Intriguing problem. Shower and Cooker tripping CB not RCD

[Andy C]

Opinions on this fault please, I have never come across this before.

Split RCD board with a shower (40 amp) and Cooker (32 amp) both on 6 mm T&E. Both are tripping the CB, apparently whilst switched off? RCD tested fine.

Shower is a 9.5kw, so feasibly the 40a might be a bit close to max, fed from a 45 amp pull switch.

Cooker is about 5.5kw so a 32a is fine, fed from a 45 amp double pole switch in a cupboard. The cooker is definitely tripping whilst switched off at the cooker? Not straight away but a few minutes later. No other load on circuit.

I haven't tested the appliances. Both are relatively new.

Both circuits have been tested from DB to isolating points at 45 amp switches and IR results were fine.

Thanks in advance for any suggestions.

 

[Deleted member 9648]

That was a reply to Wirepuller, who seems to think testing the RCD is pointless.

It's a fair point.

 

[Deleted member 9648]

If testing has not found any issues I would be inclined to try replacing the MCB, I have known 'tired' MCB's to trip randomly before on light loading.

 

[Toneyz]

Just to clarify does these MCB's trip with the cooker and shower switched off but isolators switched on?
What is the insulation resistance in this state (note the caution L-N)
What make is the consumer unit & MCBs? open mind on the tired MCB's theory never experienced it myself not saying its a possibility on cheaper brands.

 

[Rpa07]

I experienced the ‘tired’ MCB on a shower circuit.
You go there armed to the teeth with test gear and after testing and proving good, disconnected at the shower, you knock your ladder and the MCB trips! Reset, tap the CU and it trips - I made the most of the situation and sourced the correct make MCB for the CU replacement instead of the BG that was fitted.
Granted this is the only time it’s happened so far but some things do come under the acronym K.I.S.S

 

[marconi]

What type of oven and hob are connected?

 

[Deuce]

How come no feedback on the oven and shower appliance testing yet? That seems like the obvious thing to rule out.

I had an isolator switch near a hob causing tripping of CB, when investigated it was due to grease that had found its way in and was arcing. Seems plausible this could be happening here, or possibly grease ingress into the cookers internal connections.

NB: it wasn't immediately apparent in my case that the cause was grease, as the switch plate had been cleaned frequently. Very close inspection showed signs of grease ingress down the side of the switch itself, probably more forced in each time it was wiped over.

 

[Wilko]

Hi - some thoughts from afar -
If MCB is tripping with no appliance load, then there is either short circuit somewhere (trapped conductors in a back box?) or as RPA has mentioned it's the MCB.

 

[NeilC]

Hi all, I am the person who owns the oven and shower appliances. Thanks Andy for sorting some stuff out and is endeavouring to sort this issue. The shower started to trip the MCB about 4 months ago. Again it only trips when switched off never when using. It doesn't do it all the time. The oven ( an 8 yrs old Rangemaster twin oven fitted by qualified spark etc) started to do the same thing about 2 months ago. At first very occasionally but now getting worse. Mostly it trips the MCB when off but has tripped on the odd occasion when on. Its doing the roast dinner as I write this and has been on over an hour. I replaced the heating element before calling Andy just in case that was at fault but no difference. Both the MCBs for shower and cooker are right next to each other RCD unit if this helps. The RCD unit is about 8yrs old.

 

[Strima]

Loose connections on the circuits or switches causing arcing? Let through current of the arc causing the MCB to trip.

 

[Deleted member 9648]

If there are definitely no fault readings on either circuit when an IR test live-neutral is carried out with appliances isolated then check the board carefully for heat in adjacent devices, which may cause other devices to operate. Otherwise with no fault readings (definite?) it can really only be faulty MCB's if there is no load current as you suggest.
I emphasize the word 'definite' because so often on here it turns out that very limited testing has actually been carried out.

 

[ipf]

For the sake of it, change both mcb's AND the rcd. Although probably ok, it's common to both circuits.
Once you've eliminated those from your problem points, you're narrowing it down plenty.
Appliances or connections.....

 

[marconi]

Ask the electrician to check the polarity of the supply at the terminals of the shower and the oven.

From #23:

Again it only trips when switched off never when using. It doesn't do it all the time.

The oven ( an 8 yrs old Rangemaster twin oven fitted by qualified spark etc) started to do the same thing about 2 months ago. At first very occasionally but now getting worse. Mostly it trips the MCB when off but has tripped on the odd occasion when on.

 

[telectrix]

i can cover the wirral if needed. the arse end is close to me. the knob end is not too far if need be.

 

[Shoei]

Is the shower switched by a two-pole cord switch? If so the switch may be faulty. I've seen them fail like this when they get a bit dirty or moist inside. Contamination on internal surfaces can impair the creepage distances, producing an arc between live and neutral when current is interrupted.

If that's suspected you can confirm by temporarily linking out the neutral wires, disconnecting them from the switch and using it as single pole. If the CB no longer trips then it points to the switch.

 

[marconi]

I hope the OP employs telectrix to hunt the problem and fix it - then the EF might might find out what it was.