pizza oven elements

[Richard]

Fitted a supply for a 4.5kw 3 element pizza oven. I'll add at this point, a second hand pizza oven from a pizza shop.

Turned out the oven was faulty in that it kept tripping it's 30ma RCD after running for a couple of mins.

I opened up the panel and isolated each element. All three elements showed approx 0.02Mohm to earth. I could imagine one or even two being faulty, but it seems a strange coincidence that all three should be faulty. To look at them they look fine, and the oven looks tidy.

It had crossed my mind that maybe it was sold faulty, as I guess pizza shops go through quite a few elements and maybe they banged 3 dodgy ones in this oven and flogged it.

Anyway what i'm asking, so far as i'm aware, I think these elements should have a far higher value IR then that?

Thanks

 

[Marvo]

Yeah, I would expect to see elements at least 5 Megs at 1000v and even that is on the low side.

Are you sure the elements were isolated from each other ie all the wiring removed during the IR tests?

Machinery thats been standing for long periods can develop low IR characteristics due to the hygroscopic qualities of the packing material used in the element tube. This would improve if the elements are run at full temperature for several minutes or baked awhile in an oven.

 

[Rockingit]

Erm, are you sure you're not mixing your measurements/testing here? Each individual element should have a resistance of 1500w/230v = 6.5A = 230/6.5 = 35 Ohms or thereabouts.

0.02M = 200 Ohms. To consistantly get the same value across three 'duff' elements would be a strange coincidence indeed. If you test them at 250V, is the value the same? They might simply not like having 500V up them.

 

[Richard]

Erm, are you sure you're not mixing your measurements/testing here? Each individual element should have a resistance of 1500w/230v = 6.5A = 230/6.5 = 35 Ohms or thereabouts.

0.02M = 200 Ohms. To consistantly get the same value across three 'duff' elements would be a strange coincidence indeed. If you test them at 250V, is the value the same? They might simply not like having 500V up them.

Hello, and thanks both for your replys!

Yes, deffo isolated from each other. To be honest, I only megged them at 250V. Didn't see the reason to go any higher!

Your right, I tested the element on low ohms and got about 35ohms odd.

I have heard of baking them, but i'd be worried i could just be prolonging a problem....?

Anyway, I'm not actually responsible for the oven, so i'm only advising them.

 

[telectrix]

you could try isolating 2 elements and powering up the 3rd. if that's ok, try the others on their own

 

[Richard]

Machinery thats been standing for long periods can develop low IR characteristics due to the hygroscopic qualities of the packing material used in the element tube. This would improve if the elements are run at full temperature for several minutes or baked awhile in an oven.

Do you think there's merit it trying this?

To look at them, the elements look fine so possibly this could be worth trying.

 

[Rockingit]

If you're in controlled conditions, try firing it up for a few minutes without the RCD, let the elements run flat-out then do the test again.

 

[Deleted member 26818]

These elements are often known as black iron elements. They're used in a variety of situations, cookers, ovens, storage heaters, vacume packing machines, etc.
They're well known for suffering from damp when not used for long periods, or even for short periods. Even brand new cookers suffer with this.
I would try heating them up, without the RCD, and then consider whether the RCD is actually required.
You would need to look at the elements when they are heating up, to see if there are any bright spots, which would indicate that the element is on it's way out.