RCD tripping overnight

[jj4091]

It seems as though you have elininated everything, except external influences or the RCD, which I would think is very unlkely to be faulty. I think it is maybe time you spoke to your supplier, I don't see how they can charge you to investigate this fault, so you have nothing to lose.

 

[JohnD]

you say the "main" RCD tripped, you mean the time-delayed one?

When you press the "test" button on the not-time-delayed RCD, what circuits go off, and what stay on?

I am now thinking about an external lighting circuit that might get rain or dew on it.

 

[runci21]

i also have a tt system in a rural farm .
do you know what kind of heating your neighbour has ?? or does he/ she have an appliance that comes on through the night .
my neighbour complains that his lights dip when my heating kicks in .

 

[bernardgreen]

every other appliance in the house was switched off. Not sure where this leaves me as by a process of elimination, it doesn’t seem to be related to any circuit or any appliance; any advice would again be greatly appreciated as although I have a good basic knowledge I’m not experienced & running out of ideas!

Switching off may not be enough as the neutral will remain connected and a neutral to earth fault can trip an RCD. Un-plugging or double pole switches are necessary.

 

[jj4091]

every other appliance in the house was switched off. Not sure where this leaves me as by a process of elimination, it doesn’t seem to be related to any circuit or any appliance; any advice would again be greatly appreciated as although I have a good basic knowledge I’m not experienced & running out of ideas!

Switching off may not be enough as the neutral will remain connected and a neutral to earth fault can trip an RCD. Un-plugging or double pole switches are necessary.

Just in the middle of the night, Bernard?

 

[JohnD]

you say the "main" RCD tripped, do you mean the time-delayed one?

When you press the "test" button on the not-time-delayed RCD, what circuits go off, and what stay on?

I am now thinking about an external lighting circuit that might get rain or dew on it.

IF you mean that the Time Delayed RCD trips, and the non-time-delayed doesn't, then we should be looking faults on one or more of the circuits that is not on the non-time-delayed RCD. I would expect these to include lighting (including perhaps outdoor lights with timers or PIRs), the cooker, boiler, immersion heater and any alarms. there might even be storage heaters or thermostatically controlled heaters that come on when it gets cold during the night.

So I am very interested to know which circuits are on the other RCD and which aren't.

Pressing the "Test" button on the other RCD will tell us if it is jammed.

 

[bernardgreen]

Just in the middle of the night, Bernard?

That appears to be when the trips are occuring

In pure theory NO external effect can affect the balance of currents in the live and neutral conductors through the RCD sensor.

In practise the capacitor effect between live, neutral and earth conductors provides a sneak circuit that will un-balance the currents.

A sudden change of earth potential relative to the supply potential will, via the inter-conductor capacity, create a current in both live and neutral that is in the same direction in both conductors through the sensor of the RCD. These will be additive and may be enough to trip the RCD. ( normal currents are in opposite directions and cancel each other out in the sensor ).

Therefore a system using an earth rod as the reference for the earth conductor may be susceptible to "ground bounce" causing a trip where there is significant capacitive effect between cable conductors and in equipment. A "ground bounce" occurs when a nearby electrical incident creates large currents into th earth and causes the local potential of the earth to change. Lighting strike. heavy earth fault on equipment, railway traction supply using grounded rail return circuit, phase un-balance on a local transformer are some of the things that can cause ground bounce.

The effect can be agrevated by metal water supply lines extending remote ground bounce into a building's earthing system.

 

[Richard C]

To answer your questions in turn:

you say the "main" RCD tripped, you mean the time-delayed one?

Yes

When you press the "test" button on the not-time-delayed RCD, what circuits go off, and what stay on?

Upstairs/downstairs ring mains, cooker, emersion heater (although I don’t think this is on a separate circuit), garage, conservatory u/floor heating; Circuits remaining on are upstairs/downstairs lights & a separate circuit that feeds the lounge & an external light; this was part of a previous rewire job done (not by me) to incorporate an earth for some metal wall lights & an outside security light.

I am now thinking about an external lighting circuit that might get rain or dew on it.

We do have them but these have been eliminated by the isolation of those circuits during one of the night time ttests & it still tripped!

i also have a tt system in a rural farm .
do you know what kind of heating your neighbour has ?? or does he/ she have an appliance that comes on through the night .
my neighbour complains that his lights dip when my heating kicks in .

He has a large industrial multi fuel burner in a barn; we have not had any probs. since last week but of course he may have only just brought something on line so it’s on the list of things to check

Switching off may not be enough as the neutral will remain connected and a neutral to earth fault can trip an RCD. Un-plugging or double pole switches are necessary.

Hmmmm! Unfortunately, that didn’t occur to me, looks like it could still be one of those fridge motors causing the problem; I will have to re-check tonight & make sure I remove all the plugs & throw the DP isolators. But as jj4091 says, why does it run as happy as Larry all day only to trip in the middle of the night!

Thanks to everyone for your continued interest & comments, I will get to the bottom of this if it kills me (pun intended!); I’m supposed to be plastering 2 walls today but this is starting to get like one of those silly murder, mystery, suspense games you play at dinner parties.

 

[bernardgreen]

Thanks to everyone for your continued interest & comments, I will get to the bottom of this if it kills me (pun intended!); I’m supposed to be plastering 2 walls today but this is starting to get like one of those silly murder, mystery, suspense games you play at dinner parties.

Richard, enjoy the hunt. As an example of the really obscure events that could be involved.

A radio trans-ceiver station would power down at about the same time on three days a week. Not always the same day. It came back on air automatically and the log showed a earth related fault. It was not obvious what was causing the fault and it was only when an engineer sat waiting in the power room was the fault identified. A nearby train track had a branch into a factory siding. When an electric locomotive was heard to be straining to pull a train out of the siding the power monitors tripped on earth fault. Trains passing at constant speed or running on braking into the siding were not pulling as much power so did not cause a ground bounce large enough to trip the power monitors in the trans-ceiver station.

 

[JohnD]

To answer your questions in turn:

you say the "main" RCD tripped, you mean the time-delayed one?

Yes

When you press the "test" button on the not-time-delayed RCD, what circuits go off, and what stay on?

Upstairs/downstairs ring mains, cooker, emersion heater (although I don’t think this is on a separate circuit), garage, conservatory u/floor heating; Circuits remaining on are upstairs/downstairs lights & a separate circuit that feeds the lounge & an external light; this was part of a previous rewire job done (not by me) to incorporate an earth for some metal wall lights & an outside security light.

OK then. If the Delayed RCD trips and the non-delayed doesn't, the fault can't be on any of the circuits protected by the non-delayed RCD.

If you want to experiment, you can trip the non-delayed RCD on its test button when you go to bed, and (you will still have lights) and see if the delayed one still trips during the night.

The freezer will stay cold until morning.

I would suspect the outside light.

If you can post some digital photos of your consumer unit, main switch, main earth bonds and service head, and also of the outside lamps and any switches or controls on them, and the run of cable going into the lamp and into the house, that may give us some more ideas.

 

[Richard C]

OK then. If the Delayed RCD trips and the non-delayed doesn't, the fault can't be on any of the circuits protected by the non-delayed RCD.

If you want to experiment, you can trip the non-delayed RCD on its test button when you go to bed, and (you will still have lights) and see if the delayed one still trips during the night.

The freezer will stay cold until morning.

I would suspect the outside light.

I have had the suggestion to concentrate on the lighting circuits from another source; the theory being that as these circuits are NOT wired through the non-delayed 30mA RCD & as this is not tripping, the most likely culprit.

Only problem with this is that one of the night time tests I did involved isolating every circuit at the C/U EXCEPT the upstairs ring main which IS protected through the non delayed RCD. This circuit had just the freezer plugged into it (although I did not remove the rest of the plugs on this circuit) & only the main, delayed RCD tripped!

Considering Bernardgreen’s comments, it looks like I should repeat the test I did with just a night light on line but this time make sure to remove all the appliance plugs & isolate all the DP switches. Beginning to wish I’d stuck with my old C/U, fuse wire & all!

 

[JohnD]

50 says you isolated the other circuits by switching their MCBs off.

This breaks the P but not the N of those circuits.

Hence my suggestion that you trip the non-delayed RCD with its test button (not its big switch) as the RCD is a 2-pole device.

And sooner or later you need to leave the freezer unplugged overnight.

 

[jj4091]

In view of some of the things Bernard has said, it made me wonder if might have nocturnal pests chewing your cables under the floor anywhere, especially as you are in a rural area

 

[Richard C]

In view of some of the things Bernard has said, it made me wonder if might have nocturnal pests chewing your cables under the floor anywhere, especially as you are in a rural area

Some of the cable is recent & some original; I have been checking junction boxes & for evidence of chewing in all the usual places. I still have some to look at but so far have found nothing; I would have thought that Mr Ratty would only chew through it once before being fried anyway & then I would have a permanent fault.

As an update; I removed all the plugs in the house last night except for the freezer & the little night light/timer; it tripped at 02:20, exactly the same time as the night before! I am going to dis the freezer as well tonight to see what happens. I have an electrical engineer friend who is going to loan me an RCD tester, a Mega & a decent Multi-meter so I’ll be running some tests on the circuits & cable installation over the weekend; the saga continues!

 

[bernardgreen]

Tripping at exactly the same time suggests there is a definate reason relating to something happening at that time.

If it trips at that time again then you have to find what happens at 2:20 in your house or nearby.