IR test sets off gas hob igniters - WTF?

[NickD]

As per the title really, IR test @250V DC, (L+N)-E, whole-installation test of domestic property, main isolator switch open (obviously), testing caused the piezo igniters on the kitchen gas hob to spark even though no-one was pressing the knob to ignite them, and failure of IR test at about 150kohm. Switching off the appliance FCUs in the kitchen made it stop, IR test then passed at 2.2Mohm. What the hell's going on there?

 

[malcolmsanford]

Not sure about this model but most sparking units have a connection with the hob frame to complete the circuit and produce the spark. This is why if you energise the spark and touch the case you can get a tingle.

 

[telectrix]

yeah. it's because the spark goes to earth.

 

[Knobhead]

So you’ve shoved 250V DC up an AC appliance and wonder why the readings were strange?

It’s a wonder you’re not buying a new cooker.

Was this an EICR or a specific test on the cooker?

 

[davesparks]

Why were you testing with appliances connected?
Why were you testing at 250V?
Why were you testing the whole lot at once?
What results did you get testing between L and N?
How long did you apply the test for?

 

[Marvo]

Not sure exactly why it caused the igniter to operate but you're taking a big chance IR testing with appliances of any kind still connected. You're lucky the control board and the ignition board is still working TBH.

 

[Broken handle]

Not sure exactly why it caused the igniter to operate but you're taking a big chance IR testing with appliances of any kind still connected. You're lucky the control board and the ignition board is still working TBH.

Tested live & neutral connected together and that connection tested to earth. On circuits previously working fine (i.e. no live - earth reversal) that should not cause a problem, esp at 250 volts.

 

[edexlab]

The op says he's linked L-N and tested to earth I was told to do this by lecturers and Nic said its ok to do it as a global IR on an Eicr
Personally I'll do it on individual circuits this way on an Eicr just in case something is still connected as a way of highlighting any items still connected before I do the 500v test if that makes sense.

 

[Plonker 3]

The op says he's linked L-N and tested to earth I was told to do this by lecturers and Nic said its ok to do it as a global IR on an Eicr
Personally I'll do it on individual circuits this way on an Eicr just in case something is still connected as a way of highlighting any items still connected before I do the 500v test if that makes sense.

Well that's it the gospel has spoken.

Have you thought what if you have equipment that leaks to earth (eg PCs etc) is still connected your test results could be flawed?

 

[trev]

Oh well, if NIC says it's ok then who are we to even consider questioning them.

 

[Broken handle]

Well that's it the gospel has spoken.

Have you thought what if you have equipment that leaks to earth (eg PCs etc) is still connected your test results could be flawed?

Yes, he then has to spend time unplugging items that may have caused dodgy readings and then test again.

 

[edexlab]

Yes I know what your saying and you are correct they should say something to that effect when recommending this method.
But in my experience usually any item such as a pc is connected to the ring in a visible place and you'd be pretty daft not to check the obvious things that may be connected, I did say it was "just in case there are items still connected"
And no I don't believe everything the Niceic tell me but I listed them amongst persons more qualified than me who have given this advice out, I reckon the op got this method from a similar source

 

[edexlab]

Also if you read the post I did say "before I do the 500v test" I wouldn't do it as a sole IR test as many do

 

[jimmymac]

Oh well, if NIC says it's ok then who are we to even consider questioning them.

NIC who? Nic Clegg? St. Nic?

 

[davesparks]

I wonder how much better 250v DC is for an AC appliance than 500v DC?

 

[Plonker 3]

Also if you read the post I did say "before I do the 500v test" I wouldn't do it as a sole IR test as many do

So 250 volts DC won't do any damage to any equipment rated for 230 volt AC use?

 

[edexlab]

Lets be honest here hands up who has done an IR test on a circuit and then realised they've just damaged an item
Oops too late now

 

[Marvo]

I'd say linking the L+N together with appliances still plugged in to do an insulation test to earth is bad practice. Many appliances have electronic control boards and often the boards themselves or their associated power supply board will have surge arrestors with one leg on earth. If your test voltage exceeds their clamping voltage they'll conduct and give you false IR readings. Many motors with speed control inverters also have output filtering circuits that use the earth as a functional circuit and these are also increasingly seen in a domestic environment and could be damaged by the high DC voltages used in IR testing. There may be people that disagree but I'd recommend you disconnect rather than relying on a short circuit L-N to prevent damage.

 

[Plonker 3]

I wonder how much better 250v DC is for an AC appliance than 500v DC?

Just half as much damage lol

 

[edexlab]

Marvo what your saying is true but here we're talking about something which is common practice for domestic installs not commercial or Industrial where judging by the majority of test sheets I've come across the IR testing never happened

 

[edexlab]

It seems to me that people are jumping the gun here a little thinking that I'd deliberately test a circuit with appliances connected if you read my first post you'll see this is not the case, I am very careful and deliberate about testing and take it seriously .
I'm just saying that this is a common practice as it's seen by most (incorrectly) as a quick way to test a circuit/s and as we all know a lot of jobs are tested these days, very quickly and with most of the results made up.

 

[Broken handle]

Lets be honest here hands up who has done an IR test on a circuit and then realised they've just damaged an item
Oops too late now

On more than one occasion I have had a sinking feeling upon realising I have tested a circuit L-N at 500V and not isolated the boiler circuit. On only one occasion did I do some damage and that was to blow a 1A glass fuse on the PCB board.

 

[Lucien Nunes]

What's wrong with doing 500V DC IR tests on appliances with L+N linked? That's exactly what a PAT tester does. Yes, as per Marvo if there are varistors you have to limit the test to 250V or you will get spurious results because they go (harmlessly) into conduction. But otherwise, if something won't stand 500V DC to earth for a few seconds it's not fit for use on any voltage. I routinely poke 500V into expensive, sensitive lab equipment just as I would into washing machines and televisions.

I am not saying anything about doing an EICR with appliances connected - 'global PAT' - but I don't see why people are predicting wholesale damage!

 

[edexlab]

Yup everyone has had that same feeling hopefully on something fairly insignificant mine was the dimmer switch in an" attic" don't ask why
But its never been a problem since as I try to make sure everything is disconnected first , I also check for continuity between conductors first before doing anything else

 

[davesparks]

Lets be honest here hands up who has done an IR test on a circuit and then realised they've just damaged an item
Oops too late now

I've meggered a few appliances by mistake in the past, but never managed to actually damage one yet!

I just do a simple continuity test between L and N to see if there is any load connected that I have missed, it saves shoving a higher DC voltage up them!

 

[edexlab]

Good point Lucien re 500v and Pat testing. ...
However I half wish you hadn't come up with the "global pat test" concept
You just know some idiot will read that and think "ker ching"

 

[Lucien Nunes]

never managed to actually damage one yet!

Me neither. It is almost impossible to damage any sound piece of electrical equipment with 500V to earth, even the ones (and Davesparks will know which model I am referring to) that have a red and white warning label stuck on every module saying 'Do not use high voltage insulation testers on this equipment'. Of course I megger those as well and have been meggering the same ones for 25 years without damage. What they really mean is don't flash test at 2,500V, which is a different kettle of fish.

 

[Knobhead]

Yes I know what your saying and you are correct they should say something to that effect when recommending this method.
But in my experience usually any item such as a pc is connected to the ring in a visible place and you'd be pretty daft not to check the obvious things that may be connected, I did say it was "just in case there are items still connected"
And no I don't believe everything the Niceic tell me but I listed them amongst persons more qualified than me who have given this advice out, I reckon the op got this method from a similar source

Last time I looked, I.E. rescuing my evening meal earlier, the cooker is just a tad bigger than my PC.
How can you miss it?

Hope you recommended a full rewire due to the IR test being low. I need someone to restore my faith in UK tradesmen.

 

[PEG]

What's wrong with doing 500V DC IR tests on appliances with L+N linked? That's exactly what a PAT tester does. Yes, as per Marvo if there are varistors you have to limit the test to 250V or you will get spurious results because they go (harmlessly) into conduction. But otherwise, if something won't stand 500V DC to earth for a few seconds it's not fit for use on any voltage. I routinely poke 500V into expensive, sensitive lab equipment just as I would into washing machines and televisions.

I am not saying anything about doing an EICR with appliances connected - 'global PAT' - but I don't see why people are predicting wholesale damage!

Excellent point,Lucien. I had this argument with a "electrician" some time ago,regarding the devastating destruction wrought upon RCD's and RCBO's by accidentally leaving them connected,i invited him to attempt to "burst",(his words...) a selection of said devices,using my MFT,which has 1000 volt IR test facility. Whilst he was trying,using every permutation of test scenario,i asked just what component would fail. He wasn't sure,and give up,remarking they must be "tougher" than other manufacturers devices. This is NOT an excuse for leaving these devices connected during testing,but an understanding of what and how components fail,can greatly assist testing,and the results obtained. So,thanks Lucien,for saying it isn't witchcraft,it's science! :stooge_curly:

 

[Lucien Nunes]

There are potential snags (pun intended) if you can't be sure of the test conditions. For example, an appliance that has been overlooked plugged into a socket outlet with SP switch that is off, will be subjected to test voltage without its L+N connected together. Under those conditions, if there is a varistor connected L-E then you can subject the live functional parts to a DC voltage, because the varistor completes the circuit Tester - N - guts of appliance - varistor - E - Tester. That could theoretically cause damage but I've never heard of it in practice, because the voltage across the critical bits probably won't be over 200-300V and the current will be very limited.

And what do you know - if that appliance happens to be a hob ignitor (which might run on DC and is a very small load) I bet it would fire the spark gaps!

 

[Marvo]

There are potential snags (pun intended) if you can't be sure of the test conditions. For example, an appliance that has been overlooked plugged into a socket outlet with SP switch that is off, will be subjected to test voltage without its L+N connected together.

Depending on the circumstances borrowed neutrals could also be a problem as well as single pole switched sockets and lighting circuits. Guess we'll agree to differ, I'm not predicting wholesale damage and destruction if you know the risks and fully understand all the possible test paths but given what I've seen on the forum there's a lot of guys who don't and I think it's an unnecessary risk to take especially in this day and age where the LED lamping in a premises could cost a small fortune.

 

[davesparks]

And if they do manage to damage something with an IR test then they will hopefully learn a valuable lesson about their own competence and maybe the client will think twice before choosing teh cheapest price next time!

 

[Guest111]

there is less chance of damaging something by going L+n to earth,notice I said less chance! If something is left connected,lets be honest how many threads have we had regarding strange readings on an IR test with "everything off or disconnected" only to find the "fault" is a neon on an FCU hidden away in a cupboard somewhere.

 

[NickD]

Cor, I didn't expect the Spanish Inquisition.


Job was additional lighting point on upstairs lighting circuit, and a spur to new double socket (protected by RCD FCU) on upstairs sockets. Problem was that the CU internals was a proper stuffed-in spaghetti job - no rhyme or reason to the conductor positions in the earth and neutral bars, and a complete pain in the backside to get any of them out and back in again even if there had been, and risk of doing more harm than good in trying. This will not be that unfamiliar a scenario to anyone who does a sigificant amount of domestic work, I suspect.


So I elected to do whole-installation (L+N combined)-E testing at 250Vdc, to allow for connected equipment, followed by live test for polarity (including switching being of the line conductors) and Zs at the new lights and sockets, plus a Ze test at the CU (earthing conductor out) to check the source of earthing.All of my new wiring I tested at 500V between all conductors with CPCs connected to earth prior to final wire-in to the existing circuits.


Yes obviously if the result is a big fail then you can end up chasing your tail, and functional earths (e.g. on RCD protected sockets/FCUs) and filter networks can cause said big fail, but if it isn't then it's happy days...?


At risk of stating the obvious, testing L+N combined to E at 250Vdc does not inject any voltage between L and N and so nothing is getting supplied L-N with 250Vdc. This is the same as an IR soft PAT test. I wasn't inclined to spend forever running round the house in advance unplugging, switching off FCUs, taking out bulbs, removing neons and dimmers etc. etc. and then missing something anyway. Didn't test L to N as connected loads would have caused test to fail and exposed items to the test voltage L to N.

 

[NickD]

PS I have yet to blow anything up testing L+N combined to E at 250Vdc. In fact the only thing I've ever blown up to my knowledge was an RCD FCU accidentally tested at 500Vdc L to N.

 

[Lucien Nunes]

testing L+N combined to E at 250Vdc does not inject any voltage between L and N and so nothing is getting supplied L-N with 250Vdc.

Not directly, however as per my previous post there can be internal paths via suppression components often from both lives to CPC. If these do not conduct symmetrically, for example when an SP switch defeats the L-N connection at the tester, then DC voltage can develop across L & N.

It's unlikely to cause damage, hence there's no reason not to do IR in a PAT test. The DC current through the appliance is limited by the tester's source resistance, also by one VDR in series with the rogue path to CPC plus there's the opposite VDR shunting the tester from the connected live (probably N if it's an SP switch that's off causing the assymetry) directly to CPC. So it's most unlikely that any damaging voltage would occur, even if the load is vanishingly small as is the ignition generator.

Obviously I can't see the ignition unit in question but I would be surprised if it deliberately passes line current to earth, in which case it could only have been powered by test voltage appearing L-N for a reason such as the above. The fact that the spark gaps use the cooker frame as one terminal is immaterial, because it is a transformer stepup arrangement where the primary is driven by impulses when a cap discharges through a GDT. I would expect the primary to return its current to N, but of course it rectifies the mains and will run on tester DC. Battery powered versions will operate from one AA cell for years of regular use, so you can imagine how little load they present to the mains.

If anyone knows of ignition circuits that do pass primary current to earth, I would be interested to hear how the circuit is arranged.