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Comment by exar0815 | Hacker News Reader

When I was young and stupid, I did this. Suddenly felt some tingling sensation in my fingers, threw the leads away and jumped back. Most certainly saved my life and drilled in my head how dangerous it is. Still get a sinking feeling in my stomach whenever I am at some switching station below the 380KV leads, remembering that day. Especially after knowing people who got into 120KV, and what happened to them...

Electricity is dangerous. In every form. Period. Keep your fingers away from it when you don't know what you do. Household appliances can not only kill you on the mains side. Microwave transformers, flyback circuits, burner igniters, HV Car Ignition Systems...

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>Electricity is dangerous. In every form. Period. Keep your fingers away from it when you don't know what you do

An additional problem here is that the risks are different in different forms. Here, it sounds like people are approaching the technique based on their experiences and assumptions about mains electricity, or even car batteries.

2 kV is high enough voltage that arcing is a risk. Things they'll assume are insulators and safe, or even usable as protection, may be conductive through dielectric breakdown: in the case of wood, that's the whole point of the technique. Even air won't necessarily be safe if they're handling things so closely, especially if it's humid, in which case they could get breakdown at well over 1 mm. Meanwhile, GFCI/RCD is useless, because of the transformer.

I had one the other week where I was replacing an outlet in the kitchen. I flipped the kitchen breaker and the lights and appliances all turned off. I skipped the important step of actually testing the outlet and stupidly assumed it no longer had power. Turns out it was on a different circuit from the rest of the kitchen. The outlet was stuck in the wall box so I reached a flathead screwdriver in to try and pry it out, and turned my screwdriver into an arc welder for a second. Fortunately I wasn't hurt and only got a melted screwdriver tip and a smoky smell out of it, but I'm definitely not going to let myself make that mistake ever again.

I made the mistake of not testing _all_ of the wires in an outlet a couple of months ago.

I used a non contact voltmeter, and had shut off the breaker to the outlet I was working on. Little did I know that the jackass that did some previous wiring in the house had run wires from a different breaker in to this box. It was done in such a way that I didn't fully realize it could have separate live.

My palm below my thumb touched the live and neutral and gave me a pretty solid shock.

I'll definitely test every single freaking wire in a box from now on. F people who try to play electrician without knowing what they are doing!

I saw an electrician who did know what he was doing change a fuse board incorrectly. He accidentally swapped one end of two different ring mains so that they were in different breakers! Same result as yours. Always test every live wire. Those glowing voltage sniffer screwdrivers can be pretty handy.

It isn't against code to have different lines inside one box (at least at any of the places I've lived).

Easiest way is to flip the mains everytime. I wouldn't trust those pencil testers though (the non-contact ones, the light ones yes).

I discovered an outlet like this in my house. Had a lamp plugged in to the outlet (since I could see it from the breaker box) and was sequentially going through and flipping each breaker off, then on again. The light never turned off...

Look at the slotted screw heads on outlet face plate. Are they both aligned vertically/horizontally, or are they at odd angles?

If they are both aligned, the outlet was probably but not necessarily last worked on by a real electrician. If the screws are at odd angles, the outlet was probably but not necessarily last worked on by a DIYer.

Real electricians align the screws, as a sort of professionalism and a nod to any future electrician who might work on that outlet.

Where? Here electricians leave cable offcuts, stripped insulator, plaster around on the floor so consistently it’s a meme (I know because for some reason I was getting tradie reels popping up flipping through Facebook).

We have a joke at work that you know that an electrical engineer (and not a licensed electrician) did the wiring if it’s too neat (for example, a colleague of mine seriously has cable trays through his roof cavity)

I have one of those little screwdrivers with a small light to detect current; I always use it on everything electric before I actually touch anything.

It amazes me how often strange anomolies slip through safety procedures. I use a voltmeter to check before working on wiring, and like you, have still managed to grab a live wire more than once. Last time, it was a situation much as yours - had the breaker off for the circuit I was certain I was working. And I checked with the voltmeter. But like a damned fool, I only checked the hot to ground voltage, and not the neutral to ground, or hot to neutral. Yep, you guessed it - the outlet was on a circuit for a different room, and was miswired in a junction box, reversing neutral and hot. I got the full benefit of the 120v. Fortunately, standing on a dry wooden floor in rubber soled shoes, I was a lousy path to ground, but still a dangerous situation I have no desire to repeat.

I have one even better, just to show how you can't be sure...

Replacing an outlet. Shut off the 2-pole breaker for the shared-neutral circuit it was on. Outlet tester showed the outlet was off. Pull the receptacle out. Used a multimeter to test each wire contact to ground; all were 0v.

Only when I disconnected all the wires did one of them develop potential because somebody had apparently tied a neutral wire to a hot in another box, but I still don't understand the situation and had an electrician address it.

So even testing each individual wire isn't certain unless they're all hanging free, unless you make the assumption that things were wired correctly in the first place.

When I'm working with home circuits I use indicator screwdriver all the time and I'm touching everyting metal with it at every step of the way.

I'm very rarely surprised with what happens but it still happens occasionally.

For example today I was surprised when RCD tripped when I shorted ground and neutral on a circuit that was disconnected by its own breaker.

I had to think for a brief moment to understand that yeah, it's perfectly reasonable that this happened and no, I'm not in any danger.

> The outlet was stuck in the wall box so I reached a flathead screwdriver in to try and pry it out, and turned my screwdriver into an arc welder for a second.

I've got a screwdriver that incorporates a non-contact AC current sensor. Mine is a Gardner Bender SDT-354 [1] that I think I bought at Home Depot or Lowe's.

When the current sensor is turned on if you bring the tip of the screwdriver near a live AC wire an LED in the handle starts rapidly blinking and a beeper starts rapidly beeping.

It once may have saved me from a nasty shock. I have an outdoor light over the garage that is controlled by a light sensor, and the light sensor was going bad. It would turn the light on when it got dark but not turn it off in the morning. So I'd flip the breaker for that light off and then back on every morning.

When morning when I went to open the electrical panel in the garage I got a shock from the panel door. I went in and got the current sensing screwdriver and went back to the garage to see if I could get an idea of what the heck was going on.

As soon as I stepped into the garage the screwdriver started blinking and beeping. A little experimenting showed it would go off anywhere in the front half of the garage that was more than about a meter above the floor.

It turned out that the metal garage doors and the metal rails their rollers rolled on and the metal tracks the garage door openers pulled the doors on were all connected to live AC.

I got the fuck out of there and called an electrician.

What happened was that in the shed next to the garage that houses my well some critter had chewed through some of the insulation on some of the wiring for powering the well pump. I'm not sure how but that somehow made the cover and door of the electrical panel and all those metal parts of my garage door system electrified, but without stopping the well pump from receiving power.

[1] https://www.amazon.com/Circuit-Alert-Blade-Screw-Driver/dp/B...

Yeah I have the standalone tool and it's a must-have for electric work. While replacing some switches in my house I thought I had the right breaker (sidenote: those breaker-detector tools are like magic, much better than the "is the light still on?" method). The lights went out but when I jammed my tester in the gang box it started beeping. I ended up needing to flip off 2 breakers (took me a while to find the second one) before it finally stopped beeping. From that moment on I promised myself that no matter how "sure" I was, I would ALWAYS stick the tester in before reaching in, even if I hadn't touched the breaker box between working on a switch.

I've been bitten enough times in software alone by things I was "sure" about, I don't take any chances when it comes to electricity.

An important habit when using testers: test the tester first on a known live wire. You don't want to find out the hard way if your tester has stopped working.

Cool story! I have a brand new screwdriver set from the other end of the safety spectrum.

Every one had a warning sticker saying 'non-insulating handle' on the plastic/rubber handle.

I'm not sure where they found conductive plastic, but had I known before purchase, I'd have gone with a different set.

It's not necessarily that the handle is conductive after all (it likely isn't, but there are conductive polymers out there -- I've made some in Chemistry class), but rather that it hasn't been tested and certified for its insulating properties (e.g. the VDE 1000V classification). They're covering their ass by saying that it's not intended to be an insulating handle because they didn't design it to be.

> Still get a sinking feeling in my stomach whenever I am at some switching station below the 380KV leads

If you would like to get into the psychology of someone like this I am a big fan of watching photonic induction: https://www.youtube.com/user/Photonvids

He is, seemingly, a power company tech from the UK who just really loves high voltage and messes with it in his home with his buddy.

Such as a gun is always loaded, my take is electricity is always dangerous.

Even professionals can get into accidents, moreover us uneducated plebs.

I remember watching some electricians rewiring some stuff at my old workplace. We were moving into a new office, and had to have some light switches changed for the new zone.

The electrician popped open the switch plates, yanked out the old switches, clipped the wires, stripped the wires, wired up the new switches, and screwed everything back together.

It was all live (like 300 volt live), as the lights turned on (queue spark), as he wired the new switches. He was not wearing gloves. It was all barehanded.

It was quick, though. Took, like, a minute.

It obviously can be done, and I've heard folks more experienced than I say "all ya gotta do...", and I'll admit that I might have even wired a switch/outlet myself without flipping the breaker. And a pro could probably get away with it for a long time.

But it just sounds like something that is one distraction away from a bad day.

In the UK, as far as the Health & Safety Executive goes, who are the people who will be hauling you up in court if your screwups cause injury or death, they go to great lengths to discourage working live if at all possible.

That said, it is sometimes necessary. I've replaced a circuit protective device in a live electrical panel in a commercial setting before. As you point out, avoiding distraction is a very important factor. That means no chit-chat, no-one overlooking and critiquing what you're doing, noise to a minimum, all of the tools you'll need prepared up-front, etc.

Meh, for 120V or 220V worst can happen is you get a little shock or trip the breakers. I don't bother shutting off power to household circuits either.

I heard from HV vets that one acquires a habit of keeping one hand in the back pocket when working on substations, industrial installations, and such. :-)

Not sure where there would be 300V for light switches/fixtures though. Some bespoke factory setup?

The lights were using some higher voltage. The Operations Manager mentioned it to me, while the work was being done. Apparently, it's standard. Something like 320 volts (don't remember the exact number). I had not heard of it, beforehand.

230V AC is the RMS (root mean square) voltage; if you rectify it to DC, it comes out to 325V. Put another way, the /highest/ absolute potential difference between line and neutral in a 230VAC system is 325V.

Every electrician I have met, has had some kind of shock in their career. Mostly it is a small induced current issue, which can easily kill you if it knocks you off a ladder....

No matter how many times I see someone changing computer parts while the power is still connected, if I'm doing it I always unplug the power then pressing the power button to ensure it's unpowered

You won't get hurt working on a live computer unless your power supply is dangerously engineered (stick to well known named brands) or wide open (don't open it up unless it's been unplugged and sitting on a shelf for months) for the same reasons pointed out elsewhere in these comments; the voltage simply isn't high enough. It won't even induce a perceptible amount of current (unless you do something stupid like stick your tongue on it), much less a harmful amount.

You'd be surprised at all the stuff you can get away with on a live computer. For example, did you know that AHCI (SATA) and PCIe (and thus NVMe) are both hot-pluggable and hot-swappable? ... and that you can hotswap RAM (with a cooperating motherboard and OS) and even CPUs (on multi-socket boards)?

Hdd or ssd are hot swappable? thats cool, thanks for sharing.

as for ram I have tried it once and it freeze the pc, I forgot if I have another plugged in or not though.

Sata is hotswappable. IDE was not. And I believe m.2 would be if the connector was designed a little different, because that’s all u.2 is. Compact Flash Express is, but again, that’s NVMe and splitting hairs between u.2.

Typically on hot swappable devices it’s been common to make the ground pins a little longer than the rest, ensuring that those will make contact first. The issue with hot swappable devices aside from a sharp initial draw is “grounding” the circuit to an IO pin before real ground is made. Most default low potential IO pins won’t enjoy taking the full load of the device while the ground plane is missing connection.

Oddly to me, many new PC BIOS ship with SATA hot swapping disabled. IDK why. You can plug the drive in it’ll spin up, but no data transfer will happen until you reboot. Safe, just ineffective. I recently had to learn this the hard way.

> no data transfer will happen until you reboot

I think this is something the OS can override. If I remember correctly I could discover a hotplugged SATA device in Windows by using "Scan for new hardware" in the Device Manager.

More specifically, the system's firmware AHCI hotplugging configuration only applies to the system firmware.

That is to say, if hotplugging is disabled on your SATA controller, then the firmware won't see a new SATA drive if you plug it in while the machine is running. So, for example, you won't get a UEFI boot menu entry for it if you go to the boot override section that presents all of your storage devices.

Once the operating system takes over, that's all a moot point. Whether hot-plugging was disabled in your firmware settings or not, if the OS supports it, it will work. Both Microsoft's AHCI driver (msahci.sys) and the Linux "ahci" kernel module (or built-in) support hot-plugging.

On my Windows desktop with an Intel SATA chipset, hot-plugging a drive did cause Windows to briefly drop all other drives on that controller before re-enumerating all of them, which lead to a BSoD because one of those drives was the drive with Windows itself on it. However, this was solved by installing Intel's chipset-specific driver (Intel Rapid Storage Technology, iaStorV.sys) and tweaking the registry to have that one start at early boot instead of msahci.

While that's a good idea because of potential part damage, I don't think that 5/12 volt supply counts as dangerous in any meaningful way.

The idea is also that pressing the power button will also help discharge some capacitors as the power-supply tries to supply power. Not sure to which degree this works to make things safer, but trying it won't hurt.

(Note this will _not_ make a powersupply safe to open up)

Unlikely to have effect on a PC.

On a dumb device where the power button is part of the actual circuit, let’s say an old radio this will work.

On a PC, the power button isn’t hooked to anything but the chipset or some micro or an upstream circuit that tries to start a micro. Which means to have any effect, that processor needs to be executing code, or needs to have enough power to start the chipset itself. Both of these are unlikely to have any effect on CPU capacitors or caps in the power supply.

I’m fairly certain this is placebo at best on a modern PC.

It definitely kicks the power light on for a bit under a second on my 2010 era desktop. The (high end, low wattage) power supply is delivering enough voltage to keep the motherboard power controller up.

I have no idea if it discharges all the different voltage rails. I don't like having the fans try to spin up while I'm mucking about, and it stops that from happening.

Of course, from an equipment damage perspective, grounding yourself to the case ground is much more important than discharing the PSU capacitors.

I usually physically unplug the computer, push the power button, then open / touch the case (I'm in a humid climate).

I've never had an issue (ignorging fires, but those happened when the computers were intentionally energized).

You need about 24 volts to overcome the skins resistance. Most PCs use some combination of 3, 5, and 12 volt rails so you should be fine, as long as you avoid licking anything.

Edit: Some PSUs have -12 +12 rails, I suppose if you were really unlucky and grabbed both at once the 24v differential could give you a zap.

Typical recommendation for DRY skin is ~50V.

You can wet your fingers and get way lower, but I’ve never had anything much at 24V even when trying. At that low a voltage it may come down to frequency of the source.

Obviously your tongue gets way lower, as is 9V lickers know already.

Licking 9V batteries should be a rite of passage for EE majors. That and playing capacitor hot potato.

The -12v rails are typically low current though. The real danger is a poorly built supply or an electrolytic capacitor exploding in your face (had this happen once).

Incidentally, does anyone know what the -12v rails are used for? Some kind of reference?

If memory serves, the -12V rail is useful for onboard audio op-amps.

RS-232 drivers like the old MC1488 were run off plus and minus 12 volt supplies. DRAMS like the MK4116 also required a low current -5V bias supply. And there were other chips that possibly needed a -5V supply. Since the -5V was low current you could generate it off the -12V with a simple linear regulator.

And remember that RCDs or GFCIs are not the panacea they may seem like at first. All it takes is an unreferenced secondary and voila, ground faults are invisible.

Can you explain what you mean by unreferenced secondary in this context?

a galvanically isolated transformer, i.e. a voltage that is not referenced to ground, meaning an RCD/GFCI will not be able to trigger if you touch it and it starts zapping you

> Electricity is dangerous. In every form.

Well — so is an empty room, because there is no air to breathe.

Pretty much everything is dangerous in some form. You can die from excessive water ingestion. You can die falling from a bicycle.

I don't think blanket statements like yours are helpful in actually making things safer. Safety low-voltage electrical systems are designed to be safe - not perfectly safe, but comparably safe to other ambient risks of life. We can't bubble-wrap everything. There is no 100%, only a question of how many nines you want after the dot on 99%.

But regardless, the videos at issue are quite obviously not in the "99% safe" category and YouTube's behavior is egregiously inacceptable.

I had a similar experience in completely different circumstances. I was hiking in the mountains with my dad and some friends when a storm cloud rolled over us. It wasn't raining yet, but one of us noticed that if he raised a metal hiking pole above his head there was a tingling feeling. We all were trying it when my dad caught up and said "what are you doing you idiots, drop those poles, crouch down, and start praying".

Thankfully there were no strikes, and after a couple of minutes the cloud had passed. Electricity is dangerous!

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