Why multimeter safety matters

[joe72205]

Just ran across these articles. Multimeter accidents hurt people.

http://www.bnl.gov/userscenter/Immediate/cen_tech.asp

'On Wednesday, December 7, 2005 an electrician at the Fernald Closure Project was injured when the multimeter being used to check voltage in a cabinet experienced an electrical short. '





http://www.iaei.org/magazine/?p=556

"A recent survey ESA conducted with 5,000 electricians from across the province of Ontario revealed that 11 per cent of respondents had experienced a “violent failure of a multimeter.”"





http://ecmweb.com/ops_maintenance/defective-test-equipment-20100101/

"As he made this measurement, a fireball erupted from the panel, severely burning all three individuals and causing significant damage to the electrical equipment and surrounding area. Although witnesses and medical professionals provided rapid response and assistance, the injuries proved fatal for the electrician and the fire official."

[Bored@Work]

You can show these pictures a thousand times. And yet there will still be advocates for cheap multimeters, because they are so cheap.

On Wednesday, December 7, 2005 an electrician

And it beats me why an electrician would use such a meter. They should know better. It also bets me why five years later one can still find these rubbish meters with their fake CAT II 600 V ratings in the market.

[Kiriakos-GR]

Just ran across these articles. Multimeter accidents hurt people.

And your point are ........??

That you are safer in the street ?  

There is no bulletproof protection in anything .  
Even the called bulletproof glass has limits.

[joe72205]

I thought the point is obvious.  I don't really understand the intent of the reply.

When the limits are well known and backed up by testing, it means a lot more than unknown or untested limits.

Your safety in the street is certainly enhanced by proper street lighting and markings, properly operating signals, etc...

What is your point? Is it that safety doesn't matter and should be ignored because an asteroid could strike you dead any moment?

[quantumfall]

How tragic, these incidents you would think should never happen with people from the electrical industry.  The training is so important.  I worked as an electrician for a few years and never understood "Arc Flash".

I did only installation work, not any commissioning or live maintenance work so only worked with it being isolated from power. Edit:  I worked on installation of "Medium Voltage" switch gear and the phase conductors on three phase systems always had Flash barriers / guards between the bus bars and conductor with rubber boots on the lugs etc as far as I can remember. Maybe the switch gear is better in the UK.

[Mechatrommer]

my youngr bro is an electrician, but yet he never touch a DMM (or even bought one) like the one shown above (yellow), the company already provides them with more rugged one that i never have a gut to ask for the price. then i wonder what a such an electrician to use that $5 DMM?

and the clamp meter? its like burnt to hell all over! i think its just a publicity crap! Dave has tested $5 DMM with KVolt, and all we have is an exploded parts at certain area. and then this is a clamp meter? non contact one?! urghh! give me a break! IMO its just exagerration of marketing trick!

what do you think guys? we are all the real practicioner here. we dont hear it, but actually feel it.
and if its for real, i think those electricians are soo sooo stupid that they dont know whats "electrocuted" means! i think it should be included in any electrician certification including the big bang boom! to teach safety

[PetrosA]

Thanks for posting these links. The reality is that many professional users and most amateur users of multimeters have low end ones that may not be great with safety and many electricians aren't very well educated on using their meters safely. The local electrical supply houses in my area have lower end test equipment than the box stores. The average person has no clue how much energy can be released when you take a piece of wire or even a trace on a PCB and change its role during a dead short from that of a conductor to that of a resistor under high current. boom.

[EEVblog]

and the clamp meter? its like burnt to hell all over! i think its just a publicity crap! Dave has tested $5 DMM with KVolt, and all we have is an exploded parts at certain area. and then this is a clamp meter? non contact one?! urghh! give me a break! IMO its just exagerration of marketing trick!

You can always argue that these type of photos and the exploding multimeters catching in fire etc are extreme examples and part of "shock'n'awe" type marketing, but the fact is it can happen.
Anything that involves high capacity electrical installations (start from the household powerpoint) has the potential to cause this sort of damage. So that's why you should aways use a properly rated and designed unit when dealing with the mains and up.

The clamp meter will also have regular probes and voltage ranges etc, that's why it likely exploded in this case.

An analogy to this is that when I go canyoning/mountaineering etc I only use certified and tested name brand rope and equipment that is rated and tested to break at many tons. Yet even with a heavy pack I weight under 100kg, so why not just use a $1 200kg OneHungLow brand steel shackle from K-Mart?
I hope the answer is obvious!

Dave.

[Bored@Work]

I thought the point is obvious.  I don't really understand the intent of the reply.

Don't wast your time. He is just the resident troll and attention seeker. He is attacking everyone when he sees his imaginary opinion leadership about "multimeters" threatened. Ignore him.

[Bored@Work]

what do you think guys? we are all the real practicioner here. we dont hear it, but actually feel it.
and if its for real, i think those electricians are soo sooo stupid that they dont know whats "electrocuted" means!

No, I don't think they are stupid. That's why I am surprised an electrician was using that joke of a multimeter.

Where I come from electricians need three years of training and have to sit two exams before becoming an electrician.  And that is not the end, they need several years (I need to look up how many exactly) practical experience, yet another year of full time training (or years of part time training) and another exam before they are allowed to run their own shop and employ and supervise other electricians.

As an EE, when I have an electric problem in my house I call an electrician, instead of doing it on my own. Simply because they know it better. It is the same in my job. A bunch of EEs, but an electrician is called when there is an electric problem.

Should I, however, ever see an electrician pulling out such a meter in my house or in my job, he'll first an earful and then will be thrown out of the house or I'll make sure he'll be thrown out of the company. You don't pay a specialist for rubbish work.

i think it should be included in any electrician certification including the big bang boom! to teach safety

I know that electrician training here includes watching horror videos and pictures of real world accidents.

[alm]

and the clamp meter? its like burnt to hell all over! i think its just a publicity crap! Dave has tested $5 DMM with KVolt, and all we have is an exploded parts at certain area. and then this is a clamp meter? non contact one?! urghh! give me a break! IMO its just exagerration of marketing trick!

Dave's test was a low-energy circuit with probably a pretty high output impedance. Power distribution circuits are lower voltage, but have an almost zero output impedance and can deliver huge bursts of energy (which is what those IEC61010 CAT ratings are about). I'm not surprised that a high-energy circuit can cause much more damage. About the isolated clamp, probably either Dave's right and they were using it for voltage measurements, or the isolation of the clamp got damaged.

what do you think guys? we are all the real practicioner here. we dont hear it, but actually feel it.

Not many of use are industrial electricians, or have experience with circuits beyond low-voltage mains. These were all failures at distribution level or medium voltage installations I believe.

[Zero999]

So far I've kept quiet on the cheap meter argument but I'll just throw my opinion in.

Cheap meters are fine for a measuring low current, extra low voltage sources, such as a project hooked up to a small battery, a wall wart, a LM317 power supply etc. but not for hazardous voltages and currents.

For measuring a circuit with a hazardous voltage or short circuit current, use a decent quality meter, not a cheap and nasty £5 piece of shit. Also note the fact that it's not voltage which does damage but energy, a cheap meter can cause you a serious injury if you use it to measure the voltage on a car battery and it short circuits it, causing it to catch fire and spray you with boiling hot sulphuric acid and molten lead.

So yes, buy as many cheap meters as you like for measuring voltage and currents around low current, extra low voltage projects, they're very handy but if you want to do something hazardous such as measuring the mains, use a proper meter.

[david77]

I have seen meters like these a few times. Usually when someone tries to measure how
many "amps" there are left in their car battery  .

I work in a small electronics shop, so I do sell meters. The fact is: People, even the pro's, are
not willing to spend more then 40.00 EUR on a meter.
The cheapest meter for 9.90 EUR is the one I sell most. That thing is utter rubbish.
I have some nice meters in my cabinet, but they cost 80-100.00EUR and no one is prepared to spend
that much money on a multimeter. Sad, but true.

The 40.00 EUR meter has some nice features but is still shite. That's what my boss gives me to work with.
Usually that's OK, because we don't do repair work, I only need it to check wall warts, batteries and fuses.

Recently I accidently had the leads plugged into the Cx/Rx sockets and wanted to measure 230V on the primary of a transformer... BANG! SPARK! SMOKE! That made me jump, I can tell you. Luckily I didn't hold
the meter in my hands.

David

[McPete]

This is a great illustration of what a high-energy source can do to your test equipment when, if you'll pardon the expression, "Shit gets real".

One of the great virtues of my employer is their insistence on replacing older, non-IEC61010 (LV input protection) rated equipment... take the old prove de-energised sets; the test lamp. Nothing more than two 240V 40W globes in series with bare copper probes and single-insulated cable. You get a transient voltage as you're measuring, those will go off like a bomb, never mind all the pitfalls of intermittent elements giving false-negatives...

Most of those have been replaced by Fluke T140s- Admittedly, not what I'd call an ideal solution, but hell, every linesman and electrical fitter-mechanic can take a genuine voltage reading, prove de-energised and test continuity.
Safety and functionality from modern test equipment- Why wouldn't you pay a little extra for it?

[Mechatrommer]

ooo ok, i saw the link already. i posted too early before making judgement. i take back my word on the marketing thing. but one thing i dont understand, is how such a big bus bar melted by an arc flash. i thought the whole pcb tracks of the DMM will become the "limiting" fuse that burst first before the big bus bar does, even if the DMM does not have its own safety fuse?. maybe coupled with user error i think.

this is an arc flash video i found during the googling. luckily he survived, and i think he will become 5 stars electrician after that
Arc Flash Video

[Mechatrommer]

I have seen meters like these a few times. Usually when someone tries to measure how
many "amps" there are left in their car battery  .

@david77 & hero: i can assure you the car battery is still rated as low energy circuit. why? i did exactly as david77 mentioned about $5 piece of shit DMM (analog actually) i firstly bought to test A on V car batt many years ago. and luckily i'm still alive and there was no explosion or even a single click AFAICR, its just the AMM straight away went dead.

[Time]

25,000 A!

Thats a real man's current.

[joe72205]

ooo ok, i saw the link already. i posted too early before making judgement. i take back my word on the marketing thing. but one thing i dont understand, is how such a big bus bar melted by an arc flash. i thought the whole pcb tracks of the DMM will become the "limiting" fuse that burst first before the big bus bar does, even if the DMM does not have its own safety fuse?. maybe coupled with user error i think.

shafri:

Regarding PCB traces:  What WAS a PCB trace is now a live arc.  The vaporised metal plasma becomes the conductor.   The arc creates more plasma by consuming the materials around it.   In other words an arc plasma is a NEGATIVE resistance - the current just keeps going up and up until things are physically blasted away by explosive force.   This can happen in 1/20th of a second (like the video you posted).

The HRC (High Rupture Capacity) safety-rated fuses are specifically designed for this situation by snuffing out the arc upon overload.   They achieve this through internal geometry and choice of materials.  (I believe the fusible link takes a zig-zag path through a packed dieletric such as sand.)

See http://en.wikipedia.org/wiki/Breaking_capacity

http://canteach.candu.org/library/20050708.pdf (see page 9)

[Neilm]

The reason cheap multimeters are cheap is that they don't use HRC fuses, or worse they use the cheapest they can find. i.e. cheap knock off fuses that have the correct ratings on the outside. In the UK the mains plugs contain a fuse, on moulded plugs this fuse is located between the prongs and accessed by a small cover. A few years ago there were several incidents of explosions when there was a short circuit. They found out that the manufacturers were "saving a bob" and going to a cheap supplied from the far east. Click on the link http://www.era.co.uk/services/devices.asp and press play to see what happened when this was tested.

I have worked at a company that imported multimeters and we insisted that any new meter (or minor variation) be retested - and we upgraded our own facility to do it on site. Admitadly we can't get the correct power availablity as the sub station for the site is not up to the job but we are able to put an 8kV spike onto 600V. Also great fun closing the contactor when you are putting it across 600V in current measuring mode.

I know that the video that Time posted was part of an investigation done into multimeter accidents done by a national safety althority. I believe that it was presented to the IEC commitee responsible for 61010 as justification that 61010 didn't go far enough. I think they then did further testing and concluded that the meters had either been "designed to" 61010 (but not tested) or just claimed that they had. Certainly I know that several meters we got to test could not have passed 61010 even with a cursory glance. For example, a varistor that was supposed to protect against surges had a metal screw head between the legs, there was barely even basic clearance.


Neil

[lhc]

I know a guy that was a witness of such thing. He was working with guy that used some cheap chinese no-name multimeter to check circuits in wiring cabinet. He accidentally left it on resistance range (probably after checking some fuses) and connected to live circuit. The meter was blown to pieces and the probes were glowing red and burned his hands.  If he would use a good certified meter he would see overload on the screen and nothing else would happen.

This is the difference between cheap crappy instruments and high quality ones.

Those things do happen in reality and safety matters the most.

[Kiriakos-GR]

I thought the point is obvious.  I don't really understand the intent of the reply.

Don't wast your time. He is just the resident troll and attention seeker. He is attacking everyone when he sees his imaginary opinion leadership about "multimeters" threatened. Ignore him.

I think that you need a good vacation period , at list for a week ..    


To all :
Its my personal rule , or style of life ,  to walk in balance , and so my vote does not go at the low class instruments , or either at the one that screams about how safe it is  ..

[Mechatrommer]

The reason cheap multimeters are cheap is that they don't use HRC fuses, or worse they use the cheapest they can find. i.e. cheap knock off fuses that have the correct ratings on the outside.

can i buy this super quality HRC fuse, install it in my $100 Uni-T or even the $20 No brand DMM and considered it settled (safe)?

[Bored@Work]

can i buy this super quality HRC fuse, install it in my $100 Uni-T or even the $20 No brand DMM and considered it settled (safe)?

No.

[Zero999]

You can use fused leads but that only protects the cheap DVM against short circuits, not overvoltage which can cause shocks and fire.

[McPete]

Besides your fuse acting quickly and extinguishing the arc, you're really hanging on two things- Your PTC/MOV/Whatever activating quickly (which should blow the fuse/input traces), and the case, leads and terminals being able to contain a lot of current and a small explosion...

The latter is why I really wouldn't want to use my BKP 2709B on a high energy source- I doubt the case would contain the explosion of the internals...