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Comment by bonoboTP | Hacker News Reader

Even if you're a "computer person" who is comfortable messing with the internals of a PC, be very careful with the power supply. Opening it up should really be left to professionals. Capacitors can remain charged for a long time after disconnecting the power cord. If despite all the warnings, you open it up to fiddle with internals because you think you understand the components and can pay attention not to touch things, at the very least, use only one hand at a time to avoid creating a circuit through your heart.

In most other parts of the computer the worst that can happen in most cases is that you damage the computer, the power supply is different, there you also have a good chance of getting shocked.

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This happened to me as a young teenager. I tried to figure out which part of the power supply was broken. The case was opened, I measured some parts, connected it, turned it on and tested if it was working again and disconnected it before continuing to work. After the fourth time, I forgot to disconnect it before touching something with both hands. Fortunately, the GFCI was working as intended, but the cramp in my arms and a very strange feeling around my heart is something I will never forget.

So, to add another advice, be sure that you have a ground fault circuit interrupter or residual-current device and test it regularly.

Question for any medical professionals around: If you experience a substantial electrical shock, but it didn’t cause you to go into cardiac arrest and you don’t have any severe burns, is it likely that the danger is over? Or should you still seek medical attention as soon as possible?

I’m not a medical person but I am an electrician.

If you receive a shock, if can affect the rhythm of your heart and cause problems later. We’re taught to go and get an ECG after receiving a shock. Like jump in the car and go to the doctor or hospital and have someone else drive you. If you’ve received a shock - go get checked out.

Are you part of a union, and is this recommendation feasible because of "worker's compensation"? I imagine I might not go get checked out if I felt like the risk was low enough and I was worried enough about being able to pay the bill.

It is certainly a known fact that electrical shock can lead to death even hours after the event occured, which is why there is a legal requirement in most industrial nations with a working healthcare system that you get yourself checked even if you are feeling good.

Your heart muscle is controlled by a tiny voltage. Suddenly forcing it to follow 50/60Hz of a huge voltage isn't precisely natural, and something that can leave lasting damage even if your heart seems to work just fine immidiately after.

There is a reason in electrics a lot is done just to prevent electrical shocks (e.g. RCDs), because you know: it kills people.

I am legally allowed to connect houses to the electrical grid in Germany.

I have experienced some shocks from ground loops, where the shock was only perceptible when the current density was small enough. If I placed my whole palm on the conductor, I couldn't feel it, but if I placed just my finger tip on the edge, I could.

That was an electrical shock that in my naive opinion does not pose a cardiac risk.

I imagine that there is some judgement call to make about what is "electrical shock", and when does it pose a cardiac risk.

If all is going well, shocks are rare, and so I could imagine as a rule "if you felt a shock, go to the doctor". But in some [yes, of course, unfortunate, avoidable, and unacceptable] contexts, a rule like that might not be followed. That is why I was wondering about what guidance electrician's unions provide.

I am sorry for you if you live in a country where you have to chose between paying the bills or making sure you won’t end up in cardiac arrest because of something that happens as a part of your job. It hasn’t got anything to do with unions, it is basic respect for human life.

Thanks, I'm sorry too!

"basic respect for human life" and "unions" can have a lot in common, especially in such a country.

I received some electrical safety training even though I'm a software engineer and was told the same thing. An electric shock can lead to cardiac arrest for hours after it happened, with no prior warning. You should always see a cardiologist even if you feel completely fine. Without an ECG you can't know the risk.

>An electric shock can lead to cardiac arrest for hours after it happened, with no prior warning.

That's common in the way shark bites and lightening strikes are common. Dropping dead hours after a minor electrical shock is so rare you shouldn't even be worrying about it.

>You should always see a cardiologist even if you feel completely fine.

It was included in your training so the company could cover its ass. By including a comically over the top warning and recommendation like that they are ensuring that statistically nobody will follow the training and the company will have leverage if you get hurt on their watch and sue.

It's a really shame that this kind of disingenuous ass covering behavior gets picked up and parroted by people as though it were honest advice because it moves the overton window and leads to a world where everyone acts like step-stools are a serious threat to public health.

A friend who still works at that company has seen two minor electrical accidents during his career so far. An ambulance was naturally called and copious amounts of paperwork filled out. Workplace procedures were adapted so that such accidents will be less likely in the future.

Everybody follows the training, because you get fired if you don't, or your manager gets fired. I don't know where you live, but here we have unions that make sure workers are as safe as reasonably possible.

Am medical professional, if you receive a substantial electrical shock you should seek medical attention ASAP. There is the potential for a lethal arrhythmia to evolve even if you feel relatively fine immediately afterward.

Quite. And the time period for changes to evolve is comparatively long -- up to 12 hours -- if the heart muscle is damaged and a scar forms later on, leading to the potential for life-threatening arrhythmias. Bradycardia is also sometimes reported long after the event. On top of that, there are reported "cTNI +ve without pain" events indicating significant cardiac injury detected only via ECG or blood tests (for cardiac troponin).

A very good summary article is here: https://www.bmj.com/content/357/bmj.j1418 .

Take home points:

-- Arrhythmias (superventricular and ventricular) are common

-- Exposure to high voltage can cause asystole directly

-- Delayed [life threatening!] ventricular arrhythmias have been reported for up to 12 hours post incident, both with high and low voltage shocks

-- Delayed, serious bradycardia can result months or years after the accident

-- The SA nodes are particularly vulnerable; patients may effectively give themselves an SA node ablation

-- Direct myocardial injury may occur without chest pain (due to nerve damage)

-- MI may also occur via coronary spasm or thrombus.

That neglects the rest of the body -- where the effects can also be significant.

Electricity in large amounts is bad for you – c.f. the electric chair!

> Exposure to high voltage can cause asystole directly

Yep. This is what defibrillation is. When you use a defibrillator, you're shocking the heart into asystole in the hopes that it will restart itself. Shocking an already "flatlined" heart will not restart it, contrary to what many movies would have you believe.

> The SA nodes are particularly vulnerable; patients may effectively give themselves an SA node ablation

This I did not know. Wow!

If it is not inconvenient I would at least call your doctor - there have been cases (involving very high voltage equipment to be sure) where no outward damage was seen but some internal was done.

For normal household circuits I’d just say learn from your mistake.

A huge number of regular household objects are more than capable of killing you with the voltage they're capable of exposing. Go to A&E. The risk being low isn't the issue, it that the consequence if it's a problem and you don't get it checked out is: you die.

Think of it as an inverse lottery you really really don't want to lose. The outcome, not the odds is what matters in this case.

> at the very least, use only one hand at a time to avoid creating a circuit through your heart.

The Alanis Morissette method; I've got one hand in my pocket, and the other's probing a live circuit.

Keep your left hand back (unless you are one of those rare people with your heart on the wrong side).

Especially if you wear a wedding band!

dude, the heart is in the middle of your chest.

Anybody care to explain the downvotes on this one? I mean the poster isn't technically correct, but a constructive and informative response might include why that actually matters in the scenario under discussion.

I mean, we're talking about an organ that runs off of millivolts. Surely the heart is close enough to the middle when we're talking about running 120 or 240 volts of AC through it?

This is the best thing I've read in a long time.

When I was a teenager I opened a disposable camera to explore its contents; the flash was charged when I touched the circuit and I got the SHOCK of my life. I learned about capacitors the stupid way.

When I was ~14 I used to take out the caps of disposable cameras, bend the legs down its sides, and convert the rest to be just a charger. It was a big hit in school! You could leave the caps at friends desks and they would curiously pick it up. Toss them at somebody at yell “catch” ect.

IIRC they were rated at 600v, and would leave black indents in the skin.

Kids are effing stupid…

The stupid but relatively safe way - it's rather difficult to discharge that capacitor with BOTH hands, and the one in disposable cameras is not that big compared to power supply caps.

You'll get a nice, painful reminder to respect electricity that'll last a few days, and not be in any actual danger.

(I also did the exact same thing as a teenager)

I did this exact same thing!! I was probably 10. It scared the hell out of me and I’ve respected capacitors ever since.

I did that too, disposable camera shocked me when I took it apart as a kid. There's a lot of us that always took things apart to see how they worked.

I did the exact same thing after my camera fall on a floor and broke apart :)

I’m glad I’m not the only one! Looks like there’s at least a half dozen of us!

half dozen who survived :)

A CRT enters the room...

Seriously thought electrolytic capacitors need to be respected but not feared.

How about the electrolytic capacitor of a camera flash circuit?

Knowing that they could deliver a nasty shock, I discharged one once by shorting the terminals with a screw driver.

The sound of the spark, and the scorch mark left on the PCB and screw driver left me fearful of them.

Or was that caused by the knowledge that I had a friend who said he put one of those capacitors in a highlighter, thereby making a makeshift, single-use, "shelf-unstable" taser?

Many moons ago I worked at a computer service shop that also repaired CRTs. Once, as a practical joke, someone asked me to hold a lead... Which was live with around 4kV. Wasn't pleasant experience.

You probably saw the tool used. A long flat blade screwdriver with a wire wrapped around it and on the other end an alligator clip chassis ground wire. It's used to pry up and remove the high voltage lead and when doing so you'd hear a loud snap!

I opened quite a few, and I wonder if they have bleeder resistors around the tube..

it's also interesting to witness a bit glass thing acting like a capacitor

ps: for long I wanted to make a discharge rod but never finished it

it's also interesting to witness a bit glass thing acting like a capacitor

The very first capacitors were glass jars:

https://en.wikipedia.org/wiki/Leyden_jar

Also a good tip to touch things with the back of your hand first, as the current can't force you to grip harder. This is more for line connected stuff though.

An even better tip would be to use an insulated screwdriver instead of your hand. Or even a capacitor discharge tool, since those apparently exist.

When I had to do an air conditioning capacitor, I used a piece of wood with a nail. I couldn’t tell if adding steel wool to the mix would be a good idea or not.

Wouldn't steel wool catch fire when exposed to an electric current? It might be useful to indicate there was still some charge in the capacitors, though

That was my worry. The resistance might be too high and get too heated. The nail avoids that issue.

In a power supply, you'd also have to worry about shorting and shocking other ICs with steel wool, but not with a big capacitor.

My father was an expert class HAM radio operator. I remember him teaching me this as a child and illustrating it with one of his amplifiers to cement it in my head at a young age. He took the cover off his amp which was unplugged from the wall, took a screwdriver, touched ground and then touched one of the massive capacitors. The arc created a loud bang and scarred the surface of the screwdriver.

But that didn't stop him from encouraging me to experiment while taking precautions. As a teenager I destroyed two ATX PSUs trying to add 110vac compatibility to an 12vdc RC battery charger. Third time was the charm.

Around '93 I tried to fix a friend's power supply. I don't remember why, but I was trying to remove a short jumper(solid copper wire about 1cm long) by wiggling it back and forth. The wire touched something hot, vaporized in my hand, and left me with a copper coated thumbnail.

Even after that, I continued to do dumb things around electricity. I'm not all that smart.

At least you are being honest about it.

Simplest solution is to discharge the power supply: i.e., flip the switch on the power supply and then try to turn on the PC/electronic/whatever. You'll discharge the vast majority of the energy while it attempts to power on, leaving a relatively safe working environment. No tools needed.

Unless it doesn't, because the power supply is faulty.

This.

Additionally there are typically bleed resistors to drain capacitors to safe levels, but they can also go open.

Dont use this as general rule. Its quite easy to find electronics that uses a master/slave approach. If master is down, slave won't connect, but you may still have the capacitor bank charged. Usually there should be a shunt resistor to discharge capacitors, but you'd be surprise how often is missing.

Or better yet, just unplug the computer while it's on /s.

As someone who has been in the computer building game for quite some time like many others, I have no appetite to touching the inside of my PSU's. Absolutely none.

They're inexpensive for desktop/gaming machines and if there's a fault its either in warranty or its time to purchase a new one.

seconded, not to be alarmist but anything mains voltage is dangerous (caps are rated 400V~), shit happens fast. it can be a tough jolt or it can burn your skin, or pierce it or burn nerves or maybe shock you into worse cases

i repeat danger

It’s not even the mains voltage.

There are components in there that can exceed mains voltage when probed the wrong way.

When you start getting into the higher voltages, the body’s resistance is much lower. It only takes a few milliamps to stop the heart.

> There are components in there that can exceed mains voltage when probed the wrong way.

Not in a normal power supply.

In the described power supply, you'll find voltages of about 340 volts DC, for two reasons. First, AC is measured in RMS, which is considerably lower than the peak voltage. I.e., 120 volts from the wall swings between -170 volts and +170 volts. Second, this power supply uses a voltage doubler, so you end up with 340 volts DC inside.

Doubling 120 to 240 is still mains voltage in my book, if a supply these days even does double.

Sure it's peak rather than RMS, but a warning that something "exceeds mains voltage" to me sounds like a description of a significantly bigger danger than that.

Ok, so this thread, with all the replies of people unaware of this in teenage years - seems to imply we should teach this stuff in schools early on. As teenagers are in general low on money and the most likely to try to repair something that get them back to gaming, while ignoring warning stickers ...

I noticed some of them include a simple light, which slowly gets dimmer after disconnecting the power.

Is it to discharge that capacitors?

That's discharge + indication all in one. It's a great solution, elegant, effective, and one of my favorites. But you have to find the cents for the neon (usually) bulb.

Be careful, that's (probably) just on the DC side...

Isn’t the AC side connected to the wall prongs? I touch/step on those all the time after unplugging things. Or is there something isolated from both the DC side and prongs where charge could be stored up?

Those big-ass black capacitors (470 uF each) are on the AC side. They are the bit that converts 230AC to 320DC, and they might keep their charge for some time.

Those are after the bridge rectifier, so there's no path for current to flow back out the plug from them. That also means you can't discharge them via the plug, of course.

Yes, there's bridge rectifier.

I worked on electronics for the military, one of which was a giant 2000 Watt HF amplifier that was stock to a MRC-148. When we got new Marines one of the first things we'd show them was when you remove the cover, turn the lights off, and have a florescent light bulb nearby it will activate. The purpose was to demonstrate the distance which electricity, unabated can travel. That was followed by explaining:

- Always ground yourself to an actual building rod or a rod planted at least six feet deep.

- Lock out, tag out everything. Our tools were metal, go work on ONE truck with a battery array without an extension and you will almost certainly arc something.

- Voltage isn't what kills. It takes an insanely low amount of amperage to stop a heart.

- Always have another person nearby with a wood or rubberized object to disconnect the circuit. Being stuck being electrocuted is a surprisingly slow death that you will be entirely conscious for.

Mr Carlson covers this in

https://www.youtube.com/watch?v=CyKHVl3xF70

Personally I would probably not want to mess around inside a AXT supply.

Got a nice zap from an iMac power supply

Or just short out the capacitor terminals with a metal object like the screwdriver and you're fine. But still, if you don't do that and you get chocked it will just be a little painful jolt.

You know, you'd think that would work, wouldn't you?

But dielectric soakage runs in reverse, too. So even if you short out the capacitor once, it can recover some voltage after days to months left open again. (The energy comes from the molecules in the dielectric relaxing, very slowly.)

This isn't a problem if the designer has put in a bleeder resistor across the capacitor. But some designers are cheap, and don't want to pay the extra few cents to make their products non-lethal to technicians. Some are just stupid (yes, dumbfuck "senior engineer" coworker, I am thinking of you here, you colossal waste of oxygen).

What you want to do is short it out, with a screwdriver or otherwise, then keep it shorted with a gator cable or even resistor. Then you'll be safe even if you set the project aside for a week. Or three months.

Had a pallet of military surplus capacitors, packaged as 3 big (soda can sized) caps on a blade frame and wrapped in conductive foil, plastic, then heavy paper. From the 70s and they were fine when i got them in 1997.

I wired up an unwisely huge bank out of them for some fun HV experiments. They were in a shed outside and could self charge to useful levels in the course of a windy day. Dont recall the actual numbers but it could vaporize dimes handily.

Reading that, I wonder how much solar power you'd need to charge them to that level? And if that could be applied to complement solar/wind power?

I tried some halfass "comb" antennas but wasn't seriously attempting to harvest static electricity to useful purpose. There's some "joule thief" circuits available cheap and easy now that make me wonder if its possible.

Dielectric absorption is a hazard for HV capacitors that are charged to over 1kV in normal use, since they can recover to dangerous voltages.

Mains filter caps will be lucky to recover a few dozen volts, which isn't going to do much (and inside a PSU, if the voltage is high enough it will try to run and quickly discharge them anyway.)

And yet, if the voltage is low enough, it will linger for minutes to hours. Plenty of time to kill someone.

Maybe I'm just salty about having had this argument too often recently. But I strongly believe that every mains voltage or higher system needs bleeders across its capacitors. If you don't want to pay for the resistor, fine... but leave the footprint for it.

This goes 10x stronger if the thing is open-frame, sits on a bench, and is regularly being probed by engineers. Just put the goddamn resistors in there before someone gets hurt.

Ahem. Apologies for the intermission.

I recommend against this. Most supplies have a bleed circuit, but if it doesn't or it doesn't work (at all or not fast enough), then shorting primary-side capacitors can make a really nice bang and also damage your "shortening implement". That bang can startle you and make you hit something. Been there done both. Not a great idea IMHO.

Check for voltage - If not present / marginal, Then shorten, Else discharge through resistor and GoTo Check. A "capacitor discharge tool" automates this cycle.

I still don't think it's a good idea for the average tinkerer to take apart a power supply. Almost never worth it.

It requires some background in electronics. Personally I use gloves when working on electronics and soldering. There is nothing sophisticated about power supplies. Pressure vessels is where it gets real.

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