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]

It seems like a lot considering above 1/2 I∆N can trip them

The IR reading on heating elements should be high if there's no fault or damp

 

[SparkySy]

However I checked the earth leakage with a clamp meter, each ovens leaks between 10-25ma depending on what elements are on

Was this tested at the oven end or round the main L-N into the RCD? It does sound a lot but maybe why they are on separate RCDs.

If the element was in use for that long it will not be moisture, that will be long dried up.

I would say it is either a faulty RCD or more likely there is a fault on 1 or more circuits on that RCD and with the leakage from the oven it's enough tracking to earth due to the high load it trips the RCD.
Or it could be accumulated leakage from other things plugged in to the other circuits on that RCD.
Sy

 

[Steve T]

Was this tested at the oven end or round the main L-N into the RCD? It does sound a lot but maybe why they are on separate RCDs.

If the element was in use for that long it will not be moisture, that will be long dried up.

I would say it is either a faulty RCD or more likely there is a fault on 1 or more circuits on that RCD and with the leakage from the oven it's enough tracking to earth due to the high load it trips the RCD.
Or it could be accumulated leakage from other things plugged in to the other circuits on that RCD.
Sy

I clamped the main tails into the CU to check leakage, my 10-25ma reading is just for each oven, not the overall reading. Overall was much higher!

Thats the odd thing, I thought after 1 hour the elements would surely be dry, but appeared to cause a trip anyway, and 3 hours later no more tripping but still the same 10-25ma leakage depending on which elements are in use (for each oven!).

Haven't had time to get the MFT out yet but tomorrow I'll check both rcds and check insulation resistance of the elements. But just based on the earth leakage which seems high, if its moisture causing it that's understandable, but I would think it very unlikely for 6 elements in total to all be faulty!

 

[SparkySy]

Then it is already close if not over the threshold of the RCD at that mA.
Yes they will be well dried out after an hour. And rare that all six would be faulty!

Definitely get the MFT out and check both the RCD's both tripping current and times.
Insulation test all the circuits on that RCD just to be sure it's not current finding its way to a fault on another circuit. Or if your feeling inclined test all the circuits on both RCD's
Also unplug or switch off other circuits on that RCD with the incoming cables to the RCD clamped and see what changes mA wise with each circuit.
I have had dual RCD boards where there has been a lot of IT kit plugged into one circuit resulting in a lot of leakage, it only took the washing machine as well to push the combined leakage over the RCD's threshold!
Sy

 

[littlespark]

How many circuits on each rcd? Could just be cumulative earth leakage from all circuits.

If there’s a fridge, that could cause problems if left for too long.

Remember, unplug appliances, don’t just switch them off

 

[Steve T]

Some testing has revealed a problem: checked the Zs at the oven and got 815ohms! Zs db is similar, it's a TT system but the rod is under the floor so the soil will be very dry - not an effective earth connection. Clearly this needs rectifying.

Both rcds have good trip times, and ramp test at 21ma and 24ma respectively.

Pic of the board and earth rod attached

Leakage with everything i can find turned off/unplugged is around 4ma so some non perfect insulation resistance, but no one circuit adds a significant amount. Highest leaking appliance was the dishwasher with 2ma leakage, the boiler and fridge added maybe 1ma each.

Lighting circuits with no lights on add about 2ma, with all the lights on its another 4ma.

So it seems like it's just cumulative, so don't think theres one appliance or circuit of particular concern.

This morning the ovens are both settled on a more comfortable 5-6ma each, one of them peaked at 10ma, but settled down again, I suppose it must just have been a case of drying them out, perhaps there was moisture somewhere other than the elements.

I asked the neighbours this morning and the house has actually been sat empty for 20 months so enough time for moisture to build up where it shouldn't!

Haven't done any insulation resistance testing just yet, an eicr done a few months ago claims all circuits passed >200meg on a 50+ year old house with plenty of red and black wiring, including on L/N testing, which I highly doubt was carried out due to the number of LED downlights lights that would need to be wired out and stuff in the attic etc.. Also says Zdb was 150ohms, again sounds unlikely would it really drop to around 800ohms in a few months!?

 

[UNG]

Have you done an IR test on the ovens

Given the positioning of the earth rod I do wonder how deep it goes into the ground given that there will be foundations for the wall that will spread beyond the wall

 

[Steve T]

Yep, just done the initially problematic oven now, the worst element of the lot which is the fan element is testing at 1.4Mohm, just about passable! Top and bottom elements are at 16 and 12Mohm. All the elements combined are reading 1.3Mohm.

 

[Steve T]

Yep, just done the initially problematic oven now, the worst element of the lot which is the fan element is testing at 1.4Mohm, just about passable! Top and bottom elements are at 16 and 12Mohm. All the elements combined are reading 1.3Mohm.

But something weird happens where after running all the elements for a few minutes, the insulation resistance drops I've seen it as low as 0.26Mohm, but this is when the oven is off so nothing trips. A few mins later the IR rises up to acceptable levels again.

 

[timhoward]

an eicr done a few months ago claims all circuits passed >200meg on a 50+ year old house with plenty of red and black wiring, including on L/N testing,

I share your scepticism!

But something weird happens where after running all the elements for a few minutes, the insulation resistance drops I've seen it as low as 0.26Mohm, but this is when the oven is off so nothing trips. A few mins later the IR rises up to acceptable levels again.

Heating up the elements tends to drive trapped moisture to both ends, lowering IR. When elements cool down the trapped moisture tends to settle along the length of the element, raising IR. Would that explain anything?

If you didn't have a TT system I'd suggest baking each element from a non-RCD feed for several hours, and if that didn't work resign myself to changing elements or the whole appliance.

Regarding the Zdb of 800 ohms, if it were my house it would be getting a new earth rod and a new RCBO board as a priority, so as to spread the natural leakage between devices. It also makes future testing and fault finding much easier!

 

[Steve T]

I share your scepticism!

Heating up the elements tends to drive trapped moisture to both ends, lowering IR. When elements cool down the trapped moisture tends to settle along the length of the element, raising IR. Would that explain anything?

If you didn't have a TT system I'd suggest baking each element from a non-RCD feed for several hours, and if that didn't work resign myself to changing elements or the whole appliance.

Regarding the Zdb of 800 ohms, if it were my house it would be getting a new earth rod and a new RCBO board as a priority, so as to spread the natural leakage between devices. It also makes future testing and fault finding much easier!

Ah, the moisture settling in different parts of the element depending on temperature sounds like it could be consistent with what I'm measuring! Both ovens are currently fine running on an RCD, the leakage has decreased significantly from yesterday, but IR readings still aren't great. Perhaps they just need more 'baking', that certainly has seemed to improve the situation so far.

Agreed that the earthing issue needs attention ASAP, at the very least I can get another earth rod in, hopefully outside if I can fish a cable under the floor. An rcbo board would definitely be nice too if I can convince my mother to go for it!

I'll check the elements over the next few days and see what happens

 

[pc1966]

If you didn't have a TT system I'd suggest baking each element from a non-RCD feed for several hours, and if that didn't work resign myself to changing elements or the whole appliance.

The OP could get a 100mA or similar RCD fairly cheaply for testing - maybe to become the incomer later so not wasted.

But it really needs the earth rod sorted out!

 

[pc1966]

Agreed that the earthing issue needs attention ASAP, at the very least I can get another earth rod in, hopefully outside if I can fish a cable under the floor. An rcbo board would definitely be nice too if I can convince my mother to go for it!

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

 

[James]

But something weird happens where after running all the elements for a few minutes, the insulation resistance drops I've seen it as low as 0.26Mohm, but this is when the oven is off so nothing trips. A few mins later the IR rises up to acceptable levels again.

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