Author Topic: One hand in pocket/behind back when measuring mains voltages?  (Read 5303 times)

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Offline fubar.gr

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I've seen several times in this forum the aphorism that when measuring mains voltages you should use one hand only (apparently to avoid hand-to-hand electrocution).

But how do you measure stuff with one hand only? I can see only two possible ways.

1) Clamping one probe (with a grabber/crocodile clip) to a specific point, then doing all measurements with the other probe.

This could work if everything is earth referenced. You can connect your clip to earth ground and make all measurements you want. But what happens if you have to have one probe clipped to some voltage, eg when measuring line to line voltage? Then the other probe tip will be at the same voltage too. Granted, there will be a 10Gig resistor and whatever safety features the meter has between the voltage and you, but still.

2) Juggling both probes with one hand.

I tried this once and I almost made the probe tips touch together. You might avoid hand to hand electrocution that way, but in my opinion this is outweighed by the fact that you have limited control of what your probes are going to touch.

I believe that the best practice is to first make sure that the probes are clean and the insulation intact, then make sure that the multimeter is set up correctly for the type of measurement and simply use both hands.


Any opinions on this?

Offline AG6QR

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #1 on: June 06, 2015, 01:30:30 pm »
You can use one hand to move the clipped reference lead as needed.  Then, when the reference lead is where you want it, hold the other lead and put it where you want it.
 

Offline Deathwish

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #2 on: June 06, 2015, 01:57:12 pm »
If i use both hands i start singing ... nearer my god to thee...
Electrons are typically male, always looking for any hole to get into.
trying to strangle someone who talks out of their rectal cavity will fail, they can still breath.
God hates North Wales, he has put my home address on the blacklist of all couriers with instructions to divert all parcels.
 

Offline G0HZU

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #3 on: June 06, 2015, 02:04:16 pm »
The one hand in a pocket doesnt just apply to DMM usage.

A typical hazard scenario would be that you accidentally touch a high voltage point inside a unit. If you happen to be touching/adjusting some grounded chassis test gear (or the shield of a cable leading to it) with the other hand then you will receive an across the chest shock.

Some people insist on using metal work benches so the other way you can get a shock is to rest one hand on a metal workbench whilst probing inside the unit with the other hand. Very easy to do! You could even get a shock via your torso this way if you are pressed against the metal bench.

I very rarely work on high voltage stuff but I try and obey the one hand rule and I also wear rubber gloves on both hands when there are potentially lethal voltages exposed and my benches are all made of wood.
« Last Edit: June 06, 2015, 02:07:32 pm by G0HZU »
 

Offline Wim_L

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #4 on: June 06, 2015, 02:29:46 pm »
It all depends on the situation and accessibility.

One hand probing would be highly recommended if probing in a cluttered device where you could easily bump your hand into something dangerous while probing circuitry that lies behind other conductors. It's especially risky when the conductors are corroded and you have to put a lot of force on your probes to pierce through the corrosion layer. Though, if you have to do this in the first place, it's a sign that device was designed in an unsafe way. And you'd be much better off getting a longer probe or probe extender, than just sticking one hand in and hoping the likely accidents won't hurt too much. If you really must do this kind of thing, see if you can get insulating gloves rated for whatever you're likely to encounter in there.

The one-hand rule is not, by itself, about doing things safely. It's about how to change risks you probably shouldn't be taking anyway from being likely lethal to being 'merely' quite painful injuries. It does become much better protection when combined with other safeties (insulating gloves, dry soil, insulating mat to stand on, rubber shoe soles... Those make it much safer by eliminating other current paths too).

There's always a balancing act when introducing inconvenient safety measures. Most safety features serve to reduce the severity of accidents. But if the safety measure is a nuisance, it might increase the odds of causing some other type of accident. Ever try using two probes in one hand, like chopsticks? Don't, very clumsy and likely to bump into the wrong things. The one-hand rule is a basic trick. It's better than nothing. But it's no substitute for thinking about what you're doing, thinking of alternative options to do the measurement, and considering the safety advantages and drawbacks of each. If you're working in such a hurry you can only depend on one quick rule of thumb like this, you're already going down a dangerous path.

If possible, the safer option is to power down (making sure no charge remains on any high voltage capacitors too), clip leads on, then power on. But this is time-consuming and requires cycling the power for each measurement. The one-hand rule is a compromise between safety and efficiency. It's a compromise you can't always afford to make: an example would be doing high-side current measurements on a high voltage power supply. Power it all down, put the multimeter, in current measurement mode, into the high side line. Place the meter on sufficiently thick insulators (also make sure all test leads are separated from anything grounded!). Put the meter on the right current range. Then turn the power on, and observe the display from a distance. You don't want to touch any part of the probes or the meter itself while doing this, don't even touch the range switch.
 

Offline Pillager

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #5 on: June 06, 2015, 02:44:15 pm »
Well, if I measure high voltage I use CAT IV gear, because it's required. If I have a device on my bench under test, I put it on an insulating material and arrange it, so that I can easily access all points I want to measure. If not possible, I cover live parts with insulating material.

But I always use both hands. It's easier to cover up dangerous portions and work with both hands, than having that kind of limitation.

Of course, it's part of my regular job, so we have the insulating rubber matts, clips to hold them in place, personal safety gear and tools suitable for working on live circuits up to 1000V AC. But it really pays off to have these things, if you need to measure high voltage even on a semi-regular basis. At least the rubber matts and the clips.

I realise it might be a bit expensive, but you might find other materials to use for covering hazards. Just make sure you have decent test gear.
Greets

Tom
 

Offline German_EE

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #6 on: June 06, 2015, 04:24:36 pm »
I've been working with electricity for so long that measuring with one hand in the pocket just comes as second nature. If I can't do this then I find a way that I can, such as placing the meter somewhere else or connecting the ground lead using a clip.

One other thing, it's best not to work on live circuits alone.
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

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Offline Pillager

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #7 on: June 06, 2015, 04:43:37 pm »
One other thing, it's best not to work on live circuits alone.

Right!

Or even better: Don't work on live circuits above ELV. Unless you are specially trained to do so.

I know, it's pretty restrictive, but it's better to be safe than sorry (or dead, in the worst case).

But if you really want to, get some decent equipment and read up on the subject. Or better, take a course.
Greets

Tom
 

Offline KeepItSimpleStupid

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #8 on: June 07, 2015, 09:59:47 pm »
Where I worked, I used to work on some really nasty power supplies.  If I did not use the one hand rule, I'd probably be dead.

My experience probably started as a 10 year old with vacuum tube TV';s.  It took me a long time before I had the courage to remove the anode wire from a CRT.

Some supplies I worked on:
100 kV at 100 mA
15 kV @ 1.5 A;  That system used 60 A of 3 phase 208 for power.
1000 W RF transmitters with 3000 VDC plate voltages.

We had a 6 V 3000 A AC supply around too.

Then there was the 22  V 40 A with a 40 kV AC spike power supply for a lamp.

I also worked at the other end of the spectrum, 15 kV in the pA range setting up EBIC and beam current measurements on an SEM.

Then of course designed fixtures to measure < 2 PA at +-100  V. 

At the other end of the extreme 0 V at 100 mA .  Yep 0 V.

I built a 4-terminal current pre-amp with bias for a front-end of a lock-in amplifier.

You always do a job hazzard analysis and work appropriately.

Cheers!
 

Offline Pillager

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #9 on: June 07, 2015, 10:32:07 pm »
You always do a job hazzard analysis and work appropriately.

 :-+
Greets

Tom
 

Offline kingofkya

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Re: One hand in pocket/behind back when measuring mains voltages?
« Reply #10 on: June 07, 2015, 10:44:38 pm »
Another thing with the one hand rule, you can use clamp of some kind to hold the probes as well then just move one clamp at a time.
I only play with 120v stuff but still. Sharp probs are best though they will stick where you put them.

http://www.amazon.com/Bessey-BVVB-Vacuum-Base-Vise/dp/B0057PUR88/ref=sr_1_3?s=hi&ie=UTF8&qid=1433716926&sr=1-3&pebp=1433716941112&perid=1AAQD9B4955QVAPP43WQ



 


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