cooker. Disconnected cooker and fault gone from DB. The landlord tried to get the cooker fixed , but appliance engineer couldn’t fix, so he brought a new cooker. Today he had cooker installed but when it was switched on it tripped the rcd. I went back and the cooker was disconnected and I found no fault from the DB, however when I did an ir from switch back to DB I had a dead short neutral to earth. It’s a flat so can’t pull boards up from upstairs so am going back to run a new leg of cable. Has anyone got any idea why it would show a good ir from DB to switch but not the other way around.
Thanks in advance
Steve

 

[Lucien Nunes]

If the cable is continuous the IR will measure the same from all points. If the cooker worked we can assume the neutral is continuous, but did you check the CPC continuity? A nail or screw through the cable could just possibly sever the CPC and short the switch side of the break to the N, leaving the DB side disconnected.

Another explanation is that the fault is intermittent and located near the switch, where any movement of the cable can make or break the contact. It actually reads the same IR from both ends at any one moment, but the act of opening and moving the switch to carry out the test can provoke the fault to come and go, so you get different results each time you test.

 

[Rockingit]

Scenario: drill a hole through a cable due to muppetry, bang. Remove drill bit / screw = you now potentially have a broken conductor from the supply side and a shorted conductor on the load side.

 

[Steviea]

Ah of course. Then when the cooker completes the circuit, the rcd detects the imbalance and trips. Makes perfect sense now. Thank you both for your replies.

 

[Taylortwocities]

If testing back towards the board, you need to isolate at the origin if testing between E and N.

remember that N is connected to E somewhere. For TNC-S it is connected at the property, for others it’s done at the substation, but they are always connected on the supply side.