RCD (sometimes) trips on switching OFF an ELO

[charlhu]

I recently replaced an old electric cooker with an up-market multi-function ELO and ceramic hob that I had kept in storage for 4 years - a simple swap at the cooker switch. The Wylex domestic CU was put in 4 years ago and incorporates a WRS 80/2 30mA RCD and has never given any problem. There is never a problem switching the ELO "ON" and operating it but every now and then (but not always) when the programme selector switch is turned back to the "OFF" position the RCD trips. This can also happen if the cooker switch is turned off. The ELO and hob go through the same cooker switch via a junction box but the hob never causes an RCD trip. I don't want to have expensive call-ins on a problem that cannot be reproduced at will especially when it is unclear whether it is an electrical or an appliance problem. As I see it, if there was a leakage problem with the ELO it would always trip the RCD when switched on. I wonder if it might be a cable fault? I used the existing cables still connected to the two units to feed the junction box. Have rechecked the connections half-a-dozen times.
Grateful for any advice/suggestions!

 

[claire_r]

Neutral isn't switching as quick as live when you turn it off, imbalance, rcd trips

 

[sparkinlee]

Neutral isn't switching as quick as live when you turn it off, imbalance, rcd trips

I was thinking of something like that, but it still must indicate a fault condition somewhere. Ie there must still be a leakage to earth to allow the unbalanced current to flow.

My guess would be an issue with the appliance, but it could be with some of the new cables / junction box I think the OP said had been added.

And (responding to something in the first post) no, a fault with the appliance would not necessarily appear all the time because it may be related to a particular function or action, such as turning off.

Get someone in with a test meter, who can also check that the cables which were up to supplying the old electric cooker are up to powering a ceramic hob and ELO (Electric Light Orchestra??!)

 

[Had8Lives]

I was thinking along the same lines Claire_r . Could be a sticky relay or thermostat. You did say it has been in storage.

 

[charlhu]

Many thanks - but if that is the problem - what is the solution?

 

[charlhu]

I did think perhaps there is now something "sticky" in the controls due to storage but it only happens with a simple on/off operation - even if the oven has been left to cool down first (even overnight) so that rules out a relay or thermostat?

 

[sparkinlee]

The solution would be to get an electrician in who can fully test the circuit RCD itself, and also PAT test the appliance to see if the speculations above are correct...

After that, repair whatever turns out to be faulty!

 

[charlhu]

Thanks for your contribution. ELO = Eye Level Oven (Sorry!).
Cable from board to cooker switch is the built in wiring. I put this on to the junction box and from there fed the two units using the cables that were used in their previous location and still connected up. I wondered whether there might now be a partial internal insulation breakdown in the one cable that is causing it?

 

[sparkinlee]

I wondered whether there might now be a partial internal insulation breakdown in the one cable that is causing it?

Yes, it might be that, although if it was that the tripping would probably be more random and not seeming to be related to a particular action.

My main point is... it could be any of these things, so best to get it tested rather than keep speculating!!

 

[topquark]

Not sure I understand what's meant by neutral not switching as quickly as live, the neutral won't normally be switched, only the live?

There is a potentially dangerous appliance that should be checked out. What's unusual in this case is the tripping happening when the oven is turned off.

Obviously if there is an imbalance the current is going somewhere, that's either to earth (via cpc) or to a device capable of charge storage (most likely a capacitor) or via a sufficiently inductive load.

Assuming the circuit supplying the appliance checks out OK. I'd then start by contacting the manufacturers technical dept and see if there are any compontents in the ELO controls that might explain the behaviour. Then IR testing the oven (once it has been warmed up, don't forget to isolate it after warming though ).

 

[charlhu]

Thanks but, as stated in my post, that is the costly solution that I am trying to avoid to what may be a simple problem. Logically,the RCD is fine since, as also said in my post, there was no problem before I installed the oven in question. If I can first identify whether the problem is wiring or the appliance then in the first case I can fix it myself and in the second case probably need to call in an appliance technician but at least that is only one call-in.

 

[topquark]

Not sure how you can assert that the RCD is fine. Just because it didn't trip before. The RCD will trip at a certain current, nominally 30ma of leakage (or imbalance between line/neutral) ..... some will trip as low at the low 20ma's and still deemed to be working correctly.

Do you have the necessary equipment (and skills) to do an insulation resistance test on the wiring?

 

[charlhu]

You are right that, being a domestic installation, it is switched on the live only. Unfortunately, this is a "Scholtes" top-range unit that I bought and used in France and the brand (now owned by the Merloni Group) is no longer distributed in the UK. Any consumer who has had experience with the technical support for any of their products (e.g. Indesit) would probably recommend the hemlock option!
I would have more confidence in a competent independant repairer who can tackle any brand but finding one is also a problem. Any dicky component is likely to be a "bought -in" item and findable. BTW, I am in London E18.

 

[topquark]

I wasn't suggesting that the componet would be faulty ... it may be a valid part of the control design, but may explain the occasional nuisance tripping.

If there is no socket on the cooker isolator switch then you could consider taking the circuit from a non rcd protected side of the CU, but I wouldn't recommend it.

Another option to consider would be a more fault tolerant RCBO (type C) for the cooker circuit. These are nomally used for circuits supply motors and other largely inductive loads. However! You need to prove the cooker is OK for use before going down that route.

 

[charlhu]

Your theory would be correct if the RCD is inclined to trip at a level below it's 30mA rating and nothing else in the house has a low-level leakage but this oven does. That would explain the "new" problem. I don't have the kit for insulation resistance testing or testing the RCD but would modestly claim the "skill". I am a pro mechanical engineer and that requires a good working knowledge of industrial electrical installations but do not claim to be an "electrician" beyond the commonsense DIY stuff.
I now think it is unlikely to be the wiring in the light of another helpful post which pointed out that if that were it the trips would not occur only on the On/Off function.

 

[topquark]

Your theory would be correct if the RCD is inclined to trip at a level below it's 30mA rating and nothing else in the house has a low-level leakage but this oven does. That would explain the "new" problem. I don't have the kit for insulation resistance testing or testing the RCD but would modestly claim the "skill". I am a pro mechanical engineer and that requires a good working knowledge of industrial electrical installations but do not claim to be an "electrician" beyond the commonsense DIY stuff.
I now think it is unlikely to be the wiring in the light of another helpful post which pointed out that if that were it the trips would not occur only on the On/Off function.

Can I surmise that you have a clamp meter and have tested the trip current present on the cooker circuit?

 

[sparkinlee]

...in the light of another helpful post which pointed out that if that were it the trips would not occur only on the On/Off function.

As he who posted what I think you're referring to, I'd point to the use of the word 'probably' in that post!

 

[charlhu]

OK - if there is not a faulty component to replace then a total frustration and waste of time to contact the moronic world of Merloni.
I did think of taking the cooker circuit off the RCD. As far as I understand the regulations, it is not compulsory for it to be on RCD - only ground floor sockets that may be used for appliances outside and water hazard locations such as near showers .... Yes?
No, not an ideal "solution", but I am confident there is no wiring problem with the oven which makes it dangerous to use otherwise it just would not operate at all and I have been using it without problem for several months now with only this occaisonal RCD trip when switching off after use. The "now you see it now you don't" nature of the problem is typically a control one (the business side of the oven either works or it doesn't) and will probably defy identification. As you will know the "expertise" of manufacturer's technicians is limited to replacing an expensive motherboard. More sense to be gleaned from a forum like this as has already been the case in a couple of hours.

 

[telectrix]

one thought has just occured to me. it may be that the swich on the unit is double pole and one pole is breaking a fraction of a second before the other on switch off, thus causing a momentary inballance,

 

[telectrix]

sorry missed out and/or arcing.