315 VAC?

[Softus]

I went to replace some reported dud LED 'ankle' light units, in a flat, and found that the supply to the daisy-chain of three accessories appeared to be 315 volts.

I isolated at the MCB to undo the switch, then tested again and found the same thing there. Two of the light units didn't work when connected to a test lead, so I removed them to get replacements and put blanking plates on.

This is a brand new flat with all kinds of niggly faults. I'm about to request sight of the inspection and test record, but am I being stupid, or is there an obvious reason for the higher-than-expected voltage?

 

[JohnD]

I once came across an overvoltage on one phase, in a building with a 3-phase supply. It seems the supply cable had been damaged and the jointers had made a mistake when fixing it. I remember all the emergency lamps got fried.

I don't know more.

If it is a building with flats fed from alternate phases there might be something putting the neutral up I suppose.

 

[Softus]

I'd wondered about something like that.

I believe that the lifts run on a 3-phase supply.

There was one period where the measurement was up to about 430VAC, but it had been ages since I changed the DVM battery, and after changing it the value was 315.

Given that the enclosure housing the SWA termination was hanging off the partition wall, and the earthing bar in the CU was so badly installed that the CU cover wouldn't fit on properly, I can't help but have grave doubts about everything else.

When I isolated the lighting circuit at the MCB, the voltage reading was zero. However, I'm also wondering (a) why the switch was two-way, and (b) why there were black/grey/brown insulated conductors terminated at both L1 and L2.

Letting my imagination run unfettered for a moment, if the communal timed lighting was inter-connected with this ankle lighting circuit, am I right in thinking that this would be a horrific scenario, with this flat (and possibly others) being interconnected and not isolatible at the CU inside the flat?

 

[Softus]

There was one period where the measurement was up to about 430VAC, but it had been ages since I changed the DVM battery, and after changing it the value was 315.

Apologies - number dyslexia.

Re-reading my notes it seems that I had 415 before the battery change and 330 afterwards.

 

[JohnD]

if "Neutral" is carrying another phase, there may be a N-E of 240V or so

but it might be nil if TNCS depending where it goes wrong.

I hope they have plastic water pipes.

 

[Spark123]

What was the supply voltage reading across the main switch? Could be a broken neutral on the dno side.

 

[Softus]

According to my notes that was 248V.

I have to go back with the new lights, so I'll have more time to go checking, including down in the service room.

Excuse the stupid/naive question, but would a broken DNO neutral make the single-phase voltage appear higher?

 

[NotHimAgain]

According to my notes that was 248V.

I have to go back with the new lights, so I'll have more time to go checking, including down in the service room.

Excuse the stupid/naive question, but would a broken DNO neutral make the single-phase voltage appear higher?

Yes, a broken neutral can cause the phase voltages to change, sometimes to destructive values i.e. the full phase to phase voltage can appear across a single phase fitting. If you suspect this you has a serious problem that must be resolved without delay.

Measure all phase to neutral and phase to phase voltages. If possible install a temporary earth electrode (a screwdriver pushed into the ground will often do) and use this to measure the phase to 'true' earth voltage.

If the phase to true earth voltages are all around 400 volts but the phase to neutral voltages are all different you have a broken neutral and much damage may result.

 

[Softus]

Thanks for all of those helpful answers.

When measuring the voltage on all phases, would the earth on a PME system satisfy the requirements of a 'true earth'?

 

[NotHimAgain]

Thanks for all of those helpful answers.

When measuring the voltage on all phases, would the earth on a PME system satisfy the requirements of a 'true earth'?

It would if the broken neutral is within the installation but not if it is on the supply service cable or the distribution main.

 

[Softus]

Yes of course - now that you point it out I realise that I'd failed to do the necessary joined-up thinking.

Thanks again.

 

[Spark123]

Do you have a loop tester?
Are all the main equipotential bonds in place?
Can you confirm the service type?
It does sound suspiciously like a floating neutral, the varying voltages can depend on what is on load on different phases (connected phase-neutral after the break) of the same circuit pulling the neutral voltages up and down.
One thing for sure, it won't be doing any of the appliances or fixed equipment any good.

 

[NotHimAgain]

According to my notes that was 248V.
If the phase to true earth voltages are all around 400 volts but the phase to neutral voltages are all different you have a broken neutral and much damage may result.

Whoops - should have said 230 volts to true earth - not 400 volts[/u]

 

[Softus]

Do you have a loop tester?

Yes, but was on a different mission on the day I was asked to fault-find the ankle lights, so I didn't have it with me. I'll be going back though.

Are all the main equipotential bonds in place?

I didn't have access to the water stop cock, but I'll check when I go back.

Can you confirm the service type?

Yup I'll examine that too.

It does sound suspiciously like a floating neutral, the varying voltages can depend on what is on load on different phases (connected phase-neutral after the break) of the same circuit pulling the neutral voltages up and down.
One thing for sure, it won't be doing any of the appliances or fixed equipment any good.

I've asked the letting agent to obtain the test results from the installation. How quickly they appear will be revealing in itself.

 

[JohnD]

ink may still be wet.

Or tipp-ex under the address.