UK Oven element insulation resistance readings

[andrewjameshoward]

I have a dual grill element from an electric oven, one circuit of which trips the power when switched on. I have removed this to replace it and have since tested it with an insulation resistance tester. For the good circuit, I get a reading of 500MegOhms, for the supposed "bad" one, 20MegOhms. I am puzzled by this since (a) I read on the internet that element insulation resistance readings are OK if above 1 MegOhm and (b) my calculations suggest to me that 20MegOhm is nowhere near low enough resistance to produce sufficient current to trip an RCD.

Now it gets more puzzling. I bought a replacement element and fitted it. Now the oven trips on the other of the 2 grill circuits. What's more, it trips not just when that grill circuit is turned on, but when any other oven circuit is turned on. I think this is easiest enough to explain - the fault on the new element, if there is one, must be towards the neutral, whereas the fault on the old one was towards the live.

Before I attempt to get the new element replaced, I need to establish that there is actually something wrong with it and thus understand what is an acceptable reading for insulation resistance.

The live to neutral resistance of both circuits on both elements checks out fine against their power ratings - no open or short circuits.

 

[Avo Mk8]

I have a dual grill element from an electric oven, one circuit of which trips the power when switched on. I have removed this to replace it and have since tested it with an insulation resistance tester. For the good circuit, I get a reading of 500MegOhms, for the supposed "bad" one, 20MegOhms. I am puzzled by this since (a) I read on the internet that element insulation resistance readings are OK if above 1 MegOhm and (b) my calculations suggest to me that 20MegOhm is nowhere near low enough resistance to produce sufficient current to trip an RCD.

Now it gets more puzzling. I bought a replacement element and fitted it. Now the oven trips on the other of the 2 grill circuits. What's more, it trips not just when that grill circuit is turned on, but when any other oven circuit is turned on. I think this is easiest enough to explain - the fault on the new element, if there is one, must be towards the neutral, whereas the fault on the old one was towards the live.

Before I attempt to get the new element replaced, I need to establish that there is actually something wrong with it and thus understand what is an acceptable reading for insulation resistance.

The live to neutral resistance of both circuits on both elements checks out fine against their power ratings - no open or short circuits.

A few questions if you don't mind.
1. Is the cooker new, or has it been in service with the grill working, and tripping has suddenly started?
2. Is it a residual current device that is tripping?
3. What voltage is your IR tester using during the measurement?

 

[James]

Heating elements are a little tricky to test with any degree of certainty.

The problem is, you can test them when cold and all is good, energise them and the heat makes things expand, move and generally mess things up!

I have in the past, bench tested them using an isolating transformer and an earth clamp meter.
the results are often considerably different than you would expect from doing tests when cold.

I would not recommend bench testing unless you have the right equipment and an understanding of how to do it safely.

it may be possible for you to disconnect all the elements (both live and N) then connect just one element.

let it heat up and if possible measure the earth leakage by putting an earth leakage clamp meter round both the live and N cables.

note the results for each element in turn.

if none of them appear to have a large leakage then swap the polarity to each element and repeat the test.

It is also worth noting that a new element can sometimes cause issues with leakage if it has been stored for a long time as they can absorb moisture. so if possible, before going to all the effort of testing, leave the new element switched on for a couple of hours to drive out the moisture.

I am making some assumptions about your competence based on the fact you own/have an insulation tester.
this is not a device in most diy toolboxes so I am assuming you know how to respect electricity and work safely with it for both live and dead testing.
if this is not the case then i would suggest that you get someone qualified to come and assist.

 

[andrewjameshoward]

Hi

Thanks for your thoughts. The tripping takes place instantaneously, so it is not a question of expansion on heating altering the electrical properties. I'm still in the dark as to why a 20 Megohm resistance produces enough leakage current to trip the RCD. This is in respect of the old element which worked for several years in a frequently heated oven and thus also, is unlikely to have taken in moisture. Then in respect of the new element - I have read that they can take in moisture whilst in storage - but if this is the case, how do you leave them switched on for a couple of hours when they trip the power immediately?

 

[Avo Mk8]

I'm still in the dark as to why a 20 Megohm resistance produces enough leakage current to trip the RCD.

It wouldn't. The fault impedance leads to the current in the circuit that trips the breaker.
As James mentions, that may vary depending on the temperature of the element etc.
If the element 'resistance' was measured by a multimeter (using low voltage DC), that might not be representative of what leakage current flows when 240V AC is applied and the element is hot.
Even an MFT measurement at 500V would be DC, would not measure 'impedance', so would not predict the fault current exactly, though I can't imagine the capacitive or inductive components would be very large!
I would suggest the only sure way of establishing what current is flowing is to measure it directly when the element is hot, eg by the method described by James.

To bake out the leakage, if that's possible, I think you would need an isolating transformer, a Variac or equivalent, or a non-rcd-protected source of power!

 

[andrewjameshoward]

What you suggest could apply to the fault with the old element. The new element trips immediately though, ie when cold. It trips with only the neutral connected. I have not tested the insulation resistance of the new element and I think this is a missing jigsaw piece in the puzzle atm so I will do that next week, when it will be convenient to take the back of the oven again.

 

[mainline]

I have a dual grill element from an electric oven, one circuit of which trips the power when switched on. I have removed this to replace it and have since tested it with an insulation resistance tester. For the good circuit, I get a reading of 500MegOhms, for the supposed "bad" one, 20MegOhms. I am puzzled by this since (a) I read on the internet that element insulation resistance readings are OK if above 1 MegOhm and (b) my calculations suggest to me that 20MegOhm is nowhere near low enough resistance to produce sufficient current to trip an RCD.

Now it gets more puzzling. I bought a replacement element and fitted it. Now the oven trips on the other of the 2 grill circuits. What's more, it trips not just when that grill circuit is turned on, but when any other oven circuit is turned on. I think this is easiest enough to explain - the fault on the new element, if there is one, must be towards the neutral, whereas the fault on the old one was towards the live.

Before I attempt to get the new element replaced, I need to establish that there is actually something wrong with it and thus understand what is an acceptable reading for insulation resistance.

The live to neutral resistance of both circuits on both elements checks out fine against their power ratings - no open or short circuits.

Other circuits now tripping suggests that there is earth leakage but not enough to trip until other elements are switched into play.

I would measure the total leakage and see if switching individual circuits make a difference this would may also give an idea of the trip current of the rcd.
Having a decent Mft and clamp meter would be your friend in this circumstance.