bizarre fault

[siroccomanuk]

phase - neutral = 205
phase - earth = 205
neutral - earth = 50 - or 120 under load conditions

 

[securespark]

So, as I said before, that's 205V P-E without putting the system under load??

That is outside the tolerances and must be investigated by the DNO.

 

[conny]

Just a suggestion.
Check your meter on another installation, such as your own, to make sure it is calibrated correctly. Reason being we had some clamp-on ammeters calibrated a few months ago by a new company. Testing a motor and the current seemed high so used another meter, current dropped to what I expected. Decided to check with 2 others and got odd readings with them so calibration service was a load of cobblers. We sent them to our usual checker and when all tested on return they were within tolerance.
I know this doesn't help with your original problem but it should be considered.

 

[bernardgreen]

phase - neutral = 205
phase - earth = 205
neutral - earth = 50 - or 120 under load conditions

That does seem to suggest a defective neutral in the supply network.

When you say earth do you mean the CPC electrical earth ?

Can you measure Neutral to GROUND and Earth CPC to GROUND. ?

Could be a serious phase unbalance in a building on the same network feeder cable together with a neutral conductor in poor condition.

 

[ColJack]

odds on it's a 2 phase street now with a dodgy earthed neutral at the sub?

some idiot repaired a cable in the ground and lashed L1 and L2 to one cable due to an unfound break in L2?

it's been done before...

knock on some neighbours ( 3 either side to be sure at least ) and ask them if they've had any similar problems and if they'd mind you doing a quick voltage check...
explain that you think there may be a network problem and if it is then it would benefit them all to find out so it can be fixed..

 

[bernardgreen]

some idiot repaired a cable in the ground and lashed L1 and L2 to one cable due to an unfound break in L2?
it's been done before...

They did it here and the results were similar to those described. It was not such an idiotic idea as the alternative was to dig up the High St at the time of most inconvenience. As soon as the road could be dug it was repaired.

 

[Space cat]

phase - neutral = 205
phase - earth = 205
neutral - earth = 50 - or 120 under load conditions

If you draw the phasor diagram for the off-load voltages you'll see that the neutral-earth voltage is almost 90° out from the other two. That would make sense if you've got a high resistance neutral connection somewhere and the principal return path is neutral-earth capacitance.

Next question: What are you using as a test load? Have you made measurements using different loads? To keep things simple, stick to resistive loads only. A filament bulb and a heater will do nicely.

PS: You mentioned earlier that your 205 was a misprint and you meant to write 225 volts. Is this still true?

And finally ---

If there's a fault upstream of the CU, why are all the lights still working? What about a dodgy RCD?

Edit:

I've just spoted this:

If anything gets plugged in to the sockets circuits and switched on (3 seperate rings) the voltage drops to 120 across all circuits.

Does this include the light circuits or just the sockets?

 

[nobby5637]

Hi there, have come across this problem 3 times in my career and every time it was a pme fault and every time the road came up

 

[siroccomanuk]

Sorted DNO turned up (again) it eventually turned out to be a dodgy connector on the O/h lines neutral at the pole end and with the strong winds last night was coming in and out It didnt help as the muppet who put new CSU in about a year ago DIDNT disconnect the original TT earth spike when it was upgraded to TNC-s at the same time so neutral was 'leaking' to earth when the wind blew! Bloody dangerous. Anyhow, it is now sorted and house is booked for a full test & inspection to see what other disasters he has made:frown;
Thanks again guys for your help

 

[Lectrician]

I have seen plenty of dodgy neutrals in the supply. Nasty on PME - the earth rod that was left in could of actually prevent deaths and fire! (I know that sounds extreme, but the extreme could happen).

Neutral faults on PME or on other installs with different earthing systems where a N to E fault exists can cause very odd and dangerous faults.

Do I get the points for sorting the fault on the early part of the first page?

 

[securespark]

Hey! Nobody likes a smart-@rse!

 

[plugwash]

DIDNT disconnect the original TT earth spike when it was upgraded to TNC-s at the same time so neutral was 'leaking' to earth when the wind blew! Bloody dangerous.

It will do that through the boding anyway even if the spike is removed (unless all your service pipes are plastic). Anyway it's better that it does that than your earth rising up to 240V relative to real earth.

 

[siroccomanuk]

Very true - BUT it didnt help the fault finding not knowing there was an old spike buried!