Oven trips but not once warm

[Moggy82]

Hi all, I have a Hotpoint oven which trips RCBO.
There was a fault 18 months ago which was traced to a faulty solder on the main circuit under a relay. That was re-soldered and was fine for a couple of months and the main oven element (2kw) failed and I replaced with a genuine Hotpoint element.
About 6 months later this started to trip on heating, maybe 30 secs to a minute.
I discovered with some playing around that if I used the grill for about 7 minutes, turn off then back on the oven will work fine.
If the oven is turned off but then back on again (still hot) it will trip.
The element is testing at 25.2ohms.
Any ideas why this is happening?

 

[Harry Bloomfield]

Hi all, I have a Hotpoint oven which trips MCB.

Did you really mean MCB, or might it be an RCBO/RCD which trips?

 

[Moggy82]

Sorry yes, rcbo, tired! Have updated.

 

[RandomGrinch]

The element is testing at 25.2ohms

As you have a multimeter, have you attempted to measure the resistance between the element connection and earth?

 

[Moggy82]

As you have a multimeter, have you attempted to measure the resistance between the element connection and earth?

Have just tested with element removed, nothing between element contacts and element bracket. I'm assuming that's what I should be testing. Also tested with element in situ and connected and get nothing between oven case and the element.

 

[Harry Bloomfield]

Have just tested with element removed, nothing between element contacts and element bracket. I'm assuming that's what I should be testing. Also tested with element in situ and connected and get nothing between oven case and the element.

It might, well, not breakdown under test of such low voltage, but almost certainly, it will be the element which has become faulty, causing the trip. A proper test, requires a high voltage insulation tester. One way to help confirm it, is to disconnect the element at either end, insulate the wire ended terminals well, then do what you normally do to which results in it tripping. If it doesn't trip, it's your element which has gone faulty.

The element, is surrounded by aluminium oxide, an insulator, then the steel outer jacket. The oxide, if the outer jacket does not form a perfect seal, it can allow moisture into the absorbent aluminium oxide, which then becomes slightly conductive.

 

[Moggy82]

It might, well, not breakdown under test of such low voltage, but almost certainly, it will be the element which has become faulty, causing the trip. A proper test, requires a high voltage insulation tester. One way to help confirm it, is to disconnect the element at either end, insulate the wire ended terminals well, then do what you normally do to which results in it tripping. If it doesn't trip, it's your element which has gone faulty.

The element, is surrounded by aluminium oxide, an insulator, then the steel outer jacket. The oxide, if the outer jacket does not form a perfect seal, it can allow moisture into the absorbent aluminium oxide, which then becomes slightly conductive.

I'm just trying to work this out, but what I think you're saying is run the oven minus the element, but obviously live connections insulated eg electrical tape?

 

[Harry Bloomfield]

I'm just trying to work this out, but what I think you're saying is run the oven minus the element, but obviously live connections insulated eg electrical tape?

Yep, that's the idea.

 

[ericmark]

@Harry Bloomfield is correct, the element insulation is hydrophilic, any damage to the seal, and although heating it can drive out the water, once it cools it will draw it in again, so replacement is the only cure.

There are two ways to test, one is with an insulation tester, these can use 250, 500, or 1000 volts, so will find faults a multi-meter will not pick up, it however uses DC and will not show all faults, but should work OK with the fault you have, the other is while still live, clearly only if the RCD will hold in

and shows the actual leakage, it is considered total leakage should not exceed 30% of the RCD rating, so 9 mA is the limit.

This is the problem with RCD's, until 2008 we would not have put the cooker on a RCD, but at that date it was decided cables buried in a wall needed protection.

The problem with the RCD is a neutral - earth fault can give unusual results, but with a cooker the element going faulty is to be expected.

 

[Moggy82]

Thanks both. I'll try running with connection isolated first, I don't have access to insulation tester and don't feel confident in testing live. Looks like element developed a fault quickly if it does turn out to be the issue.
I'll report back.

 

[Moggy82]

Yep, that's the idea.

Still tripped with element isolated!
It's on a standalone RCD so I can also rule out the rcbo for the oven circuit.
I did though discover that the oven was still getting warm.
Turns out there is a secondary element in the bottom of the oven which is also running have just isolated that to see if that's the culprit!
It's also a cheaper element but looks to be a pain to change.

 

[Moggy82]

Still tripped with element isolated!
It's on a standalone RCD so I can also rule out the rcbo for the oven circuit.
I did though discover that the oven was still getting warm.
Turns out there is a secondary element in the bottom of the oven which is also running have just isolated that to see if that's the culprit!
It's also a cheaper element but looks to be a pain to change.

New bottom element ordered. Again thanks both.
For future reference what brands would you go for for longevity of element, I'm assuming you've both got plenty of experience with ovens.
Not been impressed with the quality of Hotpoint poor soldering and two elements in 4 years.

 

[RandomGrinch]

New bottom element ordered. Again thanks both.
For future reference what brands would you go for for longevity of element, I'm assuming you've both got plenty of experience with ovens.
Not been impressed with the quality of Hotpoint poor soldering and two elements in 4 years.

I'm afraid I can't help with element choice, but it would be interesting to see if you get a resistance measurement with your DMM (although as has been said, an insulation tester would be better) between terminals and earth on the bottom element - just a final nail in the coffin!

 

[Harry Bloomfield]

New bottom element ordered. Again thanks both.
For future reference what brands would you go for for longevity of element, I'm assuming you've both got plenty of experience with ovens.
Not been impressed with the quality of Hotpoint poor soldering and two elements in 4 years.

Sorry, I have zero experience of such elements, I only use gas.

 

[ebee]

It is absolutely amazing what goes into a cooker unit once in use in some households and spillages and crumbs can get into the most unlikely of places.
If you have ever been left in wonderment how mice and insects can get into holes a fraction of their own size then cooking appliances still makes you wonder.
And do not even think of toasters - left with 100 years or something of crumbs piled inside and almost full to the brim and never removed or the accidental silver foil milk bottle top or five down amongst the elements, folk trying to dig them out of a toaster by using a metal knife whilst the toaster still plugged in.
RCDs might have actually saved more lives than we realise.