Continuity between neutral and earth?

[HappyHippyDad]

Hello all,

I have just done a pre-works visit for a CU change and found continuity between neutral and earth!!

This is a TT so I didn't think I needed to disconnect the main earth but after getting continuity between N-E I did, I also disconnected main bonding but still getting N-E continuity. Main switch is open.

There is an voltage operated earth leakage circuit breaker and I had disconnected the earth link to the CU from this.

I've looked through past threads rather than just posting up but all I can see is advice about TNCS.

Am I overlooking something simple? I hope so but I suspect I have a fault?

 

[Marvo]

Are you testing between the neutral bar and the earth bar in the CU with the main switch off?

 

[HappyHippyDad]

Are you testing between the neutral bar and the earth bar in the CU with the main switch off?

Oh Marvo.. You've worded that in a way that means I've done something really silly!!

A hesitant.... 'yes'

 

[telectrix]

could be a N-E fault on any of the circuits. my next move would be to test each circuit with the N's and E's out of the terminal blocks.

 

[HappyHippyDad]

could be a N-E fault on any of the circuits. my next move would be to test each circuit with the N's and E's out of the terminal blocks.

Thats my fear! Just hoping I'd overlooked something and I wasn't too sure how the earth is connected inside the ELCB, but I'd disconnected anyway! Will start breaking it down when I return to do the job.

 

[stitch]

That what I was wondering, if the ELCB is at fault.

 

[Engineer54]

Once you have found and corrected the fault, replace that VOELCB with a 100 (or 300mA) S type RCD. This will backup a failure on the new dual 30mA RCD CU that you intend to install. What was the Ra value on the existing TT system by the way??

 

[HappyHippyDad]

Thanks E54,

The homeowner needed to leave so I didn't have time to test for the Ra value. However when I found the rod, the connection was made by a standard earth clamp (not an earth rod clamp) that just pulled off. The present main earthing cable is also in poor condition (sheath broken in places and strands broken in places) and could do with replacing.

I'd be grateful for your knowledge on the size of conductor in relation to a TT? I would have used a 4mm (the present is 2.5mm) and I'll fit a proper earth rod clamp and if needed put an additional rod in if I get a poor reading.

 

[KAS1]

Even in the 1970s with an ELCB I'd of expected to of seen at least a 6mm earth to the rod.................

 

[Engineer54]

First ascertain the Ra value, it would be pretty pointless installing 4mm if you just happen to have (unlikely i know, lol!!) a TN or sub 1 ohm value... I personally would use 6 to 10mm as a minimum especially if the cable is unprotected on any lowish double figure values!!

 

[telectrix]

4mm is fine if mechanically protected

 

[Marvo]

Oh Marvo.. You've worded that in a way that means I've done something really silly!!

A hesitant.... 'yes'

Lol, it wasn't a pointed question, honestly. I just wanted to make sure exactly where you were testing.

As Tel suggests you'll just need to disconnect circuits one by one and IR test each. I'm not sure if certain VOELCB's could appear as an IR fault due to their internal sensing system but you could always remove its earth if the power is off and eliminate it from the equation. There are some who'd advocate leaving it as an isolator but I'd personally retire it and install an appropriate RCD.