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.

 

[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).