Heating element earth trip

[Alex1M6]

Hi,

Can a heating elements resistance to earth depend on its temperature? As I have one here that trips the RCD but when checking the resistance cold there is around 1.5M ohm-7M ohm to earth which shouldn't trip a 30mA RCD. Putting pressure on the element doesn't change it and there aren't any obvious burnt spots where arcing could have occurred.

Could the element still be faulty but only when it heats up? This is from a Kenwood bread maker. Between the element terminals it reads about 90 ohms so it appears to be intact, maybe the elements resistance wire only makes contact with the outside when it heats up.

Thanks.

 

[polo1]

Simply, yes!

 

[Alex1M6]

Right thanks. So faulty elements don't always show up with a standard multimeter resistance check?

 

[telectrix]

what he said.^^^^^. try heating it up , either till the RCD trips, or temp. bupass the RCD then megger it. go to 1000V if necessary. only other way is to swap the element then see if the fault is cured.

 

[ImpededLoop]

As the element heats up the metalic parts expand which can then result in a fault occuring that wasn't present when the element was cold.

 

[tazz]

A multimeter is no good for testing here, you need to use a megga for insulation

 

[Lucien Nunes]

Another common problem is that any moisture dispersed through the insulant can condense at the cold ends when the element is first heated. You get a very sudden drop in insulation resistance that slowly rises again as the moisture is driven off. But seeing as it is the bread that should rise and not the IR, sounds like you need a new one.

 

[Deleted member 26818]

Black iron heating elements a well known for causing RCDs to trip.
Because of this, I don't like to use RCDs to protect cookers or storage heaters.
Often the fault can be sorted just by re-setting the RCD a few times.
In some instances (especially if the element has not been used for some time), it may be necessary to by pass the RCD, until the element has dried out.
Another option is to use an IR tester, hold the button down until the moisture is boiled off by the voltage used by the tester (the reading should improve as the button is held down).

 

[Marvo]

If the IR is a problem when it heats up you can try baking the element at 300 Celsius in an oven for 12-24hrs. If there's a silicon seal around the ends of the tube remove them first and reseal after baking. If that doesn't improve the IR then I'd replace it with a new one.

 

[Micklane]

Disconnect the earth temporarily, and make sure there's nobody else near, plug the unit in and heat for a few minutes. Replace the earth and test IR to earth.

Regards

 

[Badged01]

...... bupass the RCD then megger it. go to 1000V if necessary .....

...... you need to use a megga for insulation

Can we use the correct terminology gentlemen, as intimated below by Mr Spinlondon. There are DIY users on the forum who can be easily confused by such statements ...... not to mention the trainees and 'old' sweats :biggrin:

.......Another option is to use an IR tester ......

 

[Engineer54]

If the IR is a problem when it heats up you can try baking the element at 300 Celsius in an oven for 12-24hrs. If there's a silicon seal around the ends of the tube remove them first and reseal after baking. If that doesn't improve the IR then I'd replace it with a new one.

It'll be cheaper to go and buy a new element than to run an oven at 300C for 24 hours!! lol!!

 

[Marvo]

You're probably right with the price of electricity nowadays we tend to work with batches of elements rather than a single tube. We also use an oxy-acetelene torch sometimes as a short cut but it's a bit hit and miss.

 

[PEG]

You're probably right with the price of electricity nowadays we tend to work with batches of elements rather than a single tube. We also use an oxy-acetelene torch sometimes as a short cut but it's a bit hit and miss.

My mum gets very cross,when i fix her bread-maker,as the rubber tyres on my oxy/acetylene trolley,leave marks on the kitchen floor...:conehead:

 

[DPG]

Can we use the correct terminology gentlemen, as intimated below by Mr Spinlondon. There are DIY users on the forum who can be easily confused by such statements ...... not to mention the trainees and 'old' sweats :biggrin:

I hope you never use the term 'Hoover' then, or 'Tannoy'. Daz

 

[telectrix]

wids my lady up when she asks me to get the hoover and i ask "do you mean the kirby or the vax. we ain't got a hoover.?"

 

[Alex1M6]

Thanks for the replies and advice everyone.

Well I don't have a megger so this is what I've done.

I removed everything else from the bread maker (yes PEG, it is my mothers lol) and then using a 48v DC power supply let it slowly heat up. After a minute I felt the heat rising so I disconnected power and put my ohm meter across one of the terminals and earth tab.

There was now less than 50kohm between them, when I leave it to cool down it slowly creeps back up. If its dropping this low with just under 25 watts of heat (48v/91 ohm=0.52 amp*48v=24.96 watts) then I guess it must be dropping low enough to trip the RCD when it reaches cooking temperatures.

 

[telectrix]

from that, i'm certain that a IR test would show it's faulting to E. your ohm meter puts about 3V across it. imagine what 500V would show. buy a new element. sorted.

 

[Alex1M6]

Thanks, I'll go ahead and order a new one then. Luckily a replacement element is still cheaply available.

I left it running a bit longer to heat up more and saw less than 5kohm when I put the meter on it, more than enough to trip the RCD.

Just for giggles I placed an LED with a small 9v battery between the earth tab and used one of the terminals of the element as common, and sure enough it lights up nice and bright when the element is hot (the 48v supply is first turned off).

 

[Marvo]

Agreed. If it's got an insulation fault you can see with a multimeter then just replace the element.