Zaps and bangs

A reader writes:

Hi, Dan! 

Though I didn't follow all of the details, I did enjoy your writing about electrocution and car batteries.

Do you know the odds of getting electrocuted if one is standing in a wet shower with wet skin using a cordless (battery-powered) sander? I don't know what kind of power I'll need to work on residential showers for hours at a time, but the electric chorded sander I WAS using (until I decided that I'm tired of risking electrocution) says it's a 120 Volt, 10 Amp model. 


There's probably no danger, but there could be some.

Cordless tools all run from low-voltage DC, although the voltage has risen in recent tools that use one or another flavour of rechargeable lithium battery. Higher voltage is better, from the tool-makers' point of view, because a given power from a higher voltage requires less current. This means thinner wires, less beefy switches, and generally speaking a cheaper, lighter tool with the same power.

Cordless tools are also, in general, significantly less powerful than corded versions. It's normal for corded drills and saws and sanders and such to draw peak power of at least several hundred watts. The ten-amp 120-volt rating of the sander you mention makes it a 1200-watt unit (so I presume you're talking about a belt sander, not an orbital one), though it'll only draw that much when it's working hard. You can expect even big heavy cordless tools to have no more than half the power rating of a similar corded tool.

Discovering exactly what that rating is can be difficult, partly because cordless tools can have a larger range between their "spinning freely doing nothing" and "working so hard it's barely turning at full power" power consumption than corded tools do. Mainly, though, cordless power ratings are harder to find because consumers think more watts are always better. So a cordless tool that costs three times as much as the wall-powered version, yet has a third the power rating, won't sell well, unless the manufacturer conceals that latter number.

I'm telling you all this just to explain my original wishy-washy "possibly dangerous" statement. If you're using a 12V tool then you probably won't be able to do yourself any electrical harm with it, even if you smash the thing on the wall until it breaks and then smack yourself in the chest with the pointy bits.

A 36-volt tool, on the other hand, is edging up toward the kind of voltage that actually can harm you, if only indirectly. (Direct harm: Current through your heart stops it, you die. Indirect harm: Current through some other part of your body causes you to spasm and dig a tool into yourself, fall off a ladder, flop out of the shower recess and smack your head on the toilet, et cetera. This sort of secondary injury following a non-fatal shock is a lot more common than injury or death caused directly by electricity.)

In the real world, even crappy bargain-basement cordless tools have enough plastic between you and the wiring battery terminals that no matter what voltage they run at, you pretty much have to make a specific and deliberate project out of killing yourself with one. Working in a wet environment is still dangerous, but only because it makes it easier to slip and then drill, saw or sand yourself instead of the workpiece.

Brand-name tools are generally safer still, and adding water to the situation may ruin the tool but is unlikely to hurt the user. Even the commonly-recognised-as-lethal "dropping a hair-dryer into your bath" situation is actually not terribly likely to kill you, though I don't recommend you try your luck.

If it's possible to electrocute yourself with cordless-tool gear in any way at all, here is I think your best chance of doing it without specifically running wires from the inside of the tool to nails driven into your chest. There are plenty of battery designs with exposed terminals of one kind or another, so suppose you eject the battery from the tool by accident, and then somehow grab that battery with both, wet, hands, so positive is touching one hand and negative is touching the other.

Even then, the resistance of human skin is way up in the tens of thousands of ohms - I found the resistance between two closely-spaced points on my tongue to be 70,000 ohms. So even with a 36-volt battery it'd be surprising if one whole milliamp managed to flow across your chest, and not all of that would go through your heart. I think you'd be an easy order of magnitude away from enough current through the heart for there to be any risk at all.

(I'm sorry to say that I'm not about to conduct heart-stopping experiments on myself. I have, however, previously zapped my arm for science.)

If both of your hands had bleeding cuts on them then 36 volts might be enough to at least give you a shock you could feel and it might have cardiac consequences, but this is really pushing it. And any sort of work gloves not made of chain-mail would erase the risk completely.

And, of course, back in the real world it continues to be downright difficult to actually touch the positive with one hand and the negative with the other. If you just grabbed both terminals of a 36-volt battery with one wet bleeding skinless lightly-salted hand then it'd sting like a bugger, but once again the only real health risk it'd present would be if the pain startled you enough that you then hurt yourself in some other way.

I won't be surprised if cordless-tool voltages rise further, though. There are already cordless mowers that run from 48-volt packs, for instance. So it's possible that a few years from now there'll be cordless tools running from voltages high enough to pose real electrocution risks.

It'll still be a lot less dangerous than it was in the olden days of corded tools, though, when casings were still commonly made of shiny cast aluminium. Then, the user's life was in the hands of the manufacturers and electricians who're meant to keep earth wires connected, and prevent live wires from touching the tool chassis.

With modern plastic casings and other construction improvements, even a theoretical 96-volt cordless tool is not likely to be an electrocution risk, even if you use it in the rain or, more realistically, get all hot and sweaty while working.

There's a lot of energy in a cordless-tool battery, though, and they definitely can hurt you if that energy is released very quickly because of, say, a short circuit...

...or severe over-charge...

...or physical damage...

The reason why drills and laptops and iPads aren't exploding all over the place is that the naturally excitable personality of lithium-ion technology, in particular, is kept calm by strong casings and protection circuitry ranging from simple fuses to smart current limiting:

If one of your cordless tools manages to puncture the battery of another, though, your life may still become quite exciting.

So I suppose I've allayed your fears of one kind of injury and then given you a new one to worry about.

There's no need to thank me.

Psycho Science is a... sort of... regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

11 Responses to “Zaps and bangs”

  1. pfriedel Says:

    Yeah, I've recently gotten into RC quadcopters because the barrier for entry has fallen below my financial threshold for new hobbies ($60! with a camera and remote and batteries and everything!) and so now I've gained a new paranoia: LiPo explosions. It's weird because even a couple of years ago they were still considered to be highly dangerous things, and they still are, but there's a certain amount of familiarity with them now. My quad comes with a 1C charger that clearly works, but for the life of me looks like nothing other than a switch mode power supply that can only provide 500mA of current at 4.2v or thereabouts. And it works! I suppose that 1C charge current is basically gentle for batteries that are designed for 20C discharge curves. But it still scares the hell out of me.

    And then there's parallel charging individual LiPo cells using a bundle of wires. There's no charge controller, it's still just dumping 500mA / number of cells until the charger decides they're full. It works, and broadly I kind of understand that if you trust the charger to only output a gentle current at a voltage below the danger point, and have faith that the charge protection circuitry will prevent the cell from overcharging you won't burn your house down.

    I'm still looking to get a fireproof container for them just because I like my house - it has all my stuff in it.

    • TwoHedWlf Says:

      Basically, lipos are pretty safe. The reason everyone worries about it in the RC hobby is that about 99% of the explosions are due to incorrect charging or physical damage.

      And RC batteries are unprotected, use cheap manually set chargers, connectors that can be easily shorted are common and they're flung around at 100 MPH, smacked into the ground, poles and trees then you hammer the dents out and do it again.

      I've had a handful of smashed lipos, like this one: [img][/img]

      And it just warmed up a little.

      And it's rarely charge rate that is the issue, it's charge voltage. Charge it as a 4s rather than 3s, or in your case a 1s and it WILL blow up eventually.

      • dan Says:

        Now I'm thinking of the good old "charge cords" from back in the day when a Tamiya Hotshot was pretty damn competitive racing hardware.

        A charge cord charged your 7.2V NiCd stick pack by connecting it directly to your 12V car battery.

        Sometimes there was a resistor in there somewhere.

        Surprisingly few cars burned to the frame.

        But some did.

        • TwoHedWlf Says:

          People still do that with A123s, they're Ahhh...Lithium Iron phosphate? Less sensitive than lithium polymer, but not as high of capacity. Put them in a 4 series with a set of long jumper cables for a bit of resistance and connect them straight to a car battery. Charge time depends on how much resistance the cables have. Just keep a close eye on your watch. I think normally around 4-5 minutes?

          • Fallingwater Says:

            A123s are seven shades of awesome. Cheap, too! I got a five-pack of 18650, 1.1Ah A123s on eBay for... I think something like €6.

            I didn't expect them to be genuine, so to check I hooked them up to a portable air pump that traditionally runs from a very hefty SLA (and taxes even that with its ginormous current draw).

            It spun up faster and blew more air than with its original battery. Wouldn't have lasted too long, of course, but it was damn impressive that something this tiny could withstand the sort of current that big ol' SLA did.

            30C continuous, 60C burst - for those 18650 that's 33 and 66 amps respectively.

      • pfriedel Says:

        Yeah, I always wonder how _grossly_ wrong the videos are doing things. And how many of the bigger incidents are the bigger 3S and 4S packs with enough lithium in them to make a chemist excited. So my 500mAh 1S cells might spark and pop like a firework, they won't turn into a roman candle of lithium fun either. And my quad keeps them relatively well protected, so while they get slightly excitingly warm in use they're cradled up deep in the structure compared to my microheli where they're, uh.... Down in the skids, waiting to kiss a rock on every landing.

        I think a lot of it is the gap between the blasé RC enthusiasts and the slightly panicked microcontroller enthusiasts' perceptions of LiPo, too. On the microcontroller side of things, they're still pretty nervous about them.

        • Fallingwater Says:

          Needlessly, too. A small LiPo cell that isn't meant for a RC model (that is, not built for dumping a whole lot of current in a short time) will barely even start a fire at all if punctured or abused (as Dan himself discovered while taking apart that weird flat flashlight). Sparks and smokes, yes, and perhaps a small flame, but you really need a RC high-current battery for the jets of dragonfire you see on Youtube.

          Tangentially, I facepalm a lot when advocates of electric vehicles say that somehow having a giant slab of a high-current LiPo battery under their seats makes them feel safer than a tank full of gas...

    • trialex Says:

      Link for the good quality $60 quadcopter? Not doubting you, just want in on the fun!

  2. RichVR Says:

    Ceramic knife review?

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