Problem neutrals

[gdog]

Problem with neutral.

The other day we had a problem with a circuit not working correctly.
The circuit is a radial,wired in 4mm singles,in metal trunking powering 4 switched fused Spurs.
The circuit had previously been dead tested,with no apparent problems.
However when the circuit was energised, only 2 of the spur units worked.After investigation we found that there was 240volts at the 1st and second spur,and 160volts at the 3rd and 4th spur,between live and neutral,and 240volts between live and Earth.We have proven that the neutral between 2nd and 3rd Spurs is the same one,and has an end to end resistance of about 0.11ohms.
Any ideas please,we have had little time to pursue the fault at the moment,but need to resolve next week.
Cheers G.

 

[Pete999]

Check the connections, you may have the N in the load side of the FCU

 

[gdog]

Check the connections, you may have the N in the load side of the FCU

Thanks for your interest.
I thought the same,as if it is passing through something else,but from the Spurs I have taken off,the connections seem fine.I have even removed one spur an put on a connector block.
Cheers G

 

[Deleted member 72666]

Check to make sure the copper is in full contact with the terminal screw and not the insulation.

 

[David M]

Are you testing the voltage at the incoming terminals of each spur?

 

[David M]

The reason I ask is that I've come across worn/faulty/dirty switch contacts before that have produced similar problems to what you are experiencing. Where you get 240v at the incoming terminals but less voltage at the out going even when switched on. Obviously with double pole switch gear the problem can be on either pole, in your case possibly the neutral.
But as you say its the last 2 spurs that seem to be the problem, without any further details, then I would guess at it being a poor neutral connection somewhere?

 

[telectrix]

could be a joint in the cable as well. when measuring continuity, you'e only shoving ELV down the line. as soon as you up the voltage to LV, it could just be finding a high resistance joint or connection. try end-end with a megger.

 

[David M]

could be a joint in the cable as well. when measuring continuity, you'e only shoving ELV down the line. as soon as you up the voltage to LV, it could just be finding a high resistance joint or connection. try end-end with a megger.

I was just about to post this same thing.

 

[gdog]

could be a joint in the cable as well. when measuring continuity, you'e only shoving ELV down the line. as soon as you up the voltage to LV, it could just be finding a high resistance joint or connection. try end-end with a megger.

Thanks to you for replying.
As said,the first two Spurs work fine,it's the last two that don't.They are only about 6metres apart,but the cables travel down different runs of trunking,making it about 25metres from the 2nd to the 3rd spur,and same from 3rd to 4th.The trunking is metal,300/100 so quite big,with only about 4 circuits in it.I have tested the cable from 2nd to 3rd spur using long lead method,and it is about 0.10 ohms.
Cheers G

 

[Richard Burns]

So long as the cable is not damaged it must be a terminal connection problem. A visual inspection and re termination should resolve this.
Your initial IR test results would have shown any cross connections.
If you measure the end to end resistances of the line and neutral then you will have a comparison as they should be the same, assuming the neutral is higher then this is the area to investigate; so measure the resistance of neutral from the circuit start to each point of use, either on a long lead basis or by joining Line and neutral at the start and measuring between line and neutral; where the resistance rises indicates the two points between which there is a problem. Run a temporary cable between these points (disconnecting the "faulty" section) and see if this resolves the issue.

 

[gdog]

So long as the cable is not damaged it must be a terminal connection problem. A visual inspection and re termination should resolve this.
Your initial IR test results would have shown any cross connections.
If you measure the end to end resistances of the line and neutral then you will have a comparison as they should be the same, assuming the neutral is higher then this is the area to investigate; so measure the resistance of neutral from the circuit start to each point of use, either on a long lead basis or by joining Line and neutral at the start and measuring between line and neutral; where the resistance rises indicates the two points between which there is a problem. Run a temporary cable between these points (disconnecting the "faulty" section) and see if this resolves the issue.

Many thanks.
Your interest was much appreciated.I will try and resolve this when we return to site on Monday.
Cheers G