Comment by onetimeuse92304 | Hacker News Reader

Comment by onetimeuse92304 | Hacker News Reader

onetimeuse92304

> It's also a safety issue, since the Cathode voltage might be substantial.

When I was a small kid, my father had a TV set repair side hustle. I was 7-8yo but I would sometimes be called to help him a bit. Once he asked me to clean a bunch of CRT tubes from substantial amount of dust. What he forgot is that he was just testing one of them and the charge wasn't properly dissipated and it did not have time to dissipate on its own.

I got hit with a spark so powerful, that it must have temporarily disrupted my brain because for a brief moment I was completely stupefied, and then I just resumed cleaning it. Then I got hit for the second time, then for the third time. Only after third time I realised that me cleaning the tube has something to do with it. Probably the discharge was much less powerful the third time and did not stun me completely.

I can tell you the voltage and amount of charge on these things is no joke. Multiple tens of thousands of volts is enough to stun you temporarily. Probably not enough to cause lasting damage unless you are extremely unlucky about where the discharge went (stun guns usually have higher voltage and capacity), but enough for you to lose control of your body, fall and hurt yourself.

I got zapped once as a kid. My grandfather's TV, I was pulling the tubes to take them to the testing machine. Fairly small screen but I knew it probably had some residual charge that I most definitely didn't want to come into contact with. The layout was such that I could pull the tubes while staying far away from it. It never occurred to me that safety would require keeping him from looking on in curiosity and bumping my arm. (He was impatient about getting his TV fixed, I saw that whoever designed it had had the courtesy to put the tubes in an area where even if your hand slipped it wouldn't approach the tube. That was no protection against my hand getting moved in a direction I was not applying force, though.)

I don't think such things are actually capable of direct lasting damage but you do lose control--I slammed my hand into the cabinet pretty hard, completely involuntary. My understanding is that tasers are AC, thus involuntary movements will be back and forth rather than just in one direction.

> My understanding is that tasers are AC, thus involuntary movements will be back and forth rather than just in one direction.

Tasers AC output is at low frequency but still significantly faster than human muscles can move, if you watch any video of a tase you'll see it causes the victim's muscles to seize up and not oscillate.

I was thrown across a room twatting around in the back of a TV. Many years later I still wonder if there were any sdiefeftcs.

"I don't hire programmers who spell correctly, I hire programmers who misspell consistently" or how does it go

>Probably not enough to cause lasting damage unless you are extremely unlucky about where the discharge went

You don't have to be that unlucky. Discharge across your heart can stop it, no problem. It is not at all difficult for that energy to route through your chest. You were lucky.

Well... one difference between a stun gun and CRT tube is that in a stun gun you usually have both terminals relatively close to each other. Little chance of the main pulse going deep across the chest. On the other hand working with the CRT you are likely using both hands and that makes the signal go through your upper chest, by default. So yeah, there is something to it.

One thing I learned as amateur EE is to always use just one hand to probe the device under test if there is any high voltage or AC involved. You put the other hand behind you so that there is no chance you create a short right through your chest.

And have someone nearby with a 2x4 to beat you off whatever you might be gripping/fell onto.

Just to add a bit more color here: most flexor muscles in your body are a lot stronger than the corresponding extensors, so when an external command comes in telling both to contract, for example from electricity in the thing you're working on, gripping wins. You can't let go. Great for grabbing a branch as you fall out of a tree. Not so great for repairing a power line.

Late edit: I've heard you can use this factoid to your advantage to avoid being eaten by an alligator or crocodile. Wrap your arms around its snout and hold on for dear life. Their jaws are strong enough to snap you in two, but not enough to force themselves open when you're holding them shut. I don't know what Step Two is, unfortunately.

> I don't know what Step Two is, unfortunately.

step two is waiting for the aforementioned 2x4 wielder "to beat ... whatever you might be gripping/fell onto"

"Mother" on speed dial maybe?

The bit about holding an alligator's mouth closed is correct and I remember Step Two from an alligator wrestling demonstration: flip it over. They lose consciousness if you can hold them in an inverted position.

So do chickens, but they're a lot easier to wrestle.

Related fun fact - On many industrial robots that I've worked with, the teach pendant (the handheld controller you drive it around with), requires you to hold a 3 position spring-loaded switch in a middle position for the robot to operate, which requires you to hold with a rather precise amount of force. Squeeze it either too loosely or too tightly and the robot disables.

The idea being that not only will you dropping the pendant disable the robot, but it will also disable if you accidentally touch energized equipment and your hand clenches, or (more likely) you panic and squeeze the controller too tightly.

https://us.idec.com/idec-us/en/USD/Safety-Components/Enablin...

> One thing I learned as amateur EE is to always use just one hand to probe the device under test

I was taught to use the back of my hand to avoid gripping the damn thing and never letting go.

Same. And I do it like a superstition now.

You're describing the well-known Alanis Morissette technique for electrical work:

"I got one hand in my pocket, and the other one holding a DMM"

> One thing I learned as amateur EE is to always use just one hand to probe the device under test if there is any high voltage or AC involved.

I remember in some EE course, the prof randomly stopped in the middle of a lecture and said roughly

"Ok now that you all know this theory, I need to remind you that you should not go do your own home electrical work, because you will get ahead of yourself and die from some kind of electric shock"

The implication being:

1. You're trained, but for EE not electrician work

2. You'll think you know what to do, even if you don't it'll cloud your judgement

3. As an EE you will make enough to pay a real electrician to do your electrical work

Needless to say in the real high power EE courses (t lines, grid, etc), this wasn't even mentioned, because it was just implicitly known that all that stuff was in the realm of "Don't physically touch this stuff in the real world ever because it will kill you and it will hurt the whole time you're dying".

I did not mean doing home electrical work. That I do leave to professionals.

But there are times I have a device on my bench that is consuming 230V AC.

I have a 3kVA separation transformer set up, which makes it safer for me to work with the device (there is no potential between the DUT and the ground).

Technically, even without the transformer the residual current protection should kick in and prevent you getting killed between device and the ground. I have tested mine couple of times to check whether it works. But I somehow still feel better with a separation transformer.

But there is still the problem that I could touch different parts of the device that are at different potential and then I could get zapped this way. There is not much protection there. Solution -- If you must, don't ever touch more than one place at a time.

That said, direct current is "safer" than alternating current; AC makes your muscles (and heart) spasm/contract 50 or 60 times a second like it's fibrilating, DC locks everything up like a defibrilator.

I recommend neither though.

high frequency alternate current is the safest, in the sense that the skin effect enters in effect, and the current doesn't touch your muscles (or at least your hearth). However, the danger of burns persists.

The nasty thing about HF burns is that you often do not notice that something is wrong until you either smell the burnt skin or the burn starts to deep enough to become acutely painful. It is perfectly possible to burn 1mm dia hole completely through you thumb and nail by inadvertly placing your thumb over output connector of CCFL inverter without you noticing that this is happening.

> Discharge across your heart can stop it, no problem. It is not at all difficult for that energy to route through your chest. You were lucky.

As my dad used to say "It's volts that jolts, but mills that kills!"

(you can survive a jolt very large voltage, but only a few milliamps across the heart can kill you)

Not just heart-stopping-dangers! I had a dishwasher whose wiring was damaged by a rat. I unplugged the dishwasher, and started working on the wires - maybe 1mm thickness. Next thing I know, I've punched myself in the face and knocked myself to the floor from something discharging, and need an ice pack on my lip.

I had a dishwasher whose wiring wasn't installed properly, and it eventually caught fire. Seeing smoke and flames come out of a running dishwasher is something I won't soon forget!

But that takes current, not merely voltage. It's very hard to do with the discharge of static energy.

Stopping the heart is no problem --- that's what defibrillators do, after all! The heart stops, and then starts again with all its muscles in sync.

The problem is when the shock is not enough to stop the heart but causes it to lose sync and go into fibrillation.

I also touched a CRT anode when fiddling with a TV set when I was 12. I was aware of the danger and scared of that thing (it was huge!) but still forgot to discharge it. Despite the ~26kV nominal voltage there was only enough charge to make me jump to the ceiling, or at least I thought so. It's a lesson you never forget.

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