Phase to Earth fault on a 3 phase motor supply

[Jonathan S]

I had the 20 amp 3 phase breaker off and was testing for voltage at the motor terminal box. I had nothing between phases but had 230 volts from each phase to earth. Why might this be.

 

[pc1966]

What did you check the voltage with? A high impedance multimeter, or a 2-3mA voltage tester?

One phase welded shut so energising all 3 at the same voltage via the motor windings?

My first priority would be to verify the MCB is isolating all 3 phases, or when open check with voltage tester (carefully, this is live and dangerous) between the L1-3 supplies and the L1-3 out to see if you get 0V (contacts presumably welded shut) or ~230V (leak to earth) or 400V (one other phase not open).

Then check if there are any auxiliary supplies for control, etc, that might have faulted and are energising it.

 

[James]

Possible that one phase is still live
what test equipment and procedure are you using?

 

[Jonathan S]

What did you check the voltage with? A high impedance multimeter, or a 2-3mA voltage tester?

One phase welded shut so energising all 3 at the same voltage via the motor windings?

My first priority would be to verify the MCB is isolating all 3 phases, or when open check with voltage tester (carefully, this is live and dangerous) between the L1-3 supplies and the L1-3 out to see if you get 0V (contacts presumably welded shut) or ~230V (leak to earth) or 400V (one other phase not open).

Then check if there are any auxiliary supplies for control, etc, that might have faulted and are energising it.

I just tested with my Fluke Tester, not a multimeter. It was the same reading when the breaker was on, have not disconnected and taken out and changed the breaker yet.

 

[westward10]

There must be some kind of 'control' between the motor and the breaker.

 

[Jonathan S]

There must be some kind of 'control' between the motor and the breaker.

Yes , there is the isolater switch .

 

[pc1966]

Just an isolation switch, no start/stop contactor control and/or overload relay?

Was the isolator open?

 

[snowhead]

What was the reason for being at the motor initially and testing?

Is there a VSD in the circuit?

 

[Jonathan S]

What was the reason for being at the motor initially and testing?

I am doing some work at a block of flats, and was told the air extract was not working in the bathrooms. I checked the fan on the roof and it was not working. With the breaker on I was getting nothing between phases and 230 volts from each phase to earth, this was the same with the breaker off. I just used my Fluke tester, not a multimeter.

 

[westward10]

You need to do some systematic testing and work through the circuit. First check the output from the breaker and move on from there I would have thought there is some kind of overload relay at some point and if this has tripped you need to know why.

 

[Jonathan S]

Just an isolation switch, no start/stop contactor control and/or overload relay?

Was the isolator open?

The isolator was on, got to sort it out on Monday.

 

[pc1966]

The isolator was on, got to sort it out on Monday.

Let us know how you get on!

 

[Jonathan S]

What was the reason for being at the motor initially and testing?

Is there a VSD in the circuit?

No I dont think there is a VSD. I think it just goes from breaker to isolator to motor.

 

[westward10]

Just work your way through the circuit and it will show itself you often find there is more to the circuit than you think, I would have thought there is an overload relay somewhere for motor protection.

 

[Jonathan S]

Thanks for all your replies gents.

 

[Jonathan S]

Let us know how you get on!

Hi, it was a Vent Fan Shut Down Relay from the Fire Alarm monitoring circuit that was off.

 

[westward10]

Result.