Title kinda says it all. I was talking to someone a few days ago and remarking on some exposed contacts and he said not to worry, it was only DC.
Was he right (within reason…doubtless DC can be dangerous too given enough power)?
You were talking to Thomas Edison?
I remember that fight and him electrocuting an elephant.
Um, I guess it depends - something about Augie Ampere and Victor Volt - one of them - I never really got it, ask my brother. There’s also watts and other stuff involved. Ohms too. Oh wait, mom has ice cream, bye.
As a general statement, no. Without more details, one can’t say which is more dangerous.
Sometimes people mean DC as a synonym for low voltage DC. Say, a 1.5 V AA battery, or a 5 V USB connector. Those are indeed not dangerous, but it’s because of the voltage, not because it’s DC.
Low voltage is almost always DC, because AC is mainly an advantage with power distribution, where transformers are useful (transformers only work with AC). At low voltage, there are other means of voltage conversion, plus batteries are inherently DC.
One advantage to AC is that there’s a point where the voltage is zero. If an arc forms, there’s a chance it’ll extinguish itself during the zero crossing. That doesn’t happen with DC. But that’s also a pretty narrow scenario.
I’d guess that your friend just meant the contacts were low voltage. Anything below 20 volts or so is perfectly safe. You can grab a 12 V car battery with both hands and despite it being capable of hundreds of amps, you won’t feel a thing since your skin resistance is so high (though the situation might be different if you’re wet).
It was low voltage DC (12v) in my case.
If you stick your tongue on it, it may tingle a little bit
It’s really hard to injure yourself with 12 V. Not quite impossible, but you’d have to do something like puncture your skin with the electrodes. You could burn yourself with a 12 V arc, I suppose.
AC at the same voltage is less dangerous than DC, from what I recall. DC can cause you to spasm onto the electrode and not let go, but AC can throw you off. So, you’d rather touch 120V AC than DC.
I live by the principle that touching electricity is a bad thing. Period.
Same here.
This is DC.
Touch it and you will be vaporized.
Not that it will throw you off but the zero point of AC breaks any clamp spasm that can happen with DC as I understand it.
I’m not so sure about that. As I understand it, plenty of people get electrocuted when they are unable to let go of an AC mains source.
Sure AC has a zero volt crossing points but they are extremely brief. Could you really pull away during the few hundredths of a second the voltage is low near the crossing points?
I can tell you getting crosswise of 120 VAC can hurt about the same as 120 VDC.
If my mathematics is right (and there is every chance it isn’t) 110V 50 Hz mains is below 20V for less than 1000th of a second on either side of each zero crossing point. Needless to say I’m sceptical of the idea that is long enough to allow you to pull your hand away.
Electricity tends to kill you in one of two ways. Either it screws up your heartbeat or it literally burns you to death.
Screwing up your heartbeat can happen with a surprisingly small amount of current. Most safety regulations are built around 5 mA being the “safe” level, though there hasn’t been a lot of human testing done to confirm it for what I hope are obvious reasons. Get up around 50 to 100 mA and it gets a lot more likely to throw your heart into fibrillation. Your heart has a really funny design where the fibrillation state is stable, so if you can get it into that state it generally won’t come out of it on its own. That’s what they make defibrillators for. Without a defibrillator, your heart will stay in fibrillation, and instead of pumping blood like it is supposed to, your heart will just basically sit there and shake rather chaotically. Since it’s not pumping blood, you pass out fairly quickly, then die a short time later.
The thing about AC vs DC is that the most reliable way to throw a heart into fibrillation is to hit it with AC at right around 50 to 60 Hz or so. From a safety point of view, we somehow managed to land on the most dangerous frequencies to use to transmit power. DC can still throw your heart into fibrillation, so DC isn’t exactly the “safe” alternative here.
The thing about fibrillation is that it is very hit and miss. Your heart is more sensitive to disruption during certain parts of its cycle than others. Shock the heart at exactly the right time, and you are significantly more likely to throw it into fibrillation.
One weird thing is that a higher level of shock is less likely to kill you. Instead of throwing your heartbeat out of whack, if you pass enough current through the heart then all of the heart muscles will just clamp. The heart still isn’t pumping blood, so if no one removes the source of the current you’re still toast, but if the current is removed, then your heart is fairly likely to just go back to a normal rhythm.
Of course, at even higher current levels, then you get into the second way that electricity kills you. Electricity through anything that isn’t a superconductor generates heat, and the electricity will literally cook you to death. This is how the electric chair works. While a low level shock is rather hit and miss as to whether or not it will throw your heartbeat out of whack, high current levels aren’t so hit and miss. Nobody survives a properly functioning electric chair.
At these high current levels, it doesn’t really matter if it’s AC or DC.
That was all propaganda on Edison’s part. Edison did his best to convince everyone that his competitor’s AC system was much more dangerous. Edison not only recommended AC for the electric chair over DC, he also strongly recommended that everyone use Westinghouse generators, his competitor. Edison wanted to convey how dangerous his competitor was, which was why everyone should choose his power system instead. At those high of current levels, it doesn’t really matter if it’s AC or DC. Edison was just spreading propaganda to sell his own systems and steer people away from Westinghouse.
DC is horrible for electrical distribution systems. There is no simple DC equivalent of the AC transformer. This is why we use AC for power systems, despite Edison’s vigorous campaign against it.
High voltage DC actually has a lot of benefits of AC for power transmission. For the same size wire, insulation standoffs, etc. DC can carry more power, because the insulation etc. for AC needs to be designed for the max voltage, and the actual power you can shove through the line depends on the RMS voltage, not the peak. DC always runs at peak voltage. Like I said, though, there is no simple DC transformer, so transforming the voltage up and down at either end of the line is significantly more expensive. DC will also draw a very long arc, where AC will naturally extinguish any arc since AC drops to zero volts twice during its sine wave cycle. This means that switchgear for DC power transmission also has to include arc suppression.
Since DC isn’t well suited to power distribution, most people only experience DC for battery powered things and AC for much higher voltage power distribution. So in the common things that you are likely to encounter, DC is safer, since 24 volts and below won’t penetrate your skin. Once you get about 50 volts or so, then the electricity will punch through your skin and the electricity becomes significantly more dangerous. Note that it’s the voltage that matters here though, not whether it’s AC or DC. It’s just that most folks don’t encounter higher DC voltages.
As for a DC hazard that a lot of folks will at least recognize, your typical electric car operates at 400 volts DC.
If it’s summertime, you better hope you have AC if you’re in DC.
Do amps never figure into this?
When I was in high school the whole class touched a Van de Graaff generator which the teacher said was around 100,000 volts. No one died (we all got a shock and our hair stood up).
If volts is what kills you why didn’t everyone in our class die?
Because it’s amps that kills. Otherwise the human race would have died out from static electricity shocks.