Help - RCD intermittent tripping

Could it be a filter capacitor connected between live and earth, which takes an hour to discharge before the RCD will reset?

No

with AC its constantly charging and discharging anyway two of the *possible options that spring to mid are:

- something cooling down and shrinking and no longer shorting

- something damp drying out. I had this. everything OK until the CH came on...and dripped water under pressure.

Yes, I mentioned that back near the beginning but never saw a response.

I didn't mention the discharge bit though...I was thinking more of the slight leak heating it a bit.

RCD#2 tripped today. That's a first in recent times though my wife now tells me it went a couple of years ago. Wait an hour and it can be reset.

RCD#2 has the kitchen sockets, including a couple in the adjoining lounge but excluding the oven and nothing to do with the recently serviced boiler.

Starting to think somethings are crawling around the back of the CU?

Should I hit the main switch on the CU or push the Test button? I don't think I can easily turn off the mains to the house so I can give the back of the unit a vacuuming.

The main switch. Bus-bars would still be live otherwise.

Do you have a cordless vacuum cleaner?

I think you have an earth neutral short somewhere.

If you cant stand the pain of disconnecting every neutral (or every earth) to check this, get an electrician in and take the pain of a two day bill.

If the oven/cooker is served by the tripping rcd and if it has a double pole isolation switch turn that off and see what effect that has. It's possible you have a failing element that has a pinhole in the sheath and it dries out after tripping.

No, but why?

More likely moisture or a partially charred mouse or spider. A fine wire whisker tends to get vapourised PDQ if it shorts live to anything.

A neutral to earth short is just possible and might explain the peculiar sensitivity in that if neutral has drifted slightly high then there might be enough current imbalance to prevent a reset for a while.

It is highly unlikely.

Filter capacitors have a bleeder resistor across them to ensure that you cannot get a belt by unplugging a unit at just the wrong moment of a mains cycle. Timescale for such filters to discharge is well under 10s.

I don't know what is causing the cannot reset for an hour. All the RCD's I have seen will only refuse to reset if the fault is still present. (which it may well be)

The big question is why does it recover on that sort of timescale?

Intermittent faults that cannot be reproduced reliably to order are always hell to find.

A question which I don't think has been covered. Is the RCD one of the ones which trips to a "halfway" position and has to be turned fully off before it can be turned on? I've found they can be sticky - especially if old and have not been tested routinely.

Durr...

Moisture is plausible, it it's a supply of moisture that is only there when powered (like CH that leaks when running, or a washing machine when filling or heating etc)

Yup, there is nothing that should stop a "normal" RCD resetting unless there is still too much leakage present.

With a RCBO that tripped on the thermal part of it's over current trip mechanism (rather than it's current imbalance one) - then you may see a delay before being able to reset. (although an hour sounds excessive)

Neutral the earth faults can give results like that and be very hard to find without testing. They can manifest for all kinds of reasons - many of which can be outside or your control, and may not even be related to events in your property.

So automated systems are worth checking - anything that is controlled by a timer or thermostat (or any other type of stat).

However things like time of day can have an effect since you will only usually see trips when the neutral voltage in property drifts too far from the earth voltage. This will normally related to the total load on the electrical system - higher current draws pulling the neutral voltage up and further away from the earth voltage. With properties having TN-S earthing (rather than TN-C-S) this means that the load caused by other properties in the area can also make the effect more noticeable.

Indeed

Oh dear :-)

Do you have a long extension lead to get power from next door? Or a portable genny?