Double insulated cooker hood trips RCD

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.

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.

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).

Is it perhaps the switch itself getting carbonised or polluted by gunk from the cooking. Brian

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

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.

Just to cheer you up even more, we've been trying to track down a similar fault for 2 years now.

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.

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))

Our cooker hood is plugged into a 13A socket with a built in RCD and that trips every time we try and change the speed. So we always turn it off move the speed switch and turn on again - clearly not a serious fault!

Peter

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.

Interesting that while it seems a rare issue, it's not unknown.

That way you've got 30mA of allowed leakage per circuit, rather than

There I was thinking finding my faulty oven element was a boring/unsatisfying fix on the grounds it looked in good nick ...

No fault found. Put CU back together. Trips again the following day.

Rinse and repeat.

I had one that gave a flash and a bang on switch on once and the gave all appearances of being dead. Checked the fuse and it was ok. Then realised there were now vaporised tracks on the small PCB the switch was mounted on! Repaired those (this time with a wire that would take the full motor inrush current in every situation!) and all was well again.

Can you get RCBOs that are a straight swap for MCBs? We have a Legrand CU that was installed in the late 1990s.

Yes FSVO, we used NOS MK 6936S which are a straight swap in our older MK CU.

The jury is out on whether the current MK 7936S RCBOs are are straight swap or not.

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.

Or these maybe:

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.