Interesting and sad case of electrical injury

[calzap]

Encountered this story in recent issue of EC&M magazine.  Although similar stories are easy to find on the net, this one caught my eye because of the before and after pictures.  At an industrial job location, the foreman of an electrical crew assigned an apprentice to tidy and clean-up some switchgear.  He told the apprentice not to work on anything “hot” and walked away.  Apprentice was to determine what was hot with a multimeter which was not rated for some of the voltages in the facility.  Apprentice was not wearing any PPE.  Everything went well until the apprentice opened a cabinet with 4,160 V potential transformers.  See first picture.  Given the small size of the transformers, I assume they dropped the voltage for sensing and control purposes.  There was no high voltage sign on the cabinet.  The apprentice dutifully put the meter probes on the primary conductors.  The conductors, probes and meter exploded in a ball of fire.  See the last two pictures.  It’s obvious which two conductors were probed.  Can anyone identify the meter?  Looks like the plastic shells of the transformers burned or vaporized.  Apprentice sustained severe burns and spent several days in a hospital burn unit.  Was not stated whether there were permanent injuries.

The contractor and facility owner were both to blame, but the bulk of the blame is on the contractor.  The foreman knew that some of the cabinets had high voltage.  The apprentice should have never been allowed to work on the equipment alone.  Anyone working on it should have had appropriately rated equipment and should have been wearing PPE.  Unless functional testing of the transformers was needed, power should have been locked-off upstream.  Was not stated whether fire-fighting equipment or first aid supplies were present or what penalties were imposed.

Mike

[Gyro]

... Apprentice sustained severe burns and spent several days in a hospital burn unit.  Was not stated whether there were permanent injuries.

Several days? I would say that he was incredibly lucky! People can spend months in those burns units.

That meter does look vaguely familiar from old magazines, amateur grade.

[Kim Christensen]

The meter looks like this one:

[Stray Electron]

    I think there must be a LOT more to this story than what the article is telling!  First. what exactly did they mean by "tidying up"?  It already looks pretty clean to me.  Also how did they just happen to have a "before" picture?   Also any meter should be fused and even if it wasn't, 4,000 volts should have just fried the meter and not caused all of the damage outside of the meter and to the meter leads and inside of the electrical cabinet.

   I spend a few years working with 12,000 volt systems and later spend 4 years working on U.S.A.F. Search RADARS that operated at 15,000 volts at 200 amps and I've never seen anything like this.  4,000/ 12,000/ 15,000 volts  really aren't that dangerous IMO as long as you follow the proper procedures.

   I did see one high powered 480 3 phase AC electrical cabinet burn up, similar to this, after the cleaning crew sprayed it directly with a VERY high pressure water hose!

    But IMO the damage in the pictures wasn't caused by someone probing with a meter. 

[Gyro]

The meter looks like this one:

Yes, that's the one. A 2 socket special with current ranges! 20mm glass fuse at best. Ancient.

[ejeffrey]

Does that meter have a 20 amp range on the same jack as the volts?  That's nuts.

[thm_w]

    I think there must be a LOT more to this story than what the article is telling!  First. what exactly did they mean by "tidying up"?  It already looks pretty clean to me.  Also how did they just happen to have a "before" picture? 

The before photo is a different cabinet.

[Gyro]

Does that meter have a 20 amp range on the same jack as the volts?  That's nuts.

200mA but it doesn't matter at that point, it's as near a HV short as makes no difference. Yes, absolutely nuts to be in that environment and a fuse eater on an amateur's bench. It was a Radio Shack brand!

[calzap]

    I think there must be a LOT more to this story than what the article is telling!  First. what exactly did they mean by "tidying up"?  It already looks pretty clean to me.  Also how did they just happen to have a "before" picture? 

The before photo is a different cabinet.

You're right.  No wires entering the top of the cabinet in the before photo, but there are in the after photo.

[coppercone2]

Well I noticed if you probe a high voltage circuit you get sparks on the leads for a meter (i.e. vacuum tube transformer), even a 34401A when you probe it near the limit of the voltage. something is getting charged


I think the problem is cheap meter. I see what the radio shack meter has is like a little bead MOV on the input. It does not have any special box for containing and directing plasma etc. The protection element shorted and blew up the meter. MOV turn into plasma when they get hit hard enough, I saw it happen in a power strip.

The better things use protection to help contain the blast, heat shrink, kapton, and its own container. I took a RS dmm apart, it just has a blue little MOV sitting out there in the PCB. they give it a bit of spacing from the other parts, but thats it. Chances are the plastic is cheaper too.

I think if you don't contain it, the plasma keeps spreading and the 'load' impedance keeps decreasing. so even as it cools, more stuff decides to short out... as the explosion propegates through the PCB. Suddenly you have char and metalization short circuits connecting the MCU as a 4000V load. its like not letting it extingush because more and more shit is connnected to the circuit as it burns. So the R of the load goes down as more and more of the meter is burned up, not to mention the plastic if cheap might char and turn conductive.


Whats going to break the circuit if its 4kV connected to a expanding charcoal load that keeps finding more 'short circuit' to heat up? It needs to be contained so the R increases not decreases over time during the fire



What I think is that they should have a meter that can take probing anything in the room. Locks can fail, signs can fall off. Lets say a sign falls off and a lock breaks. Someone can think "they opened this for me". IMO lockout is kinda BS, people and nature always messes with it... you gotta be ready for anything.

The only thing thats certain is what is likely to be in a room based on the design of the building LOL, no way can you reliably predict WTF kind of BS is going down in there since then.


Especially a  random ass contractor. they should equip HV CAT IV equipment to all the soldiers of fortune that work for them. this ALWAYS involves contractors because they basically regularly walk into traps. 

If you have a contractor come into a building, chances are you don't know WTF is going on and its considered mad dangerous/tricky/hard




But the box damage is interesting. I wonder if thats just residue from the burning meter or what. Maybe the wrong kind of paint or plastic was used on the transformers? or dirty?  too tightly packed ? you would think it would just burn up the meter and stop

[calzap]

    I think there must be a LOT more to this story than what the article is telling!  First. what exactly did they mean by "tidying up"?  It already looks pretty clean to me.  Also how did they just happen to have a "before" picture?   Also any meter should be fused and even if it wasn't, 4,000 volts should have just fried the meter and not caused all of the damage outside of the meter and to the meter leads and inside of the electrical cabinet.

   I spend a few years working with 12,000 volt systems and later spend 4 years working on U.S.A.F. Search RADARS that operated at 15,000 volts at 200 amps and I've never seen anything like this.  4,000/ 12,000/ 15,000 volts  really aren't that dangerous IMO as long as you follow the proper procedures.

   I did see one high powered 480 3 phase AC electrical cabinet burn up, similar to this, after the cleaning crew sprayed it directly with a VERY high pressure water hose!

    But IMO the damage in the pictures wasn't caused by someone probing with a meter.

An arc flash from using an underrated meter on high voltage, high amperage equipment can destroy the meter, severely damage the surrounding equipment, and injure or kill the operator.  The video linked below describes a tragic accident where a 600 V rated meter was used on 2200 V switchgear.  The operator was killed.  In this case, the operator was an experienced electrician, who in a temporary lapse of judgement, violated several safety rules.
Mike

[coppercone2]

Do you ever get the feeling the equipment damage is a sign that maybe something could be designed better? i understand the meter blowing up, but sometimes you figure maybe more spacing or something would have prevented it from getting so bad... like I think sometimes you would think it can be designed to kinda prevent the plasma fireball from building up so much

I generally noticed if i try to engineer anything to a standard (i.e. rackmount) it  always pisses me off and I gotta make compromises that seem very inane to meet some specific size requirement.

I think something must be wrong with the cabinet for it to get that damaged in the picture. and that there should be something more substantial then a label for HV transformers. Like a front facing Plaque with the voltage on it, like engraved acrylic, not something that can fall off. I can imagine now, the door hinge corroded, so they made a DIY replacement for the door without a sticker. Seems that you should label more then the cabinet

[amyk]

    I think there must be a LOT more to this story than what the article is telling!  First. what exactly did they mean by "tidying up"?  It already looks pretty clean to me.  Also how did they just happen to have a "before" picture?   Also any meter should be fused and even if it wasn't, 4,000 volts should have just fried the meter and not caused all of the damage outside of the meter and to the meter leads and inside of the electrical cabinet.

   I spend a few years working with 12,000 volt systems and later spend 4 years working on U.S.A.F. Search RADARS that operated at 15,000 volts at 200 amps and I've never seen anything like this.  4,000/ 12,000/ 15,000 volts  really aren't that dangerous IMO as long as you follow the proper procedures.

   I did see one high powered 480 3 phase AC electrical cabinet burn up, similar to this, after the cleaning crew sprayed it directly with a VERY high pressure water hose!

    But IMO the damage in the pictures wasn't caused by someone probing with a meter.

It's not just the voltage; a static shock can be over 4kV. In this case it's 4kV with a huge amount of current available, so the smallest short turns into a huge ball of plasma.

[Stray Electron]

   Another thing that I noticed was that each of the three legs in the box appear to be fused but in the After picture, the metallic center conductor of the fuses appears to be the only parts of the fuses that survived!   It appears that their fuses were massively over rated and that they offered no protection at all and that is why the electrical box suffered so much damage.


    I wonder when and where (what country) where this happened?  For a box that is supposed to contain 4,000 VAC there are no warnings on it, no insulating shields and the wires are small and appear be have only 600 volt (at best) insulation and their conductors are clearly not designed to carry the amount of current that would have been required to do this amount of damage.

   There's clearly a lot more at fault here than just the fact than just the fact that somebody used a shoddy meter on an HV circuit. I'm still deeply skeptical of the entire event as described.  This looks more like a lightning strike on a small modest electrical panel than what was described.

   I for one grew up using those crappy RS meters and so did millions of other American kids but I've never seen anything like this happen because of using one.  If the meters caused this kind of accidents there would have been thousands of dead wannabe electronic types. 

[Stray Electron]

It's not just the voltage; a static shock can be over 4kV. In this case it's 4kV with a huge amount of current available
[/quote]

   Well that's the thing, there wasn't that much power available.  Go back and look at the picture and look at the size wire coming into the top of the panel. It looks like 3 each two conductor and possibly 16 gauge so at the maximum it would have only been rated for about 15 amps of current.  It could carry more current in the event of a short but not many more times more and not for more than a fraction of a second before the wire itself would have exploded.  And of course, the fuses should have blown long before that. 

[Haenk]

So an unexperienced apprentice is left alone working on electric HV installations, with no safety measures in place, neither on the guy nor on the installation? Plus his tool does not meet safety standards of the last 30 years (I assume the US has rather similar safety standards for professionals as in Europe.)
This is indeed sad, and I certainly hope the contractor and the business do get shut down for good.

[robert.rozee]

the article referred to by Mike, dated March 16, 2024, appears to be here:
https://www.ecmweb.com/test-measurement/article/21283647/is-troubleshooting-considered-energized-work
but is only available to site members. however, the content seems to be from an April 1, 2020 article here:
https://netaworldjournal.org/is-troubleshooting-energized-work/
and can be read by anyone. this 2020 article refers back to a case history presentation:
“Who’s at Fault — Owner or Contractor?” presented to the 2002 IEEE Electrical Safety Workshop by Joseph Andrews.

given the story is true, it is very likely the apprentice applied his meter probes to the top ends of the outermost two fuses, and thus the fuses were not 'involved' in the arc event. however, the resulting explosion destroyed the outer shells of the fuses while leaving the inner fusible elements intact. had the apprentice applied the probes to the bottom ends of the fuses, the result may well have been less severe.

like others, i am (a) concerned that the apprentice was left in the situation unsupervised, and (b) horrified by the standard of multimeter he was using. while it was 22 years ago, i would still expect any electrical apprentice to be issued with a Fluke or similar grade of multimeter by their employer.


cheers,
rob   :-)


addendum:
a reworked version of (part of) the 2020 article is located here:
https://netaworldjournal.org/looking-at-nfpa-70e-2021/ (see about half way down the page)
this states "The apprentice technician was told only to tighten things “that were not hot.” The apprentice mistakenly used a 1,000-volt-rated tester to test a 4,160-volt potential transformer (PT) to see if it was energized". i am pretty sure the meter as identified by others is not rated to 1000 volts!

[CaptDon]

Stray Electron, With nearly unlimited energy from the 4160 mains once the arc flash starts it just goes and goes because the plasma has enough resistance to not trip the mains protective devices so the burning continues!!! Yes, that looks exactly like the meter initiated the event!! Your 15,000 volts at 200 amps was the pulse current not the sustained current. That would be a 3 megawatt peak pulse. What was the duty cycle, perhaps .005 max? That would be 15KW average which would be believable for a U.S.A.F. search radar from years ago. With today's quieter MMIC front ends you could search the same area with about 1/10th the pulse power of the old rigs.

[calzap]

As mentioned by Robert, the “short”, i.e. the underrated meter, occurred before the fuses.  No reason they should have blown.  Their outer casings were shattered by the plasma blast.  It is probable that upstream fuses or breakers did react.  But when what is effectively a dead short is applied to conductors at 4 kV with 100’s of amps available, the usual types of fuses and breakers aren’t going to prevent an arc flash.  If you’ve caused a short at more than 12 V than on anything that that was fused, you probably saw a spark at the short as the fuse was blowing.  That small bit of plasma was likely harmless.  Now, imagine it 100,000 or a million times larger.

There is a huge safety difference in applying our common multimeters to high voltage sources like a cattle prod or even a microwave transformer versus industrial switchgear and motor controllers.  Usually, on the lower current equipment, only the meter will be damaged internally.  Long ago, I foolishly tried to measure the voltage of a cattle prod with a Beckman multimeter.  Yup, it died but was fixable.  On high voltage, high amperage sources, ordinary multimeters are bombs.

 Hard to know if the insulation on the three supply wires entering the back of the cabinet was rated for 4 kV.  However, in the “before” picture, they are lying against the back cabinet wall, which was presumably grounded, and they look OK.   The other wires in the cabinet were probably carrying relatively low (240 V or less) voltage for sensing and control.  Can anyone identify the transformers?

The case was presented originally at an IEEE symposium on safety and then discussed in EC&M magazine.  I believe it’s a fairly accurate depiction.  Watch the video; it’s happened before.  On youtube and elsewhere, you can watch an arc flash occur.  They can be incredibly powerful.  This is why working with this kind of equipment live should be done with strong PPE and tools that can be used at a distance.  And never done alone except in emergencies involving a threat to safety of people.

Mike

[jitter]

    I think there must be a LOT more to this story than what the article is telling!  First. what exactly did they mean by "tidying up"?  It already looks pretty clean to me.  Also how did they just happen to have a "before" picture?   Also any meter should be fused and even if it wasn't, 4,000 volts should have just fried the meter and not caused all of the damage outside of the meter and to the meter leads and inside of the electrical cabinet.

   I spend a few years working with 12,000 volt systems and later spend 4 years working on U.S.A.F. Search RADARS that operated at 15,000 volts at 200 amps and I've never seen anything like this.  4,000/ 12,000/ 15,000 volts  really aren't that dangerous IMO as long as you follow the proper procedures.

   I did see one high powered 480 3 phase AC electrical cabinet burn up, similar to this, after the cleaning crew sprayed it directly with a VERY high pressure water hose!

    But IMO the damage in the pictures wasn't caused by someone probing with a meter.

I wonder if those photos were actually correct for the article. Is that wiring on the primary side of the transformers even rated for 4 kV? Somehow, I doubt it.
Having said that, even low voltage can cause impressive electrical arcs as this very old but still relevant PPE demonstration video shows: 



Maybe not so relevant to the thread, but this shows what a failing buried MV (probably 10 kV) cable can cause:


And as an extra, this video shows the impressive arc on a HV station after a human error and a failing protective device (look at how the power lines expand and sag, they even hit overhead lines of the railway network, damaging so much equipment that the rail network couldn't be used for many months):

[CaptDon]

Even a modern day issued DMM, lets think about it?? The probe leads don't seem like they can sustain 4KV safely. Also, what if the arc flash starts inside the meter? The fuses inside the meter probably cannot interrupt 4KV. The internal fuse would probably be a plasma ball and sustain and grow!!! I have seen a plasma arc that I estimate to be over 4 feet in length travel the length of a city block between 2 of the 3 phases of an overhead 4160 neighborhood 3 phase feedline. Most folks can't even begin to understand the available plasma energy available on 4160 and 13,200 feedlines. The big problem with plasma on three phase when all three phases are part of the plasma ball is that energy is ALWAYS available unlike a single phase feed where the voltage will go through zero allowing extinguishing time. With 3 phase the arc sustains as it migrates from phase to phase following the highest available voltage. The arc rotates just like a syncronis motor!!

[jitter]

Agreed. Even those incredibly expensive Bussman KTK fuses that you will find in the better multimeters (like Fluke) are rated to break 100 kA, but only at 600 Vac or less.
I guess even a cat IV multimeter will blow up in your face if you were to connect it to 4 kV or higher.

[robert.rozee]

[...] those incredibly expensive Bussman KTK fuses [...]

those fuses are only active when the meter is measuring current. when measuring voltage it is just a couple of MOVs and a fusible resistor in circuit. for example, from the schematic for the Fluke 77 series II, see below...


cheers,
rob   :-)

[Marco]

What actually happens to a CAT-III 600V Fluke if you connect continuous 4kV to it?

[CaptDon]

And remember, we are not talking about some wimpy 4KV supply from a CRT flyback or CCFL inverter!! We are talking here about (looking at the picture) a cabinet fed from mains 4160 three phase that maybe could sustain 30 amps or more and on a dead short could produce probably over 1000 amps for a short duration. You could have been staring down an arc flash of over 4 megawatts or over 5000 horsepower even if for a second or two. That will ruin anyone's day!!