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Testing for isolation

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Mike Poole01/02/2019 14:07:06
2546 forum posts
60 photos

I the book “Electric Motors” in the workshop practice series the author recommends using a screwdriver blade to short the supply that you have isolated to prove you have got the right circuit. If you have not succeeded in isolating the supply you could be showered in small bits of molten metal. I cannot support this technique of testing and if you do not have the means to check properly should you really be doing the job? I have had practical experience of converting a pair of cutters into more of a crimper than cutter and shorting a screwdriver does blow the end into oblivion. Accidents do happen even to people who do know the risks and what they are doing but no matter how careful you are the unexpected can still bite you.


Howard Lewis01/02/2019 15:34:23
3149 forum posts
2 photos

A Multimeter, set to the correct range, would be a safer and less destructive test method, surely?

Even flashing a lamp in a holder across the supply would be better than what the book advocates.


Neil Wyatt01/02/2019 15:42:02
17740 forum posts
697 photos
77 articles

You could use a pair of wirecutters. If there is a bang, sparks and part of the cutting edge of the cutters disappears accompanied by a burst of bad language, the supply is still live. A bonus is that you can now use the cutters as wire strippers for mains cable. Worked for me but I don't plan to repeat the exercise.

Or much more safely, what about good old mains tester screwdrivers that glow when touched to a live wire (or the modern type with an LCD display)? They have the advantage you can test a single wire, you don't need to complete the circuit.

Do get a reputable one (about £2.50 from Farnell) not a cheap one.


<edit> With the tester I always test before isolating to prove it is working, then disconnect and if it doesn't light up all is OK.


Edited By Neil Wyatt on 01/02/2019 15:43:29

Phil Whitley01/02/2019 15:43:21
1152 forum posts
145 photos

Quite agree Mike Poole far to dangerous doing it that way! use one of these No connection other than a very satisfied user for many years, and they are under £20, and test from 12v to over 600v safely. Although they have got a bad name for no reason at all, a neon screwdriver costs pennies, and is better than nowt, and much better than shorting across the terminals!

Martin Connelly01/02/2019 15:45:48
1253 forum posts
152 photos

I've got a non contact voltage indicator (Iso-Tech IVP-1) and would recommend them for anything to do with mains voltage electricity.

Martin C

Georgineer01/02/2019 15:54:08
340 forum posts
16 photos

Like this, Michael? That one took out the company's 100 amp fuse and spoiled my favourite screwdriver. Embarrassing it was, but no harm done except the bright green glow in the middle of each eyeball which took an hour to disappear.

Father taught me that with dry shoes on a dry floor and dry fingers (and your other hand in your pocket) it is quite safe to touch a live wire. If it is live, you feel a very mild tingle. It's never done me any harm - yet! Obviously if you touch the other wire at the same time, all bets are off!

There is of course the neon mains tester as an alternative, which can even detect mains voltage through a single layer of insulation if applied with a dab of 'monkey juice'.

I came across an American book of about the 1940s which recommends testing for mains voltage by putting the wires to your tongue, though it admits that some sensitive souls might find it a bit much. Of course, their mains voltage is lower than ours.



Mike Poole01/02/2019 16:16:39
2546 forum posts
60 photos

When apprentices came to our department they would be in their third or fourth year, it was remarkable how much they would struggle to explain the isolation and testing procedure for single and three phase circuits and many would say use a multimeter when a proper Martindale tester and proving unit were available. The argument against using a multimeter is that it is easy to select the wrong range and the design of the meter does not make a failsafe device. I feel there is a definite pecking order of devices to test with and the purpose made testers have to be at the top, anything else has some reservations but are better than nothing. Getting it wrong could be fatal.


Andy Carruthers01/02/2019 16:18:54
274 forum posts
23 photos

Half a lifetime ago I worked on a valve Non-Directional Beacon and managed to get 3 belts within 30 minutes, each one hurt more than the first

Not doing that again

Old Elan01/02/2019 16:51:39
92 forum posts
34 photos

Sadly quite a few of 'that' series are no longer fit for purpose and should be removed from sale in my opinion.

On the subject of the cutters, my builder employed a 'qualified' sparks who left live tails in my garage from the utility room next door. Silly me, I thought they were from the CU that he had installed for ALL the garage/workshop power which was isolated.

Melted cutters as above.

Qualified? Hmmmm

Edited By Old Elan on 01/02/2019 16:52:42

Howard Lewis01/02/2019 17:03:30
3149 forum posts
2 photos

As an Apprentice, during my year in the Electronics and vibration lab, with a floor insulated to 5 Kv, before going home for the night, I pulled the top off a 13 amp plug. Fingers and thumb neatly across Live and Neutral (240 Volts AC ) Obviously not fatal, but a shock in all senses of the word! Made sure that the plug top was secure next morning!

And, Yes it is possible to ruin a multi meter by putting across the mains when set to the wrong range!


David Davies 801/02/2019 17:24:56
87 forum posts
8 photos

If anyone wants to read further on this subject I suggest searching on the internet for ''Electrical test equipment for use on low voltage electrical systems'' Guidance Note GS38 (Fourth edition). This is a 9 page HSE document, available free as a PDF, which is aimed at the workplace but is still relevant at home.



Jeff Dayman01/02/2019 17:50:20
1793 forum posts
45 photos

If tests are planned to be done with screwdrivers, the trick is to use your neighbour or co-worker's screwdrivers! devil

SillyOldDuffer01/02/2019 18:03:32
5640 forum posts
1159 photos

Before touching the bare 3-phase overhead wires in my street the men always seem to hook a hefty earthed pole with an insulated handle on the wires to make sure the power really is off. Once they did it and there was a loud bang when one of the giant fuses at the top of the pole exploded. Yes they were embarrassed!

Friend of mine who repaired valve TVs for Radio Rentals used to test for mains with his thumbnail. Reckoned he could sense the wire was hot without getting an actual belt. I chose not to try it...

As for father saying it's OK to touch a live wire, I agree 250Vac isn't very dangerous but surely it's not worth the risk? So much depends on your state of health, age, weight, skin resistance, time the shock lasts, and the path taken by the amps through your body.

Make a good article for MEW though. devil

Can I have 100 volunteers to sign a liability disclaimer please? After strapping these heros into a wet chair and connecting the mains via crocodile clips for a full 30 seconds I'll write up the results. There will be interesting graphs and colour photographs. I think there's a good chance of stopping a few hearts, plus perhaps a death or two due to complications resulting from burns. Most likely all participants will suffer psychological damage, and some will be permanently affected. There's a world of difference between briefly touching a live wire and what happens if the victim is held in contact until someone cuts the power.

The advantage of using a screwdriver rather than relying on a sophisticated test-set is that the instrument can't go wrong! When working on the mains, I check first with a neon screwdriver and then short the wires as well.


Mike Poole01/02/2019 18:05:49
2546 forum posts
60 photos

Glad there was no lasting damage with the screwdriver modification George. Dave that document sums things up nicely, much better info than the advice to short a hopefully dead circuit. The electricity at work act is quite a brief document but is quite onerous if taken seriously. When it came out all electrical personnel in our company were trained on its impact on electrical maintenance, around 130 people were trained. I noticed over the years that personnel, both managers and shop floor changed and the importance was forgotten. I suppose when the accident happens as it surely will they will wake up with a little kick from the HSE.


Edited By Mike Poole on 01/02/2019 18:10:06

An Other01/02/2019 18:15:38
161 forum posts
1 photos

There was a time I could have done with some insulation - more years ago than I care to remember, I was working on an old rotating coil PPI radar display. (This generated the rotating 'sweep' on the tube by mechanically rotating the coils carrying the sweep generation signal - it was a long time ago!). I was following all the accepted safety rules of the the time - one hand in pocket, nothing dangling loose, etc, etc.

The testing required the power to be on - about 18 kV supply to the tube if I remember correctly. A colleague walked past behind me, and accidentally pushed me - I touched the HV. The resulting shock flung me back against a wall so I cracked my head, and I bit deeply into my tongue. It took about an hour or so to stop shaking, and seeing red and blue spots. I think I was lucky to to suffer more serious injury.

Nothing to do with insulation, but I guess it shows a need to take care with high voltage.


Edited By An Other on 01/02/2019 18:16:29

Brian G01/02/2019 18:19:00
674 forum posts
26 photos

My father (a dockyard wireman) taught me to touch potentially live wires by sweeping my (moistened) finger down quickly at arm's length so I touched them in passing but the momentum of my arm would take my hand away. I still do that even after testing the wires with a neon screwdriver, voltage tester or multimeter as I would rather not find out that the neon etc. had failed by getting a proper jolt.

Even after testing it is still not a bad idea to short the circuit, as it could just be dead because of an intermittent fault and on domestic circuits you cannot just put a padlock on the breaker. You can also use a live circuit just to make sure that whatever you tested with really works. (I have a healthy paranoia about electricity ever since I first encountered busbars instead of cables in the pit at a substation).


EDIT: And whatever you do, don't reach down to a potentially live cable!  If it gets you, you want to fall away, not toward...

Edited By Brian G on 01/02/2019 18:21:27

Mike Poole01/02/2019 18:33:13
2546 forum posts
60 photos

I have worked with quite a few people who lick their finger and do the dab test, not a check I would endorse. I expect when they do it on a live circuit they will give up doing it.


Edited By Mike Poole on 01/02/2019 18:33:37

john fletcher 101/02/2019 18:34:22
580 forum posts

It's almost 70 years since I started in the electrical industry and how I wish others wouldn't be so flippant regarding electricity and electric shock. Each and every one of us is different in make up, so what is a shock to one is not to another, I'm reliably told its hand to hand shock which are the killer and its our hearts which are vulnerable. I've always worked on the basic SWITCH OFF and test with the appropriate tester to prove it and test the tester from time to time. Some of our lads failed the AM2 test as they attempted to test 240 volts with a broken tester. Keep one hand in your pocket. Also just because you have an RCD it won't give you exemption from a shock. DISCONNECT. John

Phil Whitley01/02/2019 20:27:20
1152 forum posts
145 photos

And of course, we all need to remember that the first thing you test is the test equipment! EVERY TIME!

peak401/02/2019 20:58:26
1078 forum posts
124 photos

Someone sent me this via Facebook the other day; Don't try this at home as they say. surprise

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