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

[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

 

[Lucien Nunes]

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!

 

[marconi]

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?

 

[Midwest]

You never see Lucien Nunes & marconi in the same room, do ya?

 

[marconi]

Truth be told I'd rather be on-site with some tools and test kit working with Mr Hogg on this fascinating (to me) problem.

 

[Midwest]

Truth be told I'd rather be on-site with some tools and test kit working with Mr Hogg on this fascinating (to me) problem.

Make him an offer he can't refuse

 

[Lucien Nunes]

I don't think it will be a failing capacitor. Agreed, if it is flashing over from the wound element to the case, it will cause a leakage under voltage transient conditions. But I've tested an awful lot of capacitors, probably tens of thousands, of which quite a few have metal cans, and I don't remember seening any kind of breakdown to the case that has not also involved severe damage and total shorting or failure of the element itself.

One thing we haven't eliminated yet though, is aggregate leakage. If the suspect lighting circuit that was disconnected has high leakage, and a fault on another circuit pushes the total over the trip threshold when the motor starts, then disconnecting the lighting might prevent the trip without being faulty itself.

Occam's razor applies though, so this circuit is still the prime suspect.

 

[marconi]

Occam's razor (or Ockham's razor) is a principle from philosophy. Suppose there exist two explanations for an occurrence. In this case the simpler one is usually better. Another way of saying it is that the more assumptions you have to make, the more unlikely an explanation is. Occam's razor applies especially in the philosophy of science, but also more generally.

 

[marconi]

the scientific method:

Steps of the Scientific Method - https://www.sciencebuddies.org/science-fair-projects/science-fair/steps-of-the-scientific-method

 

[Midwest]

Phewww!

 

[Midwest]

In other words ---- happens, and normally for the simplest of reasons. Like I was just about to take the last gulp of my whiskey & coke, but missed my mouth. Probably for the best. To bed

 

[Leesparkykent]

Assuming the 100mA upfront RCD is time delayed to discriminate with the 30mA RCD's....How are you achieving disconnection times? It's a 32A final circuit so 0.2 seconds as TT. For a time delayed RCD, tested at 100% of residual operating current a satisfactory result would be the RCD operating in a time range of between 130ms and 500ms.

 

[alasdairp]

Marvo wrote: "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."
Now you have semi-sorted it I suspect he is correct and it could be high quality LED (or, less likely CFL) light fitting with inbuilt surge protection so that the 3 MOhm drops rapidly when hit by a high-voltage spike caused at startup. A large (10 or 15 microFarad) X-rated capacitor, or a proper rated suppressor, directly across the motor terminals may well sort it fully by absorbing the voltage spike.

 

[happysteve]

Assuming the 100mA upfront RCD is time delayed to discriminate with the 30mA RCD's....How are you achieving disconnection times? It's a 32A final circuit so 0.2 seconds as TT. For a time delayed RCD, tested at 100% of residual operating current a satisfactory result would be the RCD operating in a time range of between 130ms and 500ms.

Presumably he's meeting disconnection times by ensuring that Zs (or Ra) <250Ω, which shouldn't be too tricky. In this case, Ia = 200mA, and trip time will be between 60 to 200ms (ref table 3A, appendix 3).