Identifying a neutral supply problem?

[MickSmith5824]

Had a call today. Property has 400v 3 phase DB....it used to be old commercial unit but changed to residential. He's been in process of moving in but within day or two appliances starting to burn out. He's father had reported testing some sockets and voltages varied at times when other appliances were turned on on same circuit and he also reported 330v on one socket. DNO has been out but just live tested on incoming side of main switch as had been unable to find where to isolate our DB from. Customer has since found another DB in an area of this commercial unit that isolates his DB. I am due to go there tomorrow. My question is what is the quickest and most of obvious tests/readings that point to a dodgey neutral. ZS will only give me Line to earth, would a L-N loop reading potentially point to it or PSSC. My thinking is there is potential he has two crossed phases supplying a single circuit...but this would result in around 400v on sockets and wouldn't of fluctuated as much as he has said it did and also surely these problems would of been noticed by previous tenants and rectified

 

[snowhead]

If things are burning out and the voltage is up to 330v then the DB has lost the neutral, creating a floating Star point instead.
Look at the D.B and at the source or original location if the Cables have been extended, it'll be obvious what's wrong.
And that D.B needs switching off NOW.

 

[Pete999]

Had a call today. Property has 400v 3 phase DB....it used to be old commercial unit but changed to residential. He's been in process of moving in but within day or two appliances starting to burn out. He's father had reported testing some sockets and voltages varied at times when other appliances were turned on on same circuit and he also reported 330v on one socket. DNO has been out but just live tested on incoming side of main switch as had been unable to find where to isolate our DB from. Customer has since found another DB in an area of this commercial unit that isolates his DB. I am due to go there tomorrow. My question is what is the quickest and most of obvious tests/readings that point to a dodgey neutral. ZS will only give me Line to earth, would a L-N loop reading potentially point to it or PSSC. My thinking is there is potential he has two crossed phases supplying a single circuit...but this would result in around 400v on sockets and wouldn't of fluctuated as much as he has said it did and also surely these problems would of been noticed by previous tenants and rectified

Simple job for a Sparks I would have thought.

 

[MickSmith5824]

If things are burning out and the voltage is up to 330v then the DB has lost the neutral, creating a floating Star point instead.
Look at the D.B and at the source or original location if the Cables have been extended, it'll be obvious what's wrong.
And that D.B needs switching off NOW.

The DB is isolated Snowhead they haven't moved in yet. The DNO said there is no supply issues they tested upto supply side of main switch so is 230v there between L and N so not lost Neutral atleast not entirely.

 

[davesparks]

My question is what is the quickest and most of obvious tests/readings that point to a dodgey neutral.

You already have all of the information you need to identify a disconnected neutral, testing may be required to identify where exactly the disconnection is. continuity testing should be all that is needed if it can't be found by visual inspection.

The affected circuit,DB etc should be isolated until this is properly rectified.

 

[buzzlightyear]

He's been in process of moving in but within day or two appliances starting to burn out. He's father had reported testing some sockets and voltages varied at times when other appliances were turned on on same circuit and he also reported 330v

The DB is isolated Snowhead they haven't moved in yet

sorry this is confusing .

 

[MickSmith5824]

You already have all of the information you need to identify a disconnected neutral, testing may be required to identify where exactly the disconnection is. continuity testing should be all that is needed if it can't be found by visual inspection.

The affected circuit,DB etc should be isolated until this is properly rectified.

Thanks Dave. The DB is isolated. If DNO has proved supply is good to to main switch of our board though. How can anything on consumer side be causing 330v or burning out of appliances. Loose/disconnected neutrals on actual circuits won't do this will they? Surely.
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Simple job for a Sparks I would have thought.

Pete it probably is no doubt but what I don't get is if DNO has tested upto supply and said everything good what exactly can cause 330v on consumer side.

 

[snowhead]

You're looking for the main neutral to the affected DB missing , not the individual circuits.

 

[buzzlightyear]

Pete it probably is no doubt but what I don't get is if DNO has tested upto supply and said everything good what exactly can cause 330v on consumer side.

you have 85 amps adrift ,you need to test accordantly. when the DNO tested it at thier supply only on the head not on the consumer side .

 

[davesparks]

Thanks Dave. The DB is isolated. If DNO has proved supply is good to to main switch of our board though. How can anything on consumer side be causing 330v or burning out of appliances. Loose/disconnected neutrals on actual circuits won't do this will they? Surely.

Did the DNO check it with the DB isolated? Have you checked their findings yourself and proved continuity of the neutral from the incoming supply through to the neutral bar in the DB?
DNO operatives are not infallible, they can make mistakes just like the rest of us do.

No loose neutrals on final circuits won't cause this.

It could be between the main switch and the neutral bar, is it a four pole or three pole main switch?
Have you checked the neutral disconnect link in the DB is fitted properly?

 

[Lucien Nunes]

If it's not an obvious air-gap (e.g. missing link) or visibly overheated, you can check across any suspect points along the neutral with a voltmeter (if the installation is energised and there is at least some unbalanced load on, or for any hint of resistance (if it's not energised).

 

[MickSmith5824]

Did the DNO check it with the DB isolated? Have you checked their findings yourself and proved continuity of the neutral from the incoming supply through to the neutral bar in the DB?
DNO operatives are not infallible, they can make mistakes just like the rest of us do.

No loose neutrals on final circuits won't cause this.

It could be between the main switch and the neutral bar, is it a four pole or three pole main switch?
Have you checked the neutral disconnect link in the DB is fitted properly?

They must of tested it with DB isolated Dave as main rcd switch is now faulty and cannot reset. I'm not sure if it is 4 or 3 pole or if continuity good between main switch or tested link. I will test these when go there thanks for heads up.

 

[davesparks]

They must of tested it with DB isolated Dave as main rcd switch is now faulty and cannot reset. I'm not sure if it is 4 or 3 pole or if continuity good between main switch or tested link. I will test these when go there thanks for heads up.

If its an RCD then it should be obvious whether it is 3 or 4 pole.

 

[MickSmith5824]

If it's not an obvious air-gap (e.g. missing link) or visibly overheated, you can check across any suspect points along the neutral with a voltmeter (if the installation is energised and there is at least some unbalanced load on, or for any hint of resistance (if it's not energised).

Thanks Lucien. So when you say check for unbalanced load you mean check voltages between each phase to neutral? And if can't manage to get it energised low ohms test between phases to neutral?

 

[davesparks]

Thanks Lucien. So when you say check for unbalanced load you mean check voltages between each phase to neutral? And if can't manage to get it energised low ohms test between phases to neutral?

No, he is saying that if it is energised you can measure the voltage drop across the various possible points of failure in the neutral. However as the DB is isolated it would not be sensible to energise it until the fault is found and rectified.

Testing between phases and neutral is unlikely to find a fault in the neutral. Testing the resistance of the parts which could have failed will likely find the fault.