Back during the current wars Thomas Edison tried to make the argument that DC power was inherently safer than AC was he right? Is there something more dangerous about AC that at the same power level you’re more likely to survive a shock from a DC source than an AC one?
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I would consider the opposite to be true. I think of AC as being “safer” because if you grab a live AC wire, you have 60 chances a second to let go.
You know I thought this was a simple question, but it is hotly debated:
Edison had a point.
A friend of mine (who sadly is no longer with us) once worked for a researcher into early pacemakers. They needed to put animal hearts into ventricular fibrillation (V-fib) for testing, and they found that the best way of doing that was to apply an AC shock somewhere around 50 or 60 Hz or so. In that respect, we pretty much could not have chosen more dangerous frequencies for AC power.
Some people say that AC is more dangerous because it makes your muscles spasm, where DC just makes the muscles jolt and throws you from the shock. Others say that DC makes your muscles clamp and AC allows you to break free because of the zero crossings. From what I’ve read, actual experimental evidence suggests that DC is easier to break free from, though I suspect that this is another case where the exact circumstances play a role.
Electricity tends to kill you in one of two ways. At low current levels, the danger is mostly screwing up your heartbeat. It doesn’t take much current to throw your heart into V-fib. Safety standards are generally based around 5 mA or less being “safe”, and currents around 100 mA are known to be easily capable of getting your heartbeat all wonky. The human heart has kind of a funny design that allows the V-fib state to be stable, meaning that once your heart is in V-fib, it will usually stay there unless something acts to take it out of that state (like someone shocks you with a portable defibrillator). For these low level shocks, AC is significantly more dangerous than DC, especially at the frequencies we tend to use for power systems, as I mentioned above.
At higher current levels, the heart will typically just clamp instead of going into V-fib. The heart still isn’t pumping blood, so if nobody removes the current source you are still going to die. But at these higher current levels, the heart will usually go back into a normal rhythm once the current is removed. So oddly, the survival rate gets a bit better as the current increases, up to a point.
At even higher current levels though (several amps), the fatality rate starts to climb again. This is because you are now getting into the second way that electricity kills you. It literally cooks you to death. For the same amount of current (assuming RMS for AC), you get about the same amount of damage, so AC and DC are roughly equivalent here. The one difference is that the human body’s impedance is slightly lower for AC, meaning that under the same applied voltage, a bit more current will flow with AC than DC, making AC slightly worse in this case.
Some things depend on circumstances, though. High voltage DC will sustain an arc a lot better than AC, so once something starts arcing, it’s a lot more difficult to extinguish the arc on DC. AC, because it is a sine wave, naturally crosses zero twice during the AC cycle, and those zero crossings tend to naturally extinguish arcs. There are some circumstances where a DC arc will continue damaging someone, where an AC arc would extinguish itself under the same circumstances.
The implications of that fine summary seem to be that if we wanted to massively improve electrical safety with no regard for cost or the installed base problem, we’d lower the voltage to WAG 25 to 50 volts and raise the frequency well away from the 50-60 danger zone or a multiple thereof.
Given the reduction in total power requirements for lots of devices within a household or business office we’d also install low voltage * low amperage = low total power feed circuits and disaggregate them onto a large number of fast acting transient intolerant current interruptor devices in lieu of the current multi-ganged mechanical or magnetic circuit breakers, GFCI, AFCI, etc. But retain the GF & AF detection capability. We’d keep the existing high voltage high amperage distribution for things like ovens & industrial equipment.
As well, we’d update the pitiful pre-turn-of-the-last-century design of the US standard plug & socket.
Which leads to an overall question I don’t know the answer to:
How many people are injured or killed by household electricity in a year in e.g. the US? And what changes would give the greatest reduction in that number? Better plugs, better breakers, better installation & maintenance, or better education and fewer drunk/drugged adults and unsupervised toddlers?
Some quick googling gives us some parameters for the above. FWIW, here’s what I found:
http://www.ecmag.com/section/safety/alarming-statistics is about electrical workers, mostly in construction. About 150 are killed per year through electricity, falls and everything. Most electrical injuries come from skipping safety precautions and working on live equipment. The article tiptoes around the question of whether that’s from workers being macho / careless or management pushing for cutting corners on safety to save time and increase productivity.
The medical industry says: Electrical Injuries in Emergency Medicine: Background, Pathophysiology, Etiology
500 to 1000 deaths per year. It’s almost entirely either little kids at home or young adults goofing around with utility poles & lines or working as electricians.
The pictures in the media section are pretty mild as emergency doctor pictures go. Be forewarned. Interestingly the first pic is a superficially burned foot. Wearing a toe tag. Apparently the entrance wound and the internal injuries would’ve been more spectacular.
These folks Home Electrical Fires - Electrical Safety Foundation International are an electrical safety industry group. Summarizing mightily, they say a lot of house fires are electrical and a lot of problems occur at outlets.
The AC system allows use of transformers throughout the system, making it much cheaper & safer compared to an all-DC system. If we didn’t have an AC electrical system, we’d have many more fires than we do now. And fires are more deadly than electrocution.
I don’t know that that follows. Back in Edison’s day the difficulty and inefficiency of swapping DC voltage for current and vice versa was huge. As you say. And was the main reason AC won out worldwide.
Today that DC-to-different-DC swap is a solved problem using what amount to switching power supplies as voltage/current converters. Transformers are commonly used now mostly because they’re cheap. Not because they’re the only possible option.
If we were wiring up a new country today we could use high voltage DC for generator to substation, mid-voltage DC for neighborhood distribution and low voltage DC for residential and small commercial supply. Without necessarily changing the fire hazard vs. present US practice.
The question is confusing. Wouldn’t it make more sense to compare AC/DC’s safety to some other band, like say the Sex Pistols or Guns n’ Roses?
Once they’re out of condoms, they’re all equally unsafe.
I would rate the Sex Pistols as the most dangerous. Sid Vicious killed Nancy Spungen, and then later died of an overdose (which some say was actually suicide). For AC/DC, Bon Scott killed himself (so tied with Sid Vicious there), but Phil Rudd wasn’t successful in his attempted murder, so AC/DC comes in second.
I don’t think anyone related to Guns n’ Roses has died.
As was already mentioned, Westinghouse won the battle because of the AC transformer, which is just a couple of coils of wire around a hunk of iron. It’s hard to get much simpler than that.
DC does have its advantages, though.
DC always runs at peak voltage. For AC, you have to design the insulation and standoffs and such for the peak voltage, but you effectively only get the RMS voltage out of it.
DC also does not suffer from reactive (inductive and capacitive) losses in long transmission lines.
For undersea lines, reactive losses are significant, giving DC an even bigger advantage.
But then, DC has disadvantages as well.
DC “transformers” are much more complex. In Edison’s day, they could use a motor/generator set. Not the most efficient thing in the world, but it worked. Not very practical for stepping up and down voltages for household use though. Modern DC transformers use semiconductors and other more efficient methods, but still they are nowhere near as simple, cheap, and efficient as AC transformers.
DC also isn’t self-extinguishing with respect to arcs, as I mentioned upthread. This means that DC switchgear has to be designed with arc suppression, adding cost and complexity.
The long and short of it is that if we were to design the U.S. power systems all from scratch today, we would use a lot more high voltage DC for transmission lines, but we would probably leave distribution as AC.
I don’t know that we would change the frequency of the AC all that much. Higher frequencies would be safer, and in general higher frequencies means smaller and cheaper transformers, but transformer hum also gets much more annoying at higher frequencies.
Ref another current (heh :)) thread we could boost the AC frequency to, say, 16 kHz. Then only those pesky kids would be annoyed by transformer hum. The rest of us grumps couldn’t hear it a bit. 
Higher frequencies of AC also would result in smaller hardware overall. It’s the main reason why aircraft use 115/V400 Hz AC. Higher frequencies would also mean higher losses for transmission, though and that would be bad.
FWIW, I’d rather work on DC rather than AC systems any day of the week. I’ve been zapped by both and AC is…more unpleasant.
Either way, you’re AHHHH AHHHHHH THUNDERSTRUCK
Over the long term, we may be headed in that direction: