Odd NE fault.

[securespark]

Got a good one today.

Floodlight on unprotected side of S/L board trips RCD when switched on.

No neutrals crossed.

Find NE short on GF ring, along with no neutral continuity. But, when the "good" leg of the ring was re-instated, all sockets worked.

Came to the conclusion that the NE short must be "supplying" the return feed in place of the neutral.

Any other ideas?

 

[Adam_151]

Not directly related to your question, though similar in some ways, it reminded me of something I read on the IEE forum

Credit to John Peckham:

did find a potentially dangerous fault in a newly refurbished unoccupied pub. In the basement was a main distribution board feeding a sub board on the ground floor. I IR tested the main board with the the sub board connected and got an acceptable IR reading on both phase and neutral for the whole installation. I later tested the sub board and found the neutral down to earth which I did not understand as this could not be the case if the whole installation was acceptable. The sub board fed the lights in the bar of the pub and the lights and hand driers in the WCs. I disconnected the neutrals and found the feed to the handrier in gents was a dead short to earth. I left this out and taped the end up with the breaker off for the sparks to find the defect on this new circuit. I then switched the board back on and went back in to the bar to carry on testing and found none of the lights on. Thinking I had left the main switch off I went back to the board and found the isolator on. Used my volt stick to check power present at the board and the breaker all OK. Go to a light fitting and check with voltstick and find power present. Very strange I have only disconnected one neutral to hand drier, must be borrowed neutral. Go back to board and re-connect neutral and the lights work again. I note that the hand drier feed is 2.5 and lighting is 1.5mm. I then pulled out neutral for one of the lighting circuits and with a long lead check continuity to lighting neutral at one fitting and get continuity.
I then pull out all the neutrals from the board and start testing them all for separation between circuits with the breakers off and they are all clear. Check the neutral to the hand drier again and it is dead short to earth. Re-connect eaverything except both phase and neutral to hand drier. Switch on isloator and no lights. Check with voltstick phase present. In desperation I get out my test lamp and go on to the isolator terminals and find phase to neutral OK at the input side of the switch but nothing on the other side. Isolate at main board and remove main switch and find the neutral side of switch open circuit in both postions. New Wylex 100A 60947-3 duff out of the box. With no neutral the neutral return path was being provided by the neutral earth fault on the hand drier circuit.

I went back after the faults had been corrected and the sparks said that he had to re-run the supply to the hand drier as he could not find the fault and could not access the cable under the floor above.

Had I not found this I would think that it would have continued in service for many years and even passed later PIRs if the whole installation was IR tested and found accepatable.

I would think that neutral earth faults will stay undetected on installations that are not tested unless and RCD is fitted at a later date.

 

[RF Lighting]

If undiscovered that can be an incredibly dangerous situation indeed. You will have the combined current for the entire dist board all flowing down the neutral with the earth fault on it. It shows you can't be too careful and MUST test as part of commisioning an installation.

 

[securespark]

Having looked at every single outlet on that circuit, nothing is amiss.

There may be a fault under the floor in a JB, or some damage to a leg somewhere.

Must megger out each leg and see what it is.

Will keep you posted.