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

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JD Hogg

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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.
 
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?
 
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.
 
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 ****!
 
Is there physical separation between motor MCB and the RCD.

Could be magnetic influence causing the RCD to trip.
 
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?
 
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.
 
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.
 
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.
 
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?
 
marconi said:
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?
Click to expand...

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?
 
JD Hogg said:
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?
Click to expand...
A fault to earth?
 
A question that hasn't been asked yet, is the CU a high integrity board? just so I can get my head around the problem.
 
JD Hogg said:
32A MCB feeding motor, 16A MCB feeding different motor both 32A and 16A MCB's fed directly from main switch
Click to expand...
@Pete999 This quote is part of post #10, so it looks like it's a high integrity board. The confusion is that the motor is not connected to the RCD which is tripping.

@JD Hogg If it is a TT system how can you have circuits not connected to an RCD?
 
Is it possible to move the problem circuit to between the Main switch and the 1st RCD? might stop the heavy load affecting the RCDs. Or possibly fit another small non RCD ccu just for that circuit?
 
Just trying to understand the set up - has this board itself got its own means of earthing, or is it derived from the main board ?
 
westward10 said:
It is a TT system needs rcd for fault protection.
Click to expand...
Yes I appreciate that Westy, but the OP has mentioned in his post that the cb for the motor is fed from the main switch, or I think that's what he means#10.
Streamer, the reason I asked about the high integrity board was the OP mentions the Motor cb is fed from the MS, I was asking mainly because the OP may have adapted the board to suit his needs, it has been known to happen .
If he has then the borrowed neutral theory does hold up despite the OPs assurance that there is not a borrowed N, OP I'm not having a dig, just can't get my head around the problem.
 
Billsays said:
Is it possible to move the problem circuit to between the Main switch and the 1st RCD? might stop the heavy load affecting the RCDs. Or possibly fit another small non RCD ccu just for that circuit?
Click to expand...
Loading should not trip an RCD.
 
My early thinking is along the following lines:

Worth checking the motor IR and its supply IR to confirm all is well even though it is a new motor and new installation.

1. DOL motor start - large transient current through L and N.
2. TT system so N not hard wired to E in installation so N can change in potential with respect to E/cpcs.
3. 1 above means all L conductors throughout installation will experience a transient drop in potential with respect to E/cpc. This will cause a current to flow through distributed capacitance between L conductors and E/cpcs and earth mass.
4. 1 above also means all N conductors will experience a transient rise in potential with respect to E/cpcs and body or earth. Again this will cause a current to flow through distributed capacitance between all N conductors and E/cpcs and earth mass.
5. The net result of 3 and 4 is an unbalanced current through the RCDs, and in the case of one of them sufficient for it to trip. That RCD may also be 'biased' already towards tripping because of earth leakages in the final-circuits it protects.

What to do?

a. Can you discover insulation resistance faults on the tripping RCD's final circuits? Or damp, dirt, dust, detritus near live conductors which can be cleaned away.
b. Check all L and N connections are well made and low resistance. Poor connections can create transients only noticeable when high currents flow through them.
b. Can the installation be converted to TNC-S to tie N and E/cpcs?
 
Some of the cheaper RCD's can be twitchy with phase angle change from non-linear loads like motors.

Another possibility is the motor is causing a voltage spike that's being sunk to earth by surge arrestors on a completely different circuit which can get RCD's all emotional especially if they're earth referenced.

Before you get complicated, the first thing is to to do the usual barrage of tests on the RCD, IR all the circuits fed from the RCD, check for borrowed /crossed neutrals etc. Also IR test the motor @ 500v and spin the motor by hand whilst your IR testing it.
 
SparkyChick said:
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 :)
Click to expand...
Not impossible I have had rcbos tripping the adjacent device on Memshield 2 boards, rcds seem to defy logic.
 
SparkyChick said:
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 :)
Click to expand...
You stole my idea from post #6... :eek::D
 
SparkyChick said:
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 :)
Click to expand...

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.
 
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?
Click to expand...

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...
 
JD Hogg said:
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.
Click to expand...

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.
 
Last edited:
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.
 
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.
 
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.
 
Interesting find. 3MΩ as you say is acceptable and to pass even 10mA of leakage if it were a true resistance to earth would take a transient of 3x10^6 x 0.01=30kV which obviously isn't what is happening. So I think the 3MΩ is a symptom rather than a cause, and there is something more that doesn't reveal itself at a 500V DC test.
Do let us know if you find out what it is!
 
Is the lighting circuit for fluorescent, discharge or incandescent fittings? If one of the first two I wonder if among the possibilities of faults one is that the PF correction capacitor is failing/has failed creating a low impedance path to earth to step changes in potential to earth of the L(step down) and N(step up) conductors. Such a failure might have a high resistance (3MOhms) even to high dc voltages.

Aware that time is money to you, my suggestion is to unwire each fitting in turn and after each disconnection to take an IR test or attempt to start the motor- this might quickly identify the offending fitting which you can examine further.

What I still find significant is that the smaller motor causes no problem to any RCD when it starts yet it too will cause a large brief transient in voltage and current. Have you checked how each motor is switched - SP or DP?
 
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