Oven element curiosity

[timhoward]

A strange small job this afternoon.
Stoves oven : another sparks had proclaimed the element was faulty and causing the RCD to trip. Householder had already purchased a new element. The original sparks has since been unwell. Slight suspicion crept in that I'd find wires dropped down the back, or that the whole range needed pulling out.
But no, I turn up, remove old one, test it, and get 27.6 ohms, and open circuit from both terminals to earth.
I test new one, and get the same.
At this point I share my doubts with customer, and ask if I can have a nose at the dual RCD consumer unit, suspecting a N-E fault that the oven is driving current through. Unfortunately the consumer unit had been boxed in to the extent I couldn't get the front off.
We agree I'll change the element anyway which on the face of it seemed to work. No sign of tripping after 5 minutes.
It then occurs to me I can ramp test the RCD from a socket on the same side of the board, and do this - it took 25ma to trip it.
So at this point there is no sign of fault at all, and 5ma leakage, giving me some confidence that it's not about to trip.

Can an element measure a normal resistance and still be faulty?!
If not it must be something that is on a plug top or downstream of a double pole isolator that is the actual problem.
I have a feeling I haven't heard the last of this one.

 

[sxdave]

A strange small job this afternoon.
Stoves oven : another sparks had proclaimed the element was faulty and causing the RCD to trip. Householder had already purchased a new element. The original sparks has since been unwell. Slight suspicion crept in that I'd find wires dropped down the back, or that the whole range needed pulling out.
But no, I turn up, remove old one, test it, and get 27.6 ohms, and open circuit from both terminals to earth.
I test new one, and get the same.
At this point I share my doubts with customer, and ask if I can have a nose at the dual RCD consumer unit, suspecting a N-E fault that the oven is driving current through. Unfortunately the consumer unit had been boxed in to the extent I couldn't get the front off.
We agree I'll change the element anyway which on the face of it seemed to work. No sign of tripping after 5 minutes.
It then occurs to me I can ramp test the RCD from a socket on the same side of the board, and do this - it took 25ma to trip it.
So at this point there is no sign of fault at all, and 5ma leakage, giving me some confidence that it's not about to trip.

Can an element measure a normal resistance and still be faulty?!
If not it must be something that is on a plug top or downstream of a double pole isolator that is the actual problem.
I have a feeling I haven't heard the last of this one.

I would think the element could be faulty and still have the expected resistance between L & N terminals as the fault to trip the RCD is a leakage/failure of insulation resistance to E/CPC, not the resistance of the element itself. Having said this I do not know how probable this is.

 

[DPG]

Could it have dried out since the electrician called?

 

[timhoward]

I would think the element could be faulty and still have the expected resistance between L & N terminals as the fault to trip the RCD is a leakage/failure of insulation resistance to E/CPC, not the resistance of the element itself. Having said this I do not know how probable this is.

That's an excellent point. I should have IR tested the old one to earth, I only did a low current test for that.
Maybe I did fix it!

Could it have dried out since the electrician called?

Very possible, it's apparently taken weeks for them to give up and call someone else (me).

 

[Pretty Mouth]

That's an excellent point. I should have IR tested the old one to earth, I only did a low current test for that.

I changed a bottom oven element a couple of weeks back. The existing one gave the correct resistance across the element, and lowish, but not RCD-trippingly-low IR to earth, about 0.25 M ohms, while cold. Once warm, the IR dropped right down to 0.00 M ohms, tripping the RCD.

 

[timhoward]

I changed a bottom oven element a couple of weeks back. The existing one gave the correct resistance across the element, and lowish, but not RCD-trippingly-low IR to earth, about 0.25 M ohms, while cold. Once warm, the IR dropped right down to 0.00 M ohms, tripping the RCD.

Thanks, good to know.

 

[timhoward]

As a bonus this small 30 minute job has unexpectedly led directly to being asked for several EICRs in an apartment block managed by the uncle of the guy living there. I've found before that small jobs in leafy suburbs are generally worth doing!

 

[brianmoooore]

It's not only not unusual for an oven element to trip an RCD and still have the correct L-N resistance, but it's also quite common for a new element to also trip it out.
The cure for this is to bench power the element for a few minutes, either on a non RCD protected supply, or by not connecting the earth terminal. Has to be done under safe conditions of course, with no one else having access.

 

[123]

Might find that the element needs to heat up and expand slightly for it to go into fault conditions, while still testing out totally fine when cold.

 

[Lucien Nunes]

The insulation resistance of a leaky element varies widely with temperature and time, as the moisture causing the leakage moves around inside it. An element can read quite good for IR when cold, but as soon as it is powered up and starts to heat, the moisture evaporates from along the length of the insulant powder fill and condenses at the very ends on the cold leadouts. With the moisure concentrated into a small area, the insulation resistance plummets and will often cause a trip at this point. If the element is left on e.g. by not being RCD protected, as the ends warm up the moisture disperses back into the bulk of the insulant and the IR climbs again, possibly to a high figure. Some will begin to escape through the leadouts; the longer it is left on, the better the IR will be at the next temperature cycle.

So the tripping can depend not only on the temperature but on the recent history of usage conditions. A single IR measurement cold isn't guaranteed to show up a fault although it often will.