Maybe it’s the ones who know how to explain how not to get electrocuted instead of some explanation that leaves them saying
“Don’t worry, it’s the volts that kill!” Just before finding out that’s not how it works. Doesn’t matter if you’re right when no one is listening.
I learned that when I was four and stuck my finger in an electric outlet.
When I was in college, we were taught that when probing high voltage/high current circuitry, you should keep one hand behind your back. Grab onto a belt loop or something. This prevents you from inadvertently grounding your other hand.
This saved my ass a couple of years ago due to a mislabeled breaker. I had to install a new dishwasher, so I flipped off the breaker marked ‘dishwasher’. But because I learned to be careful, I still put my hand behind my back while I put the screwdriver to one of the terminals to loosen it - and got a good arc that burned a little divot into the screwdriver. Never trust the labels on a breaker.
The current has to go through your heart to kill you. The easiest way for that to happen is to have one hand on a live wire and the other on a ground. So long as you aren’t grounded, it’s much less likely that any current will make it’s way through your heart.
One of my instructors told me the story of an old power engineer he worked with who didn’t even bother with a meter. He’d just lick his index and middle fingers, and touch the terminals quickly. He didn’t die because the current path was between the two fingers. Apparently he’d sort of just swipe them across the terminals so the contact would just be for a few milliseconds. This is an insane thing to do, but the guy had been doing it for most of his career and was still alive. No word on the condition of his fingers.
Actually, floating-ground systems are a thing. A ground is usually a good idea, but not always: There are some situations where, overall, floating is safer. And even “a good idea” is not the same thing as “necessary”.
Oh, completely agree.
All else being equal, a floating system should be safer vs. a grounded system. Because if a system is floating, and you touch one of the terminals, then (theoretically) you shouldn’t receive a shock, even if you’re grounded.
But… if you float a system, you have to be careful that it doesn’t inadvertently “float up” to a high potential relative to the earth. And that’s what can happen if the primary winding of the transformer is at a high voltage relative to the earth. (A transformer can’t provide perfect isolation; there’s always a finite impedance between the primary and secondary windings.) One way to prevent a system from “floating up” is to install a resistor (e.g. 100 kΩ) between the secondary windings and the earth.
To make the wiring in your house safer, I suppose you could install a whole-house isolation transformer between your existing transformer and your house. But it would be very costly, of course. Not only that, but it might be difficult to maintain isolation, as “sneak paths” to earth ground will probably form, one way or another.
Seems I read somewhere that some of the receptacles in hospital rooms are isolated. Are those the red ones?
Orange is isolated power.
Red is connected to backup generators.
most ships built after the 60’s use AC.
I got cross wise of a boiler igniter once. That sucker was 10,000 volts. Right hand to left hand. Hurt like a mother. Felt like someone made a body cast that was about 1 inch smaller than my upper body. The they slammed that body cast on me. I had to quickly recover and get the boiler light off before the steam pressure dropped below 75f psi. If it had I would had to call up centeral supply and tell them to reset all their autoclaves. Fun day. I learner where to put the torch when lighting off on oil.
I think that small vehicles in general (including cars and small watercraft) usually use DC because, at small scale, a battery is going to be a very significant part of your electrical system.
With that kind of voltage from one hand to the other, you’re damned lucky you survived.
Every electric car on the road today uses an AC motor. They have too many advantages over DC motors. Many of them are even 3-phase. Except for uncommon double-wound, AC motors are brushless, which is one less thing to wear out. And DC motors typically have a speed ceiling that AC motors do not. And anyway, a DC motor merely converts DC to AC internally, so why not just use an oscillator to turn battery current into AC?
Now, the inventor who comes up with a battery that naturally generates AC will be highly praised by automakers and others.
With an ignitor it is a very small amperage. But man it did hurt. The second I stepped on the pedal to light the boiler and the ignitor was energized I shot straight up in the air which pulled my foot off the pedal. And on that boiler the ignition was as long as your foot was on the pedal, on the other boiler it was a timed ignition. But yes, I do consider myself lucky. I also learned how to light borh boilers off and keep the tourch clear of the ignitor.
Maybe 30 years ago they would have. Nowadays, though, a waveform generator is so cheap and easy that even if a “naturally AC” battery existed, it’d probably be cheaper just to use a DC battery combined with a waveform generator.
I was under the impression that most electric vehicles use an AC motor that is driven by a VFD (variable frequency drive, for those who aren’t familiar with the acronym). They vary the frequency to control the speed of the motor.
I don’t think that they would want a fixed-frequency AC battery even if someone could invent one.
I think we have to say DC is safer than AC because this is correlated with lower versus higher voltage. Therefore, most DC power systems and applications are safer than most AC power systems because they’re mostly at pretty low voltage, and that’d count as a “yes” to the OP. But correlation isn’t causation, and we don’t mean to mislead.
Good point. But I’m thinking the OP is asking, “All else being equal…” I think @engineer_comp_geek did a good job of explaining it in post #17.
Yeah, I think so too.
But imagine this hypothetical: somebody completely and totally nontechnical calls you in the middle of the night in a panic, because they have to move some scary looking wires out of the way to get out of a dangerous and urgent situation, and they ask you which ones they should move but they’re useless in describing them in a helpful way. Then they say that they can move either these ones over here that say “AC” on them, or they can move those ones over there that say “DC” on them. You hear the fear in their voice, and you have to take your best shot. Do you tell them to mess with the “AC” ones or the “DC” ones.
Yeah.
Which is kind of interesting when you consider that electric vehicles have that sort of (ungrounded) DC voltage available. The current decade is probably the first time in history that ungrounded, deadly, DC voltages will be commonplace in consumer level devices.
My FIL who survived a career as an electrician without electrocuting himself has a triple safety habit. First he turns things off (duh). Then before he touches them he gets the relevant wire and lightly brushes it against something conductive that might be expected to be at lower potential if the wire was live*. Then he brushes it against the back of his hand.
*note careful avoidance of word “ground” to avoid upsetting Chronos! 
It takes a combination of factors for electricity to kill you. Why oh why do these endless assertions about singular factors have to be made. It’s not volts. It’s not amps. It’s not a completed circuit. Etc. It’s a bunch of things that have to be present for electrocution to occur!
I’ve heard it said that, once you get past dry-skin resistance, a human body is 330 ohm 1/4 watt resistor. And will burn out accordingly.
Don’t let the magic smoke out. it really stinks!
There’s also a product called ground enhancement material (GEM) . If the soil has decent conductivity, and you’re just dealing with a safety ground, then pounding a ground rod directly in the soil is adequate. For soils with crummy conductivity, or a need for a better quality ground (e.g. as a reference for sensitive laboratory instrumentation), you can use GEM, which is basically conductive concrete. A 5/8" diameter rod a few feet long provides very limited contact area if you sink it directly into soil, which can result in unacceptably high electrical resistance. So instead you bore a post-hole, fill it with this stuff, and set your ground rod in it. Now instead of your 5/8" rod, you’ve got a 6" diameter cylinder making contact with the surrounding soil - ten times more contact area, reducing contact resistance by a factor of ten. Need lower resistance? Use a bigger bore hole for bigger perimeter area, and/or multiple boreholes and grounding rods. In extreme situations (e.g. in facilities where high voltages are used and/or where soil resistivity is a problem), a grounding mesh may be used, i.e. a literal grid of wire is buried in the ground and connected at many points to ground rods (on which GEM may also be used to enhance grounding contact).