The Dangers of Working with a High Voltage Supply

[jtr1962]

Really, not to discredit what you said because it makes a lot of sense, however, I thought it was the other way around. I'm fairly sure the pulses of AC will lock muscles above around 60ma.

Read this-directly from the mouth of someone who works on the RR and should know:

"DC current tends to lock your nerves up, which is why electricians touch wires with the back of their hands in case it is charged and they cannot let go. AC(without a significant DC offset) lines do not have this effect because the current runs in both directions and the average direction of current is close to zero."

Another interesting post in that thread regarding the effects of DC:

"I've peeled more than one dead, smouldering body off of the third rail only to find their skin, muscle and other soft tissue adhered to the rail. I've cut metal bars and garbage pails with a torch that have become welded to the rails. And I work with men who carelessly came in contact with the third rail, burned to a crisp and laid in a hospital bed for months.

One was sitting on the protection board and his dangling keys hit the rail. The docyor had to dig the brass key out of his hip bone."

In fact, the entire 4-page thread makes for interesting reading if you have the time. I just picked a couple of highlights. High voltage anything is bad news, but all other things being equal DC is way worse.

Oh, and the AC used in electric chairs may well have had enough of a DC offset to lock muscles. In fact, I'd be surprised if it didn't. Hard to make 2000 VAC without having a DC offset of a few volts, which is really all it would take to lock muscles once the surface resistance of the skin was overcome.

I don't know why electricians wouldn't use a multimeter instead or test probes with a light... seems safer

Agreed although you can still get shocked using a multimeter or test light (yes, it happened to me more times than I care to admit). Now I tend to wear rubber gloves even when working on supposedly deenergized line voltage circuits just in case.

Experience tells me that DC grabs, AC throws. A friend of mine suffers from the loss of the use of two fingers and two thumbs from a 12VDC lighting circuit (amperage unknown)"... perhaps DC is the more dangerous one. Either way I don't intend to test it for the forum

Yep-I've gotten mild shocks touching 12VDC right after washing my hands if I've neglected to dry them completely. Same thing also happened coming in contact with sharp test points of opposite polarity. Once they penetrate the skin, ouch! And the old 9V battery on the tougue trick vividly shows how little voltage it takes to get a detectable current flowing through your body.

 

[Alan B]

DC tends to lock muscles.

AC tends to stop your heart. 60 hz is too low a frequency to have much skin effect, so it gets plenty deep.

I have heard that 60 hz is fairly optimal for heart-stopping. Great choice for the power grid.

I have heard that more people die on 120VAC 60hz than any other voltage. I have not verified that.

 

[Mr Happy]

Where I grew up in England we have 240 V mains supply, and in my rash youth I got one or two shocks from incautious behaviour. What I experienced was a painful sensation something like golf balls going up the inside of my arm, followed by a loudly expressed OW! and a very strong reflex to pull my hand away, much like touching a hot stove. Any screwdriver in my hand at the time was likely to be found stuck in the ceiling afterwards. I didn't experience any kind of muscle locking tendency. But it goes without saying, don't ever do that deliberately!

On a more serious note, the heart danger with AC depends on how the timing of the AC cycles coincides with the electrical waves in the heart muscle. Catch it at just the wrong time and it will stop your heart, other times you can be lucky. It's really not something you want to take chances with.

 

[D-Dog]

Thanks for clearing it up, it makes a lot of sense now (still have to finish reading that other thread but that's scarry stuff). I'm glad DC burns... I'll take a burn any day over my heart stopping...

You also raise an interesting point with the 9 volt story... I remember doing that when I was younger and didn't feel like taking out my battery tester... I've even had one or new 9 volts which stung when tested in the above manner.

I believe that more people die on 120/240, however, it's probably only because of how common they are, nothing more. I bet if you were to setup a chart detailing deaths per shock over a wide range of voltages the results would be more telling Of course when you go up higher in voltage of AC as mentioned before, you can get thrown which may save you/ break a bone or two. With this in mind I would expect high voltage DC to have the highest death per exposure as jtr1962 pointed out

 

[LuxLuthor]

More great information that we don't think about enough while "playing."

Here is another interesting link, including the one hand rule, and life saving procedures at the end.

 

[SilverFox]

Hello D-Dog,

I have enjoyed following this discussion on safety. Thank you for sharing your experiences.

I am going to move this thread over to the "smoke and fire" section, and make it a sticky. There is a lot that can be learned from this discussion and I hope people will take the time to review this thread.

Tom

 

[GeorgiaDave]

Howdy D-Dog,

I've read your thread with a tight sphincter. I cannot stress enough how much you need to swap your love for experimentation for a love of yourself.

Once while making a homemade Ham Radio Amplifier, I think it was using 2-6LF6 tubes to drive 4-6LF6 tubes (I'm pretty sure that's what it was?). Anyhow, 24 hours after unplugging it I grazed my forearm on an output wire and woke up flat of my back almost an hour later. I knew full well that I should ALWAYS ground-out the collector braid, but I got careless and thought that 24hrs should surely allow the voltage to bleed off.

Experimentation is the mother of invention, however carelessness is the fast lane to regret.

Don't stop experimenting, however NOTHING is worth losing your life over. Stay safe. g.d.

 

[Cesiumsponge]

Extremely high voltages do strange things...like punch through insulators and snake around barriers.

I've worked with a lab-sized ~750kV-1MVDC Van De Graaf power supply for demos and I've been bit even using a 1 meter long chickenstick. Of course, it's about the furthest thing from lethal given it's small amount of stored charge in the collector, but even a few microamperes is enough to wake you up. Work with tesla coils as a hobby too.

Worst bite was 7200VAC 60mA on a secondary winding terminal to the center-tap transformer core, hand-to-hand, across the chest. I hit the rocker switch on the variable autotransformer to turn it off before working on it and apparently didn't hit it hard enough to disengage fully (those rocker switches have a little bit of spring-snappiness to them...like hitting a light switch but it doesn't fully snap into the "off" position). Skin offers no resistance at high voltages, and once an arc carbonizes your skin into your juicy inners, you become a pretty good conductor :nana:

People say getting shocked "throws you" when you read their recollections. In reality, it's your convulsing muscles that tosses yourself across the room. Burnt flesh from the arc on my fingers, a racing heart and adrenaline for the next 30 minutes, and waking up sore the next day...a very cheap lesson on respecting high voltages. It could have easily cost me much more.

I think everyone gets bit at least once before gaining a healthy fear...just like you get burnt on the stove at least once. Lets just hope that bite is a small nibble and your only bite.

 

[Tally-ho]

Worst bite was 7200VAC 60mA on a secondary winding terminal to the center-tap transformer core, hand-to-hand, across the chest. I hit the rocker switch on the variable autotransformer to turn it off before working on it and apparently didn't hit it hard enough to disengage fully (those rocker switches have a little bit of spring-snappiness to them...like hitting a light switch but it doesn't fully snap into the "off" position).

Even if it could save your life, you can't trust a rocker switch because of failure. EDIT: Sorry, i thought that a rocker switch was an "emergency switch". Never trust an emergency switch too.

You must do, what it is called in french, "une consignation", you must remove to service the transformer and i suppose that, like in France, you have to follow a well defined electrical safety policy.
I don't want to preach anyone but do not mess with tension above 24VAC and 50VDC in dry conditions or 12VAC and 30VDC in wet conditions. Anything above can be lethal very quickly.

Electrical Safety Policy (google search results)

Before doing anything on a transformer or exposed (unisolated part of) wires you must at least follow this:

- Cut off by disconnecting it (plug, wires, ...) from the source.

- Test each phase conductor or circuit part with an adequately rated voltage detector to verify that the equipment is de-energized. Test each phase conductor or circuit part both phase-to-phase and phase-to-ground. Check the voltage detector before and after each test to be sure it is working.
In french, it is called doing a "V.A.T." ("Vérification d'Absence de Tension").

- Properly ground all possible sources of induced voltage and stored electric energy (such as, capacitors) before touching. If conductors or circuit parts that are being de-energized could contact other exposed conductors or circuit parts, apply ground-connecting devices rated for the available fault current.
In french, it is called doing a "M.A.L.T." (Mise A La Terre)

Never ever start a work before doing a "V.A.T."...really, no kidding, a "V.A.T." takes a "few seconds" only to avoid permanent injuries or death. (Never do a "V.A.T." with a cheap multimeter = )

 

[Cesiumsponge]

Hindsight is always 20/20. That was a long time ago. Statistically. I'm way more worried avoiding the careless drivers on my daily commute

 

[theChipmunk]

I guess the rule, "always remember your rubbers" work for electricity too although a condom probably isn't rated to too high a voltage... I wonder if a static shock could get through

I wonder what our lab technician would say if I tested that in the lab on Monday...? Actually (she's a very liberal and nice woman), she probably wouldn't bat an eyelid - the PhD students do worse, I'm sure.

Whilst, I've seen the one-hand rule mentioned multiple times, I don't think anyone has mentioned that it's best to use only your right hand, the thinking being that if current flows, it will flow (hopefully) down the shortest path, through your right side and down your right leg, missing your heart. That was always what I was taught, at any rate. Not sure if it's a dodgy theory, but I always approach HV equipment with my left hand behind my back. It may look odd, but hey.

In several years of working with 10kV+ equipment (bog-standard PSUs, van der Graffs and a few X-ray tubes), I've managed to avoid a shock...however, when working with EDLCs (Electric double-layer capacitors, aka supercapacitors) a year or two ago, I got one nasty shock and, in another incident, a slightly carbonised radiator pipe and an annoyed mother. To be honest, the darn things liked to pop, which was probably more dangerous than the voltage/current combo.

Finally, for a school play several years ago, an antique looking flash was required. Funnily enough, the school weren't hot on magnesium powder being ignited on a wooden stage, so I was asked if I wouldn't mind cobbling something together and sticking in the appropriate housing.

I thus went to the nearest photo developers, asked for the disposable camera bodies with flash that had been discarded after the film had been taken out and proceeded to dissect them. I used several xenon tubes and soldered a couple of caps to each, since it was only needed for three nights and I was told it needed to be as bright as possible. (It needed to compete with the new 1kW lights that our teacher in charge of lighting had just purchased! ) Anyway, testing was going fine - I would fire them off to test and if they didn't work, discharge them through a resistor to ground and then figure out what the problem was.

Thing is, I had to get my friend to help me. It was grand up until he slipped with the pliers and connected my hand to the whole shebang before I'd discharged it! Thankfully it was only a nasty jolt!

It didn't do much for the pliers, though, which was more painful when our technology teacher whom I'd borrowed them off found out...

Moral of the story? Get a friend who has a decent attention span!

 

[Freax]

While building this Scott17 KT88 SE amplifier there was a miswritten step involving plugging the unit in and checking the voltages, without the tubes plugged in, that was the miswritten step, because of this the voltage never went down quickly but instead crept down, and I was left with a very dangerous amplifier with tons of exposed wiring laying on my desk waiting to fall off, charged up with 551 volts DC.

There was 551volts at about 120uF all charged up and ready to go.

So not having any large bleeder resistors within the ohms range available I did the only safe thing I could think of, With it unplugged and waiting for the voltage to go down I put one hand behind me while looking away (but still looking) and cowering around the corner of it to get my body and my head away from any shrapnel and grabbed a really thick screwdriver and I bridged the two terminals on the capacitors. (I was afraid one of the bridged wires would explode due to an excessive amount of current.)

I put the very well insulated screwdriver handle onto one of the terminals and slowly brung it over to the other one, contact was made and BANG!!!!! It gave off such an almighty big bang that my ears rang for 30 seconds afterwoods.

Anyway here is the end result of my (and Scott17's) hard work:

Some long exposure shots:

I'm very very happy with the amp, its given me now hundreds of hours of listening pleasure and it was fairly easy to put together.

It also has 0% hum, 15 watts per channel, 2 modes Triode or Ultralinear, and custom-wound transformers throughout.

All for $1,000 in kit form.

 

[325addict]

that first power supply must have been a bad one. Indeed, it takes just ONE diode to protect the regulator (being a MOSFET or a BJT), even the smallest PSU I will design for 50mA audio purposes has such a diode...

I also shorted a high-energy power supply once. I tried to discharge two huge capacitors (47,000µF/100V) that were really charged to 96V. A very safe way to discharge them SEEMED to use a 230V / 100W household bulb, and hold it so long to the screws of the capacitor that it no longer glows. Upon trying to get it connected, the lamp suddenly moved over the contacts and I shorted the capacitor with the base of the bulb. An extremely loud and sharp <BANG!!> was the result, as well as a large hole in the aluminum base of the bulb.

 

[ledlight]

Just paraphrasing some of my training.

The resistance of the body is pretty low, only the skin has high resistivity. People have died from single digit voltages in cases where the skin was penetrated.

Water soaked skin has much lower resistance. 120V and water proves to be very lethal.

..........

Is water and 120v really as dangerous as it is commonly believed ? This experiment called Electrocution in Water seems to prove otherwise.
http://www.youtube.com/watch?v=dcrY59nGxBg
And in no way am I trying to disagree with the utmost safety when using electricity AC or DC.

 

[inetdog]

Water and 120 volts AC can indeed be very dangerous if the human involved offers an alternate path for the current involved. One finger in the water will not end up carrying much current.
Someone with the other hand connected to the 120 volt neutral would have a different experience, especially with water which is not chemically pure.
Note that the video apparently requires proof of age which I could not provide from my phone, so I am going mainly from the photo.

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