Lots of earth leakage from ovens

[Steve T]

Hi all,

There's 2 single fan ovens in the house my mother recently moved into. Posh elecronic AEG units. It's been sat empty for a year, so the oven elements will have had a chance to accumulate some moisture. There's a dual RCD consumer unit, the ovens are on separate RCDs. One of the ovens worked fine on all settings from the start, but the other tripped the RCD a few seconds after it was turned on. We reset the RCD and again a few seconds after the oven breaker was turned back on it tripped again. I thought this could be down to moisture in the element. So the next day I managed to get the RCD to stay on for the oven only with all the other breakers off, to get rid of any background earth leakage. I ran the oven with a combination of elements for an hour or so to try and dry them out, didn't trip at all over the period. Then after cooling down and trying to turn it back on (with the other breakers back on as well), the RCD tripped again. So thought it needed more time to dry, repeated the process, more drying time, now it is working fine for the time being, no more trips.

However I checked the earth leakage with a clamp meter, each ovens leaks between 10-25ma depending on what elements are on. Its only the elements that leak, not the cooling fan or fan. So the highest total leakage on the tails I've seen with both ovens on and the fridge/lights/boiler/some sockets on is 56ma. That's basically the limit for the RCDs, 30ma each but of course normally they'll trip lower than that. I'll ramp test them tomorrow.

Just wondering really if that sounds normal at all or excessively high? Also are the ovens likely to leak less as the elements dry fully and get more use? Otherwise (assuming the RCDs are working right) I can definitely see some problems with nuisance tripping occurring !

Thanks for any replies!

 

[Mikegh]

Weird the way the readings go up and down. Learn something new everyday

 

[Steve T]

The high Zdb also makes me wonder if there are bonds to service pipes, unless they are all plastic of course!

Ideally dig down a bit by hand first before driving a rod in, you should find any usual services in less than 1m. One of those fence post shovels, etc, might be worth getting. Once down 1m or so then drive in two of the 1.2m rods to get 2.4m if you can. Linian make SDS bits and drive adaptor for the end to allow driving in to stony ground, etc.

Earth Rod Pro Drill Bit (Pack of 1)

Allows rapid earth rod installation when used with Earth Rod Pro drivers - sold separately [1LERPD] and [1LERPM]

www.cef.co.uk

Earth Rod Pro SDS+ Driver

Allows rapid earth rod installation when used with Earth Rod Pro drill bit - sold separately [1LERPB]

www.cef.co.uk

The gas has literally just been re piped in plastic by Northern gas, the driveway is still dug up, think the flow rate was too low so they needed to put a bigger pipe in, anyway it's plastic. Can't see the incoming water pipe or any bond there might be to it, its hidden under the floor. There are 2 bonding conductors in the CU but possibly both old and disconnected!

Those linian adapters look great will save smashing it in with a sledgehammer

 

[Steve T]

expansion / contraction of the elements or moisture moving around inside the elements?

Must be something like that, really hoping they'll dry up/fix themselves with more use, otherwise that's 6 to replace or 2 new ovens to get! All the elements read lower than they should on IR. They're probably 8 years old, but as mentioned earlier have seen no use at all for the last 20 months.

 

[freddo]

In the shop we used to heat the elements with a blowtorch starting in the centre and slowly working towards the terminals to drive out moisture in elements that had been in the stores for years and failed insulation test after initial normal heating.

 

[Steve T]

Just an update for anyone interested, I got another rod in outside today, which has lowered the Zs db to a more healthy 53ohms. The oven elements IR still fluctuates quite a lot, still occasionally dropping below 1 Mohm after being switched off, but hasn't caused the rcd any trouble, and numbers do appear to be gradually improving, so I'll probably just leave it and hope that continued use will drive any remaining moisture out, or if I can find my blowtorch maybe use that! No visible damage to them anyway. Worst case scenario we will just have to replace the fan element, its the worst by far.