neutral-earth voltage on 3-phase sytem

[eds_taff]

A machine in work has been intermittantly stopping, and when conducting some tests today I found a voltage between neutral and earth of around 100Volts (cant be 100% on the accuracy of these values as it was late, and I didnt write them down).

When I tested phase to neutral I got
L1 - N 360Volts
L2 - N 120Volts
L3 - N 360Volts

Phase to earth values
L1 - E 240Volts
L2 - E 240Volts
L3 - E 240Volts

And phase to phase values were all around 420Volts.

Any opinions on what could be causing this?
Some sort of L2 - N fault?
An imbalance in the phase loads?
Loose neutral connections somewhere?

 

[eds_taff]

A bit of other information.
It is on a saw which uses 415V 3-phase for the motor, and the neutral is needed as the control is 240V.

It is plugged into a 32Amp Ceeform socket. I also unplugged the machine and tested at the socket and got the same values.

 

[ColJack]

you need to test at the board supplying it and back as far as you need to to find where this 120V appears on the neutral..

it's a posibility that the neutral is no longer tied to earth..

get phase to phase readings toworrow..

 

[bhm1712]

L2 to N should not be 120 volts

there should be O volts between earth and neutral . seems you have some leakage and a circuit with bad protection.

there is an in-balance in the phases and this can cause unwanted potential between ground and neutral.

where did you measure these values? at the isolator or at the motor terminals?

incoming side of the isolator or outgoing?

usual suspects can be bad or loose connections.

 

[bhm1712]

i see now that you tested at the socket with the machine unplugged

in this case you need to find out where the socket is fed from at a dist board and test here, and check for the obvious loose connections etc

 

[bhm1712]

also, i would have thought that the phase to neutral voltages for a 415 volts supply were a bit low, forgetting L2 for a minute, i have never seen voltage like this between phase to neutral on a true 415 volts supply. is it 415 volts??

 

[eds_taff]

Thanks for the replies.

I located the dis board and the breaker supplying the socket today, but it was too late to do anything so first thing tomorrow I was going to have a look and check for loose terminals.

If there is nothing obviously wrong though, any other suggestions at what could be causing it?

Do you think it is then likely to be a (big!) unbalance in the phases?

 

[electronicsuk]

Floating neutral?

 

[RF Lighting]

It sounds to me like a neutral somewhere which has gone high resistance.

It would be worth doing an earth loop impedence test and also a neutral impedence test just to see what the values are looking like.

As has been said, test the supply at the DB which this socket is fed from and also at the main incommer, and see if the readigs are the same there too.

 

[ColJack]

i see now that you tested at the socket with the machine unplugged

in this case you need to find out where the socket is fed from at a dist board and test here, and check for the obvious loose connections etc

I said that allready..

also, i would have thought that the phase to neutral voltages for a 415 volts supply were a bit low, forgetting L2 for a minute, i have never seen voltage like this between phase to neutral on a true 415 volts supply. is it 415 volts??

want to re-think that?

phase to neutral should be 230V ( 240 reality ) ..
phase to PHASE would be 400V ( 415 reality ) ..

 

[bhm1712]

sorry ive not got me head on the right way..... yes 230 between phase and netural.

i am a dunse.

sometimes i suprise myself

ive been up since 3 am and cant think straight now

 

[ColJack]

it is almost definitely a loose or disconnected neutral somewhere.. likely the incommer to the CU..

it's getting to earth through another neutral somewhere which sugests it's ok as far as the board..

 

[eds_taff]

Thanks everyone, your help is much appreciated

I will check out the distribution board tomorrow, and post back on here to let you know the outcome.

 

[ColJack]

there's no transformers on the 3 phase anywhere is there?
no new machines?

 

[eds_taff]

Don't think so, but I'm not 100% sure as I only started working there today!!