RCD trip scenario.

[The Natural Philosopher]

I mentioned that I had had an RCD tripping issue, and today decided to probe into it more.

I had isolated a kitchen cooker circuit and left it off at the consumer unit - I use the Aga to cook mostly - so today I took off te wall plate, which is a double unit featuring a double pole cooker isolator and a switched 13A socket, both equipped with neons.

Probing around with a resistance meter at that socket/isolator showed no conduction live to earth at all - maybe a capacitor as I saw a brief hint on probe connection.

Neutral to earth was about 40 ohms, on the incoming side, which is consistent with the run back to the earth stake and so on. And nothing on the cooker side.

The main RCD is 100mA as there is a lot of electronics here. Yes I should fit RCBOs..

When the trouble first manifested itself in the middle of the night, it was taking several minutes to trip, then it developed a much harder fault, so I switched every circuit off and then on again until it tripped, and that was the cooker circuit.

Now I am a bit stumped. I haven't yet switched the cooker back on but the RCD is holding up with it live to the isolator.

I have never had an RCD trip that didn't show either a neutral earth short or reasonably serious - 3k? or less - path to earth from live.

I have had a lot of mice eating away at plastics. And I have had a mouse eat through live cables elsewhere once. And cause an MCB trip. But I cannot see how that particular piece of cable could be mouse accessible.

I haven't yet screwed it back together , but can anyone come up with a plausible scenario to explain it?

I have two (fairly unlikely) ones to date.

1/. some kind of suppressor cap in the cooker is breaking down at high voltage. But shows no issue at test voltages. 2/. There is an electrocuted mouse somewhere, behind the cooker, that has since dried up and become non conductive.

You know you love armchair theorising....

[The Natural Philosopher]

Ah well. THAT was conclusive to a point.

I screwed the plate back to the wall, put power on it and switched the cooker circuit on and then switched on the cooker isolator - instant BANG as the RCD tripped.

Well at least I know its downstream of the isolator and there is only a meter of wire and a cooker thereafter....and its no longer a sometimes trip, but is a good solid reproducible hard trip

Looks like that cooker needs to come out, but that is a job for another day....

[GB]

Nothing on the cooker is switched on?

[The Natural Philosopher]

Nothing on the cooker is switched on. The original problem occurred late at night with te cooker off...Furthermore, I looked at the circuit diagram and there are no suppressor caps or anything from live to ground.

BUT what I didn't realise till I looked again is that the upstand at the back has two large mouse sized slots to act as fan exhausts, and I know a rodent has been on the worktop, eating plastic like mad, and I am about 97% certain there's a carbon arced mouse across the incoming terminals down at the back.

I've emailed an Aga engineer to see if he can get the cooker out and fix a thermostat issue it has on one oven, while I look at the cable...

[John Rumm]

Sounds that way. Could still be a mouse that dries out a bit with enough current through it :-)

[mm0fmf]

I had a lamp in a cooker hood that kept blowing a 3A fuse in the plug feeding the hood fan and lamp. It was an SES lamp fitting and I thought maybe it was grease that had been sucked in as the lamp holder was greasy. I cleaned it up and *bang* it blew the fuse after a few seconds. After much swearing I was able to remove the lamp holder and further examination show there was a blackened wasp body touching the tip and screw. Removing it and more cleaning and it's been fine since.

Your carbon arced mouse sounds very plausible from how a wasp body could blow a 3A plug fuse.

[The Natural Philosopher]

Well Aga engineers have not responded, so I will have to try and remember how to pull the unit out to sort out the cable. IIRC it has a long tail to allow that which probably means there was/is a loop at around ground level.

[John J]

You didn't mention what fuel your aga uses but from your mention of a looped tail I'm guessing it's gas fired so you don't have a solid chimney or rigid oil pipe attached? Is it bolted down to the floor or would two or three metal skid strips enable you to drag it forwards sufficiently to access the rear? Maybe some hired equipment skates would help. Trying to track down earth leakage paths with a simple multimeter isn't an easy option. Can you get your hands on a 250/500 volt megger and test L&N together to earth?

[Brian Gaff (Sofa]

I'm assuming that the loosened switch was disconnected, Its not as simple as a tiny hole in the insulation shorting or high resistance to a bit of sharp metal when the box is fully screwed in, I suppose that would be far to convenient!

I had this on a dimmer switch once. A teeny little point on the pcb was flexed as you did the screws up. Brian

[Brian Gaff (Sofa]

I'd have thought with a posh cooker like that some mesh would have been over the exhaust holes to stop rodent access. Might be worth fitting something if it transpires to be something biological. Brian

[Brian Gaff (Sofa]

I thought 40 ohms sounded a bit high to my mind. I think mine is a quarter of that last time it was checked when they put in the so called smart meter. grin. Brian

[The Natural Philosopher]

Oh dear. You missed the whole scenario. Its not the Aga that has the problem., it's an all electric 'Aga companion'. Basically an overpriced Aga branded and themed electric cooker bolted on the side of a two oven oil Aga.

And yes, probably all I need to do to fix the trip is to unbolt the coupling bracket and pull it forward enough to remove da hypothetical mouse. And add some kind of connector or insulation where it has presumably chewed through the cable loop.

But it hasn't moved in 20 years....

Testing it with a megger really isn't an issue now. It's instant trip every time the (switched off) cooker gets mains so the problem is between the wall switch and the cooker - not in the cooker itself.

The only piece of wiring that is exposed is the loop behind the cooker itself, so my guess is that SuperMouse has gotten his teeth into that and bitten off more than he can chew.

Got some super new traps, and caught one more mouse - total is around 4 or 5 I think. I lost count. So I think they are all gone now one way or the other.

Apparently PVC is just right to trim mouse teeth down to size - rodent teeth grow continuously.

[tony sayer]

I reckon you could do with a good cat there mate;!....

[John Rumm]

Might have ended up a bit warmer than bargained for!

[SH]

Absolutely shocking!

The mouse was cleaerly a livewire.....

[Tim+]

More of a dead short?

Tim

[The Natural Philosopher]

I called him Barbirolli.

He was a pretty good conductor.