Teen electrocuted while salvaging parts from a computer

By Shawn Knight · 98 replies
Oct 12, 2012
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  1. Per Hansson

    Per Hansson TS Server Guru Posts: 1,957   +214

    No it does not work like that.
    When you press the power button on a PSU not connected with a power cord the power stored in the capacitors is discharged and put to use, for example spinning the fans which you can see if you discharge a PSU this way.
    However I must stress that this is not a safe way to discharge a PSU, because it assumes the PSU is working properly, which as I said before it most likely is not if you feel the need to open it up and fix it!
  2. Gamesinner

    Gamesinner TS Rookie Posts: 86

    Darwin was right.
  3. dotVezz

    dotVezz TS Booster Posts: 112

    Oh man... Power supplies can be dangerous if you don't know what you're doing. Hell, I know very well what I'm doing when I open them (Rarely... very rarely) and take all the precautions, and I'm still a little intimidated.

    Poor kid... That could easily have been me when I was younger, taking stuff apart and playing around with electricity and fire without fully understanding the dangers...
  4. I think it takes AC current 2 to 3 sec to dissipate. The indication of LEDs on the power supply. The capacitor stores DC current the deadly kind. It is not the one that permanently damages you. It is the benign kind that just stops your heart. So if no CPR is available you are just out of luck if your dad comes home late. Death by misadventure.
  5. Capacitor Facts. The only way to discharge a capacitor is to touch a small screw driver to it's lead wire on the negative terminal. Or the 2 leads from it you touch a 400 watt light bulb. Yes capacitors may contain 400 volts or more. Isn't that something. A thing that is so small can store so much energy. Also fact pressing the power button does nothing. I may take days for it to discharge on it's own without being plugged in. The only way to make sure it has no energy left is with a volt meter. Do not discharge it using your body.
  6. There are three ways to discharge the large filter capacitor in a switch mode power supplies.

    Discharging the capacitor with a screw driver (not recommended).
    The reason for not using the screw driver to discharge a capacitor is because the printed circuit board or circuitry can be damaged due to the spark generated while discharging the high voltage in the capacitor. I once have blown the power area using this method. However, if you knew that the capacitor stored voltage is not too large after confirmed it with a meter, you can easily discharge it with a small screw driver.
    Just place the screw driver tip to touch the two pin of the capacitor, within seconds the charge will be gone. If the capacitor holds a heavier charge of electricity then discharging the capacitor with a screw driver may melt the tip of the screw driver and the copper on the printed circuit board. Sometimes a big spark may cause small disintegrated solder lead or copper to fly out from the circuit board and might injured your eyes or body.

    Another method is that you can use a socketed 100 watt electric light bulb and touch the two wires coming out from the light bulb on the leads of the capacitor. This method had been used by many electronic repairers around the world for the light bulb will act as an indicator to see if the capacitor still holds the charge. If there is a charge the light bulb will light and after discharged the light bulb will goes off. I still preferred the third method because you have to use both hands to touch on the capacitor lead.


    Use a resistor to discharge the capacitor leads

    Third method and also my favorite method is to place the leads of a high wattage resistor on the leads of the capacitors you want to discharge. You can use either a 1.8 k or a 2.2 kilo ohm 5 to 10 watt resistor to discharge the high voltage capacitor in a switch mode power supplies. It is very simple to use and very effective .It takes only a couple of seconds to fully discharge the capacitor.
    Using only one hand, you can do the job while the other hand you still can hold a solder gun or secure the equipment casing.

    I strongly recommend to those who are using the screw driver to discharge a capacitor in power supply to consider the second and third method as these is the safest methods. It not only protects the circuit, it also protects you. If you had discharge the capacitor and you are still not sure or no confidence whether the charge already gone, you can always use a meter to confirm it.
  7. AlbertLionheart

    AlbertLionheart TechSpot Chancellor Posts: 2,026

    The bigger the sticker telling you not to open the thing the more tempting it is to prove you are smart enough to ignore it. Doh!
  8. Ignoring electrical safety sometimes has tragic consequences and not knowing the details makes it difficult to pinpoint exactly what happened in this case. The main factors are current, duration and pathway.

    It is important for everyone to know that as little as 60 milliamps can kill if the current flows from the point of contact through the heart muscle on its way to ground. Large shocks tend to propel us away from the electrical source, which sometimes prevents electrocution. Small shocks typically don’t do this and we don’t let go of electrical source before a lethal shock is delivered. Sustained electrical shock even at lower currents can cause ventricular fibrillation leading to death.

    Electrical paths from arm-to-arm, or arm-to-foot are the most likely to traverse the heart and cause electrocution.

    There are many other factors such as, whether the current is AC or DC, the conductivity of the skin, whether we are perspiring, if the conductor somehow manages to puncture the skin, etc. that can come into play.

    Capacitors must always be treated with respect because the stored energy they hold can be lethal. Computer power supply capacitors normally bleed off fairly quickly, but should always be properly tested or better yet discharged by holding a screw driver by its insulated handle and then touching the metal part across the both contacts at the same time. Wearing eye protection is a good idea.

    Doing any kind of electrical work when you are alone is never a good idea.

    I hope readers find this information useful.
  9. avoidz

    avoidz TS Guru Posts: 460   +56

    Hard to say what really happened. Maybe he was a curious teenager who opened up a PSU to have a look inside or to clean it. Usually it's safe to unplug and handle a power supply unit and transplant it to another PC. I just replace them when they get too dusty.
  10. cliffordcooley

    cliffordcooley TS Guardian Fighter Posts: 9,715   +3,693

    Here is an interesting concept.

    Why does the electrical resistance of the human body varies according to the Composition of the Blood?
    An answer that was marked as the best answer. I personally don't know anything about human physiology.
    Electricity will always travel the path of least resistance. So the interesting concept here would be, was the path of least resistance using blood as a highway straight through his heart? Where the conditions in his body perfect for even the slightest of jolts to cause major continuity?
  11. Pushing the power button has absolutely no effect on the remaining charge in the capacitor. The boy opened up a tightly sealed box (the one that the power cord plugs into directly) ignoring the giant yellow 'WARNING' stickers that say not to do that.
  12. treetops

    treetops TS Evangelist Posts: 2,072   +219

    One thing I do know is when my resolution was so high my screen was stuck black, unplugging my computer and holding in the power button fixed it. I would never advise a none pro to open a psu, but it sure seems like holding in the button uses the electricity, which for some reason beyond my meager understanding of computers was supposedly the cause of my video card resolution changing on its own correcting my problem, going back to default perhaps?

    Meager in comparison to a lot of these techspot pros :).
  13. Zoltan Head

    Zoltan Head TS Booster Posts: 247   +27

    I think that will have discharged some capacitors, but not the capacitors (I.e. the big meaty ones in the PSU!):)
  14. wiyosaya

    wiyosaya TS Evangelist Posts: 1,920   +755


    I read your post, and found a good schematic on the net that covers supplies with APFC. I tried to post it Friday, but for some unknown reason, I was unable to post it. http://resources.vr-zone.net/uploads/11366/ATX_power_supply_schematic_PFC.png

    Because in that schematic the filter caps are approximately 2mF and at that value with even a 400V charge there would still be only about 80J in the capacitors, I am still having trouble with the reporting that states the kid had unplugged the supply. I highly doubt it was accurate.

    Personally, I think it would be interesting if someone connected a modern, high-impedence volt meter across the filter caps in a supply that is powering a working computer, then either shut off the main switch or pulled the plug from the wall, and watched how rapidly the filter caps discharge. My bet is that they would discharge in a relatively short period of time which would be further evidence that the kid had not unplugged the supply.
  15. Per Hansson

    Per Hansson TS Server Guru Posts: 1,957   +214

    Wiyosaya: I tried that yesterday actually to satisfy my own curiosity :)
    I used a half-decent ATX PSU, without PFC though.
    I shorted Pin 14 to ground to turn it on.
    Next I measured the voltage at the primary capacitors, it was 200v DC
    When I unplugged it from the grid it took less than 5 seconds for the fan to stop spinning, and the voltage across the primaries where then 30v DC.
    Of course with a bigger load than only the PSU's own fan connected it would discharge much quicker...
    Tried right now with another PSU (without PFC) but with a computer connected.
    It drains the caps in less than a second.
    Even when pulling the cable from standby mode it took less than 4 seconds to drain the primaries completely (0v measured across them)
    This one had 165v across the primaries which is more common
  16. Emexrulsier

    Emexrulsier TS Evangelist Posts: 574   +72

    There is a sure fire way you can prevent things like this happening ... when you open your psu ask your mate to touch all the components removing any risk to yourself :D. I have opened PSUs in the past normally to change fans or add some new cabling to power non pc items :D
  17. Per Hansson

    Per Hansson TS Server Guru Posts: 1,957   +214

    Yes, and if you care for your friend you can both hold hands when touching the components.
    So he will touch the minus leg of the primary capacitor and you the positive leg while holding hands.
    This way the load will be shared by the both of you and thus you will only get half the zap each :D
    cliffordcooley likes this.
  18. cliffordcooley

    cliffordcooley TS Guardian Fighter Posts: 9,715   +3,693

    May the best man live!!! This game would be energizing for both parties at the very least.
    Per Hansson likes this.
  19. Simon wadey

    Simon wadey TS Rookie

    When I was doing my aircraft electrical degree the best fun was charging a large capacitor bending taggs close together and throwing them to unsuspecting airframe guys they got lots of buzzes outta that LOL
  20. Did the Police check if the father recently did anything to upset the bankster cartel, like that guy who reported on the 43 Trillion dollar lawsuit against them and had his kids killed the next day as a warning to other Journalists?

    Cause the capacitors don't keep power long enough to give you time to unscrew the four screws it takes to actually open it up and get your hands inside...

    I've worked on enough of them to know...
    pidjones likes this.
  21. I read recently that the average human heart is pulsed electrically within the body and it is extra susceptible to being stimulated into fibrillation during about 1/7th of the cycle of each heartbeat. This is from the book "Overcurrents and Undercurrents" by Earl W. Roberts. When this happens a defibrillator is required to stop the heart and initiate the correct beat.
  22. pidjones

    pidjones TS Rookie Posts: 31

    Most of these comments are about as full of stuff as the original story. Someone posted about the human heart beating at 60 Hz? Really? Are you the FLASH?

    If I were the police there, I'd be checking the father's story. I suspect someone plugged the supply back in. Intentionally or accidentally is the question.
  23. SlyDrJ

    SlyDrJ TS Rookie

    I own an Electronic Waste Recycling company. This is a very real unspoken danger I see a few times a year, normal just a tiny zap. But in larger commercial equipment, they have much larger batteries than the quarter battery on a mainboard...can get scary- MONTHS after it's been unplugged..
  24. captaincranky

    captaincranky TechSpot Addict Posts: 12,967   +2,524

    I'll reply since this has been bumped to the top of the stack. What you're saying is only partially true. In most circuits, the + terminal of the capacitor is connected to the load. If that is not the case the cap just charges to full and stays that way. They can hold charge for quite a while, and it doesn't matter if you pull the plug, or shut off the switch, they'll nail you big time, if you're anyhow grounded when you touch the positive lead.

    The lethality of any given electrical charge is determined by the amount of charge delivered. On dry skin with a poor ground it takes quite a charge to kill a person. delivered close to the heart and subcutaneously, it becomes a matter of microamps.

    Capacitors don't sustain the delivery of the charge. However, when a human touches a live wire with continuously flowing current, there exists a "no let go" threshold, when the current causes the muscles themselves to contract involuntarily, and out of the their owner's control. These are the most often causes of electrocution, a sustained contact with continuously flowing electricity. Although, lightning kills quite a few people a year. It's a short term event, but with millions of watts seconds of charge, it's fast acting! (Citation required).

    It's also the operational principle behind the defibrillator, which clamps the heart muscle solid, and hopes that the heart itself will resume normal sinus rhythm, after that event. Keep in mind, to deliver that charge, a conductive paste is applied to the chest, and the charge delivered to the chest can be as high as 360 watt seconds. That's why they yell "clear". Cause if you're touching the body, you're going down.

    And that boys and girls, concludes our feel good, necro-bump, bedtime story for tonight Nighty, night.
  25. Darth Shiv

    Darth Shiv TS Evangelist Posts: 1,811   +472

    Total energy doesn't really mean anything... you could sit there for years with a trickle current running through you and it won't kill you.

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