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

[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).