Double insulated cooker hood trips RCD

Double insulated cooker hood with the usual plastic cased motor is tripping the RCD it's connected to, but only intermittently and only when changing the fan speed.
Thing I don't understand is how? Arcing maybe? I did find a couple of discussion threads, but they were more about new installations. This has worked fine for the last ten years*...
It's definitely not earthed (dire warnings against doing so in the manual) and there is no measurable DC resistance from any metal parts of the unit to mains earth. There's no DC resistance from L OR N of the mains plug to any metal parts either - in case any one asks :)
* At ten years old I'm more than happy to swap it out, if it really is the cause of the problem.
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Lee wrote:

It is not likely to be the problem in itself. More likely to be other things on the same RCD taking the cumulative leakage nearly up to the trip threshold and switching spikes on the hood taking the total over the limit.
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Check for cobwebs. I had a double insulated desk fan trip the RCD. Turned out to be a spider in the plug, which had created a nest bridging the live and (unused) earth pin.
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Andrew Gabriel
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There was a case in a kitchen where slightly carbonised deceased Ants got into a switch causing intermittent problems like this. A wag I knew called it the Pink Panther problem. Deadant deadant, I'm sure you remember the tune.
Brian
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On 28/12/2017 21:35, Lee wrote:

It could just be an indication that you are close to the trip limit through cumulative leakage elsewhere on the circuit. The cooker hood will have an induction motor that may have a reasonably large inrush current. That transient switching can cause enough leakage elsewhere (typically through mains filters on other connected appliances) to nudge the RCD over the limit.
Cable damage elsewhere could be another cause (say a neutral earth short in its supply somewhere).
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Is it perhaps the switch itself getting carbonised or polluted by gunk from the cooking. Brian
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wrote:

The hood may have little to do with it. It could be the result of a Neutral to Earth fault anywhere in the house. This can cause a leakage current just insufficient to trip the RCD. When the cooker hood is switched on or speed changed there is enough (harmless) inductive coupling to surrounding metalwork/cabling to trip the RCD.
A friend once had a house where plugging an extension lead in was fine - but as soon as you walked out with it and put it on the grass (with nothing plugged in) the RCD tripped. The fault was eventually traced to a nail through the upstairs ring which bridged neutral and earth.
Just to cheer your holiday up these faults can take ages to find.
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[18 lines snipped]

Just to cheer you up even more, we've been trying to track down a similar fault for 2 years now.
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On 29/12/2017 11:18, Huge wrote:

Great. So the consensus would appear to be no point in changing the cooker hood because it's something else.
Fair enough. At the aesthetic cost of a bit of trunking I'll put it on a non-RCD circuit and I'll wait until the next time to see if the real culprit shows up. At a no doubt inconvenient time.
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On 29/12/2017 11:41, Lee wrote:

It would probably only take 15 mins to go through each circuit in the CU and do a quick N to E resistance check...
(i.e. turn off MCB, disconnect circuit neutral(s) from bus bar, and measure resistance to earth (of if you have an insulation resistance tester measure N+L joined together to earth))
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On 29/12/2017 12:20, John Rumm wrote:

Did this a few months back when we were chasing an intermittent tripping fault with the oven, which turned out to be a grill element.
Only change made as a result of trying to fault find that issue* was to replace the MCBs with RCBOs, of course that could well be the issue in itself but it's been fine for the several months since then.
*I posted about it tripping the RCD as the oven was turned off. The suspicion from the start was grill element but it never showed up on IR checks, hot or cold. Put cooker on non RCD circuit and the grill element eventually failed in a nice shower of sparks and melted metal :)
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Lee wrote:

That way you've got 30mA of allowed leakage per circuit, rather than 30mA combined for half (or all of) your circuits.

There I was thinking finding my faulty oven element was a boring/unsatisfying fix on the grounds it looked in good nick ...
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On 29/12/2017 12:51, Andy Burns wrote:

should pass half their rated current, but some only just manage that.
So a 30 mA nominal RCD must trip if there's 30 mA leakage. It should just allow 15 mA leakage. Some RCDs can be close to the 15 mA limit so it's best to keep a comfortable margin below 15 mA to avoid nuisance tripping.
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On 29/12/2017 12:51, Andy Burns wrote:

Several people did suggest using a non RCD protected circuit as a test:-)
And that is not the same as the suggestion of disconnecting the earth!
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ARW wrote:

I could have run an extension reel from the shed back to the kitchen, I suppose.
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Can you get RCBOs that are a straight swap for MCBs? We have a Legrand CU that was installed in the late 1990s.
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On 29/12/2017 13:00, Huge wrote:

Yes FSVO, we used NOS MK 6936S which are a straight swap in our older MK CU.
https://uk.rs-online.com/web/p/residential-rcbos/4253719/
The jury is out on whether the current MK 7936S RCBOs are are straight swap or not.
https://uk.rs-online.com/web/p/residential-rcbos/7116001/
I believe they are the same same electrically but they are different mechanically. Some people have said they can be made to fit, others have said they will not.
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Or these maybe:
http://www.cef.co.uk/catalogue/categories/rcbo-s-10ka-sp-b-curve-rcbo-s
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OK, thanks.
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On 29/12/2017 13:00, Huge wrote:

Probably easier and cheaper to swap the CU for an all RCBO installation.
I did see an ad for shortened RCBOs not so long ago that might fit your CU.
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