DIY Junction Box for TAB / Insulation Testing

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TLD

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#238043 30-Jun-2018 16:09

I have just got hold of a Digitech QM 1493 insulation tester that I intend using for checking cables and electrical equipment at my Menz Shed. It turns out that TAB testing is not a legal requirement for organisations/groups like a Menz Shed in New Zealand, but we thought we'd show willing and play safe. The tester usually sells for $289 on Jaycar, and there is bugger all information on line for it, so I am guessing it is rebranded. I paid just $103 for it on Trademe, and it still had the original batteries wrapped in plastic, and the leads in the sealed plastic bag.

What I'd like to do, is construct a junction box with a mains socket, and 4mm banana sockets so I can hard wire the tester to the box, and plug in the lead being tested, and switch in the test connections (live/neutral, live/earth, neutral/earth) and the whole thing has to be good for the 1000V test voltage without affecting the test results. So I am wondering if anyone has experience with this sort of voltage, and what switches might be suitable? Maximum test current is 1mA.

Trevor Dennis
Rapaura (near Blenheim)

TLD

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Ultimate Geek

#2046476 30-Jun-2018 16:57

Just thinking since posting this... I don't actually need to use switches. I could use multiple 4mm sockets and plug and unplug the leads to the tester. That would definitely eliminate any chance of issues with the high voltage bridging switch connections and messing up the results. Yes, I might go that way, but I'd love to hear any other ideas?

Trevor Dennis
Rapaura (near Blenheim)

solutionz

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#2046530 30-Jun-2018 19:02

Do you mean PAT testing leads / appliances like?: https://www.portableappliancetesters.co.nz/pac3760dl

Either way yes I've built test rigs for this sort of thing. You'll find most mains rated gear (switches & cable etc) will test in excess of 1000v anyway (obviously not designed for prolonged use at that voltage but ok for a test rig).

Otherwise http://www.krausnaimer.co.nz do some high voltage/current multi pole/throw switches suitable for that purpose but pretty pricey.

solutionz

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#2046532 30-Jun-2018 19:06

The main thing is not to build anything that could possibly be used in a mains outlet providing a hazard (exposed conductor). E.g. Something with a 3-pin plug & banana plugs etc on the other end.

gregmcc

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#2046539 30-Jun-2018 20:09

Be very careful here.

Do you know which standard you are testing to?, AS/NZS3760 - in service safety inspection of electrical equipment - this is testing of appliances etc that are currently in use, the multi-box under your desk, the electric drill you use at work, the dishwasher at home.

or.....AS/NZS5761 :- In service safety inspection and testing - 2nd hand prior to sale - so this would be the used appliance that you are selling, regardless of it been repaired or not

or....AS/NZS5760 :- In service safety inspection and testing - repaired equipment - this would be a repaired item such as a faulty washing machine that has just been repaired prior to it been returned to the customer.

AS/NZS 3760 is not a compulsory standard, BUT is a good way to prove safety (Electrical appliances tested to this standard are deemed safe by the Electrical safety regulations)

not 100% sure on 5760 & 5761

Anyone who wants to test to 3760/5761/5760 must be deemed competent, there are courses for non electricians to attend, electricians are already deemed competent to test.

Judging by they way you have said you want to test, you most likely don't realise that using and insulation resistance tester between phase and neutral on an appliance can easily destroy any sensitive electronic components inside appliances (Variable speed control in an electric drill, battery charger for battery drill), so a bit of training would be the right way to go.

elpenguino

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#2046541 30-Jun-2018 20:13

Don't forget that using an appliance tester only partially meets the requirements under the testing rules.

gregmcc

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#2046542 30-Jun-2018 20:13

Also forgot to add, There are several other tests besides an insulation test that need to be done to pass the test tag standards.

Testing a 230V appliance at 1000V is a sure fire way to kill it.

elpenguino

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#2046557 30-Jun-2018 21:17

Generally true for anything with electronics but 1000v is ok for ur average toaster, electric blanket etc.

The rules say use a higher voltage unless that will kill the device.

gregmcc

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#2046573 30-Jun-2018 21:36

elpenguino: Generally true for anything with electronics but 1000v is ok for ur average toaster, electric blanket etc.

The rules say use a higher voltage unless that will kill the device.

the rules say at least 2 time the nominal RMS voltage.

As far as testing at 1000V on a 230v appliance, typically the insulation rating on wiring inside an appliance would be 600V, by putting 1000V you are over stressing it beyond its rating and damaging it.

Testing a 230V appliance at 1000V is a very bad idea.

SomeoneSomewhere

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#2046582 30-Jun-2018 21:58

gregmcc:

elpenguino: Generally true for anything with electronics but 1000v is ok for ur average toaster, electric blanket etc.

The rules say use a higher voltage unless that will kill the device.

the rules say at least 2 time the nominal RMS voltage.

As far as testing at 1000V on a 230v appliance, typically the insulation rating on wiring inside an appliance would be 600V, by putting 1000V you are over stressing it beyond its rating and damaging it.

Testing a 230V appliance at 1000V is a very bad idea.

Do you have an actual reference for either 2x the nominal voltage or 'highest voltage possible'? 3000 and 3760 both say 500VDC, unless there are surge suppressors bridging the insulation, in which case 250VDC. I haven't seen anything requiring 400V systems to be tested at 1kV. I think it might be a myth left over from old rules or 'good practice', because I have heard it before.

Short-term stressing at 1kV should be fine, though, at least for plastic insulation (not electronics/capacitors etc.). I believe manufacturer's testing is required to be at 3kV, for example.

Testing live-neutral is definitely a no-no, at least at IR voltages. Shorting them together and testing to earth halves the number of IR tests you need to do, and further protects the electronics.

A basic continuity test to verify there's no shorts is fine and probably a good idea, but bear in mind single-digit ohms can be normal for the cold resistance of large lamps.

And finally, a reminder that you need to do an earth continuity test on class 1 appliances.

TLD

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#2046596 30-Jun-2018 22:47

Blimey guys. I was wondering if I'd even get a couple of replies, so stoked to get this sort of response and so much information.

Definitely nothing live connected to any rig. It's only going to be extensions leads, and unplugged appliances like kettles, and workshop equipment.

The box would be a convenience so that the mains plug would not need to be held while holding probes to the terminals.

Thanks for the heads up regarding the different standards. I knew they existed, but had no idea what they were. We initially thought we had an obligation to test ALL leads twice a year, and had been quoted about $10 per item, so probably a couple of $k per year! I asked the lads on the Menz Shed Facebook group, and was told that we were exempt.

(Search for 'How do other sheds go about electrical equipment Tag Testing?' at the link below if interested)

https://www.facebook.com/groups/917022518450093/

GreggMCC, I'll check out the standards you mentioned. We don't intend doing more than tie-wrapping a small label with the date on near the plug end of all leads. As well as staying safe, it is also to show that we are taking it seriously. While we may not be required to do PAT TAG testing, Menz Sheds can be shut down if deemed not to be safe, and it is a constant battle trying to convince members to play the game.

Trevor Dennis
Rapaura (near Blenheim)

SomeoneSomewhere

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#2046598 30-Jun-2018 22:51

There's a common quote that 90% of faults can be found with visual inspection alone, and it's true. Lead damage is the one that always crops up.

Actual insulation resistance failures are rare, and broken earths... more common than insulation failures, but still not common.

Supply everything from an RCD, check everything every time it's used, immediately condemn or fix stuff that fails, and you'll do better than pretty much everyone else.

gregmcc

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#2046599 30-Jun-2018 22:51

SomeoneSomewhere:

gregmcc:

elpenguino: Generally true for anything with electronics but 1000v is ok for ur average toaster, electric blanket etc.

The rules say use a higher voltage unless that will kill the device.

the rules say at least 2 time the nominal RMS voltage.

As far as testing at 1000V on a 230v appliance, typically the insulation rating on wiring inside an appliance would be 600V, by putting 1000V you are over stressing it beyond its rating and damaging it.

Testing a 230V appliance at 1000V is a very bad idea.

Do you have an actual reference for either 2x the nominal voltage or 'highest voltage possible'? 3000 and 3760 both say 500VDC, unless there are surge suppressors bridging the insulation, in which case 250VDC. I haven't seen anything requiring 400V systems to be tested at 1kV. I think it might be a myth left over from old rules or 'good practice', because I have heard it before.

Short-term stressing at 1kV should be fine, though, at least for plastic insulation (not electronics/capacitors etc.). I believe manufacturer's testing is required to be at 3kV, for example.

Testing live-neutral is definitely a no-no, at least at IR voltages. Shorting them together and testing to earth halves the number of IR tests you need to do, and further protects the electronics.

A basic continuity test to verify there's no shorts is fine and probably a good idea, but bear in mind single-digit ohms can be normal for the cold resistance of large lamps.

And finally, a reminder that you need to do an earth continuity test on class 1 appliances.

Testing any phase to earth would of course be @500V, but testing between phases at 500V would be pointless as the peak voltage of 400V RMS is 692V, so testing at 500V is less than the peak voltage that is normally on the wiring making.

The point of testing to earth is to prove that the touch voltage of any earthed metal work will no exceed the maximum allowed value (typically 50VAC), 3000 also says that the test voltage shall be at 500V +/- 20% for live conductors to earth, testing single phase to earth @1000V may give a low enough insulation resistance that indicates a failed test, but when tested at 500V is a pass test.

IR testing between live conductors isn't required (as far as I can see) the only reason is to prove that there isn't a fault between them, or a low enough IR that will cause a heat issue.

gregmcc

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#2046600 30-Jun-2018 23:00

TLD:

Blimey guys. I was wondering if I'd even get a couple of replies, so stoked to get this sort of response and so much information.

Definitely nothing live connected to any rig. It's only going to be extensions leads, and unplugged appliances like kettles, and workshop equipment.

The box would be a convenience so that the mains plug would not need to be held while holding probes to the terminals.

Thanks for the heads up regarding the different standards. I knew they existed, but had no idea what they were. We initially thought we had an obligation to test ALL leads twice a year, and had been quoted about $10 per item, so probably a couple of $k per year! I asked the lads on the Menz Shed Facebook group, and was told that we were exempt.

(Search for 'How do other sheds go about electrical equipment Tag Testing?' at the link below if interested)

https://www.facebook.com/groups/917022518450093/

GreggMCC, I'll check out the standards you mentioned. We don't intend doing more than tie-wrapping a small label with the date on near the plug end of all leads. As well as staying safe, it is also to show that we are taking it seriously. While we may not be required to do PAT TAG testing, Menz Sheds can be shut down if deemed not to be safe, and it is a constant battle trying to convince members to play the game.

If you are going to do testing of your appliances, you will need to use a tag that meets the standard, easily purchased from electrical supply companies.

testing to 3760 is an easy way to achieve the "Deemed safe", you could go with a custom test solution, but this could easily cause more headaches proving it is compliant so it's easier to use 3760.

Ideal solution would be to find an electrician mate to do the testing for you.

Don't forget to keep a record of things been tested and the results, if there are any issues you have evidence to back up what you are doing and not just a tag stuck to a lead that could have been put there by anyone.

SomeoneSomewhere

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#2046602 30-Jun-2018 23:03

It's required between all conductors of consumers mains and submains (8.3.6.1), presumably because these are the only conductors that can easily be isolated at each end and don't have any other loads anywhere on them.

gregmcc

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#2046604 30-Jun-2018 23:09

SomeoneSomewhere:

It's required between all conductors of consumers mains and submains (8.3.6.1), presumably because these are the only conductors that can easily be isolated at each end and don't have any other loads anywhere on them.

That's right, but we were talking about appliances and why they are IR tested - touch voltage rise.

For consumer mains it's potential faults between live conductors that would cause a break down in insulation due to heating - fire risk, keeping in mind most domestic consumer mains are protected by a 63A fuse, that's 14.49kw, that's a lot of heat energy available for a insulation breakdown fault.