AC or DC power transmission.

the original patent war was between Edison and Westinghouse. one had invested in end use equipment for DC, and the other in AC

as Greg says, there is some extra power loss in AC, but at the time there wasn't enough to offset the advantage of being able to very easily adjust the voltage.

proprietary fittings are a pain in the (posterior)

For transmitting power over wires, it doesn't matter if it is AC or DC.

Edison came up with DC and 110V. 110V because he kept touching higher and higher voltages and 110V was the limit where he didn't receive an intense electric shock. The human skin is a very good insulator but at a certain voltage, the skin "breaks" and turns into a very good conductor. The problem is that the voltage depends on the condition and "thickness" of the skin. Edison who worked a lot with his hands was very tolerant to high voltages. Different humans have different "break through" voltages. This voltage is somewhere between 50V and 200V for dry, healthy skin. That's why Edison planned his power system for 110V and produced 110V light bulbs. Since Edison had the patent for the light bulb and he came up with a lot of electric hardware like fuses, wires, switches and even power meters also all designed for 110V, the very first power stations around the US all used 110V.

Edison used DC because it had many advantages. If the load changes, you need to adapt the power of the power plants to the load. In case of DC, the RPM of the steam engine driving a DC Generator is proportional to the power generated. By increasing or decreasing the RPM, the power can be regulated very easily to keep a constant voltage. Also you can simply connect more generators in parallel and control them independently. Also you can hook lead-acid accumulators to the system which charge when more power is generated than used and discharge when less power is generated. A very simple and highly effective method to balance the power grid. Something we badly need nowadays with all the unreliable alternative power sources like wind power.

The competition had to use AC because Edison's patents were all restricted to DC, so the only way to enter the market was using AC.
Using AC has its own advantages. You can use transformers. You can step up and down the voltage very easily with a simple and passive system. Losses, like Greg had already pointed out come from resistance of the wires. When you double the voltage, you need only half the current to convey the very same amount of power. Half the current means half the voltage drop over the wires. Half the voltage drop multiplied with half the current means ¼ of the losses over the wires. 4 times the voltage gives you ¼ of the current which in turn causes 1/16 of the losses. In a modern power grid, they use even more than 1000 times the voltage which translates to less than a millionth of the losses using the same wire gauge. This allows long distance power transmission with reasonable losses and reasonable costs for the transmission lines. You just don't need less copper, you can even use the lighter and cheaper aluminum. And lighter wires allow less and cheaper power poles which also cuts costs dramatically.
But AC has lots of disadvantages, too!
The major disadvantage is that switching the polarity in a sine wave causes gaps in the power with little and no voltage at all. To compensate, you need to increase the voltage to convey the same effective power and make light bulbs glow as bright. Whenever AC is used, the voltage rating is measured in "effective". So 110V_eff AC is not the same as 110V DC! The peak voltage (Û) is 1.42 times higher than the rated Voltage (U_eff). And Û is what breaks the resistance of the skin, so Edison’s "Voltage which is safe for working class human hands" turns "lethal to almost everybody". This was Edison’s main argument against using AC to keep his monopoly of the market.
And there are more disadvantages. The impedance of coils in electric motors is frequency dependent. You need bigger coils using AC and the efficiency of the motors decreases. And much worse is the lower torque of slow running motors. So you have bigger motors with less efficiency and unable to start high loads turning. That's why all railroad and streetcars operate with DC motors or related. Originally by using DC on the feeder wires and rails, nowadays using rectifiers and other systems to solve the problem of AC.
And even worse, you can't just connect AC generators in parallel. If they are not in sync, they fight against each other which is often worse than a short circuit. You need to keep all AC generators of a power grid running in sync. And even then they still partially fight each other causing losses, losses which can't happen on a DC grid.
Also using AC opens another whole can of worms, currents can become out of sync with the voltage. Those currents won't convey any useful power but those additional currents cause more losses on the wires again, decreasing the performance of transformators and generators and lowering the performance of electric motors. Compensating those "blind" currents requires to generate more power than is actually used.
ALso capacitors can let AC currents pass. And long power lines are a real capacitor since everything conducting which faces something else is an electric capacitor. The more surface you have, the bigger the electric capacity, and length increases the surface as good as width. So there is a massive blind current "bleeding" out of long power lines.

While using DC is much better for generating and using electric power in most respects, it can't be used for simple long distance power distribution and to create a real power grid.

So in General, AC is real bad but it used to be the only method to allow different voltages in the same system so AC had to be used to create a power grid.

Nowadays, high power semiconductors allow transforming DC voltages up and down just fine so there is no reason to keep using AC on the power grid any more. You still need to provide AC to households for compatibility reasons but there is no need to use it for power distribution any more.

So finally, DC has won the battle of currents. Most household applications run on DC anyway, switching power supplies are everywhere, in computers, most electronic devices, CFL, washing machines, even some modern stoves use DC internally. First thing those devices do is to convert the AC to DC using a rectifier and THEN convert the voltage and make use of the power.

There is only one disadvantage using DC really has. In combination with moisture, DC makes conductors "rot" by galvanic processes. That's why Jamie had much better results using a small DC current dissolving his prison bars than Adam who used a massive AC current. Because if you use AC, most of the damage caused by galvanic processes is reversed 30 times a second (25 times in the EU). This is a great problem for HVDC power lines, but easy to fix. Just reverse the polarity of the line every now and then. Households and all common electric outlets will remain running on AC. First of all to protect the wiring and last but not least for compatibility reasons.

Actually, 120VAC is still considered to not be lethal to a healthy person with clean dry skin. human skin is resistive, not dielectric. resistive substances resist the flow of electricity, and greater voltages drive greater currents through it. Dielectrics block the flow of electricity until they reach breakdown voltage, at which point they essentially lose all resistance.

It is still annoying as hell to me. 240 hurts, and 277 leaves a mark.

well, yeah - when I blew a hole in my finger with 277V it stopped being dry unbroken skin. that is ablative damage, not dielectric breakdown.

overwhelming resistance is when I tried to put a cover on a shorted HO fluorescent fixture, and got lit up between the fixture and the aluminum ladder. dielectric breakdown is when I tried to put the cover on using a pair of 600V rated pliers and got lit up again.

there are two ways current can be limited in a system:
by limiting the voltage: I.E. a 120VAC system sill not provide a RMS value above 120V so the current will not exceed voltage/resistance.
by limiting the current: I.E. a transformer with an AFC (Available Fault Current) of .5MA will only deliver .5mA of current regardless of how low the resistance gets. at that point, you get voltage drop, just like you get pressure drop in a kinked hose.

electrocution is considered to be cardiac arrest caused by electric shock - which is calculated at .5mA for a healthy heart. other forms of electrical related injury have their own defined terminology, with the most common being electrical burns usually caused by the current flash heating bodily fluids or causing arc flash burns. Addendum: second is mechanical injury caused by involuntary muscle spasms.

the three factors commonly considered for electrocution are magnitude of current, current path, and duration of contact.

further AC vs. DC data: AC electrocution is considered more dangerous thn DC electrocution, because AC electrocution will essentially reset your heart rate to 60 hz (50 in europe) whereas DC electrocution simply stops the heart until current is removed.
guess which current a defibrillator uses. (to spell it out, Fibrillation is the term for when the heart is trying to beat so rapidly it cannot effectively move any blood)

Just because you didn't notice the split second your skin was able to block the 277V isn't proof that the skin didn't protect you until it had failed.


120V AC is measured in RMS, as RMS is the effective voltage. The peak voltage of AC is by the factor of the squareroot of 2 greater than it's AC rating. AC is rated as "effective" so a light bulb and a heater will have the same power throughput. To compensate the "gaps" in AC, the peak voltage of AC is made greater to have the same effective voltage.


Correct. And just like with a hose, the pressure to burst the hose is a lot greater than the pressure you have afterward.


That doesn't change the 3 major effects how electricity can kill you. You don't survive by just using a different name. I avoided to use the term "electrocution" for a reason.


As I had said, what matters is the current density in your body and this depends on numerous factors including the ones you had listed. The "safe current" of 0.5mA is only a DIN-EN rating for currents entering the body under normal, "DIN-EN certified" circumstances. Of course it can take less or more.



Fibrillation is one cause of death as I mentioned under 1). The greater danger is the greater peak voltage. The heart isn't a frequency storage device. Fibrilation is never anything near 50Hz since human nerves can't convey much more than 12Hz. There is no way that any nerve could run with 50/60Hz! Fibrillation is actually less than 10Hz and has nothing to do with adopting to an external frequency at all. Even DC shocks can cause fibrillation, the trick of the defibrillator is the massive amount of current density exhausting the nerves instantly.

It can take less than 100 micro amps applied directly across the heart to cause fibrillation. That's why hospital ECG (EKG to C64) monitors must be tested to have less than 10 microamps leakage when connected directly to a 120 volt AC line.

The "perfect weather" quote wasn't mine, but I do agree with it and what you said.