RCD tripping - but this seems an unusual one to me

[njhuz]

Hi there

Have a Wylex RCD consumer unit for the whole house. Every once in a while the whole electrics circuit (not the lighting) trips. It is generally when I try and switch something on (not always on the same socket or even in the same room) but not always e.g. it happened the other morning at 7am when we were still in bed, although it could have been the CH boiler firing up that tripped it that time.
Once tripped I can only reset it if I turn one of the circuits off. That circuit controls all of the sockets at the front of the house on the ground floor, so eventually I have to turn it back on again. Then, inevitably at some point (not just when I turn that circuit back on) the whole thing will trip again - but it could be an hour, a day or a week later.
It seems temperamental to me, as if the slightest surge is tripping this one circuit, which means the RCD then steps in and shuts the whole block down. Does that seem possible? If so, can I do anything about it like replace a bit of fuse wire in that circuit's block (if so, how do you do that???) or is it something I need to get a sparky in to do?

Cheers all

 

[Steve]

Once tripped I can only reset it if I turn one of the circuits off. That circuit controls all of the sockets at the front of the house on the ground floor, so eventually I have to turn it back on again. Then, inevitably at some point (not just when I turn that circuit back on) the whole thing will trip again

You have a fault on that circuit then.

What appliances are on this circuit? Is anything running constantly, like a computer, heater, fridge, freezer?

I'm thinking something on that circuit is producing a constant earth leakage (which may be perfectly normal), of say 20mA. Then when you turn on another "leaky" appliance such as your central heating, it tips the RCD over the edge.

Earth leakage is perfectly normal in appliances with pumps, water, motors, heating elements etc, but several leaky appliances will trip an RCD.

 

[njhuz]

Thanks for coming back so quickly Steve.

There's nothing on that circuit that is on permanently at all. There is a computer but that is always switched off while not in use, and yet the thing still trips whilst it is switched off. There are no meaty electrical appliances at all. The odd lamp but nothing else - it's a REALLY light load on that circuit. The circuit for the back of the house does the fridge/freezer/microwave etc
The only thing that is on there, but that I have switched off at the 'isolator' switch in the front room is a circuit for some outdoor lighting, which I know is dodgy because the junction box buried in the ground outside has some water in it. But, as I say, I have switched that off from inside the house, so I'm sure that can't be interfering with it.
Cheers
Neil

 

[Steve]

But, as I say, I have switched that off from inside the house, so I'm sure that can't be interfering with it.

How sure are you?

An RCD monitors both live and neutral currents and if they imbalance by 30mA, it will trip.

If the switch for these outdoors lights is single pole, ie. it only switches the live, it will still trip the RCD if its leaking current from neutral to earth.

Unfortunately RCD problems can be difficult to diagnose, its usually a case of eliminating circuits and parts of them.

 

[njhuz]

Steve

Not sure at all - I assumed that by switching it off I was isolating the problem. So it looks as though I should sort out the dodgy outdoor junction box then and see if it helps?
Or is another solution to replace the outdoor light switch with a double pole switch to isolate the neutral as well? Could that sort out the RCD tripping at least so that I can then at another time sort out the junction box outside?

Thanks again
Neil

 

[bengasman]

Water in an outside junction box in the ground? Disconnect that lot, the sooner the better.

 

[DESL]

Water in an outside junction box in the ground? Disconnect that lot, the sooner the better.

I'll second that. The main purpose of RCD is to protect against electric shock and in your case it's doing it's job.
The neutral conductor is live in any cct as it is the return path for the current so any leakage to earth through damp or worse through your body will trip the RCD and keep you alive.

 

[njhuz]

Thanks guys for all the replies - looks like that's a good place to start.

Neil