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Topic Title: Ovens and RCD's
Topic Summary: Nuisance tripping
Created On: 14 November 2017 01:46 PM
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 14 November 2017 01:46 PM
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mikejumper

Posts: 2451
Joined: 14 December 2006

Is there really always something wrong with the oven when nuisance tripping occurs or are ovens just prone to go leaky over time?

With a view to minimising nuisance tripping should we always consider putting them on an RCBO in a dual RCD unit for example so that no other circuits are contributing to earth leakage? Does anyone do this?

A 4 year old Electrolux dual oven in pristine condition (EOD 3410 AOX) I looked at yesterday is already causing nuisance tripping and it's only been used 5 or 6 times.
 14 November 2017 01:53 PM
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Christofer

Posts: 11
Joined: 11 September 2017

"A 4 year old Electrolux dual oven in pristine condition (EOD 3410 AOX) I looked at yesterday is already causing nuisance tripping and it's only been used 5 or 6 times. "

Hi

That's possibly the issue, from my experience the elements are pretty good at absorbing moisture, so when not used for a while leakage occurs.
 14 November 2017 02:00 PM
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mikejumper

Posts: 2451
Joined: 14 December 2006

Thanks Christofer, do you have a way of dealing with that?
 14 November 2017 02:11 PM
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dustydazzler

Posts: 1449
Joined: 19 January 2016

We have a neff single oven at home which is fine like 90% of the time.
But every now and then when you switch it off at the oven after using it , it trips out the board rcd..
I would say maybe 1 in 10 uses it trips out the rcd.

My sister has an immersion heater that is similar. It works fine nearly all the time. But once in a while it bangs out the house rcd.

Spooky
 14 November 2017 03:22 PM
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mapj1

Posts: 9568
Joined: 22 July 2004

It is down to the high temperature insulation, which is a powdery mineral and exactly like the old pyrotenax cables, that often struggled on the meggar testing, being rather prone to absorbing moisture by diffusion up from the ends.
There is less likely to be trouble if the heating element gets above a hundred degrees once a week as this will drive off the water.
Ovens and cookers can have the additional problem of things like gravy spilt into the works, and sometimes covers off and a good clean helps.

I have known folk to run the things with the earth off for a few minutes to get over the tripping problem when recommissioning a cooker in a deserted property or similar but that is NOT a practice to be recommended, as if there is enough leakage to trip an RCD, it is also enough to kill.

A better solution would be to have the thing on its own RCD,or to dry it out with some other source of heat first.

Tripping on switch off suggest either switch arc, or a switch where neutral breaks firsts, and there is some earth leakage is at or near the neutrally end, so only sees a low voltage until during switching when it sees the full supply voltage.

-------------------------
regards Mike
 14 November 2017 03:31 PM
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broadgage

Posts: 2435
Joined: 07 August 2007

Originally posted by: mikejumper

Thanks Christofer, do you have a way of dealing with that?


There are several ways, most of them a bit dubious from the electrical safety point of view, but arguably acceptable short term under skilled supervision.

1) Disconnect the CPC at the oven, and run it for perhaps an hour to dry out. Take great care that no one can touch the oven whilst doing this, and also make sure that no parallel earth paths exist..

2) Disconnect the oven CPC and connect the frame of the appliance to neutral, again a bit risky and great care should be taken.

3) Disconnect phase and neutral from the public supply, but leave the CPC connected and run the oven from a portable generator, the output of which is floating WRT to true earth.

4) Supply the oven from an isolating transformer with a floating output, until it had dried out.

5) With oven turned off, attempt to heat and dry it by use of an electric hot air gun. DO NOT use a gas blowlamp as the combustion of the gas produces water vapour.
 14 November 2017 03:48 PM
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mapj1

Posts: 9568
Joined: 22 July 2004

Unless you are on a TT supply, there is a better option than lifting the CPC or neutralling the chassis, that is less effort and cost than isolating transformers.
An lead of suitably heavy rating, run surface wired, i.e. over the floor, direct from the CU to the offending load, not RCD protected. let it run nice and hot for 15 mins, and then reconnect and see if now happy.

-------------------------
regards Mike
 14 November 2017 04:50 PM
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mikejumper

Posts: 2451
Joined: 14 December 2006

It's a PME earth.
House is 4 years old (oven came with it)
Dual RCD CU fitted. (No non-protected ways available)
 14 November 2017 05:17 PM
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mapj1

Posts: 9568
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Its quite possible the oven is perfectly in spec, just that a small rise in leakage through the element is the 'last straw' as far as the RCD is concerned if there is already a lot of stuff on it from other circuits.
Changing the element will probably also fix it, for a while, but if it really is only used once every 3 months or something, a repeat is likely.. Ideally you need to trick it into staying on for long enough to get it hot and see if it then works on the RCD. I presume you cannot easily measure the leakage current as it is always off..
Pity about the lack of spare ways really.

-------------------------
regards Mike
 14 November 2017 05:48 PM
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davezawadi

Posts: 3905
Joined: 26 June 2002

There is the option of not RCDing it at all if it is not fed form a plug and socket with a TNC-S supply. RCDs with everything seems to be a modern trend which has a downside, and does not increase safety for fixed items. (see comments on RCDs on SWA post). The same goes for immersion heaters, there really is no point as long as the heater and tank have a satisfactory earth, unless the earthing method is TT when you obviously need to RCD everything.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 14 November 2017 10:45 PM
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sparkingchip

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The first job of the day was to find out why the RCD safety switch tripped when the cooker was being used.
A loose neutral connection had caused a terminal to overheat in the fuse board neutral bar and the plastic support melted, allowing an electrical fault to develop between the two neutral bars. This fault caused the RCD safety switch to trip when the cooker was being used due to its heavy load increasing the current through the fault.

The cooker itself seems in tip top condition, despite the RCD tripping when it was being used.

Andy Betteridge
 15 November 2017 01:33 AM
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MHRestorations

Posts: 40
Joined: 22 October 2017

Magnesium Oxide insulation tends to behave in this odd way... MICC cable used to be (maybe still is?) notorious for breaking down under any voltage spikes. (BICC used to sell surge protectors for their L series cable when used with fluorescent lights to prevent this). In the days before RCDs,/RCBO's, the breakdown could destroy the cable. Maybe the one time in 10 dustydazzler mentioned was due to the point at the half cycle the load was switched on?
 15 November 2017 09:15 AM
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mikejumper

Posts: 2451
Joined: 14 December 2006

Thanks all.

Looks like it's going to be worthwhile getting the oven heated up to see if moisture in the elements is causing the problem, this is not difficult to do as there is a DP isolating switch next to it where I can isolate the earth.

Mike
 15 November 2017 09:26 AM
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mikejumper

Posts: 2451
Joined: 14 December 2006

Originally posted by: mapj1
I presume you cannot easily measure the leakage current as it is always off..

It ran for a minute or so from cold then tripped the RCD.
Not difficult to see an indication leakage, I put an ammeter in series with the earth at the DP switch adjacent to the oven. Oven is on it's own radial circuit.
 15 November 2017 09:41 AM
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ady1

Posts: 832
Joined: 19 April 2005

HI
Had a similar thing where the oven had not been used for months and we called an engineer. He dissed the cpc, ran it for 15mins re-connected - All fine. . Standard trick - he said.
Regards
Ady

-------------------------
Resistance is futile.
 15 November 2017 06:18 PM
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alancapon

Posts: 6915
Joined: 27 December 2005

I had a similar thing with a dual fuel cooker. It had an electric warming plate on the top next to the burners. The wires had detached from the element and were sitting on the gas pipe work. . .

Regards,

Alan.
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