Low resistance Neutral to Earth on existing wiring

[pcboffinuk]

Hi,

Spent most of today fitting some new sockets in what will be a small kitchen and struggling to persuade at least some of the existing plaster to remain on the wall!

Adding a new circuit to an existing MK fuseboard which provides power to existing sockets, lights and doorbell and has AFAIK been working happily for some time.

After completing all my tests and connecting my circuit up for live testing one large pop and the 40A mcb supplying this CU (CU in another location) tripped. (No RCD fitted).

Further investigation showed that the existing lighting circuit has a very low resistance N > E (0.23Ohm). Checked my new cabling again and it passed all dead tests.

So for now my circuit is disconnected until I can go back to diagnose the problem properly (didn't have full access today).

My confusion is the following:

1. I would have expected one of the existing mcbs (2 * B6, B32) to trip before the B40A in the feed CU?
2. With a low N>E resistance like that, I would have expected the existing MCBs to trip? The cabling on the existing sockets is fairly short so presumably the Neutral resistance is lower. However, my new circuit is only 4m or so of cabling, so the Neutral resistance should be lower and hence no more likely to trip than the existing cabling.

Am I missing something really stupid? If so I blame the plaster dust for slowing my mental processes!

 

[equitum]

Just checking the obvious - your N-E reading of 0.23Ohm - is that with the main switch on or off ?

 

[JohnD]

"With a low N>E resistance like that, I would have expected the existing MCBs to trip?"

why?

My PME doesn't

 

[pcboffinuk]

Just checking the obvious - your N-E reading of 0.23Ohm - is that with the main switch on or off ?

Hmm, I was convinced it was off, but now you've said that I may have to eat my words - will see when I return and have another go.

 

[Softus]

I sense that you're not Part P registered - have you notified your LABC?

 

[briwire]

your neutral will be connected to earth at the substation, hence the main switch in your CU must be off to isolate it.

 

[lookinn]

Think you do not understand neutral and earth connected at sub station. You much check for short circiuts or neutral earth paths after the rcd.

 

[Space cat]

Whatever may or may not be wrong with your neutral and earth wires, it's not relevant - UNLESS that 40A breaker in the other CU has RCD built in. Have a look for a test button on it.

If there is definitely no RCD in there you are looking for a short from LIVE to either earth or neutral. Start with all breakers in the new CU off. Does the 40A one go when you turn on only the main switch in the new box? If it does, look for a short between the live busbar and one of your new earth wires. Did you sleeve them?

If it doesn't, turn on the other breakers one by one leaving your latest addition until last.