The Importance of Electrical Safety

Everything you do bears some risk of getting you hurt or killed. That’s just the way life is. Some people drown in the bath, and others get kilovolt AC across their heart. Knowing the dangers — how drastic and how likely the are — is the first step toward mitigating them. (We’re not saying that you shouldn’t bathe or play with high voltages.)

This third chapter of an e-book on electronics is a good read. It goes through the physiology of getting shocked (DC is more likely to freeze your muscles, but AC is more likely to fibrillate your heart) and the various scenarios that you should be looking out for. There’s a section on safe practices, and safe circuit design. It’s the basics, but it’s also stuff that we probably should have known when we started messing around with electrons in bulk.

There were a couple of things that seem common sense, but are worth repeating. Since your arms are essentially resistors in parallel, touching something at high voltage with both hands passes twice the current through you as doing the same with one hand. Hence the keep-one-hand-in-your-pocket rule.

Rings, jewelry, and anything metal that’s connected to your body dramatically decrease your resistance. While 30 V is generally the threshold where skin resistance alone isn’t enough, you can get shocked badly by only 12 V if you’re sweaty and wearing a tight metal watchband. Do the math.

For more practical tips, focusing on mains voltages, we really like [Jenny List’s] two-part series on working with mains voltages.

77 thoughts on “The Importance of Electrical Safety

    1. low voltage dc can still sting pretty bad. i was a mechanic for around a decade. pretty shit job. lost the wife but still have the scar from the wedding ring that i somehow shorted across a battery.

  1. Elliot, that’s a pretty good read (I didn’t read it in depth). But it’s also timely as we have a new member of the hacker space who wants to play with high current (140A) toys (and some high voltage (>48v) toys. We’re not trying to dissuade him but make sure he stays on the correct side of the components. ;-)

    1. Teaching the best practice and precautions is always the best idea, People that frequent hacker spaces are the type of people to give it a go either way. So dissuading isn’t really any good, Glad you are trying to help out.

    2. Its intersting how your background determines what you see as dangerous. Us welders frequently wave around 200+ amps 90+ volts (not counting hf start) both ac and dc

      1. i have got jolted many times when hot, sweaty or damp while welding, you really have to watch open circuit voltages, when welding the grounding lead and the lower welding voltages reduce the risk greatly

        1. That assumption doesn’t sound right to me. I’ve often heard that your heart is on the left side but a while back when researching something I discovered it’s actually centered between your lungs and tilted a little to the left. Perhaps the “left” thing comes from the fact that the large pulsating chamber is left of center https://en.wikipedia.org/wiki/File:Real-time_MRI_-_Thorax.ogv. More relevant to this discussion, the blood vessels that go to the heart are connected to the top half and go to both arms. My understanding is that those blood vessels are the main pathways for electrical current from the arms.

          1. The location of the heart is largely irrelevant. Blood is an amazing conductor, and as you said, both arms connect to the same spot. The trick is keeping the lethality from crossing the chest (more importantly, the heart). Ideally you’re going to keep the lethal away from your body, but sometimes accidents happen (or too much risk is accepted).

    1. If you hold your (earthed) equipment with your left hand you just created a path from your right arm trough your heart and to earth (or other potential) via the left hand. So the left hand may be closing the circuit, that’s what I learned about the left hand in pocket rule

      1. For the record, the “across the heart” story is the way I was told the two-hand rule as well. I’m just not sure it holds up. I bet you’re just as dead with one foot in the bathtub and a hand in your back pocket.

        The scenario in the book — grabbing a “hot” pipe with two hands — puts no differential voltage across the chest. It’s all hands to feet, which will probably get you in the heart. And there it’s safer to have 2x the resistance (provided by one hand).

        The one-hand rule also prevents you from resting your free hand on something grounded if you’re in a lab with isolation transformers.

        Which is all to say, even if the “across the heart” logic is flawed, it’s a darn good practice.

    2. I believe the reasoning is that when you have both hands in the cabinet you are more likely encounter live wiring. Depending on the circumstances it would be possible for the current to enter a hand and exit through the feet.

  2. “(DC is more likely to freeze your muscles, but AC is more likely to fibrillate your heart) ”

    Incorrect. Muscles are insensitive to DC because the nerves work by pulses, not by steady state currents.

    1. “Hence the keep-one-hand-in-your-pocket rule.”

      Also incorrect. The hand in pocket rule is so that you aren’t grabbing the (earthed) chassis of a device while touching the internals, which would lead to a shock across the chest and a cardiac arrest.

      1. As someone who woke up under a bench on the opposite side of the workshop because I accidentally came in contact with the capacitor bank of a 10kV 30kA 8/20us lightning test generator – yes it can indeed throw you across the room.

    2. Where did you get that garbage from? DC is widely known to cause people to hang up far stronger than any AC source. It is also more likely to kill you through blood clots than stopping your heart mainly because it doesn’t disrupt the signals to your heart as much and it causes people to hang up longer.

        1. For example:

          http://electrical-engineering-portal.com/what-psychological-effect-does-an-electric-shock
          “How AC affects the body depends largely on frequency. Low-frequency (50- to 60-Hz) AC is used in US (60 Hz) and European (50 Hz) households; it can be more dangerous than high-frequency AC and is 3 to 5 times more dangerous than DC of the same voltage and amperage. Low-frequency AC produces extended muscle contraction (tetany), which may freeze the hand to the current’s source, prolonging exposure.”

          “DC is most likely to cause a single convulsive contraction, which often forces the victim away from the current’s source.”

          No mention of DC causing tetany, or “freezing”.

          1. However, the article then contradicts itself on the conclusion, saying DC is more likely to cause tetany.

            The confusion seems to be over whether by “freezing”/tetany you mean incapacitation by paralysis or by cramp. DC will paralyze a heart, or any muscle, but AC will make it contract continuously by repeatedly reversing the nerve polarizations.

            That means, when you grab a wire with DC on it, your hand paralyzes and you can’t open your fingers, but you may be able to pull your hand away. If the wire is carrying AC, your hand will actively grab the wire and you may not be able to pull free.

        2. I question that too having been zapped at least a couple of times by 240V DC. and many times by 240V AC. The most noticeable thing about DC is it feels like your flesh is burning from the inside out. AC is pleasant in comparison to DC.

    3. Nerves actually work by polarization. DC polarizes your nerves so they no longer function. That’s why you can’t let go of a high voltage DC cable as it cooks you.

      1. Yes, and muscles require continuous nerve -impulses- to keep contracting. That’s why DC doesn’t make them “freeze”.

        DC knocks out the signal path, so you lose control of the muscle. DC is most likely to cause a SINGLE convulsion on the intial shock, and then the muscle goes limp.

        1. No please see my earlier comment directed at you…

          DC bypasses neural signalling and depending upon the level of voltage &
          resulting current can cause seizure – which locks up ie clamps upon the
          conductor depending also on posture & circumstances & reflex actions…

    4. Dax stupidly claimed “..Muscles are insensitive to DC…” – ugh, RUBBISH !!!

      Yet again you Dax FFS are so off beam its scary with your facile flakey random
      thoughts without any foundation based only upon immense ignorance of
      straightforward neural biochemistry & frankly now Dax you are supremely
      guilty of scientific misconduct which immaturely puts people at risk by such
      willfully misplaced idle claims negligent of potential consequences !

      Please people, disregard the ramblings of the mindless anonymous Dax
      who has minimal cognitive understanding of risk & consequences & beware
      DC even at as little as 12v in automotive voltages as I will articulate later on
      & especially relevant if you have any sort of heart condition whether present
      or familial. DC has rather more danger than AC & not just academic as I
      have experienced in the jungles of East Malaysia etc More later…

      For the time being, to address anonymous Dax’s utter ignorance:-

      Neural signalling is NOT electrical its based fully upon the minimal energy of
      a Na/K concentration differential traveling along neural axons/dendrites
      membranes submerged within a soup of inter-cellular electrolyte buffers
      which enable repeated pulsing without expensive chemical refreshing…

      I repeat NOT an Electrical Pulse by any means as Dax badly claimed, its
      purely a local (traveling) electrolyte concentration change which is why it
      doesn’t travel anywhere near the speed of light, travels according to a max
      speed in which the electrolyte balance is restored which is rather slow ie mS !

      Applying direct current to nerves & muscles of sufficient voltage to impress
      even minimal current flow drastically overwhelms the Na/K balance resulting
      in catastrophic & cascade changes in membrane permeability in that the
      current can pass by direct electrolytic conduction inside & outside the axons
      far faster than the Na/K “pulses”. This also overwhelms the muscle activation
      receptors causing a cascade as the significant electron current spreads
      through the muscle causing seizure & which often does not necessarily release
      quickly upon removal of the current.

      Such seizure is easily sustained so it is true that, depending on the voltage
      & resulting current flow & the physical position of the person (eg reflex reaction)
      that the subsequent victim cannot release the conductor – such as if their
      hand were clasped around the conductor & the other point of contact were
      not removed due to seizure reflex. Occurs also where the limbs seize upon
      being brought to the torso which can also “hold” a conductor causing
      sustained exposure – even whilst the victim is conscious & cannot release
      their posture and suffer from the immense pain as tissues can subsequently boil !

      Should this DC be continued it results also in various secondary polarisations
      which can cause irreversible damage to not only axons/dendrites but also muscle
      contraction receptors resulting in longer term weakness in those muscles exposed
      as well as secondary effects re denaturing proteins & consequent infections.

      I have specific training in this area as a qualified electronic engineer & qualified food
      scientist (which included microbiology) from Curtin University, Bentley, Western
      Australia, my student number is 7602128 & anyone can check my background.

      I take a very dim view of the likes of Dax who spout rubbish here & under his other
      nickname on phys.org & will address their ignorance head-on whenever possible
      and especially so as it can so easily put people at significant risk !

      Please Dax, own up & apologise for misleading on this forum ?

      In respect of my specific experience re 300+ V DC in East Malaysia I can report
      DC will (generally) more easily find a sustained path due to its polarising nature
      not generally present with AC unless certain chemical thresholds are exceeded.

      As such please beware DC beyond the vaguely accepted danger threshold of 48V
      and that even far below this such as at 12V can be a danger to some which is easy
      to demonstrate – if you dont have a heart condition that is, more later & response
      dependent upon replies here

      Cheers for now…

      Mike Massen

      1. What a colossal ASSHOLE you are. Please stop inserting opinions and claims onto other people and then “debunking” them by rattling off a wall of irrelevant junk. All the rant about nerve damage and such may be correct, but wasn’t under question in the first place! – only the notion that DC current makes your muscles “freeze”. It doesn’t.

        >”I repeat NOT an Electrical Pulse by any means as Dax badly claimed”

        I didn’t claim so!

        I said just “pulses”. A muscle contracts upon recieving a continuous train of nerve impulses, and if such impulses stop coming then the muscle stops contracting. DC by its very character causes ONE impulse to travel down the nerves and then it polarizes the system so no further nerve impulses may travel – the muscle is therefore incapacitated. It is not “frozen” or cramping, but out of control, limp.

        >”Such seizure is easily sustained”

        But not by the DC current. The muscle cells will eventually release after the initial shock dissapates. This is in complete contrast to continuous AC which will cause your muscles to cramp and hold on indefinitely so that you can’t even pry your hand open.

        1. Dax,
          The knowledge of neural operation is not “junk”, its easily verifiable.

          Making simplistic banal statements ignorant of such knowledge is not
          useful or helpful, it merely misleads & puts people at risk, such as your claim:-
          “..Muscles are insensitive to DC because the nerves work by pulses,”

          is demonstrably FALSE, Muscles ARE sensitive to DC, you should be
          more careful making such claims when people’s safety is at risk, geesh !

          And one of your further idle claims:-

          “and if such impulses stop coming then the muscle stops contracting”
          is not necessarily true, there is no such absolute, please read up on it,
          learn to be more precise & swearing doesnt help your case one bit…

          1. >The knowledge of neural operation is not “junk”, its easily verifiable.

            Indeed, but that wasn’t junk part of your “rebuttal”.

            >”is not necessarily true”

            Is too. Muscle cells will not continue pulling indefinitely unless the signal potentials are reset. With a steady state DC, the muscle will inevitably go limp. DC eventually results in the paralysis of the involved muscle – not a sustained cramp.

          2. Plus, the reason why I said that muscles are insensitive to DC is because they are – it takes a significantly larger DC current to disrupt the relevant nerves. The sensory treshold for AC is 1 mA and a complete tetany of a muscle happens above 20 mA whereas it takes up to 300 – 500 mA to do the same with DC.

            It’s on a different order of magniture entirely. It’s very difficult to pass half an ampere through the human body from relatively low voltage sources. For example, from 48 Volts it would require your body resistance to be as low as 100 Ohms which is very unlikely.

        2. He came across quite harsh before the fact dump….
          I have heard some first-account stories about HV-DC:
          Sometimes they grip-stick to the source,
          sometimes they go limp (complete body limp) but most of the time…
          a large muscular energy movement occurs:
          I.E. they literally throw themselves across the work-space (Probably due to source disconnection and sudden gain of brain-body control).

          Personal experience though: Built a 48v (Nominal, 50v+ when charged split for 2x 24v center-tap), I was stripping the wires with my teeth, I’ve done it before without incident, but the last time I stripped wires for such a battery pack a wire must of brushed the cheek and I saw a bright flash, I couldn’t throw 8KGs+ of battery any harder than I did that day.
          My cheek went numb for a few hours, ached for a couple days and I decided on a less Darwinian approach at building battery packs from that point onward.

          1. The let-go-treshold for AC is 22 mA and is considered to be 88 mA for DC. It takes about four times as much DC to cause involuntary grabbing.

            The reason why people grab the wire is because all the muscles in the arm are activated, but the muscles on one side are stronger. This doesn’t always happen.

          2. On a personal note, I was troubleshooting a camera with a dodgy flash and had the covers off. I didn’t realize that the capacitor voltage was present in a mechanical switch and touched the contacts, and instantly smashed the camera to a wall.

      1. I agree fully Nate B :-)
        Just took a peek prompted from your comment as I had initial difficulty
        with this issue decades ago appreciating its consequences regarding
        design criteria – especially re electromagnetic compliance (EMC) & radio
        frequency interference (RFI) & disturbance management at board
        level with simple things like distributed RC networks adjacent to CMOS etc

        Although the chapter is very much complete in many respects, I do like the
        challenge of augmentation therefore I feel it might be worth adding a paragraph
        re an aspect of the momentum of current flow as a mechanical (mass) analog.

        I guess primarily for those who have a grounding in Newton’s laws as well as
        reflecting upon Maxwell & Einstein’s relativity given the velocities involved,
        then this might well induce some to develop the capacity to resist traditional
        views which might shed light on conducting improved educative intent ;-)

  3. Twice over the years I have sat on health and safety committees where we have had to investigate a fatal electrocution in an industrial setting. In both cases, sloppy procedure was the proximate cause of these accidents, not ignorance. Everybody directly involve was a trained, certified professional. While understanding is important certainly, discipline is what is going to keep you safe.

    1. “fatal electrocution” – is there another kind of electrocution?

      I was “that” worker who everyone gets annoyed with because I would badger other workers to follow safety procedures and *NOT* cut any corners. If I have to put my life in their hands at times then I am going to make sure they have safe working habits. Complacency is the biggest killer.

        1. Survived many so far. More luck than judgment. Many as a child, dismantling tvs, radios, vcrs, microwaves, vacuum cleaners, you name it. These days, I prefer a nice rcd. Still test pp3s on my tongue.

        2. “Electrocution” was originally defined as death by means of electric shock (originally actually “execution” by means of electricity). But now it’s ruined and means “I felt some static electricity discharge” to most people.

          1. Formally, the words electrocute and electrocution always implied fatality. Informally, however, these terms are rather often used to refer to serious but nonfatal electric shocks and are used in that sense in accident and incident reports. Keep in mind that English assigns meaning to words by usage – there is no proper definitions for words as there is in French, for example.

        3. From the wording I always understood “electrocution” as fatal – the word stem “-cution” comes from “execution”. “Electric shock” or similar I understood as non-fatal incident.

      1. Generally speaking, people that don’t understand electricity approach it with a very healthy caution, and it is rare that they have an accident tampering with it. Those that know enough to feel confident working on a live circuit usually have some degree of understanding of the risks, and the basic notions of not letting themselves become a conductor. But like the pros, they get sloppy too and it is this inattention to detail that almost always is at the root of any incident, not ignorance per se. The point here is that, very much like firearms, safety in electrical matters depends less on knowing the physics, and more on paying attention in the firm knowledge that you are not likely to get a second chance.

  4. Part of my career was working with high voltages up to about 50kV.

    Now I just work with low voltage stuff so the highest voltage is the mains voltage (power point) before a step down to under 20 Volts.

    I don’t bother making PSU’s any more. I just use a pre-made plug pack (wall wart). That way I don’t have any personal risk and best of all, I don’t have any liability for other people as long as the plug pack or wall wart is certified and or approved.

  5. I recall learning extra hand in your pocket and elbows bent (when arms in use).

    Isolation transformers are very important on live ground equipment.

    Also I’ve had good luck with GFCI keeping me from blowing up a laptop that I forgot to pull the power supply from. I really need to add that to my test bench in the garage.

  6. Throws a charged capacitor… Catch. :D
    I enjoy building Tesla coils and various other high voltage or high current things.
    I try to avoid shocking myself too often. Note to self, a big stack of forty 3V lithium coin cells packs quite a punch. 230V mains buzzes a bit to feel it in your forearms for a few minutes. 415 on the other hand, just don’t even take the chance.
    Remember the skin effect. At high frequency things don’t conduct in the way they would with dc.

    1. I took 400 volts DC from hand to hand once. Was working on a military microwave preamp with ‘lighthouse’ tubes – no glass, just ceramic and metal. They have heat radiating fins at anode potential. Forgot to turn off the B+, put my left hand flat on the nice sturdy aluminum chassis, and reached in to pull one of those tubes. It didn’t throw me off my stool, but I experienced quite a convulsion, and it really rattled me. Afterwards I just lay my head on my bench and rested for a few minutes. I was fortunate that it wasn’t a Darwin moment, and I’ve been much more careful about such things since then.

  7. I’ve been tingled with 120V and 240V, no ill effects.

    I was working for a commercial electrician for a time. We were replacing a 3ph motor with a higher hp motor, and needed to change out the contactor and wires to handle the higher amps. Electric company had the power to the plant shut down at the pole while we did this. My boss saw that I was closing up the wire chases, and tole the supplier to go ahead and reconnect. I wasn’t ready for that, as I still had to reposition a few wires to close the last section. I woke up a few minutes later on the floor, with a nasty burn across the palm of my hand where the blade of the screwdriver had been, and the entire installation was black and still smouldering.

    Turns out one of the wires at the back of the chase had a section of heat compromised insulation, and the movement in the chase allowed contact.

    This was just the 480V section. The entire section – disconnects, fuseblocks, contactors, etc had to be replaced, due to my BOSS’S impatience. If I hadn’t been so pressured timewise, it is very likely I would have caught the damaged wire before closing that section.

    Arc flash is nothing to mess with! I got extremely lucky with that accident. By all rights, I should have been killed. I thought I was safe enough pushing the wires around with the handle of the screwdriver, holding the shaft/blade. WRONG! Lesson learned.

    BTW, the replacement cost of that entire room of equipment was NOT covered by his insurance, after they determined that he was at fault. That job bankrupted him.

  8. When I was younger there was an old time arcade by the local lake at a small Disney type park rides and so on.
    In the arcade was a machine with two handles. You popped your quarter in grabbed both handles and turned one to see how much current you could take.
    I often wonder how those who just returned from a swim did at the test.

  9. theres a bad trick i use when working with potentially charged picture-tubes, if the goal is scrap circuit-boards.

    instead of cracking the nip, and shorting the 2nd anode (to outside coating), i just stand on several inches worth of weak insulation, combined with several layers of weak insulation over hands and yank that 2nd anode right off, but i make sure to keep my (left) hand in the air and only stand on my right leg. i also go barefoot until im completely ready to keep my feet/socks/shoes free of traces of sweat. I ALWAYS CHECK IF THERE IS A PROPER RIMBAND INSTALLED BEFORE ANY ROUGH PLAY AS A LACK OF RIMBAND IS DEADLY.

    PS: NEVER EVER REMOVE A RIMBAND FROM A PICTURE TUBE, doing so turns a minor hazard into a impalement hazard, as glass can rocochet off of other pieces with enough force to go through walls(or you). the location and thicknesses of the tube walls and band are tied together in an attempt to reduce weight but still make it safe to break while in the same room as living persons. you have been warned.

    PPS: those stupid warning stickers talk of “mechanical implosion protection devices” as if it’s black-magic: its a freakin metal strap that LOOKS like its only for mounting BUT ITS FOR IMPLOSION CONTAINMENT!!!!!

    1. maybe i should add in the fact that the 50kv rated wire sometimes may develop a microscopic crack that eventually “fills” with air, air is not rated to 50 kv and so if the wire is defective (and the TV can work for years like this) when you grab that thing its like theres no insulation at all, once the air inside that little crack ionizes and goes low-impedeance, its the flesh that takes the heat-energy.

      so in the case where the tube’s 2nd anode does not bleed to ground when off you can get a kick just by grabbing the defective wire.

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