Can a glow plug has no continuity & resistance but still short a circuit?

[Dougie Reid]

I have an original 250w glow plug in my 8yr old pellet boiler that's used to light the pellets during the start up phase on my boiler. The boiler has been tripping the fuse on around 50-60% of the start-up cycles. Using a bit of trial & error I think I've narrowed it down to the glow plug However, the following is confusing me: when I test the resistance of the glow plug using my multimeter I get a figure of around 211 ohms & no continuity. The voltage that goes to the glow plug when the boiler is simply switched on at the mains is around 30v, & increases significantly when I press the start cycle. All seems good yeah?
However, when I connect the glow plug & try to turn on the power to the boiler, the fuse trips. If the glow plug isn't connected the boiler, it switches on just fine (lcd display etc, not a full start cycle) & stays on. I tested the glow plug for continuity both whilst fitted & out of the boiler but there's no continuity!
How can the glow plug trip the fuse when it has resistance & no continuity? Could the glow plug have continuity & short out with the voltage applied on powering on but show resistance & no continuity with the tiny current from a voltmeter? Is this what could happen if has broken down inside. It's 8 yrs old after all. Thanks

 

[snowhead]

It's a heating element so it will have resistance but not continuity of zero ohms.

BUT has it got resistance or continuity between either of the 2 wires in the glowplug and to the metal body which will be connected to earth when fitted.

 

[Dougie Reid]

Hey, thanks for your reply. I hadn't checked for continuity between the wires & the housing. Just have though, no continuity or resistance if I touch one wire plus the housing. Same result with both wires but again, i'm getting resistance between the wires to the glow plug, thanks again

 

[mainline]

I have an original 250w glow plug in my 8yr old pellet boiler that's used to light the pellets during the start up phase on my boiler. The boiler has been tripping the fuse on around 50-60% of the start-up cycles. Using a bit of trial & error I think I've narrowed it down to the glow plug However, the following is confusing me: when I test the resistance of the glow plug using my multimeter I get a figure of around 211 ohms & no continuity. The voltage that goes to the glow plug when the boiler is simply switched on at the mains is around 30v, & increases significantly when I press the start cycle. All seems good yeah?
However, when I connect the glow plug & try to turn on the power to the boiler, the fuse trips. If the glow plug isn't connected the boiler, it switches on just fine (lcd display etc, not a full start cycle) & stays on. I tested the glow plug for continuity both whilst fitted & out of the boiler but there's no continuity!
How can the glow plug trip the fuse when it has resistance & no continuity? Could the glow plug have continuity & short out with the voltage applied on powering on but show resistance & no continuity with the tiny current from a voltmeter? Is this what could happen if has broken down inside. It's 8 yrs old after all. Thanks

Have you tested the voltage with the glow plug disconnected from its supply and the boiler started up, you should get around 230 volts.
211 ohms sounds about right.
Is it tripping the MCB or RCD ? Or have you rcbo ?

 

[Dougie Reid]

Hi, thanks for your reply. Yes I tested the supply & it's around 30v with just the power on. When I pressed start to run a heating cycle, the voltage increased as expected. The MCB to the shed trips. The boiler is around 8yrs old 7 has the original glow plug in it. At first, the failed start ups were occasional & got more & more frequent. I took the glow plug out today to take it into town for a replacement. I've ordered a new one but when I came home & put the old one back in for tomorrow's attempt, it's immediately tripping which is wasn't doing before today. Cheers

 

[mainline]

Hi, thanks for your reply. Yes I tested the supply & it's around 30v with just the power on. When I pressed start to run a heating cycle, the voltage increased as expected. The MCB to the shed trips. The boiler is around 8yrs old 7 has the original glow plug in it. At first, the failed start ups were occasional & got more & more frequent. I took the glow plug out today to take it into town for a replacement. I've ordered a new one but when I came home & put the old one back in for tomorrow's attempt, it's immediately tripping which is wasn't doing before today. Cheers

Have you got the model number or a pic of it.
It does sound like the element has failed, though.

 

[Dougie Reid]

So, it trips the breaker when I switch the boiler on at the mains when the glow plug is fitted. It doesn't trip when I have the glow plug disconnected. I have resistance through the glow plug but not continuity. and between the plug lead(s) & earth, no resistance or continuity. I've ordered a new glow plug but worried I'll wait 3 days to then find out it trips with a new glow plug as well. I've physically not changed anything to the boiler at all. It's worked for 8 years & the 'permanent' tripping only started today when I refitted the glow plug after having it out all day trying to locate a new one in town. Thanks

 

[Dougie Reid]

 

[mainline]

Thanks.

Apart from trying an Insulation resistance test (which I doubt you would have) is to actually wire it to a test supply, which I wouldn't recommend.

It's highly likely to be the element shorting as it warms up, and since disturbing it has made it worse.

 

[Dougie Reid]

Thanks.

Apart from trying an Insulation resistance test (which I doubt you would have) is to actually wire it to a test supply, which I wouldn't recommend.

It's highly likely to be the element shorting as it warms up, and since disturbing it has made it worse.

Thanks Mainline, that's what I'm hoping is wrong but it threw me a bit after googling & finding lots of reports that resistors rarely short when they fail. I really can't work out how it can be anything else though. Cheers

 

[mainline]

Thanks Mainline, that's what I'm hoping is wrong but it threw me a bit after googling & finding lots of reports that resistors rarely short when they fail. I really can't work out how it can be anything else though. Cheers

Resistors aren't covered in an earthed metal jacket though

 

[Dougie Reid]

Ok...fingers crossed a new glow plug fixes the issue. Cheers

 

[Lucien Nunes]

I agree that the element is probably shorting to the case or between coils as it heats or its insulation is breaking down when 230V is applied. But, the element itself has continuity if you are reading the correct resistance on a resistance range. Your meter might not beep to show that continuity, because its continuity test function is not designed for testing elements.

A continuity test on a multimeter is simply a resistance test* with a threshold decided by the maker of the multimeter. If the resistance is below the threshold it beeps, if it's above the threshold, it doesn't. The threshold is often a few hundred ohms, in which case it will beep and show continuity through any piece of cable, contacts, connectors, anything that is designed to carry current but not have significant resistance.

But, when testing a load device like an element that is supposed to have resistance, we would say it 'has continuity' provided its circuit is not broken, whatever the intended resistance might be. If the element is above a few hundred ohms, the multimeter won't beep because that range is intended for testing the continuity of wiring rather than elements, but the element still has continuity. We would say it didn't have continuity if instead the resistance reading was infinity, or at least abnormally high, making it clear that the circuit is broken somewhere.

*On continuity range, instead of showing the resistance, some meters display the voltage drop between the probes in mV as this can be used to test transistors and diodes, but the principle is the same.