Main trip switch problem

The main trip switch on our circuit board trips whenever my next door neighbour uses his electric lawn mower. This has happened every time he has cut his grass this year. It is not that we have overloaded our system as it has happened twice when we have been away for the weekend and left only the minimum electrical appliances on eg clocks, fridge and freezer. This is driving us mad and I would appreciate any suggestions. We are in a semi detached house and it is the adjoining house which seems to be causing the problem and he is using a socket in his garage.

There are almost certainly more experienced people than me who read this forum who will be able to resolve your problem but as a first stab:-
Q1) When your main circuit breaker trips (presumably it's the 100mA RCD that's tripping) does your neighbour's lawn mower stop?
Q2) Does the same problem occur if your neighbour uses a different electrical appliance (e.g. an electric drill) in the same socket in his garage?
Q3) What happens if you borrow your neighbour's lawn mower and plug it into a socket in your garage?

Yes it is the main RCD that trips but his lawn mower doesn't stop working.

We haven't seen him use any other appliances from his garage.

We are going to try our lawn mower in his socket or use his lawn mower in one of our sockets.

He doesn't suffer from his supply being cut off - just us and he has cut his grass twice in the past week!!!!!!!!!

I'm thinking about a neutral to earth fault, and as you are in semis perhaps you are both bonded to the same water pipe (you haven't got an earth rod, have you?)

Where the supply cables come into your house, by the meter, is there a biggish green and yellow cable coming out of the side of the cable head and going to the consumer unit; and is there a (slightly smaller) g/y going to your incoming water and gas pipes?

Also, do you know if your neighbour has an RCD, either in his CU, or a plug-in one? Do you know if he has an old electrical installation?

Do you have an overhead or an undergound electricity supply?

Where the supply cable enters the house there is a largish green and yellow cable going to the consumer unit and a smaller one going to the gas pipe. (Can't trace the cable to the water pipe inlet as it goes underground but would guess that the other small cable wouild go to it and there is one on the water pipe)

There is a label in the electricity supply box advising that it has a "protective multiple earth system".

The supply is an underground supply.

The houses are approx 22 years old and I believe that it would be the original installation in his house. Would guess he has an RCD similar to ours.

Alas, I can't think what it is, you seem to have the bases covered.

But with a bit of luck someone will be along who's had dozens like it!

The neutral earth idea put forward by JohnD is what I'd suggest as well, though if it doesn't happen when the neighbour uses other applicances, then the path might not be totally resistive (though without knowing the magnatude of the starting current of the mower motor...) if the motor was 'noisy' and was putting frequencies other than 50hz on the line, then there could be increased leaking through capatance, that might make up suppression circuits on your appliances (this is what happens when a freezer tends to trip the RCD on startup)

Knowing what earthing methods are employed in both houses would help as well, you seem to indicate that yours is PME/TNC-S but then you seem to have an overall 100ma (?) rcd, Are you certain that the earthing conductor comes from the service head? A pitcure of your consumer unit and service head might be useul, and if you are on speaking terms with him, then the same in your neighbour's house

You could also try and interest your neighbour in the virtues of petrol powered mowers

It sounds like PME; if it's a 30mA RCD, I wonder if there's enough general leakage (washing m/c, immersion heater etc) to be near the tripping current, and the lawn mower pushs it over the edge?

If it's a 100mA RCD, then there would surely have to be rather a lot of background leakage to be near it?

Phil, the pictures would help, if you can lay your hands on a digital camera. In the meantime:

Does your RCD protect all circuits, or just the sockets? Can you see what letters and numbers are on it?

Have you got, for example, outside sockets/water features/pool pump/fountain/supply to shed/anything that might get damp? Or a pipe to a garden tap? The reason I mention the washing m/c and immersion heater is that in my limited experience, slight background leakage comes from the "watery" appliances, and these small leakages can add up and take you near the tripping point.

BTW one of my relations had an electric mower with an old flex, it looked all right, but when it ran across wet grass, it tripped the RCD. I didn't see it but suspect old rubber insulation, which gets little cracks if left out in the sun. Doesn't explain why the trip is jumping from house to house, if there's no earthing spike, though.

I am unable to confirm the details of his mower except that it is a "not new" hover mower. The neighbour is not the most friendly to make contact with.

The main box in the house is a MK Sentry Consumer Unit LN 5780s 80A 30mA. I am unable to provide any photos at present but will work on it.

I understand what you are saying about the leakage but just cannot understand why it is only when our neigbour cuts his grass and it doesn't affect him. Have contacted an electrician to come and take a look but would still appreciate any help from you all. Thanks.