Mystery tripping of RCD [solved]

[Alec_t]

In a house I've just moved to I have an under-counter 13A socket on a spur fed via an above-counter isolator switch from, I believe, an upstairs ring. The previous occupier of the house had a washing-machine plugged into this socket.
If I power my portable drill from this socket the solitary RCD in the CU promptly trips. If I power the drill from any socket on the downstairs ring the RCD does not trip. Both rings are protected by the one RCD as far as I can tell, so why doesn't the RCD trip with the drill on the downstairs ring?

 

[AndyPRK]

pictures help

 

[JohnD]

there may be an incorrect connection in the switch or socket, or possibly cable damage, or even (in a kitchen) damp.

Open them up and show us the wiring.

If you can work out where the spur comes from (perhaps a socket directly above) photgrapg that too.

What leads you to believe it comes from the upstairs circuit? Does it go off if your turn off the 1st floor MCB, but not if you turn off the ground floor MCB?

Does it trip if you plug something in that is not your drill?

If you unplug or isolate all your watery appliances (washing machine, dishwasher, kettle, tea urn, immersion heater, boiler, fishtank, jacuzzi) does it still trip?

 

[ericmark]

I have had odd tripping of one RCD, and not another, and using a RCD tester they both passed, however this was in the early days of RCD's and my knowledge was not as good as now. So never did find out why.

But there are a number of likely causes.
1) Wired up wrong, any item would trip it.
2) There is a drain to earth already on the socket, we are told 9 mA is the limit, but I don't have anything to measure that, so if there is already some leak to earth, the drill could be tipping it over the balance.
3) The reverse on other sockets, with type AC RCD's a DC component can stop them tripping, so maybe some thing stopping the other RCD from tripping.

So as already said try some other items in the socket, then down to testing, depends what you have, I have a insulation tester, my son has the RCD tester so in my case insulation tester first. But you need some test equipment.

 

[Alec_t]

Thanks for the replies.
I'm not at the house at the mo, so can't check inside the fittings or provide pics. Will do so when I can. And I haven't had time yet to identify which sockets are on which rings (I'm just assuming there's an upstairs one and a downstairs one, as two MCBs in the CU are labelled 'sockets' ..... if I'm interpreting the must-have-been-written-by-a-doctor writing correctly ).
When I used the drill, the only thing plugged into any socket in the house was the drill. The only other item consuming power was the gas combi boiler on standby, which naturally lost power when the RCD tripped. The boiler has its own MCB.
A test lamp (12V incandescent bulb, transformer-powered) plugged into any socket (including that spurred one) did not trip the RCD. So that seems to rule out cause 1 in post #4. I suspect cause 2 may be the culprit. Cause 3 is ruled out by there being only one RCD.

 

[ericmark]

I keep on promising myself a clamp on to measure down to 1 mA and DC, but now retired and really now just a toy, electrics now just a hobby, the insulation tester only cost me £35 new, misplaced orignal which of course turned up once I had new one.

So know not simple leakage, I blamed next door who was a welder but happened when he was away, so spike if was spikes not from him.

Son has house now, expect he will fit new consumer unit, often wondered if simply the old pre-electronic RCD more supseptable to being tripped by spikes. The RCD was around 1990 vintage so getting on a bit, but have know a couple of years between trips, then three same day and nothing found wrong.

So new house all RCBO protected, they have tripped but not without good cause. One was incorrectly wired switch using wrong neutral.

Dad's house gave up until rewired, old rubber cable.

But a drill in not a good thing to test with it can so easy produce spikes, iron, kettle, toaster much better, once you have tried a few it will build up a picture, if trips with any high power device looking at neutral to earth fault.

 

[Alec_t]

Thanks. I'll try different loads when at the house.

 

[Alec_t]

Mystery solved. I had a look inside the double-pole isolator switch today and found that someone had used an over-long screw to attach the face-plate to the back-box. The screw had nicked the insulation on the spur's neutral wire, shorting it to earth. My test lamp that I mentioned is 5W (or less on its dim setting), so was drawing only ~20mA, not enough to trip the RCD, whereas the drill current was enough to trip it.
All sorted now. Neutral wire taped and re-positioned.

 

[JohnD]

Tape peels off.

You can get heat-shrink sleeving which does a much better job. Small mixed packs if you don't expect to use yards of any one colour.

In DIY sheds or Ebay.

 

[Alec_t]

Thanks. The tape was a temporary fix as I didn't have my heatshrink with me today.

 

[ericmark]

In the past I have used silicon sealant and tape, the tape stops contact but over time as said it tends to fall off, but by that time the silicon sealant has set, maybe not to British standard, but seems to work, it is same as using a cable joint in a way.