Oven trips circuit, overheating?

[jumbuck10]

Hi

I have an oven cutout problem that I haven’t been able to solve and I am wondering what I should be looking at next.
It sounds like a cooling fan problem but for some reason I am not convinced since the fan appears to kick in – see details below.

Model: Smeg C9GMXA (free standing).
Power: The oven is on it’s own circuit direct into the switchboard circuit breaker. ie. nothing else is hooked in other than the oven.
Background: Back in March I had a blown oven fan forced element replaced by a repair person. The oven worked for about a month or so then it started cutting out the power. We use the oven about once a week. After this I did a quick inspection, removed all the oven elements to and test them – they appear to be ok. I called the repair people again and the guy said (not the same person that did the repair) that the element is likely to be ok so it could be something like the safety thermostat or something else. I am not keen on getting them out again.

Symtpoms:
Fan force mode – cold to warm up to about 125C and cuts out the power. Circuit breaker trips. ie no power to the oven
Convection mode (top & bottom element): cold to 200C – no problem – circuit breaker does not trip.
Fan and grill mode – trips the circuit as well.

Resetting the circuit breaker immediately doesn’t work as it just trips. If I wait about 1-2 minute I can flick the breaker back on and turn on the oven again (presumably the cutout thermostat has cooled down). If I keep it in convection mode it doesn’t appear to trip even if I turn it up to 200C. If I turn it back to fan forced mode the oven circuit trips again in a few minutes time.

Element test: (top, bottom, 2 x fan forced) elements. Continuity test with a multimeter and resistance test seem to indicate that they are okay.
However I did notice: Fan forced element on the left is 2200W (generic – replaced by the repair person), and element on the right is (2000W – been in the oven for a couple of years now) .
I checked around that the replacement elements should be about 1550W. I called up Smeg and they said that a 2000W element should be ok.
Both Fan force elements glow red with fans spinning.
Top and bottom element – Heat up when used in convection mode to about 200C – 220C no problem but I noticed it doesn’t glow bright red like the fan forced ones.

Is it possible that the new element is pulling too much current overall and causing an overloads or perhaps it is simply heating up too quickly?

Fan force fans: clean, spinning freely, and both are working when in fan forced mode.

Oven Temperature Thermostat: The oven light temperature LED goes on/off at the desired temperature so I presume that is working ok.
I placed an oven-proof meat thermometer to check it against the oven temperature setting. They are roughly about the same.. eg. 150C setting shows about 150-160C on the thermometer.

Cooling Fan:
I did notice that if I set the temperature at a certain temperature (eg. 125C, 150C) that the cooling fan did not turn on immediately when it supposedly reached the set temperature (LED indicator light goes off). I am not sure if this is normal behaviour but I would have thought that the cooling fan would kick in to cool oven down. If I turn the dial down to 50C (after it reaching 125C) the cooling fan still does not turn on.

Shouldn’t the cooling fan be turning on when I bring it down to 50C on?

If I the oven reaches about 175C, the cooling fan does turn on. If I set the thermostat down to say 100C the cooling fan stops at about 100C.

Note: Cooling fan thermostat is rated at 80C, safety cut-out thermostat is 180C

Safety Cut-out Thermostat – replaced. It is a dual cutout thermostat. My multimeter test had some unusual test results – showed one open, one closed at room temperature.
Cooling Fan thermostat – replaced. It appeared to work when I applied heat to it but it is a cheap part so I replaced it anyway.

Does anyone have any suggestions on what to look at next?

Thanks In Advance.

[electrofix]

need to know whats tripping

MCB like the old fuse also whats the current rating ?

RCD earth leakage protection

or is it a combined unit RCBO ?

Dave

[jumbuck10]

New style MCB 20 Amps.
Trip Interrupting Capacity 4.5kA

Not an RCBO.

[electrofix]

if you look at the rating plate in the cooker what does it give as a max current ?

tried the owners manual but it just tells you to look as the plate

Dave

[jumbuck10]

The plate says Max load 4.1kW – that is all I can find.
So by my calculation 4100 / 230 = 17.8 A – a 20A circuit breaker should do the trick and it has been for at least 8 years until now.

So if fan forced elements are 2200W (left) and 2000W (right) then we are effectively 4.2kW (yes have had a couple of service guys come through over the years). Smeg ones are supposedly 1550W each but I have also been told that certain C9GMXA models have had 2000W elements – not sure how accurate that is. This would explain why the fan force function trips the CB.

But what has me stumped is that the CB also trips if I put it on Grill & Fan function (symbol) which I presume runs the Top Grill with 2 Fans and no fan force element.

I also just checked the resistance of the fans when idle but I don’t know what the correct range should be.
Fan force fans: approx 113.5 ohms each
Cooling fan: 154.5 ohms, also noticed that the fan serial had what appears to be 125 degrees C stamped

Just found something interesting from Smeg documentation.
Cooling Fan: 20 Watts
Rear fan-force fans: 25W x 2.
[TABLE=”align: left, border: 1, cellpadding: 3, width: 500″]
[TR]
[TD]Function[/TD]
[TD]Circuit Trip[/TD]
[TD]Element[/TD]
[TD]Max Wattage[/TD]
[TD]Current Amp[/TD]
[/TR]
[TR]
[TD]Convection[/TD]
[TD]N[/TD]
[TD]Top & Bottom[/TD]
[TD]3050[/TD]
[TD]12.71[/TD]
[/TR]
[TR]
[TD]Fan Grill[/TD]
[TD]Yes, about 150-175C [/TD]
[TD]Top (inner & outer) with 2 rea fan [/TD]
[TD]2900[/TD]
[TD]12.09[/TD]
[/TR]
[TR]
[TD]Fan Forced[/TD]
[TD]Yes, at 125-150C[/TD]
[TD]2 x Rear Element + 2 x Fan[/TD]
[TD]3080 (assuming 2 x 1500W elements)[/TD]
[TD]12.84[/TD]
[/TR]
[TR]
[TD]Supercook[/TD]
[TD]Yes, 125-150C[/TD]
[TD]Top, bottom, 2 x Rear[/TD]
[TD]4480[/TD]
[TD]18.67[/TD]
[/TR]
[/TABLE]

[electrofix]

a lot depends on the elements perameters

if the elements are designed for 230v and give out 4200 watts then they draw 18.26A

now the supply can move and more often than not its high, nearer 240V and with the same elements they now pull 19A

add the fans and trip

as for your grill they use the rear elements as a boost as for as i know in fan mode

change the elements to correct 1420W and all will be well unless you want to uprate your supply

correct part no listed as 806890943

https://www.elementman.co.uk/smeg0442.html

or

https://shop.ukwhitegoods.co.uk/806890593-genuine-smeg-fan-oven-element#description

both list your model but double check

Dave

[jumbuck10]

Thanks Dave.
It sounds like I should replace the element with a lower rating in any case.
I could be dealing with multiple problems here.

I have been doing lots of roast potatoes lately as I watch the oven behaviour – the Mrs is getting a little sick of roast potatoes 😀

Last night I checked out the fan grill function and watched the rear elements – they didn’t glow hot and the leaflet I have says it doesn’t use the rear elements. (geez it was hard to find). https://www.smeg.com.au/smeg_com/docs/common_pdf/C9GM.pdf

If it doesn’t use the rear elements then the only other conclusion I can think of is that it is a heating issue rather than electrical.
rationale 1: CB trips because the safety temperature reaches the cutout temp. It takes a minute or so before the CB can be flicked back on without it tripping. So it is likely to be the safety cutout thermostat has cooled down a little.

rationale 2: CB trips because the safety temperature cut-out reaches the cutout temperature. The fact that it trips with the Fan grill function (at about 125-150C) is due to the fact that there is better air circulation (but then this doesn’t explain that the convection mode can reach up to 220C).

[electrofix]

now this is getting odd

your saying the trip goes and does not reset for a few minutes
but previously you said it was not an RCD or a RCBO so it does not have an earth leakage trip
so your trying to reset the trip and its not resetting which means its like having 20a fuse wire blow everytime you reset. its a wonder your have not blown the contacts

have you thought of the possibility that your trip may be faulty ?

what cable size is feeding the cooker ?

Dave

[jumbuck10]

I set it on fan force at 100C – an click! power cuts out.
So we confirm that it’s an electrical trip. CB flicks back up straight away.
That’s problem one. The element calculations make sense but I am wondering if the fans have something to do with the trip too.

[electrofix]

have had fans with earth leakage problems that blow rcd’s but they cant blow 20 amp breakers

Dave

[jumbuck10]

Yes, it’s really strange. The breaker is rated with 20A 4.5kA. – wonder if it is just sensitive and trips at 16A or something.

If the rear fans have an earth leakage then it should trip it straight away wouldn’t it?

As for the Fan Grill tripping – could it be cooling fan causing the trip as it switches on?
The cooling fan thermostat is rated at 80C. So if the back of the oven hits 80C it should go on. I will have to put the meat thermometer on it somehow. The problem is the thermometers are never that accurate.

Do you know what the 125 deg C printed on the cooling fan means?

I’ll have to do more grilling – might have to try chicken wings next.

“your saying the trip goes and does not reset for a few minutes
but previously you said it was not an RCD or a RCBO so it does not have an earth leakage trip”
Nope, not RCD or RCBO – I checked the specs.
Trip not resetting happens on a hot oven.

[electrofix]

if elements had earth leakage it would not affect your trip which is an MCB *minature circuit breaker) it only responds to input current and earth leakage would have to be pretty high to have any effect
I assume you have a RCD in circuit and this would trip instead if you had any earth leakage

and begining to think your trip may be over sensitive although those elements are over rated and could do with being changed

Dave

[jumbuck10]

Update:
Replaced 1 x Fan Forced Element so now I have: 1 x 1550W (new) + 1 x 2000W element (originally 2200W (6 mths) + 2000W)
The MCB trips on the Fan forced (2 fan elements) and Super cook (all 4 elements). This happens from a cold warm-up to 150C.
It doesn’t matter which of the two 2200W or 2000W I swap out – it doesn’t make any difference.

However, the Fan+Grill function does not appear to trip the MCB.
I have also noticed that on 2-3 occasions the Fan force setting (1550+2000W) element did not trip the MCB.

I am stumped. I can’t see any obvious damage to the wiring and connectors, although the plastic shielding on the end terminals have broken off due to age.

I have ordered a new oven thermometer to double check the oven thermostat settings but I don’t think this is the problem since there are no issues when fan-forced mode is not used. The cooling still appears to be running fine.

Oven Relay Circuit Board? Clock circuit board issue?

[electrofix]

problem here is your only guessing whats going on and you need to measure things

you need to get a clip on ammeter. there work by measuring the magnetic field created by the current through a wire. you need to access one core either live or neutral as you must only clip it round one
you can then see the current drawn by the cooker

https://www.amazon.co.uk/Aofan-handh…ag=googhydr-21

dont know how accurate this one is but you get the idea

there is always the chance your trip is faulty, dont rule that out

Dave

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