Motor Tripping RCD but it's not even connected to it

[JD Hogg]

Hi, I have just installed a dedicated 32A circuit to feed a motor which according to the data plate is 3.0kW and 16.6 Amps, single phase. It is fed from a 32A MCB via 6mm cable about 22 metres. I have fed it from a MK twin 63 Amp RCD board but this motor circuit is not protected by either RCD,

The confusing thing is that one of the RCD's trips when the motor is started; when the motor is running I can reset the RCD and everything is fine. The RCD that trips is the one physically located nearest to the motor circuit RCD

There are no borrowed neutrals, I have swapped the RCD's over to make sure one of them isn't faulty but the RCD nearest the motor circuit still trips when the motor is started.

Obviously there is going to be a voltage dip on start up but would this trip an RCD and if so why don't they both trip?

There is no soft start on the motor.

Any ideas appreciated.

 

[SparkyChick]

I don't... I think it could be as simple as the quick trip magnetic coil in the MCB interfering with the toroidal inductor in the RCD. Hence my question about placement of devices

But I'm happy to be wrong as then I'll probably learn something

 

[westward10]

I don't... I think it could be as simple as the quick trip magnetic coil in the MCB interfering with the toroidal inductor in the RCD. Hence my question about placement of devices

But I'm happy to be wrong as then I'll probably learn something

Not impossible I have had rcbos tripping the adjacent device on Memshield 2 boards, rcds seem to defy logic.

 

[Wilko]

Agree Marconi idea re TT vs TNCS - tempted to have a "never have I ever" test with rod in place etc.

 

[Strima]

I don't... I think it could be as simple as the quick trip magnetic coil in the MCB interfering with the toroidal inductor in the RCD. Hence my question about placement of devices

But I'm happy to be wrong as then I'll probably learn something

You stole my idea from post #6...

 

[JD Hogg]

I don't... I think it could be as simple as the quick trip magnetic coil in the MCB interfering with the toroidal inductor in the RCD. Hence my question about placement of devices

But I'm happy to be wrong as then I'll probably learn something

When I originally installed it the motor MCB was next to the RCD that trips and was in fact fed from the RCD. When we started the motor and the tripping issue arose I moved the MCB away from the RCD and as stated previously I have fed it from the main switch.

It is an MK metal clad board so not particularly cheap and nasty. The motor is made by Wonder which I think is a chinese mfr and the starting current is 110 Amps. Also as previously stated there isn't a crossed neutral and I have IR tested all of the circuits.

 

[JD Hogg]

Also I think it is very unlikely that PME will be available.

 

[SparkyChick]

You stole my idea from post #6...

Well it was the reason I asked for a picture of the board in post #3

Seems this may have just been ruled out by the OP though so we're both probably wrong

 

[Lucien Nunes]

Um, have we had an answer yet to the most important and obvious question, which if I have read the thread right only appeared with Marconi's post #27:

a. Can you discover insulation resistance faults on the tripping RCD's final circuits?

The starting current of the motor may be the highest peak load on the system, hence the only thing that shows up an existing N-E fault on a final circuit protected by the RCD that trips. It is just possible that a surge protector on one of those circuits will cause the same effect. I would have no hesitation in making tests on the tripping RCD's circuits, not the motor circuit, my next task...

 

[Leesparkykent]

Is the upfront 100mA RCD time delayed?

 

[marconi]

I don't have a photo of it but from left to right it is main switch, RCD 1 63A 30mA, 5 MCB's RCD protected, RCD 2 63A 30mA (this is the one that trips), 3 MCB's RCD protected, 32A MCB feeding motor, 16A MCB feeding different motor both 32A and 16A MCB's fed directly from main switch. Mainly old installation, new board, installation fully tested all OK, 2 new circuits for the motors. No borrowed neutrals, definite separation between busbars. Even if the three circuits breakers fed from RCD 2 are switched off the RCD still trips when the bigger motor starts, nothing trips when smaller motor starts. TT system. Brand new motors.

Opening the 3 mcbs will leave the Ns and Es connected so some electrical characteristics of the final circuits remain in situ fed from RCD2.

I suggest:

a. you completely disconnect the LNEs of the final circuits fed by RCD2 and then observe the behaviour of RCD 2 when you start the motor.
b. check the IR of these circuits one by one.
c. connect one final circuit back - operate the motor - observe what happens. Then completely disconnect it.
d. repeat c) for each of the other final circuits.

This little method might help you discover if it is one final circuit fed from RCD2 which is interacting with the motor to trip the RCD.

As Marvo and Lucien Nunes said - are there any transient filter sockets, computers, washing machines and the like which contain filters connected across LNE? If there are see what happens when you completely disconnect them.

 

[marconi]

Last for now -

Do you have access to an earth leakage clamp to measure the current in the main earthing conductor to the earth electrode? If you have you can measure the background leakage current and any change when the motor is started.

Could you also measure PSCC and PEFC to provide some information on the impedance of your supply and that of the TT earthing system?

What state is the earth electrode in? Try pouring a few litres of water around it if it looks dried out.

Please do let us all know how you get on.

 

[marconi]

Pondering further on the problem of immediate trip of RCD2 when you press the start button of the DOL contactor - have you wired this contactor to provide double pole switching of the supply to the motor?

It might be that the N is last to make and first to break which can cause nuisance tripping of RCDs because of transient voltages on the N conductors.

If you have, try changing the switching to single pole of the line only.

 

[westward10]

I did ponder this as polyhase motors on rcds though contractors can cause this.

 

[JD Hogg]

SOLVED.

Thanks to all of you who gave helpful suggestions.

I went back yesterday afternoon and tested the IR on the new motor and was all good. So I then disconnected the neutrals of each circuit that was on the tripping RCD and found that with one of the lighting circuits disconnected the RCD didn't trip when the motor started. The N-E IR on this circuit was 3 Mohm which isn't brilliant but is acceptable and it didn't trip the RCD when it was connected. So the lighting circuit has been left disconnected subject to futher investigation.

 

[Octopus]

^^ Good man ............... nice to get the feedback , so few people have the courtesy to do this