"Nuisance" trip domestic oven / hob circuit

"Intermittent" fault, oven / hob circuit has tripped 4 times this year, most recently a week ago. Customer can see no pattern other than when they (and possibly only the oven) are in use.

RCBO tests satisfactory (trip / ramp)

Comined demand of oven / hob is well below 32A of RCBO even before diversity.

All cable lengths satisfactory IR (>199M)

Oven / hob low IR (c.5M) but ok...clamp meter shows <1.8mA max leakage when running...

IR tested L / N conductor terminations (load) on "cooker switch"...0.2M

Would say this was the issue. When oven / hob have been running and conductor temps have built up suspect a short between L / N causing "trip".....any thoughts?

However, the switch was replaced a couple of months ago and the fault pre-dates this.....it's possible that both switches are / were faulty....but how likely?...

Have I missed anything?

Would you have done anything different / extra?

Edit - there is a solar PV installation with RCBO / energy monitor within the DB....RCBOs are Type A but single pole....could there be a DC leakage N / CPC from the PV setup which is causing sufficient DC leakage to cause my trip via the common N bar?....but other RCBOs on the board had shorter trip times / ramp test times than the oven circuit RCBO....I dont "do solar" (not enough sun up here in Salop!)...so would never claim to be 100% with its issues....I'm not certain how / if it's possible to measure for DC leakage with my clamp meter (Megger)....I'll have to dig out the instructions tomorrow.

How low would the IR need to be at the terminations before a L-N "short"? I usually work on an assumption of 0.008M for an earth fault to trip the RCBO....

I am working on the assumption of a faulty switch...but the fault pre-dating this switch has me doubting myself.

Thanks in advance for any thoughts.

Timotei

dodgy element. you need them hot to IR test properly

With ovens and intermittent faults I'm always suspicious of moisture in the mineral insulation of the elements. It can change the resistance drastically as the element warms up. My belief is that, following switch on, dispersed moisture is driven towards the cooler ends where it concentrates, lowering the resistance.

Andy™ said:

dodgy element. you need them hot to IR test properly

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Thanks Andy.

That was my first thought...but oven had run for 15-20mins as I took my leakage reading of 1.8mA max....admittedly did not re-take IR until a while later...but the leakage measurement peaked at 1.8mA after 5 minutes and went down to less than 1mA as time went on....so I had ruled this out...

edit - i also thought a dodgy element would likely have tripped more than "4 times this year" which is what the client is telling me.

I would agree it is most likely Oven related...

However I have seen some cheap-tat switches/sockets/fused spurs etc..
Over the years..
Assuming the switch(es) you have used are a decent brand, then I cannot see any issues..

But also have you have opened the switch to double check no melted / cracked / perished cable insulation that could be causing issues when the cables warm...

And have you have the oven/hob out to visually check cable terminations from connection plates to appliances?

I had an oven a few years back that had a loose termination on the back that gradually melted insulation back sufficient to trip the MCB while on..
But then MCB could be reset until it got warm again when more than one hot plate was turned on.

Are there any other 32A RCBO circuits on the board that you could swap over...
More as an exercise to show the customer that the RCBO is working correctly..
as I doubt there is anything wrong with it from what you say from your tests?

SPECIAL LOCATION said:

I would agree it is most likely Oven related...

However I have seen some cheap-tat switches/sockets/fused spurs etc..
Over the years..
Assuming the switch(es) you have used are a decent brand, then I cannot see any issues..

But also have you have opened the switch to double check no melted / cracked / perished cable insulation that could be causing issues when the cables warm...

And have you have the oven/hob out to visually check cable terminations from connection plates to appliances?

I had an oven a few years back that had a loose termination on the back that gradually melted insulation back sufficient to trip the MCB while on..
But then MCB could be reset until it got warm again when more than one hot plate was turned on.

Are there any other 32A RCBO circuits on the board that you could swap over...
More as an exercise to show the customer that the RCBO is working correctly..
as I doubt there is anything wrong with it from what you say from your tests?

Click to expand...

Thanks SL.

This is apparently a cheap £10.00 Amazon switch......supplied and fitted by the customer after the last electrician to investigate the fault said the switch was "faulty"....no more info than this, sorry......being self installed I expected a mess, but in fairness it had been done very tidily...there is no sign of overheating / arcing at any (known / visible) point on the circuit....unfortunately I only had 1 gang switches on board at the end of the day, and this is one of the 2 gang switches (but without a socket)

The fact it is a £10.00 Amazon job, along with the low IR between live conductor terminations (but not live conductors) on the load side termination points, all points to a faulty switch.....but the customer is adamant the fault existed before this switch was installed, although he was not clear how many of the "4 trips this year" were before and how many were after the new switch...

I removed the oven and checked the connections behind,....not how I would want it, but nothing that looks likely to be a cause of this issue....

The CU / DB is Fusebox, I tested all the RCBOs, all within expected parameters..... I could supply and install a new 32A RCBO, but as the existing one tests perfectly I am reluctant to do so....I do not want to be accused of going down the path of not finding the fault for certain, so lets throw parts at it route....I appreciate this is not what you suggested (i.e. swap with an existing 32A RCBO), but the customer watched my every move and is happy that I have undertaken a through fault finding process which, amongst other things, shows the RCBO to be within satisfactory parameters on every test....I could go down this route as a last resort...

Please see my edit to original post re DC leakage....

I wouldn’t normally recommend the random changing of parts without proving by testing but oven elements is one of those exceptions to the the rules as they can be a bit random with their tripping. Providing it is not exceptionally expensive and they aren’t normally I would change just to eliminate it, making the customer know that there is a good chance this is the fault but you can’t be a 100% sure due to the different conditions affected by temperature.

Do the RCBO trips happen to coincide with the oven thermostat clicking on or off? Had a case where sometimes the thermostat clicking OFF created a spark across its contacts that was vicious enough to create enough disturbance to fool the breaker into "imagining" a residual current fault.

roys said:

I wouldn’t normally recommend the random changing of parts without proving by testing but oven elements is one of those exceptions to the the rules as they can be a bit random with their tripping. Providing it is not exceptionally expensive and they aren’t normally I would change just to eliminate it, making the customer know that there is a good chance this is the fault but you can’t be a 100% sure due to the different conditions affected by temperature.

Click to expand...

If changing the element then change the thermostat(s) as well while at it. The thermostats are expendable items, so less time until failure and less future hassle or down time.

runningpiglet said:

Do the RCBO trips happen to coincide with the oven thermostat clicking on or off? Had a case where sometimes the thermostat clicking OFF created a spark across its contacts that was vicious enough to create enough disturbance to fool the breaker into "imagining" a residual current fault.

Click to expand...

I think maybe it was fooled into thinking it a "fault" rather than a residual current fault.

runningpiglet said:

Do the RCBO trips happen to coincide with the oven thermostat clicking on or off? Had a case where sometimes the thermostat clicking OFF created a spark across its contacts that was vicious enough to create enough disturbance to fool the breaker into "imagining" a residual current fault.

Click to expand...

Thanks runningpiglet.....I couldn't induce a trip for love nor money!....always the same with these intermittent faults isn't it?!...and with it being "only" 4 trips in the year to date it has not been often enough for the client to be able to answer that for me.

sometimes you've just got to wait until its tripping a lot more and there's something to find

Moisture in element problems often occur after a long period of non use and vanish when the element has been partially dried out.

Andy™ said:

sometimes you've just got to wait until its tripping a lot more and there's something to find

Click to expand...

Thanks Andy, but you know how it is, customer calls, and with loss of power comes great responsibility, and your friendly neighbourhood electrician swings into action......only to sometimes be left wishing he hadn't!

Geoff1946 said:

Moisture in element problems often occur after a long period of non use and vanish when the element has been partially dried out.

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Thanks Geoff, you're absolutely right of course, but as best I know the hob and oven have been in daily use.

Run tour fingers all over the element and feel ,for any rough areas or very tiny splits

kerching said:

Run tour fingers all over the element and feel ,for any rough areas or very tiny splits

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Thanks kerching....but that sounds a little too similar to what the doctor told me last time I paid him a visit....