Volt meter reading 760volts in 3phase dol starter overloads

[Sparky42]

The 110v is a waste of time. It sounds like you are testing at 2 points on the same phase. Unless the control circuit to pull in the contactor is 110v. I just can't tell from here.

You need to always test the lives with one prong on phase and the other on the earth bar.

It's not though it's across all phases.

I've even disconnected the outgoing wires that are connected to the motor and they still read 50volts to phase and they are not even connected anymore.

 

[Sparky42]

Just so you don't think I'm making this up.

This is l2 outgoing to earth from the contactor after the contactor has been de-energised

 

[rapparee]

I will add it's a TT System and the earth stake resistance is 16ohms

With a Ze of 0.25.

I can't fathom out that ive got no earth faults on the system itself but once I turn on any load I get these voltages.

Once contactor has been energised and then stopped the voltage to earth reads 550volts!!!! Unless the neutral is disconnected from the supply.

If the neutral is disconnected you would know about it.

The whole building would be "haunted" lights would be flickering off and on etc.

But you have to confirm the basics.

Have you got 3 phases, neutral and earth up to the machines isolator?

Have you got three phases going to the top of the contactor etc

 

[Sparky42]

yes I have. I've done a the normal tests.

all voltages are equal from all phases to earth, to neutral and to phase.

Only time I get these voltages is after a load has been applied and that's why I am thinking neutral failure.

 

[rapparee]

It's not though it's across all phases.

I've even disconnected the outgoing wires that are connected to the motor and they still read 50volts to phase and they are not even connected anymore.

So a disconnected motor sitting on its own is producing 50v.

I assume you are testing to earth?

 

[rapparee]

Have you verified the neutral St the distribution board.

Is anything else in the building suffering from a potentially floating neutral.

 

[Sparky42]

I have tested the neutral to earth from the mains to my main panel and it is reading 4 meg ohm.

Neutral to earth resistance at the main incoming is reading 0.

But voltages are all ok and equal

 

[Sparky42]

Have you verified the neutral St the distribution board.

Is anything else in the building suffering from a potentially floating neutral.

I have had uk power networks out and they have checked their supply and all is fine. I can only seem to find this fault on these contactors because they are the only ones which have a neutral in the system for the pilot light. The main lighting and single phase sockets in the building have 242volts but obviously cannot test anything once load has been removed like I can on the three phase circuits.

 

[rapparee]

So what does the customer make of this

2 days in, a call out from the DNO and still no solution.

You say that the neutral is only at the contactor for a pilot light.

What goes to A2?

 

[PEG]

Hi,the three meters you tested with,are all digital....have you got access to "proper" one?

 

[Sparky42]

So what does the customer make of this

2 days in, a call out from the DNO and still no solution.

You say that the neutral is only at the contactor for a pilot light.

What goes to A2?

He is worried about this voltage.

Basically this is what I have found so far.

First conveyor DOL starter is wired as normal. 3phase 415 coil fed through overload etc.

Control wire for second DOL starter comes out of T2 of first conveyor to a proximity sensor (to shut off motors when bin is full)in the storage tank which is through n/c contacts on a relay to enable a siren to be muted when bin is full then back to NO contact number 13 on 2nd starter.

What I'm finding is when the first contactor is turned off the second remains pulled in and the voltage at number13 of the no contact on the contactor is 560v to earth.

If I disconnect the neutral from the pilot light in the control box which tells the operator it's going to the bin (also fed from l3 on the first starter) it all shuts off and the interlock works fine and no excessive voltages.

My question is now this. Am I trying too hard yo find the neutral as the fault when it could be something more simple like in the relay and back feeding down the neutral to the contactor as the pilot light obviously more than doubles in brightness when this happens.


And just realised I haven't answered your originsl question

A1 is fed from L1 and A2 Is fed through this control circuit through overload etc from L3.

The neutral for the pilot light is taken from the consumer unit which supplies these starters.

 

[PEG]

Hi,i think you may need to do some alternative tests,using the appropriate equipment,for those tests.
Some of the results,you already have,may be leading you to the wrong conclusion.

Accepting the test gear you are using,and discounting any alternative supply to the building,generator supply,PV,etc have you done a full set of voltage tests,at the supply,when the machine is set-up to give that 550V reading in your picture?

 

[Megawatt]

Hi

I have been called out to a fault on a grain dryer where the motor interlocks are not disconnecting. Previous conveyors. Upon investigation I have found one starter that does not disconnect the preceding motor but when tripped I am getting 550v to earth from a1 and wait for it 760volts to phase from a1! This has baffled me as there's only 410v incoming.

Thoughts?

Is your plant a 3 phase ungrounded delta or is it a 4 wire y connection and is the line voltage 600 or 480

 

[Megawatt]

Is your plant a 3 phase ungrounded delta or is it a 4 wire y connection and is the line voltage 600 or 480

I don’t think it’s in the control aspect of it. You have a power problem not controls

 

[Lucien Nunes]

Is your plant a 3 phase ungrounded delta or is it a 4 wire y connection and is the line voltage 600 or 480

We're in the UK, our voltages are different, and we don't use delta supplies. Our normal service except for new, very large installations is 230/400V 4-wire Y. So anything over about 430V is abnormal.

Let's look at the possibilities.

1) The high voltage does not exist. The voltages being shown are not the actual voltage present, due to interference from a VFD or similar, causing the instruments to malfunction.

2) The voltage actually exists, and is coming from the supply. This doesn't make sense. If you lose the neutral on a 230/400V supply, 230V single-phase circuits can receive anywhere between 0 and 400V. 3-phase circuits will still have 400V between each phase and another, but the neutral could be anywhere from 0 to 400V from each of them. Nowhere does the voltage rise above 400V.

3) The voltage is being produced within the panel / system. I don't think it's been explicitly stated what the control voltage of these contactors is. Do some / all of the coils operate at 230 or 400V? If there is a control transformer in the panel, a fault on the secondary that puts it in contact with a line of the primary, could add their voltages. It would be possible to have say 510V from one 110V control wire to one line wire, if the other side of the control supply is in contact with another line. There are some very unlikely scenarios that we could think up, where isolated devices that can behave like transformers could add to make higher voltages still.

But all of this is just conjecture if we can't see the circuit!