Pond pump trips RCD

Ha! Seen something similar twice now. On our old Hotpoint built-in fridge, and my pal's well pump.

If it is the same, there is a Neutral wire somewhere in the motor that is nearly touching Earth. As the motor runs down, the vibration is enough to cause a transient N-E short.

Thanks,

I think I'll wire it up off the RCD

The RCD trips when there is an imbalance of current in the phase and neutral supply to the pump.This normally occurs if there is a fault to earth from phase or neutral but I am guessing the pump is double insulated and there is no earth wire (cpc).The double pole switch might be causing the problem, if both contacts do not disengage at the same time then this could cause a momentary imbalance of current.

Not the best idea you've had all year, I'd suggest. Outdoors, mains, some distance from the house, water ("impure", so probably more conductive than what's in your tap). I'm not paranoid about electrical safety, but I'd *really* not want anything mainspowered in a pond of mine which *wasn't* on an RCD.

Rather, I'd try to sort out what the pump problem is. Tony's experience suggests one intriguing possibility. You can do some faultfinding by visual inspection - is it possible that the switch you're using has some loose wiring, making either L-to-E or N-to-E contact when it's operated? Can you put another switch temporarily in (alternately) the L and the N conductors, to try to give a clue as to whether it's the pond switch itself, or the pump (and on which side the return route is?) Do you have a long enough extension lead - and/or access to the garage CU - to supply the pump through a different (but 'known' to work) plug-in RCD, to see whether the garage RCD is 'oversensitive'? Do you get the same nuisance trip supplying the switch and pump from that long extension lead (which suggests a problem at the switch-and-pump end), or does it go away (suggesting maybe a 'sensitising' fault - rodent damage? - in the buried cable - 'nearly' enough to trip the RCD, which gets added to the switch-off transients (suppressors, collapsing magnetic field, handwave ;-) to make the garage RCD trip?

Hope something in there gives you some fault-finding ideas, and allows you to avoid the 'bypass the RCD and hope' non-solution...

Stefek

I think it is to do with the way in which a motor twists on start or stop. If the motor is soft-mounted, the flying leads to it will get a bit of a workout at every start/stop twist. If one of the leads is touching the metalwork there is a chance that it will eventually chafe through.

Our fridge was a bummer..... it took months to find out what was doing the random trips. I only discovered it when the N-E short got permanent enough to be sometimes seen with the Megger.

Customer: My brakes appear to be seized on. Mechanic: No problem. I'll cut all the brake lines. That will solve the problem. Customer: Thank you!

Hint: The RCD is there is save your life. Having done its job, please don't throw it away!

Christian.

So how would that work then?

Christian.

It's not likely to be the same: fridges are metal and earthed, pond pumps are usually plastic and don't have an earth.

Well it's probably all running in to the circuit but it can't get out again. You shold notice a little bulge in the cable while this happens.

Actually it might work like this:

The pump motor is just a mains coil embedded in plastic. and say this insulation is going and water getting in to the coil at the neutral end. While both live and neutral are connected the current flow from neutral to earth is low enough not to trip the RCD but if the neutral pole of the switch opens first you momentarily have a flow from live through the coil (negligible resistance) through the dodgy insulation to earth, so the RCD trips.

contacts

happens.

Lol, good one. :)

Maybe Cable capacitance to earth/water, add inductive spike from motor being switched off tog enerate a high peak v and thus push more joules through given capacitance. Cant say Im very convinced by that explanation though. Just megger and see what fails is the sensible approach.

NT

We really need a gallery for posts like this. Trouble is, some pros are no better.

NT

I'd start by checking inside the switch housing to make sure it's still dry and clean. The travelling part of the switch mechanism may be hitting damp etc one way, but not the other.

Thanks for the support, I did feel a bit stupid after the responses to my idea about the double pole switch but feeling much better now.

But the RCD is tripping so there is likely to be a sideways path to Earth, somewhere.

I wonder if the switch is wired back to front (L-N, N-L) as well? I don't know, but I'd guess that a double pole switch is designed to break the live first. If it's wired the other way then this would give the effect described.

Hmm.... I didn't notice 'double pole switch' in the OP's original post. So my proposal of a transient N-E short can be politely ignored.

OK, OK I'm an idiot!

The other idea I had was to never switch it off!

I have changed the switch, bypassed the buried cable and connected it via a plug in rcd. The plug-in rcd trips also. I think I'll replace the pump with a low-voltage one!