RCD tripping

[scubamike]

I have a circuit protected by a 6A RCD for all power to the outside of the house.
On this circuit are two security lights, a pond pump, and a pond UV light.

The external lights are via one switch inside the house, the pond equipment via another.

When the pond circuit is on at the switch, no problems, circuit stays on without issues.

When the lights are turned on, the circuit comes on OK, security lights come on, but a short time later (a couple of minutes after the security lights have timed out), the circuit will trip. I have disconnected the lights on at a time, but regardless of which light is left on, the circuit still trips. The circuit will trip regardless of whether the pond circuit is switched on or off.

Total load on the circuit (with everything on) is no more than 4A.

Any suggestions as to what the fault might be?

No work in the loft for a long time, so I've ruled out a crushed cable (am I right to do this?).

 

[Johnmelad]

Total load on the circuit (with everything on) is no more than 4A.

????

 

[EFLImpudence]

I have a circuit protected by a 6A RCD for all power to the outside of the house.

Do you mean it is a RCBO or do you mean MCB?

 

[scubamike]

Total load on the circuit (with everything on) is no more than 4A.

????

I take it the question marks indicate you want clarification?

Total load, taking the power rating of everything on that circuit into account, is less than 4A, the MCB is rated to 6A.

Pond Pump = 500W
Pond UV light = 18W
External lights = 150W + 150W
Total = 818W = c3.4A

I rounded up.

 

[scubamike]

I have a circuit protected by a 6A RCD for all power to the outside of the house.

Do you mean it is a RCBO or do you mean MCB?

Apologies, Yes, it is an MCB.

 

[EFLImpudence]

Yes, it is an MCB.

In that case the cause would normally be an overload.

However, as the tripping occurs after the lights (or light with each disconnected in turn and only < 1A) go out I would suspect the MCB itself is faulty.

 

[scubamike]

Yes, it is an MCB.

In that case the cause would normally be an overload.

However, as the tripping occurs after the lights (or light with each disconnected in turn and only < 1A) go out I would suspect the MCB itself is faulty.

Many thanks,
Just a case of removing the old one & replacing like for like?

Do these just "go" after a certain age? (I think these have been in place for 15 years or so).

 

[EFLImpudence]

Well, nothing lasts for ever.

If you feel comfortably doing it you could swap the wire from the suspect MCB to another 6A one.

Switch off main switch and test for dead.


It would be odd for it not to trip with the pump on - if it is indeed 500W.

 

[sparkiemike]

From the loads you say you have I would not expect it to be overload current.

You really need to carry out an insulation resistance test.

I suspect you have some kind of water ingress at a joint, which would give you low resistance between line and earth, but not low enough to cause instant tripping.

 

[scubamike]

Thanks for all the responses.

I can't understand how it could be water ingress.
As I said, the pump works fine. Only part of the light which is outside (only one connected at the moment), is the short length of wire coming through the wall, which I've checked, and it is not frayed or wet. Also, when it first started tripping, I replaced one of the external security lights (it was old and rusty), and I've double checked that everything is wired as indicated on the instructions (which is the same as the original one). The connections are all in a protected little box, which is dry and no signs of any arcing etc in there.

Pump off at the switch, lights on, circuit trips.
Lights off at the switch, and pump on, runs without tripping.
Fault would certainly seem to be somewhere in the light circuit, but will try to replace the MCB first.

 

[sparkiemike]

If it works without tripping with the pump and I very much doubt it is the MCB, but try it if you must.

As you have already identified it would appear to be the lighting part of the circuit. Perhaps the fitting itself or a faulty cable. As already stated you need (or an electricain) to carry out an insulation resistance test.

 

[Mursal]

Head scratch ..........................

Fault between phase ans neutral in the light circuit. Usually at one end or the other (switch or lights) or as already stated water in a joint. Unlikely to be the MCB, unfortunately. Would the wires be snagged as they enter the lights?
Have you separate circuits going out or each security light, if so its unusual for both to have a fault. If not, its the part of the circuit that's common to both lights.

Hope this helps

 

[scubamike]

Update:
I traced the circuits.
From each switch there was a switched live, and a neutral.
One switched live was for the lighting circuit, the other for the pond.
The two switched lives went to a junction box in the garage.

Both switched lives were connected to the circuit going to the "garden".
There are some very old garden lights which have never worked in the 10 years I lived here, I presume the fault is somewhere in them. Probably water ingress as suggested earlier.

Disconnected the switched live for the lights from the "garden" circuit, and everything is now stable.

I have insulated the disconnected lighting switched live from the garden circuit to make it safe for the moment, and will have to dig up the cable which runs under the lawn. This is a job for the summer I think.

Many thanks for all the help.