RCD trips when load is switched off - any ideas ?

[robgraham]

My mates's house is an old one with the inevitable modified electrics !

The garage (free standing) and a room under the stairs share a modern CU wi th 2 16A MCB's and a normal 30mA RCD. The room under the stairs is the hom e office so contains the PC, printer and router.

I rewired the garage for him some years back including converting an old wo od turning lathe (Graduate) to run off a 3 phase inverter. It's here that the oddity occurs. Bob reported back to me some time ago that on occasio ns when shutting down the workshop at then end of the day the RCD tripped - and I was pinned down today to investigate. And of course nothing happene d !

Bob's normal procedure when shutting up for the evening is to switch off th e 2 150W halogen floodlights he uses for lighting the lathe and then poweri ng off the inverter (a small 1.5kW one) at the 13A socket. As is the way o f things, this sequence sometimes gets reversed and erratically it's then t hat the trip occurs - ie the inverter gets switched off before the floodlig hts, and then boof! he's in the dark as the RCD in house front hall has tri pped.

The usual suspect of earth leakage current is clearly the culprit, but why on switching **off** the just somewhat larger than domestically normal SMPS ? Or is one of the floodlights leaking and the switch off just creates tha t little bit of instability on the circuit ? If the latter, is there any way of measuring it ?

Thanks Rob

[John Rumm]

If the RCD is sensitised close to its tripping point in normal operation, then any transient can be enough to push it over the edge...

Might be worth adding a known quantity of leakage yourself and seeing at what point it trips. A RCD tester with a ramp facility is handy for that.

[Bob Minchin]

I expect the inverter is only having its live connection switched leaving half the internal filter connected to neutral and earth

[Andrew Mawson]

I had persistent rcd trips when turning off a set of uplighters with folded fluorescent lamps. Totally solved the issue by putting a snubber circuit (capacitor in series with a resistor) across the load

Andrew

[fred]

The professionalism here is beyond my limited grasp of electrics but this t hread may hold the answer to a problem we have. Something in the kitchen ri ng occasionally trips the rcd.(Never when I'm around) SWMBO is convinced it s the fridge but then she's looking for a new one. I was just this morning wondering if there was any device I could place between the fridge and the socket to help me prove/disprove her theory.

We had a similar problem many years ago which resulted in the replacement o f many toasters, as the assumed culprits, and the splitting of the ring in an effort to source the problem. It transpired that the main connection to the house was faulty. It eventually melted the connection and damn near cau sed a fire.

I did talk it through with an electrician we had in to wire some 3 phase so ckets in the factory but he had no suggestions and as the house is 30 miles away from the factory and his rates were a bit eye watering I didn't fancy paying him for 2 hours travelling time and two hours head scratching.

So even if I had enough knowledge to allow me talk the suggestions through with someone it would be a big help

[charles]

[Snip]

[Snip]

My initial suspect would be an electric kettle.

[fred]

Strange you should say that. She has just replaced the second Dualit kettle to fail. (They make good toasters but their kettles are sh**e). The second one ? Yes. It was the right colour for the kitchen, you see. This time com mon sense prevailed and a Cuisinart purchased.

Wonder if that was the cause.

Only time will tell.

Thanks for your tip. It has given me a bit of ammunition

[Andrew Gabriel]

Use an extension lead to plug the fridge into a different circuit, if you have another circuit which is on a different RCD.

[fred]

is thread may hold the answer to a problem we have. Something in the kitche n ring occasionally trips the rcd.(Never when I'm around) SWMBO is convince d its the fridge but then she's looking for a new one. I was just this mor ning wondering if there was any device I could place between the fridge and the socket to help me prove/disprove her theory.

Except it only occurs occasionally and I cant see her-indoors living with a n extension lead snaking around her kitchen for weeks on end.

[Bob Minchin]

Her indoors needs to understand that analysis of some problems need some tolerance on her part! Are you man or mouse in your house?

[John Rumm]

Own up, how many would answer "squeak" to that?

[Dave Liquorice]

More a case of which leads to less agro. An occasional trip and whine about wanting a new fridge or several nags a day about the extension lead around the kitchen, up the hall and into the lounge... B-)

[Jim K]

Simples - move the fridge to the lounge.... ;-)

Jim K

[John Rumm]

I would first point you at:

The combination of transients and mains input filters seems to be a bad one for RCDs - they are often either fast enough or "upset enough" to see the imbalance caused by the filters dumping excess harmonic noise to earth.

There is quite a lot of detail in the wiki that should be doable even with basic test gear, and a methodical approach.