Outside Light tripping MCB

[diynoob]

The cable can't have been drilled into, it's not buried in mortar. It goes upwards, into the wooden eaves then into the loft. It's loose and can be pulled back and forth a bit. It could be damaged in the loft of course, or loose in the junction box.

The MCB would not reset during the rain. After the rain stopped it could be reset but then tripped again after a few minutes - without any load. Now the rain has been stopped for a few hours the MCB has not tripped.

It's definitely reacting to the rain. Maybe the outside cable is a red herring. Could be rain is getting in the loft and things don't work again until it's had time to dry out. I need to find a midget electrician who can crawl through the loft

 

[JohnD]

it's quite possible that the cable end you found is not the only one. There might be one or more others, perhaps for external lamps, perhaps for sockets, perhaps for a pond fountain.

Water tends to boil away before tripping an MCB, so your fault is unusual.

You would do best to strip out all the old cable and run a new one.

 

[diynoob]

The MCB that trips is connected to the lights in one upstairs room. No sockets or other lights are affected. It is possible it's connected to sockets I haven't found yet, but it's really unlikely someone tapped into this radial in the loft in order to feed the garden when there are so many other circuits nearby.

I had an electrician round to fit a cooker circuit a few months ago. Unfortunately he didn't test this circuit.

 

[bernardgreen]

Does the MCB trip when nothing is being done. No switches being operated, no thermostats switching, no timed lights coming on, no PIR lamps switching on.

For a 6 amp MCB to trip there has to be a fault that passes at least 10 amps for several minutes. That is the equivalent of more than 2 kW of power in the fault. Water in a fitting or soaking a damaged cable cannot pass that much current. Even if it did then like a kettle the water would soon boil away.

Walking on floor board that is resting on a damaged cable may press the cable and cause conductors to touch and short out.

 

[diynoob]

Think I've found it. The outside cable was a red herring. I thought all the light switches in the room were off but one of them is backwards and was in fact on the whole time. I've turned this one off and all the other lights on and it hasn't tripped now for 4 hours.

It's an LED security light. I can't see and water in it or damaged cables. Could it just be faulty?

 

[ban-all-sheds]

Does the breaker trip the instant that switch is turned on?

 

[diynoob]

Turned that switch back on and it hasn't tripped straight away. Probably the bad weather was triggering the PIR and causing the trip. I'll leave it off for now and experiment again when its dark and/or raining.

 

[ban-all-sheds]

So the MCB trips when the PIR turns on?

 

[diynoob]

Sorry, had to wait for it to get dark. Yes it's definitely the PIR on the security light that is tripping the MCB. So with that light switched off, no tripping.

 

[Blueflash]

Well done Sherlock

Dave

 

[ban-all-sheds]

Check that light - rewire or replace.