Looking Mains Voltage In The Eye And Surviving

It is often a surprise to see how other people react to mains electricity when they encounter it in a piece of equipment. As engineers who have dealt with it both personally and professionally for many years it is easy to forget that not everyone has had that experience. On one hand we wince at those who dive in with no fear of the consequences, on the other we are constantly surprised at the number of people who treat any item with more than a few volts in it as though it was contaminated with radioactive anthrax and are scared to even think about opening it up.

We recently had a chat among the Hackaday writers about how we could approach this subject. The easy way out is to be all Elf-and-Safety and join the radioactive anthrax crowd. But the conclusion we came to was that this site is a resource for hackers and makers. Some of you are going to lift the lid on boxes containing significant voltages no matter what, so we thought we’d help you do it safely rather than just listen for the distant screams.

So here follows the first in a series on how to approach electronic devices containing high voltages, and live to tell the tale. By “high voltages” we mean anything up to mains voltages, and those directly derived from them such as the few hundred volts rectified DC you’ll find in a switch-mode PSU. For multi-kilovolt EHT you’ll have to wait for another article, because that is an entire subject in itself. We’ll mention these higher voltages in passing, but their detail is best left for a Hackaday colleague with more pertinent experience.

The Warning

Sign, reading: DANGER! NOT ONLY WILL THIS KILL YOU, IT WILL HURT THE WHOLE TIME YOU ARE DYING.The very first point to make clear when writing an article like this one is that you should disregard any ideas of “safe” voltages or currents. While the story about some significant number of people who die every year from licking a 9-volt battery is probably an urban legend and you have little to worry about from low voltage equipment, it is best to look at any higher voltages as potentially dangerous and react accordingly.

If you had a mains-voltage shock once and got away with it, you were lucky, that doesn’t mean the voltage is safe, you were merely playing a game of Russian Roulette with Ohm’s Law and a low impedance high voltage supply. Your domestic mains can dump the hefty amounts of current your home heating, your cooker, or your electric kettle demands, so if it finds a low resistance path through you then it is going have no problems dumping whatever current Ohm’s Law allows it to through that path. The chances are if it happens to you the path will be a high enough resistance that you’ll only get a very nasty jolt and live to tell the tale, but if it’s not your lucky day the resistance will be low enough that you’re just going to sit on the end of it twitching until the power is turned off, whether you’re alive or not. That’s the gruesome truth. Mess with this stuff and you can die, end of story. You are responsible for keeping yourself safe, and this is not a joke. OK? Now to work!

The Workbench

The place to start when talking about mains-voltage equipment is your bench. We realise you’ll probably be working at your kitchen table or wherever you happen to be, but it’s best to start an article like this with a baseline, of what you really should have when doing this stuff. There are three things we’d consider essential in the mains power system on a mains-voltage bench. An isolation transformer to power the item you are working on if it needs a mains supply, a residual current circuit breaker for the rest of your mains supplies, and an emergency isolation switch for all bench power.

Isolation Transformer

A mains isolation transformer. wdwd [GFDL], via Wikimedia Commons
A mains isolation transformer. wdwd [GFDL], via Wikimedia Commons
An isolation transformer isolates the mains supply from the earthing system. This does not change any of the risks inherent to the mains voltage or other high voltages in your device, but it does provide you with some level of protection should you accidentally provide a path to earth from a live component.

It’s probably worth at this point explaining for a minute how earthing works with respect to mains power. You will have a local earth connection at your house, and your utility company will connect the neutral line to an earth at the substation to ensure that line voltages are not induced at a higher voltage with respect to their surroundings than the voltage they are supposed to carry. There may be 110V or 230V between the live and the neutral depending where you live, but without that earth connection both of those conductors could end up at thousands of volts higher than their surroundings, for example in a thunderstorm. The earth connection provides a fixed relationship between line voltage and the surroundings, such as the utility poles, trees, your house, and you.

Earthing is thus a vital part of power distribution safety. The only snag with open equipment on your bench is that any earth connection becomes a valid part of the return circuit for the power, and since that earth connection can come through you, that’s dangerous. The isolating transformer breaks that earth circuit for your bench, thus removing that particular hazard. The upshot of this is that you can safely touch either one of the two wires and because there’s no path to ground, you won’t get zapped. (Touching two wires completes the circuit. You still have to be careful!) Isolation transformers are also used to lift the ground so that you can connect mains circuitry up to your oscilloscope, even though the black probe clip is connected to earth ground.

Residual Current Circuit Breaker

A residual current circuit breaker. Jimbob82 [Public domain], via Wikimedia Commons
A residual current circuit breaker. Jimbob82 [Public domain], via Wikimedia Commons
A residual current circuit breaker compares the current flowing in the live mains conductor with that flowing in the neutral. In a normal situation these currents will be identical as current flowing out one must flow back in the other. If they differ, it is likely that the missing current is due to a hazard or fault, and the circuit breaker is activated. So if the power shorts to earth through your body rather than the normal return path, it is detected and cut off. It is important to understand that a residual current circuit breaker will not protect you from circuits on the other side of an isolation transformer, however this is to provide some protection for the other circuits on your bench. You may well find that the wiring codes in your country mean you already have a residual current circuit breaker in place.

Emergency Isolation Switch

Emergency isolation switch. Santeri Viinamäki [CC BY 3.0], via Wikimedia Commons
An emergency isolation switch. Santeri Viinamäki [CC BY 3.0], via Wikimedia Commons
You will be familiar with emergency isolation switches if you have worked with large machine tools. A big red switch designed to be easy to hit to turn all the power off. You may want to hit it if something gets out of hand, or someone else may want to hit it for you to turn off the power that is electrocuting you. Either way it provides a very quick way to manually make any mains sources on your bench as safe as possible in the event of an incident. Remember the bit we mentioned earlier in about sitting on the end of the line twitching until the power is turned off? This is the big red switch that makes it very easy for someone to do that for you.

The Tools

The markings on a screwdriver insulation tested to 1000V
The markings on a screwdriver insulation tested to 1000V

Having sorted out your bench, how about your tools? The chances are you’ll already have everything you need in this department, but it’s worth saying here. If you are using a tool or an instrument on a piece of equipment carrying electricity, it has to have the appropriate insulation properties for the voltage in hand. Perhaps you have to have experienced insulation breakdown in a cheap test probe at first hand to truly understand this point.

A decent quality electrician’s screwdriver will have its insulation voltage rating stamped on its handle, though the absence of a rating is not necessarily a reason not to use it. A lot of screwdrivers have first-rate insulated handles, but without a voltage rating because they are sold for general-purpose use rather than specifically electronic. When you choose a screwdriver or other tool, consider the amount of plastic it puts between you and the metal, and pick accordingly. Wooden handles may not provide adequate protection, and metal handles should be avoided at all costs.

The IEC61010 category ratings on an Agilent multimeter. Medvedev [CC BY-SA 3.0], via Wikimedia Commons
The IEC61010 category ratings on an Agilent multimeter. Medvedev [CC BY-SA 3.0], via Wikimedia Commons
Your instruments such as multimeters, oscilloscopes, and their probes will all carry an appropriate safety rating for maximum voltage, surge voltage, and current. In modern equipment this will be denoted by specified classes each with their associated voltage defined under IEC 61010, though you may find older equipment with IEC348 or other national standard ratings (Fluke publishes a handy PDF explaining these standards in detail if you would like to know more). These ratings are important in ensuring that the instrument keeps you safe as you use it.

However there is a snag. In recent years there has been a flood of cheaper instruments arriving on the market, and some of the manufacturers of these devices have been found to put IEC 61010 ratings fraudulently on items which do not in any way meet those standards. We have covered tales of dodgy cheap multimeters and test failures more than once here at Hackaday, unfortunately this is a widespread problem. So please bear this in mind when buying a multimeter or other instrument for use with mains voltages: the cheapest may not be good enough. We have all used £5/$10 bargain meters with our low-voltage analogue or microcontroller work because a flashing LED on a breadboard is hardly hazardous, however for higher voltages those IEC class markings have to come with a believable provenance. Buy the most expensive instrument you can afford from a decent and reputable manufacturer, it’ll keep you safer and you’ll get a top-quality meter that will last you a lifetime.


So far we have discussed your bench and associated equipment. We are not however quite done though, because there is one final component in keeping your bench safe: you. If you have a piece of equipment in front of you and you are about to open it up, the most significant part of your safety routine doesn’t lie with your equipment, but in how you approach it.

Think for a minute, what are you going to find inside that box? You need to consider what voltages it takes as a power supply, what it does with those voltages, where any hazardous voltages might lie inside it, and whether or not they are likely to be accessible once the cover is off. It’s important to realise that simply because an item isn’t mains-powered does not mean you no longer have to consider this, for example if you’ve ever opened a camera with a built-in flash you may have encountered a capacitor with several hundred volts remaining on it. Where is the high voltage going to be?

The likely hazards identified, what are you about to do with it? Are you just opening it for curiosity’s sake, are you going to work on the high voltage circuit to fix it, or is it surplus that you’re scavenging for parts? If you’re just curious or simply scavenging parts, you may not have to increase the risk by applying power in the first place, so what other risks might remain?

Once you’ve  established these ground rules you can then tailor your approach based on a realistic expectation of the risk involved. This process might seem rather long-winded, but in practice it should only require a moment’s thought. It’s infinitely better to take that moment to think than get an unexpected and possibly lethal high-voltage electric shock.

Next Time

In the next part of this article we will venture inside some of the typical devices you will be working with to examine the individual hazards and techniques you might encounter.

195 thoughts on “Looking Mains Voltage In The Eye And Surviving

  1. I’ve been bit in a big way only three times. Two of the three were “Trust but don’t verify” issues, and I learned from them – always verify, no matter what anyone says.

    The first was working with a bench buddy on a military SSB Shortwave amp. My bench buddy assured me he had powered the system down and unplugged everything necessary, and I trusted him. I reached in with a pair of needle nose pliers to remove the plate voltage supply on the amp tube, and got hit by 26kv DC. Shouted and was teleported backwards about 3 feet, but no damage done (except a small melted bit on the end of the pliers).

    The second was in an industrial setting, working on repairing a 480v-supplied welder in a hard-wired installation. Plant electrician assured me the circuit was locked & tagged out. I trusted him. Sprayed some contact cleaner on the mains relay to start, and took 480v up the spray and into the can (and my hands holding it). That was an unusual sensation.

    The third incident was also while I was working for the welding repair company. I had a previously perfectly working high-pot tester and needed to test the insulation on a transformer. The tester itself failed and shorted to the case – and I took 11kv AC to the fingers just by turning the power on. That stung a bit, but I got a new high-pot out of the deal, so a net positive.

    I probably ought to be dead, or have some scars at least, but apparently I’m a very resistant person.

    1. We teach our employees never to trust anyone concerning lock-out tag-out. Everyone has their own personal lock, and most power sources can accommodate multiple locks. Fortunately you just enjoyed an unusual sensation,

    2. I’ve been “bit” three times as well…

      The first was when I was a wee, messing w/ a big 24v motor w/ weight (read: kickback voltage as it spools down && lots of amperage) and I turned it on w/ my hands on both leads…

      Second was with an old (illegal) CB amp that I was converting to HF, and I touched an unshielded feed inductor of some sort, the aftermath from the RF burn was worse than that time I grabbed the wrong end of an iron with my fingers.

      The third was 220v, it hurt like a bitch and burned down my arm, but no lasting damage.

      I play around w/ HV/low amperage PSUs all the time, that stuff will make you lose control of your muscles…

      But 110/120v A/C? that stuff is baby-play, as long as you realize that you shouldn’t use both hands, or use grounding straps, it just kinda buzzes, I’ve played around with TENS devices that had more (felt) kick than it…

      At the same time I’ve accidentally lit a moped on fire (carb cleaner + poorly sealed relay = fire)

      In retrospect how am I not dead?

    3. I too have been “bit” three times as well…

      The first was when I was a wee, messing w/ a big 24v motor w/ weight (read: kickback voltage as it spools down && lots of amperage) and I turned it on w/ my hands on both leads…

      Second was with an old (illegal) CB amp that I was converting to HF, and I touched an unshielded feed inductor of some sort, the aftermath from the RF burn was worse than that time I grabbed the wrong end of an iron with my fingers.

      The third was 220v, it hurt like a bitch and burned down my arm, but no lasting damage.

      I play around w/ HV/low amperage PSUs all the time, that stuff will make you lose control of your muscles…

      But 110/120v A/C? that stuff is baby-play, as long as you realize that you shouldn’t use both hands, or use grounding straps, it just kinda buzzes, I’ve played around with TENS devices that had more (felt) kick than it…

      At the same time I’ve accidentally lit a moped on fire (carb cleaner + poorly sealed relay = fire)

      Which is why you should keep a first aid kit, fire extinguisher, etc. near you bench.

    4. First rule of LO-TO: if you didn’t LO-TO it yourself it’s not! I will never EVER trust anyone to do a LOTO for me. If I’m to stick my hands into something dangerous the key to that lock on the switch is going to be in MY pocket!

      1. Indeed, I did a HV Course at a functioning power station where once you had isolated an 11kV switchboard you had to grab hold of the bus bars to show the other workers it was safe to carry out their work.

      1. hmm, not sure why my comment went in the wrong place. looked like it was in the
        [ GuruBuckaroo] comment, when I pecked it in.
        The “unusual sensation” quip was spot on & funny!

        ehh, best to keep me out of the electrical room for now I guess. x^P

        1. You did reply to the Gurubuckaroo comment, just a few interjections managed to find their way in first. Does mess up smartarsed replies occasionally, shame when it’s a particularly funny one. On busy articles, I often find posts popping up between pressing “reply” and “post comment”.

    5. not exactly mains but when i was a kid i enjoyed playing with photoflash caps and at one point had one fully charged to about 330V and discovered the negative post to the AA battery was also connected to the cap and put the entire shot up one arm, across my damn heart and out the other arm, my whole body locked up for only about a second (maybe 2) and i think i fell over (like people in taser videos but i was sitting). it sure shook me up and im glad it was only a small cap

    6. I’ve also experienced very similar shocks, mains flowing from one hand to another by trusting my co-worker. I couldn’t move my arms and upper body, couldn’t scream, breathe or even see during the shock. I don’t know how long it lasted and there was nobody nearby. Luckily my brain and lower body was still partially functional and I somehow managed to kick the wall in front of the, to disconnect myself from live connections. Some of my fingertips were smoking. I learned my lesson as well – never trust anyone, especially if they are repeatedly assures you that the power is down.

    1. I don’t think you can get multimeters that share the volts and amps socket, since the amps is usually a shunt to the ground terminal, very low-resistance, big and chunky high-wattage resistor. Volts is, ideally, infinite resistance, but not in practice of course.

      It does seem to be an old electrician’s habit, never leaving the leads plugged into amps. Somebody commented on one of Big Clive’s videos about him doing that. Seems like a good habit to get into.

      You could make a multimeter that had an internal switch for volts / amps. But I think electricians wouldn’t trust it. So it’d be a useless feature, as well as, probably, increasing the shunt’s resistance. And be another point of failure somewhere dangerous. Etc! Nobody wants a multimeter that killed their mate.

  2. Engineering types have been known to reduce “Preventive Maintenance” in the large “Mains” switch panels in an attempt to save budget funding. A close friend stepped up to a sadly neglected switch panel, pushed the dis-connect and the panel door blew off in his face. It seems that dust is conductive, and will explode if a large spark is present. The good news, he is alive, and was promoted to avoid a lawsuit. :cheers:

    1. Just reach for the “Flame Wrench” while working on a press, in a corrugated box plant.
      Within the next two minutes, you’ll know how many fire extinguishers are in that building.
      Box dust and the old, grease based, Letterpress ink was always a major concern when you were doing the repairs and needed to use a grinder, torch or welder, etc..

    2. “Engineering types”? – I would suspect business or accounting “types” to be responsible for “saving” on maintenance. Engineers normally know, that maintenance is important.

    3. I heard from an instructor that some guys from the national power company use full PPE when switching on/off old main fuses in domestic installations (230V, 15 to maybe 60A). These things are fully plastic and can be operated by everyone, so no PPE needed, but it seems like they explode sometimes – maybe because of the dust you mentionned.

      (Yes, it’s an older article, but i just saw it now.)

    1. Concur.

      Nothing proves lack of voltage like a dead short.

      Just be sure you’re standing back and have PPE on, in case the voltage hasn’t been removed.

      When I’m doing electrical work, I *always* short the conductors to verify no power (after flipping the breaker and testing with a meter). Better safe than sorry.

  3. As someone who started playing with electronics in the days of B+ in the 350VDC range with double that across the main xformer’s secondary, I was utterly appalled at the idea of tying myself to a hard ground when working with some of the newer (for the time) solid-state stuff that was coming out. We had a respect for high voltage then that is often missing these days, this article is an excellent reminder that this is still needed.

    1. I learned even more respect for shock hazards on the job one day when I was working for a military contractor. I forgot to turn off the 400VDC B+ supply to a radar driver assembly before I put one hand on the metal chassis and grabbed the fins of a lighthouse tube with the other hand. After that I put my head down on the workbench and had me a rest for a few minutes. Even when you know better, one moment of inattention can be the end.

      As for anti-static grounding, proper setups aren’t ‘hard grounded’. For reasons of safety they have resistances in the megohm range, (usually distributed resistance), between the technician and ground.

        1. And even today people give the advice of plugging a computer in the wall and grabbing the metal case as you’re working inside it. Bad idea – if the PSU has failed, you may very well have live voltages in the boards.

          And it’s is a bad advice from ESD protection standpoint as well because nothing else, like your table where the new components are laying on, isn’t grounded so you’ve just created a potential well.

          That’s also a mistake in the article, because the house ground doesn’t exactly ground the house. It just grounds the appliances. You can still have thousands of volts between a table and the wall socket ground in a thunderstorm. Even on a normal day, you have about 100 Volts per meter in potential difference just from atmospheric charges, which is why it’s a bad idea to ground the computer you’re working on unless you also ground the table, the chair, the floor and yourself.

          1. I think most people recommend having the PSU switched off at the wall. That way it should be pretty much safe, but there is still the other issue you mention.

          2. Yup, switched off but still plugged in, that way the case is still earthed. Assuming a sensible 3-pin plug, or whatever the hell they do in places with just 2.

          3. “or whatever the hell they do in places with just 2.”

            Chassis floats with just 2. My folks lived in the middle east before my dad retired and I visited them in 2013. You could feel 50hz on the refrigerator door handle. Touching their aluminum iMac gave a nastier jolt. I remember my dad telling me, “yeah, don’t touch that” after trying to plug in a USB stick.

          4. #PreferLinux mentioned switching off at the mains socket. Do that, but still test; that saved me once when the switch was faulty. Yes, that socket was replaced, pronto!

    2. The ESD protective wrist strap should not tie you to a hard ground, it usually has a 1Meg resistor included, just for this reason. But perhaps not every cheap chinese one has?

  4. When I was a kid I built a Tesla coil using a 12kV neon sign transformer which my mom got for me (this was the 1960s, before Americans turned into wimps). At some point I had the transformer sitting on my workbench and plugged in, with nothing connected to the output. The corona discharge made a hissing sound, but I had music on and couldn’t hear it. Perhaps eight inches away was my table radio, which had a metal chassis. I reached between the two to get something, and my arm shortened the air path enough for an arc to form. I had a couple of nifty burn marks (entry and exit) for a few weeks.

    The spark-gap Tesla coil itself was ???????????????? (I’d spent many, many hours hand-winding the secondary), and put out an estimated half a million volts. Low current, of course, but it would melt holes in thinner bits of metal that it arced to. I would get my jollies by connecting it to the doorknob of my room, to give my nosy siblings a reason to think twice before disturbing me. Considering that the contraption made a tremendous noise when running (that spark gap), they quickly learned when to avoid my room.

    And here I am telling the tale, one reason I snort whenever the momma’s kiddies post their “safety warning” comments on HaD.

    1. As you said yourself, low current. There is good reason to worry about mains power. The most common breaker is 15A, and that is about 15 times what is needed to kill you. It won’t trip right away either, unless it is so old and weak that it needs replacement anyhow.

      1. Low current (and damage-reducing high-frequency skin effect) on the Tesla coil’s HV secondary. Mains frequency penetration, arcability and cross-heart lethal current on the neon sign transformer’s secondary. So, it’s good that my wakeup call went in & out of the same arm.

        15A is more like 1,500 times what’s required to kill you, depending on what part of your anatomy it finds to traverse.

        1. “(and damage-reducing high-frequency skin effect) ”

          The skin depth varies with the magnetic permeability of the material. Iron for example reacts at very low frequencies, so much so that it becomes useless for wires at 50 Hz.

          In salt water – which approximates human flesh, the skin depth at 1 MHz is actually 25 cm which means your arm is a pretty good conductor through and through. The illusion of safety comes from the fact that the frequency is too high for the nerves to react, so you don’t get spasms or cramps and you don’t feel the heating that’s happening below the skin because there’s no heat-sensing nerves there and those at the current entry points have died already.

          That’s how you may get some serious, even life-threatening, internal RF burns without noticing anything.

        2. The problem with high voltage is that low current in itself isn’t safe.

          For example, putting 1 mA through a 1 MOhm resistor requires 1000 Volts. The current itself is barely noticeable and if it’s high frequency current you’re unlikely to feel pain – it’s just a funny spark that kinda tickles at your fingertip – but it’s still depositing 1 Watts of power into the resistance.

          If a high-frequency current is large enough to be immediately painful, it’s large enough to immediately burn you, which is why hooking a tesla coil to a doorknob is rather dubious. It hurts, because it really hurts.

  5. I stuck a finger in a light socket when I was about 3 years old back in Brooklyn NY and got a momentary buzz sensation. Nothing more. I got a few more 110V shocks as a US Army Air Force student of electronics back in 1944. No problems but I was warned to keep my left hand in my pocket in uncertain situations to not form a circuit through my heart, Here in Helsinki where I now live I have taken 220 V a couple of times. Nothing I’d recommend but no harm done. Nevertheless, I’m careful and never fool around with wet hands.

    1. I recall doing the exact same as a kid when in france, on holiday. The french also have those so seductive to interested kids 2-Prong plugs with a gap of ‘just’ that size….

      Threw me back a little bit but no damage that I’m aware of.

      Then there’s the time i went to take apart a computer PSU for the fan, without realising some how I’d left it plugged in (I’d dismantled the entire PC around it, yet left the power lead in?!)

    2. I had the same shock at 8 years old I put my finger in a lamp socket with no bulb, Living in UK I was treated to 240v. I have never had a mains shock since (thankfully). i have had lots of other shocks though mainly non discharged capacitor’s/

    3. I once started cutting into a wall which the building super assured me had no utilities in it with an old sawzall with the ground pin cut off, woke up on the floor with the saw still going in my hands. I hit a live natural gas pipe but the bad ground knocked me down before I could sever gas pipe and destroy the building, kind of like the freak accident where you get thrown clear since you weren’t wearing a seatbelt and walk away from an otherwise deadly crash. I don’t recommend using ungrounded tools but don’t take anyone’s word, verify yourself!

      1. Speaking of mains power and gas pipes, I live in a 100-year-old house which has some rather dubious wiring (though, perhaps not as bad as the knob-and-tube it started with). The kitchen’s gas range had been in place since puke green was the preferred color, and its power cord (for the clock & timer) was, unbeknownst to me, seriously frayed. The range’s gas inlet was connected to the shutoff valve on the wall with one of those scary-looking flexible tubes (not a modern, yellow insulated one, but rather bare metal). Well, one day the range got pushed back for cleaning, and the exposed hot lead in its power cord brushed that gas line. ZAP!, followed by a roaring jet of flame from the large hole now in the flimsy gas line. I couldn’t get close enough to the shutoff valve to use it, so I had the frantically seek out a big wrench and shut off the gas where it enters the house in the basement. By then the wall behind the range was well and truly aflame (way too big for my wimpy little fire extinguisher to deal with), so I threw pans of water on the fire. I figured the breaker for the outlet the range was plugged into had already tripped, but it later turned out that the contact had been too brief for that and I was throwing water on a live electrical circuit–no harm came of that, fortunately.

        So, mains power is scary enough, but mains power plus natural gas can be a real terror.

        1. When I was doing rehab work on old Chicago apartments I would often find electric wires run through abandoned gas lighting pipes, ready made conduit going to where the old gas light fixtures were, not exactly code and very confusing, especially since there were “live” gas pipes mixed up in the walls and ceilings with the ones carrying Mr. 120!

  6. The third apartment I ever rented had ungrounded two-prong outlets in the kitchen, and rather than do the old 3-prong-adapter route for the microwave (the good ones with a green wire to the center screw), I said, “let’s replace the outlet,” I switched the breaker, verified that there was no power with a lamp, and started disassembly…. and the screwdriver arced, Most likely the breaker was on the neutral instead of the hot or something stupid like that. I did one of those “cat with octopus on his face” nope nope nopes, and *carefully* reassembled everything and fetched the darn 3-prong adapter.

        1. In the US, sharing neutrals between circuits is generally prohibited in the National Electrical Code, 200.4, which says, essentially, “Don’t share neutrals, unless we say somewhere else in the code that you can.” That bit was added in 2011, though other parts of the code cast a penumbra that seemed to say that sharing neutrals was verboten before 2011. In 2011, it became clearly prohibited.

          You can share neutrals among, say, receptacle circuits, if you make sure that there’s some kind of disconnecting means that opens all the conductors associated with all the circuits that share a neutral. That might be a multipole breaker, or it might be handle ties that physically tie breaker handles together, forcing all to trip if any trip.

          Before the code prohibited it, it was common practice to share neutrals between circuits. It was customary to pull three hot conductors – fed from three different phases on a three-phase system – a neutral and a ground into a single conduit in commercial facilities. The assumption was that the neutral carried only the unbalance between the three phases, and that would never exceed the highest current in a phase conductor. Now, you can’t do that anymore, unless you use a three-pole circuit breaker, handle ties, or a three pole disconnect switch.

  7. I’ve had 240v shocks about half-a-dozen times, but the two that I recall were both back in the 80s, and were a) trying to fix a set of disco rope lights whilst they were intermittently working, and b) replacing the panel fuse in my CB PSU that I’d removed and hidden to stop my sister using my rig. Both of them involved my subsequent launching of the equipment across the room, accompanied by some suitable profanities.

    1. Once upon a time, must have been 13 or 14 at the time, was doing the annual find the blown bulb in the christmas tree lights. Must have completed said mission and marvelled how the bit of cable at the middle/end of the lights was hidden within insulation tape (lights must have been 20 or so years old, this was in the 90s) and the curious being that I am thought let’s see what the tapes hiding – obviously bare wires.

      Found said bare wires and had the ultimate Homer Simpson moment after separating them (probably when the lights weren’t plugged in), and thought ‘hey wouldn’t it be fun to complete this mains circuit across my body’. I didn’t really check with the lights lit up, but I was having a whale of a time juddering and screaming for a few seconds till somehow I managed to let go of the wires :D

      I definitely knew at the time touching bare wires wasn’t a good idea, especially not good to put yourself in a circuit, and have no idea what came over me at the time. Likely my fortune came from the fact I was in series with 20 odd low brightness bulbs so didn’t get to enjoy much current. No humans were harmed in this odd experiment.

  8. This is great info.
    In the next instalments I would love to see advice in dealing with a mains power supply and possibly a power line carrier modem on the same board or enclosure as low voltage components.

  9. The Residual Current Circuit Breaker is a tad expensive, and is probably overkill for most hobbyists. A Ground Fault Circuit Interrupter outlet, (GFCI), is cheaper to buy, cheaper to have installed, and a *knowledgeable* do-it-yourselfer can install his or her own. If you are knowledgeable about electrical wiring and codes, but don’t want to change an outlet in your house, you can buy a GFCI outlet and some regular outlets and make your own ground-fault-protected power bar, as a correctly wired GFCI outlet can protect additional regular (cheap) outlets as well. Alternatively, I’ve also seen GFCI units that plug in overtop of regular outlets and add protection to them.

    1. I certain the Residual Current Circuit Breaker is what GFCI is in North America. Not expensive at all for the units that will fit into a device box. The breaker that mount in the breaker panel are pretty pricey though, but are rarely called for. The only time they are needed if a building uses shallow device boxes for some reason.

  10. I had three electrocutions with mains voltage in about one hour. It was a dark and stormy night, when I decided to try again building my own half-bridge SMPS which had tendency to explode whenever I plugged it in. It was made of two boards: one contained EMI filters, bridge, fuse and HV electrolytic capacitors, other had high-side controller, transistors, transformer and everything else. When another transistor decided that it would rather choose new Pope instead of working, I got a bit angry. I grabbed something to drink in kitchen, returned and grabbed the board in order to smash it against the wall. I forgot to unplug it first. Ouch.
    Half a hour later, it was still dark and stormy night, I attempted to fix an ATX supply. When I was grabbing something from behind of supply, I touched the board with my forearm. Supply was plugged, of course. Ouch.
    Half a hour later, yes, it was a dark and annoying night, I was still working on the ATX supply. I unplugged the supply and was inspecting the traces, my nose and lower lip brushed solder pads of those two big caps that were charged to ~320V. I went to bed with my face hurting…

      1. Don’t those wild & crazy Brits use “electrocution” to include a non-lethal shock? Our friend [Moryc] could have learned English from Britlish speaker, as he mentions that he’s not a native speaker of our bastard tongue (though, he could have fooled me with that delightful prose).

        1. I learned British version of English, but my accent is all over the place. I also speak a bit of Spanish, Portuguese, Latin and a tiny bit of German. I can swear in Russian too, because my wife knows that language. And don’t forget my native tongue, which is Polish.

          1. No, Britlish, which is some modern bastard concoction. It’s well known that we in the colonies (American, that is) speak English that’s closer to the “original” than what the British warp the air with.

          1. The standard phrase in our repair shop was “electroplated’… as in …”I took that old CRT out and whoosh…. I damn near electroplated myself into the middle of next week when I touched the bottom of the LOPT”…. electrocution was reserved for politicians… as in “….how the hell could there possibly be enough numpties in this country to electorcute that old hag Thatcher…”

            I’m not saying it made any sense to the uninitiated though.

  11. Can you create, and how safe it would be, isolation transformer with two transformers? so, secondary of first transformer connected to secondary of second (2:11:2) ? In my naive thinking head, it should work. i have few microwave transformers with modified secondary (about 20-30V output from 220V input), and was thinking of using two of them…

    1. It works, but the losses are significantly higher, and if you’re drawing much power you’re going to need heavy, expensive low-voltage sides on both transformers. In a pinch, fine, but if you’re going to be buying them it’s better to spring for a single transformer.

      1. well, originally they were 2 kW transformers (or at least from microwaves that are rated to 2-2.5 kW), and on secondary is wounder stranded thick wire (diametar around 7-8mm)… enought power to melt nails with 3-4 mm diametar…

        or are you talking about magnetic loses in core(s)?

        I usually don’t need it, but sometimes….

    2. You can, but it will likely not be very useful. The losses in the two transformers will be such that you won’t be able to draw much current from it and most appliances won’t work.

      I have used a setup like this using two back-to-back “door-bell” transformers (220V to 9V) to play with neon tubes when I was a kid. The current was so low that even a lightbulb wouldn’t turn on. However, it was enough to teach me about electric shock from mains voltage when I have carelessly put my finger across the output terminals …

      1. Might be safer your way, since you’re transforming down, then back up again. Using a microwave oven transformer means horribly high voltages in the middle of the thing, and should, by chance, some insulation fail, or a bit of an arc, or whatever…

  12. one of my least favorite shocks is a phone ringing. I was fixing a socket one night about 11p and thought ‘no one will call this late’ and didnt bother to disconnect it. got a nice zap from that and the ringing has a nice pulse to it that really gets your jumping.

      1. I understand it’s only about 20hz, which is why it gives you that interesting jiggly feeling instead of a good Wack like mains power at 50/60 Hz does.
        When I was still a young buck in my early 20’s I worked in a mid-sized company and one of my duties was to maintain our POTS lines; my boss who might have been a sadist would often have me remap the phone lines mid day while the lines were in use and insist that I was not to busy any lines out. I can only amagine my coworkers delight at the inventive array of profanity that would come out of the wire closet when the reception switchboard would light up during these sessions. Sti got my lines man’s handset stashed somewhere…lol

      2. I worked in telephony for many years, and the ring voltage can be as high as 180 bolts at various frequencies, but, rather low current. In really old telco systems it may also be a square wave.

          1. Interesting how both the sounds and the letters on a QWERTY keyboard for V and B are close enough that it’s possible to switch them by accident.

          2. I typed “bolts” for “volts” in one of my ramblings here, but caught it before clicking Post Comment. Let this be the ROUND(e^23,0) request for the ability to edit our comments, at least for a few minutes after posting. Hell, even WaPo allows it.

          3. “180 bolts per second”? Now there’s an engineering unit I’ve never encountered in more then 50 years. No, it’s just a 150-180 volt sinusoidal or square wave at various frequencies. In past years, the coil on the bell was mated to an appropriate capacitor to allow ringing a specific phone on a “party line” based on the ring frequency. It was amazing stuff for a time when they simply hadn’t run enough wires for all of the individual “private lines”, so they had to engineer very basic things to fill the need. It was in a sense, the most basic and pure form of understanding the physics (electrical and mechanical) hacking.

    1. Stripping wire with my teeth (my front teeth have a perfect gap at the bottom for it, I could probably specify them in AWG) to fit the socket onto a phone extension cable. While rather stupidly forgetting I’d plugged the damn thing in downstairs. Yowch! Fortunately it wasn’t ringing. As a mistake, it was educational.

  13. I have been hit HARD by one leg of 440v, 3 Phase power running the 2.5 HP motor of industrial manufacturing equipment. I was on a cement floor, and wearing combat boots at the time. I am sure that helped, but my right arm and shoulder hurt with a strong, but dull ache for over a week afterwards.

    1. Let’s see, you’ve pooh-poohed my 12kV hit and [Scott_Tx]’s 90V one, so I guess 440V three-phase (was it the high leg, or one of the low legs?) is the sweet spot for you. Lucky for our edification that the boots did indeed save you. I knew someone who was quite thoroughly taken out by 220V in a basement while wearing (presumably damp) leather sandals.

  14. One of the most important rules ever: “Never underestimate your own stupidity.” Knowing perfectly well how to safely power down a circuit / appliance / machine is sadly not sufficient to actually do it, especially so if it has become a repetitive task and there are elements of distraction. If ones life is at stake, there should be check-lists written in stone, just like in aviation.

    1. One of the most important rules ever: “Never underestimate your own stupidity.”….

      Agreed…. I used to have a small plastic box in our workshop. It had two copper contacts on opposite surfaces, and a 9V battery, relay and tilt switch inside, and a Caution High Voltage sticker on it,

      Being small and shiny, it was an almost dead cert that a new engineer would pick it up, When tipped over, the relay would be energized by the tilt switch, the relay then switched off its own power, which resulted the contacts closing again, resulting in oscillation, and large back emf spikes which were presented across the two metal plates. As a result the person who picked up the box got a fairly significant belt, and let out some major expletives.

      Lesson learned… even small things can pack a punch,

      Always pay attention to the safety warnings, and never underestimate the stupidity of anyone, most importantly your self.

    2. Dunning-Kruger effect. It’s impossible to correctly estimate your own stupidity. This is the cause behind half of everything wrong in the world. The other half is psychopaths in positions of power.

      I once got an email reply from Dr Dunning, was very proud!

        1. Like many things, it’s the few psychopaths who spoil it for the rest. For most people, it’s quite possibly an emotional disability.

          But it seems our society is structured so that a lack of morals, and inability to feel empathy (but ability to fake it, when needed), can lead certain ambitious and capable individuals right up to where they can do a lot of harm. There’s so many political decisions, particularly in the UK, where “fuck ’em” must have been the answer to any qualms about the needy. Even to the point of ill people killing themselves, and even a few who’ve starved to death.

          “The Psychopath Test” by Jon Ronson is a great book about it, the guy who invented the checklist now used to diagnose psychopaths, has his pet theory that psychopaths with power are the cause of many of the world’s ills and inequities. His name’s Bob Hare, and he probably knows more about it than most other experts.

  15. Radioactive Anthrax Yeah! :-)

    The real problem with mains voltages is that somewhere, some day, some bonehead will manage to fry himself – and the media will blame the maker movement for it. I’ve already seen far too many absolute beginners planning to wire up their home appliances to an Arduino as their first project…

    1. Eh, boneheads frying themselves or setting their houses on fire because of DIY approach to mains wiring are hardly something new that can be blamed on the “maker movement”.

      I know a lot of people that have never heard about “makers” but I wouldn’t trust them near any mains wiring – like my own dad who has wired a circuit breaker between live and neutral. Only the ensuing BANG and my mom’s swearing and yelling from the now dark kitchen have disabused him of the idea that that is how it should be done.

        1. The media aren’t rational about anything. Not something you can really do much about, especially by being reasonable. Try not to think about it too much, is probably the best way to cope.

  16. Very interesting article. Being in the “treat it like radioactive anthrax” crowd, I rarely mess with mains level voltage, but I got a peek at what is required to do so. I read the paragraph about the “residual current circuit breaker” and thought it sounded very much like a GFCI (Ground Fault Circuit Interrupter). I clicked the link and sure enough, that was exactly what it was. I was surprised the article didn’t immediately describe it as such, but as mentioned in the Wiki that the link led to, only the US and Canada call it that.

      1. Your writing is appreciated.
        It’s good to learn something more than just the U.S. english terminology for things.
        Also makes for a better laugh (sometimes) at some of the anecdotes written in the comments
        that one reads in so many places online.

  17. I’ve been shocked a few times. First time, a friend and I were changing out outlets in a room in his house. He shut off the breaker for the room, and I watched him plug a lamp in and test it. I found out the HARD way, that each side of the room was ran through a different breaker… I got a quick jolt and the gerber tool I was using catapulted across the room…

    Quite often, when I was in the Air Force as a Cable Dawg, I would get zapped while doing copper cable cut splices for mission critical systems. Meaning they were left powered and came back on one by one as they were spliced. -48v DC is very tingly when it runs through a crimper and into your arm…

    I’ve done enough research about power capacitors and mains voltage that after I unplug anything, including all the TV’s that I work on, to repeatedly press and/or hold the power button until I’m satisfied that it’s discharged before cracking it open.

  18. It should be mentioned that any of us that have been working in commercial/industrial settings should be familiar and certified under NFPA70E. This includes, but is not limited to, how to work as safe as possible with hazardous energy sources, what the proper protection is for various levels and what is considered “safe” (working distance, approach distance, protection level, etc).

    BTW, one can’t just put in a big red button and call it Emergency Isolation Switch. At least in an industrial setting a Emergency Stop follows strict guidelines which include possible damage to the machine from using it. There are rules and regulations for how these are wired, where they are placed, etc. One doesn’t just run mains power through it. Typically these are low voltage switches that cut power to coils of high current contactors and relays.

    And don’t trust the 15KV gloves you pick up a Dayton Hamvention for $20. There’s a reason they’re so cheap, unless you certify them they’re just as bad as having nothing at all due to unknown defects and misplaced trust.

    Rule #1 Remove ALL energy sources*

    *Using approved LO/TO procedures which include verifying the energy sources are removed/disappated and safe

    I would like to add that it’s not just electricity you need to worry about. There could be other energy sources present from things like air, springs, hydraulic, etc that could pose a safety risk when you remove the electricity.


    1. But at the end of the training it says “you can ignore all of this if you are troubleshooting”. If you are afraid of live circuits, stay at your desk where it is safe…

  19. The use of isolation transformers that is suggested here is very dangerous as the residual current detector of the installation won’t detect anything if one gets electrocuted from the secondary of the transformer. I’d rather not use an isolation transformer unless I really need.

    In my experience, the more people know about electricity, the more cautious they are. And the more they have to deal with dangerous voltages, the more cautious they are. With few exceptions, most Youtubers you see doing dangerous stuff online have no idea what they are doing. Speak to a decent electrician and you’ll see they work more safely than most engineers and thousand time safer than amateurs. We don’t really need electrical cowboys showing off how dangerous the stuff they do is.

    1. Also, if you leave your oscilloscope floating at mains voltage with an isolation transformer puts you in danger as all earthed parts of the oscilloscope will be floating at that voltage. Again: when things are floating, it is ok to have 1 fault.. but the second fault will definitely kill you or break something as no RCD will protect you.

      1. A word of advice: if your idea of working safely on mains-powered stuff hinges on an RCD protecting you it might be a better idea to put down your tools and back away, slowly…

      2. Yes, I have seen that sometimes (floating oscilloscope) but I always thought the good way would be to power the DUT through the isolation transformer and leave the oscilloscope safely earthed. Of course you sometimes need a bigger iso. transformer this way.

    2. You can get special ground fault detection switches. The circuit at the output of the isolation transformer should be floating with respect to ground. When they detect an impedance between that circuit and ground below some configurable threshold (sometime 5Mohm) they will switch off the power.

    3. This. An isolation transformer can be a very useful thing, but also very dangerous. If you touch ONE end no problem, but if you touch both ends you will get cooked and the RCD will NOT trip. Or more directly: An isolation transformer does NOT allow you to switch off your brain! And NEVER connect a scope to such a transformer.

      (Yes, it’s an older article, but i just saw it now.)

  20. – Use isolated differential probes to measure high voltage.
    – Use current clamps or current transformers to measure current.
    – Put bleeding resistors in any big capacitor in your circuit.
    – Don’t change the position of the probes in the power circuit without power everything down.
    – Don’t work with high voltages alone.
    – If there’s more people involved in your experiment, block any circuit breaker, button or at least put some signs to prevent people from turning them on while you are messing with the circuit. Paranoids may even ground things while they are playing with the installation to direct the current to the ground in case someone makes a mistake.

  21. my first “real” job was in a video game arcade back in the 80’s
    my boss was a really cheap prick that used to buy asian cocktail machines that were “native” 110V, with a 240V to 110V step down transformer wedged in where it would fit, my second day I was fishing out a 20 cent coin that had missed the coin box, I got “zapped” hard enough to blow my finger nail in half and stop my heart.

    In the second week, I’m not even sure why I did it, I was working on a pinball machine and measured 240V between the chrome plates on the front of the 2 pinball machines to either side of the one I was working on.
    The electrician had “forgotten” to hook any of the earths for the whole building.

    the scariest thing I have ever seen/been around was in a fuel terminal at an oil refinery.
    I couldn’t figure out why we had printers, plotters, Apple II’s and monitors dropping like flies.

    After a long, bitter, loud argument with the plant “electrician” I finally got him to check the distribution board, when we opened up the “big grey box” first thing I saw was all the earths, bundled together with tape and just hanging there.
    From where I was standing I saw a fuel tanker driver hooking the earth lead to his truck, which came into the same box.

    I’d been “dead” 3 times at that point and thought I knew what it meant to be scared, seeing that tanker about to be filled, surrounded by thousands of litres of petrol, diesel, kerosene and other agents of flaming, painful death was and still is the scariest moment of my life.
    After that nearly being eaten alive by a feral pig or trapped under a hang glider under water was nothing

    1. Ladies and Gentlemen, I hereby nominate [cyberteque] as the replacement for our recently departed “most interesting man in the world”. Assuming he was, in fact, nearly eaten alive by a feral pig, or trapped under a hang glider under water. Double votes if he was nearly eaten alive by a feral pig who was flying a hang glider upside-down under the water.

      1. around the town of Wentworth in New South Wales the feral pigs are huge, there are lots of them, the scrub is very thick and even back in those days you were not allowed to use a semi automatic centre fire rifle of any kind.
        The boar in question had lower tusks that were 5″ long.

        Anyone that knows me really well, especially family members, is surprised/amazed that I’ve managed to be over 50 and still more or less bilaterally symmetrical, without too many visible scars.

  22. I work with high voltage all day everyday except for weekends, and then I’m probably doing some wiring or something at my house. 480VAC is on the low end of the voltage scale for what I do. The biggest and most important safety tip is: “Know thy equipment.”. That means knowing where the 480VAC is coming in, everything it powers, where that voltage gets stepped up to thousands of volts, what chassis and assemblies are floating at high voltage, at what point of the turn on sequence they are at high voltage, knowing what to use a chicken stick on when things are powered down (i.e. the high voltage bits – BUT NEVER AC MAINS!!!). I’ve seen the ends of chicken sticks where the unknowing tried to short 480VAC to ground… thank el diablo for fast acting circuit breakers… If you do not know what the hell you are doing or think suiting up in some silly bomb suit and silly gloves that are going to get you killed faster with than without, you need to stay the hell away from anything over 28 volts…. You cannot troubleshoot everything with LOTO, you sure as hell can’t troubleshoot with big stupid gloves and a bomb suit on. Those things were invented by desk jockeys who have never had to work on anything themselves in their lives. Great for theory, terrible for getting shit done. Knowledge is the only thing that will keep you safe. Stupidity and trusting others to keep you safe will kill you.

  23. But what to do after you or your friend gets shocked?

    1) If not breathing, give CPR. Electrocution has pretty good recovery rates if CPR is started right away.

    2) If breathing and everything seems fine, there can still be some arrythmia that will manifest only hours later. It is a good idea to visit a doctor for EKG readings, or at the very least have someone monitor you for the next few hours and if you go unconscious, be present to give CPR & call ambulance.

    1. I came here to post those two points! The corollaries to 1) are that should something go wrong, you have a chance if there is someone else there who knows CPR. I like mandatory CPR training for lab workers because of this. AEDs are good too. Rules against playing with things that can shock you while working alone are also good, but much harder to follow–I’ve spent decades working alone in research labs. CPR fails way more often than not, but electrical shocks are an important exception to that rule. And you almost got the extra credit for correctly spelling arrhythmia. :-)

      A coworker at JPL got bit by an ion pump supply. On-site health unit recognized abnormal EKG, sent him to ER. ER doctor diagnosed a congenital abnormality unrelated to the incident. But he ended up with a date with a nurse, so he considered it a worthwhile exercise.

        1. AED= automatic external defibrillator.

          When I was a about 12, my mother decided to fault find a 35mm slide viewer.
          240vac in, 6vac out of the transformer to a plug which fitted a socket under the viewer. Slide closed two metal contacts.

          “OK, you hold that small bulb across that plug, I’ll connect this 9v battery across the live & neutral pins…”


          “Why did you drop it?”

          By my calculations, that was a single pulse at 360v

  24. many years ago I had the bad behaviour to strip cables using my teeth… well, one day I was ready to strip the end of a cable that used to connected to my portable stereo (240v), its just few seconds that I noticed that the other end was still plugged-in on the mains…

      1. I think xChris noticed this just before he tried it. – “was ready to…”

        Remembers me of an incident when my mother asked me to fix the extension cord for the electric lawn mower – “please check the cable, the garden worker complained the mower does not work”. I opened the socket type end of the cable, saw a dislodged contact spring and wanted to reseat it. Then I had to feel the other end was still connected to 230VAC mains. It just did not think of thisbefore… But luckily it just stung a little. I pulled the plug and continued my work.

  25. I started my hacking adventures decades ago with vacuum tubes and high voltages, and have to say that this is an excellent and important article.
    I’m not sure of the country of the author, but one piece of translation for those just getting started. The “Residual Current Circuit Breaker” here in the U.S. is called a “Ground Fault Circuit Interrupter”; but, provides the same protection. Another more recent protection innovation is called the Arc Fault interrupter and actually contains DSP type circuitry to determine if a dangerous arc is present on the line. Flipping a light switch or running a small motor creates an arc; but, the AFCI can tell the difference between that source and a frayed power wire that could overheat and start a fire.
    The largest voltage I’ve ever worked on was an amateur radio linear amplifier that ran 2600 VDC @ 600 ma for the finals, which brings me to the main point of this post. One additional rule: Always keep one hand in your pocket or one finger hooked in a belt loop. While it makes handling equipment more tricky, it keeps the power from having a path across your chest, which is also across the heart and lungs, which are electrically operated and can be severely damaged or stopped.
    BTW, I love the image of the sign/sticker in this article which hit’s the nail right on the proverbial head.

  26. Best thing about isolation transformers is the one hand in the pocket rule.

    With no possible path including your heart or CNS you are much safer.

    Sometimes you’ll get a zap across your hand, maybe even a horrible burn. But I like those odds compared to you touched live and it routed current through your heart!

  27. Dumb question but in wiring up a bench setup as described – in what order should the isolation transformer, RCD and isolator switch be connected up. RCD on mains side of transformer along with isolator? Double pole isolator for live and neutral? Maybe add a suggested circuit diagram to the excellent article and improve readership safety even more.

  28. When I was around 10 I tried to get a plug out of a 240V AC socket with a metal spoon.

    I can still feel the hum.

    Fortunately I lived to tell the tale with no ill effects in an era where there were no ELCBs

    Later while an engineering student I worked in a factory where they built video projection systems with a 30kV final anode voltage

    So high they had lead around the RGB tubes to shield the X rays

    The guy on final test stood on a rubber block but still used to get blown across the floor once a month

    Usually on a Friday afternoon

    After which he had an excuse to take the afternoon off and he repaired to the pub for “a feed” of Guinness!

  29. The focus voltage on a CRT can wake you up. The shock may not kill you, but falling backwards over a stool can break a limb. I remember working with some Japanese colleagues a long time ago on color displays for IBM mainframes. A young Japanese guy had been sent over to watch our testing and make notes. He didn’t speak a word of English, and to be honest we didn’t warn him very well that the bared card on the tube neck he was investigating was live, and then some.

    After that small incident, he was a little less hands on with his involvement with our testing.

  30. We must also remember that just because an item is unplugged and may also of not been plugged in for a few days it can still hold a big charge in the electrolytic capacitors.
    Back in the 80’s I used to work for a service department in the uk called Vallances. When working on tv’s with a crt I found that crt screens that had not been plugged in for a week or two could still hold a big charge and the first thing I did was to short the tube to discharge it.
    I also remember one of the older engineers working on an old valve radio which was unplugged at the time, suddenly there was a bang and a big cloud of smoke and a very stunned engineer. All I an guess is that an old electrolytic capacitor was shorted and exploded.
    So remember when something is unplugged it can still be very dangerous, more so with microwave capacitors.

      1. No need, really. You just need a few mates with smartphones – just enable the wifi hotspot on each of them, place them roughly in a circle and put your coffee in the center. Make sure to dismantle the circle before you reach in for your cup though if you don’t want to get fried too – safety first!

    1. The microwave I recently tore apart had a hefty resistor connecting the capacitor to ground, to quickly drain it of power when the magnetron itself wasn’t energized. (The capacitor itself had a label saying there should be a resistor bridging the cap’s contacts for the same purpose.)

  31. Most people posting seem to have three stories, I only have two. Hard to believe I learn faster than you all; I must have just forgotten the other times.

    I was in sixth or seventh grade, playing with a transformer with a 540 V secondary, when I accidentally grabbed one cllp lead too many. The current was flowing from one hand to the other, and I could not let go. Eventually–I’m sure the subjective time was much greater than the actual time–I was able to use my legs to knock myself off of the lab stool onto the concrete floor. Hitting the floor felt so good!

    In college, I was playing with our nitrogen laser which had a huge exposed Blumlein discharge capacitor. So large that a bottle with fluorescent die had been set on it. While the laser was running, I picked up the bottle, and received several 20-30kV discharges to my hand. The exit was my bare feed on concrete. I was able to count the hits afterward, because each one left a small burn mark on my hand. That hurt.

    1. It takes volts to push amps, and if your body tripped a 1A breaker you’d be dead. For shock hazard, 110V is safer than 220V regardless of the size of the circuit breaker. But for a given power draw the possibility of overheating your wiring is lower with 220V, which is why I use it in my (American) shop whenever my equipment supports it.

  32. Bravo! Excellent topic, and you covered it brilliantly! Please keep articles like this coming!
    I look forward to more in this series.

    As an electrician (among many other things), I cringe when I see people acting like line voltage is nothing to worry about. I’ve seen photos and video of people who died gruesome deaths because they either failed to take the proper precautions or had no idea what they were doing when handling line power. I spent years training to safely handle electricity, and then spent another 15 years working with it in industrial, commercial, and residential settings. Despite all of this experience, I still have a healthy respect for electricity, which has probably saved my life on more than one occasion.

  33. Most systems now do not distribute ground (earth) from the MV / LV substation. At the sub the neutral is connected to ground through a low value resistor (<1 ohm) to a nest of earth spikes. Live and neutral are then distributed. At the house Neutral will be connected to a suitable ground point and at that point the earth connection is split off from the neutral. This means that any difference in ground potential is slowly leaked through the big resistor at the substation and it is a big resistor, looks like an oil tank maybe 2 metres high and 1.5 metres across. It is also normal that if there is any equipment at a pole it will also have an earth nest for the neutral wire.

    On an MV distribution there is no neutral or ground wire, just the three phases with a delta / star transformer to make the LV.

    1. Not always, you have to think on a global scope here before saying generalizations!
      In the UK a lot of houses are on what they call PME or protected multiple earths, where the earth bond goes back to the substation as a extra conductor. If you are in one of these properties and are not aware of it, and have your own ground system or leakage you can end up being lower impedance than the substation ground and grounding out faults in your neighbors properties which are also on the same bond.
      Where I am in europe, all the houses are individually bonded to their own low impedance earth, and the inspector comes out with a megga tester and tests its impedance before they will sign you off to connect to the public supply. It has to be below 50ohms total from any gnd pin on any socket in the property I remember from my inspection. I laid a load of mesh subfloor before pouring the floor so I had no problems with meeting spec. A nice low impedance earth will make your rcd and protection devices even more sensitive also.

      1. My post outlines the PME system. There are older systems that do run an earth / ground but they are being removed as and when they need attention. The US system is different but the number of countries using the US system is much less than the number of countries that use a similar system to the UK. I am UK based and UK trained.

        1. Which part is wrong? Lets use the IEC definitions of the different systems for clarity and you can educate me with some links to regs where I’m wrong, since I don’t do this for a living or have formal certifications in it.
          I’ve just a couple of years back installed 380v 3 phase into my barn conversion in mainland France myself and its most definitely a terra terra (TN) earthing system I put in. I had a inspection to get my certificate (certificate du consuel) to connect from a inspector, who during his 3 hr inspection tested the impedance to ground of my independant earth with a megga tester then checked all the earth pins in every socket for spec of impedance back to my earth point, all of which are wired star back to the main consumer unit (ring main is illegal here). We get presented 3 phase wires + a neutral as tails into our main consumer unit from the RCD device (called a disjoncteur) belonging to EDF (supply company) and its not permitted to bond neutral to your own earthing system under the regs.

          When we moved from the UK about 15 years ago, we lived in a PME TN-S property and my ex neighbors house is still connected thus. Newbuild and current standard might be to try to retrospectively phase that out, but its still a generalization to say all properties will be TN-C connected. If your in a TN-S property, its my understanding that ignoring that layout and putting your own earthing system in and bonding it to the supply one, can result in ground current back flowing through your system from neighboring properties.

          Wikipedia states most of mainland europe is on TN−S. “This arrangement is a current standard for most residential and industrial electric systems particularly in Europe”, if thats wrong, someone should fix it, preferably someone who has a authoritive position and can quote standards ;)

          1. There are only two conductors run from the substation to a normal one phase installation. Those are live and neutral. Though the neutral is bonded to ground at the substation through the resistor and is also connected to earth at any and all points on the cable where any work is done it is still neutral. At the house the neutral and ground become separate conductors. The neutral is treated as a live wire when working on the system, as far as I know all utilities treat neutral as another phase for safety reasons.

            If it is a three phase installation then there are four wires run. These are three phases and the star centre point or neutral.

            Whether you call it neutral or ground is (I suppose) not important, what is important is that there is no distinction between neutral and ground until you are at the customer’s meter.

          2. Perhaps a bit of an explanation as to why it is done this way may clear things up.

            If you run a separate earth wire that is hard grounded at the sub and hard grounded at your house and nowhere in between. In this situation if you get a lightning strike at or near your house then the ground potential rises. This flows through the earth wire to the sub ground and can be a massive current, enough to make the network fail dramatically. This rise in ground potential can also be caused by other things such as a faulty insulator on an MV or HV tower (pylon) or pole. Even if insulators are good then you can still get this ground potential rise. You can easily measure it with a length of wire and two earth spikes, put one near a tower and the other a distance away and you can see a significant voltage. If you also have a separate wire called neutral running form the sub then the potential between that wire and the earth wire is unknown so you have a risk of lethal voltage between neutral and earth, a situation that is not expected and potentially extremely dangerous.

            By not distributing the three wires and only distributing two you try to limit the possible potential between neutral at the sub and neutral at the house by using multiple earths anywhere you can so in this case when lightning strikes the local ground potential rise applies to the ground and to the local end of the neutral/earth wire. Massive currents may flow through that neutral/ground and eventually through the low value resistor at the substation but under no circumstance other than network failure is it possible to get a neutral ground potential of significance with a premise. The low value resistor at the sub limits the current to one that the network can handle.

            The more you think about this the more you realise that there is actually no truly bomb proof way to organise it but the methods now being implemented are probably about the safest way for most situations.

  34. It is not enough to have the proper tools, you also have to use them in a safe way.
    The basic five rules go like this:
    1. Disconnect from mains.
    2. Secure against reconnection.
    3. Verify that the system is really dead.
    4. Perform earthing and short circuiting.
    5. Provide protection from nearby live parts.

    Also take special care of any “other” sources of energy, like capacitors.

    If you really have to work on a live circuit, electrically isolate yourself and try to work with only one hand. Put the other one in your pocket. That way the risk of closing a circuit through your body is much smaller.
    Also don’t work in a hurry, carefully plan your actions and slowly do them.
    Use nonflammable clothes. Use electrically protective gloves. Use face protection.

    1. This! I just want to add:
      >3. Verify that the system is really dead.
      Use an appropriate tool. Test the tool, test if the system is dead, retest the tool. It should be broken or may have broke while testing.
      >Use nonflammable clothes.
      Yes! Synthetic materials melts, sticks to your skin and burns – you don’t want this!
      >Use electrically protective gloves.
      and check them before use!
      Do not work alone and if the second person has no clues about electricity instruct him:
      -What NOT to touch.
      -CUT THE CURRENT before trying to help somebody that has been electrocuted.
      -Where the main breaker is and how to operate it.
      Have a fire extinguisher and a fire blanket (if you catch fire) handy.

      (Yes, it’s an older article, but i just saw it now.)

  35. This is the sort of topic where it wouldn’t be a great idea to be dismissive of textbooks and/or formal instruction. Often local power line crews will put on a safety demonstration at hamfests. No doubt they would do the same for the various builder/hacker/maker spaces

    1. One of the best demonstrations I’ve ever seen, was at the Dayton hamvention. DP&L (Dayton Power & Light) set up several short poles with standard 7200 to 220 center tap transformers. By back feeding the secondaries with 220 from the breaker box, they could energize the 7200 lines and began demonstration what happens when you encounter these wires with numerous items. Antenna masts, kite strings, and even a hot dog were used to show the immense power available, and the audience was always suitably impressed.

  36. I think that one of the statements that stuck with me the most while doing LV operations training was from a doctor who said “If you are admitted to my hospital with electrical burns then I will do my utmost to make sure that your family get here before you die”

    That stuck with me. :)

    The mechanisms that take place if a big shock happens (big = high current, not voltage) is that it cooks the muscle so even though you sort of feel OK within a few hours the blood coagulates, you start to ache like you would not believe and you suffer a horrible painful death.

  37. Ive had a few shocks off 220vac but nothing that hospitalized me. But I do distinctly remember a shock recieved by a friend.
    He had a illegal cb with a zetagi 1kw valve linear amp, it tended to carbon up its changeover contacts and loose quality on ssb so I showed him how to turn it off, isolate it, drain it down and clean them up with contact cleaner. And I kept repeatedly stressing, dont do this without doing the whole isolation things as it will kill otherwise.
    Well, one day it went bad in the middle of a qso, and this was to some prized russian area, so throwing caution to the wind, he sprays contact cleaner at the relay while mid conversation, while holding a aluminium microphone in other end to talk it up.
    You guessed it, the valve hv field collapsed into the stream of contact cleaner, threw him across the room and into a wall and his glasses were blown off and twisted by the event. His girlfriend took him to hospital and it took a fortnight before they were happy with his vital signs to declare him fit to leave. All I could say when I visited him was “I bloody told you so” but his girlfriend was up for blaming me “for showing him”, and since then I’ve shied away from showing people things that if they ignore some of my warnings could kill them.

    Theres a homebrew 80vdc @150amp supply in my cnc converted bridgeport, that runs the entire dc rail for the steppers and other stuff, its always scared me slightly because I know that the big transformer its based on has a mag field and when it collapses and it has some massive smoothing capacitors to boot, so I have masses of isolation distance everywhere the dc side runs and I disable the transformer with a dedicated switch while working in the cabinet of it. I had a stepper driver supplied directly off the rail go bad and it just had a large hole where it used to be. I just imagine that was a bit of me if I get tempted to get sloppy with it.
    I’ve never put much thought into getting electrocuted by a welder, I’ve tickled my finger with my 3 phase tig set more than a few times getting too close and giving a nice easy starting path to the startup pulse. Maybe I should, I’ve got a massive 3 phase 380v transformer tig that can do 375amp dc, that tends to throw all the breakers out while its saturating its massive transformer with the inrush current…
    I try to treat mains voltage with the respsect it deserves. Its a killer, but if you take sensible precautions and steps you can avoid it being you it happens to.

  38. Nice article!

    I’d love to see a follow piece on how to work with High Voltage as well. Lots of interesting projects to do with microwave and neon transformers…

    I’m currently looking into buying some Class 2 insulating gloves for our hackerspace, for starters. Better safe than crisp…

  39. Once upon a long ago I was tuning a Tesla coil I’ve built with a friend. The coil was driven by a dirty cheap spark gap made with a screw that had to be rotated to change the distance between electrodes. I had to switch off the 6kV primary transformer, ground the terminals and rotate the spark gap, repeating this procedure until the biggest possible spark jumped out of the coil. After repeating it many times, I accidentally forgot one step. I’ll never forget that moment. My arm was thrown in the direction of the wall as if it had its own will and wanted to get through. Not good. Very painful.

  40. Actually, some places (notably Australia and the UK) go to 240V, and this is +/- 10%, so theoretically you could be seeing anything up to 264V supplied. However, the supply companies seem to have a “better high than low” philosophy, and my UPS tells me I am currently receiving 257V on a 240V line. I actually believe the UPS, as I’ve regularly seen this happen with test equipment on the line.

    Also, worth noting is that many cheap supplies out of China are designed for 110V/220V, +/- 10%. Consequently they’re designed to 121V/242V. For those of us in 240V areas, so in perfect conditions, they’re operating at the very top end of their specifications.

    About ten years ago, when the first cheap DVD players came into Australia from China, there was a huge failure rate due to this issue. The cause was almost always blown caps, and they could be fixed by up spec’ing the voltage tolerance for the caps in the supply.

    At the time I tried to get the regulatory authority in NSW to do something about it. Zero interest. Useless idiots.

    1. “If you’ve ever opened a camera with a built-in flash you may have encountered a capacitor with several hundred volts remaining on it. Where is the high voltage going to be?”

      Haha, my first brush with electrocution. Ah the foolishness of youth. I learned a lesson I will never forget though.

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