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.

 

[Pete999]

Borrowed Neutral, just re read your post which I have to say is confusing, is it an RCD that's tripping and not Circuit breaker? Are you an Electrician?

 

[SparkyChick]

A photo of the board and the connections in it may help.

 

[Devonchris]

Is there clear separation on the busbar between your non rcd circuit and the rcd circuit? As clearly current is present which is not balanced and you say its not the neutral.

 

[Midwest]

Long long time since I done anything with motors, but remember in the past motors tripping RCD's on their circuits, due imbalances or something on start up. Perhaps it that, and the neutrals are not clearly separated. OR I might just be talking ****!

 

[Strima]

Is there physical separation between motor MCB and the RCD.

Could be magnetic influence causing the RCD to trip.

 

[Wilko]

Hi JD, can I ask what is the earthing type, and is this new circuit in a new board? (describe installation?). Also, is the motor new - have you confirmed it's IR?

 

[westward10]

It is the start up causing it.

 

[Octopus]

Neutral to earth fault on one of the circuits protected by the RCD which trips which then is affected by the motor you are starting.

That's my guess

What happens if you turn off the circuits on the RCD? Can you start the motor without tripping the RCD?

A photo of the board would be useful.

 

[JD Hogg]

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.

 

[westward10]

I suspect it is saturating the rcd coil on start up or if it is some type of split phase motor it it creating a momentary imbalance when the centrifugal switch opens. Is it DOL.

 

[marconi]

I assume the motor is an induction motor. So there are two events when it starts. Does the RCD in question trip when:

a. the DOL start button is pressed?
b. the contrepetal force switch opens to disconnect the start winding?

In addition what does the motor drive?

Is it started with high or low mechanical shaft load?

Have you taken any IR measurements of the motor as wilko asked earlier?

 

[JD Hogg]

I assume the motor is an induction motor. So there are two events when it starts. Does the RCD in question trip when:

a. the DOL start button is pressed?
b. the contrepetal force switch opens to disconnect the start winding?

In addition what does the motor drive?

Is it started with high or low mechanical shaft load?

Have you taken any IR measurements of the motor as wilko asked earlier?

It is an induction motor,
The RCD trips when the DOL start button is pressed
It is for an apple press
It is started with low (no) shaft load
I have not taken IR measurements, as I said it's a new motor and even if there was a fault why would it trip the RCD?

 

[Pete999]

It is an induction motor,
The RCD trips when the DOL start button is pressed
It is for an apple press
It is started with low (no) shaft load
I have not taken IR measurements, as I said it's a new motor and even if there was a fault why would it trip the RCD?

A fault to earth?

 

[westward10]

This is being caused by the motor starting, rcds and motors often don't make good friends.