New cooker still trips rcd. Boggling me.

[Bijlee]

Hi everyone. Im on a job thats really boggling me.

So old freestanding electric cooker was tripping when hot plates are on for about 15-20mins. I did the insulation tests on the cables when disconnected at board. 999+ reading. So I thought ok must be cooker. Estate agents orders new one. I install new one and same thing happens.

So I tested cables again. Wiring is 5years old. Perfect readings all the way from consumer unit to the cooker itself when hot plates are cold.

When I put the hot plates on for 5mins, I test again at the cooker. Resistance drops o about 400ohms. When I have plates on longer about 10mins. Resistance drops to 4-5 ohms.

I have on for longer and eventually rcd trips. I test the resistance at the cooker on each individual plate and it's showing 0.

Same readings I'm getting between the earth to neutral earth to live. Is this normal for hot plates?

I have tested the rcd trip times. I've moved the cooker over to the other rcd, still tripped.

Its a Toolstation axiom dual rcd board. Maybe these rcds can't handle cookers. But then it's been working fine for 5 years. I'm thinking the new cooker is also faulty.

If anyone has any thoughts and ideas please.

 

[Pretty Mouth]

When I put the hot plates on for 5mins, I test again at the cooker. Resistance drops o about 400ohms. When I have plates on longer about 10mins. Resistance drops to 4-5 ohms.

Just want to check, are these numbers are correct, or do you mean M ohms? And what have you measured the resistance between?

 

[Bijlee]

Just want to check, are these numbers are correct, or do you mean M ohms? And what have you measured the resistance between?

Yes sorry they are in my ohms. I've tested between live and earth. And neutral and earth. Both show 0.00 when plates are on for about 15-20mins.

 

[Rockingit]

OK, so just to be sure I'm clear...
1. You've got end to end conductivity down each separate conductor in the supply cable
2. When cold there is acceptable resistance between the three combinations L-N, L-E & N-E
3. When hot, (2) changes.

When it's hot/faulting, where are you measuring the new IR readings from? The conductors with the oven removed or some place on the oven with it still in circuit?

Have you clamp tested the cpc at the board to see what leakage current is happening when it's running?

 

[Avo Mk8]

As per Rockingit above.
Not ignoring it could be the supply wiring, but it's vaguely possible that both the old cooker had a genuine fault, and the new cooker has earth leakage because it's been sitting in a damp warehouse for the last year, so has moisture in the elements. That should eventually bake out after several cycles, maybe after resorting to running one at a time.
When you checked if the rcd tripped with the new cooker, were all the hotplates on together, or were you testing with one or two? Does the oven do the same?

Edit: The topic of moisture in oven elements crops up here quite often. I seem to remember a post by Lucien ages ago explaining that when an element containing moisture is first turned on, the heat drives moisture towards the colder ends of the element, and this results in the insulation resistance falling to start with (ie the higher concentration of moisture at the live end leads to higher leakage current L to E). So far I've failed to find that post I'm afraid.