Which is more dangerous to work with and why; US domestic power at 120 volts or UK at 230 but half the amps?

Question

I know lower voltage requires higher amperage so much thicker power cables that take more resources but otherwise which system is safer to work with and which is more efficient?

Answer 1

LeAnne,

Ohm's law states that higher voltage equal lower current, given the same resistance. Neither is safe to be "hit" by, both can be fatal. I would rather have the 230 get me at a lower current. I also prefer AC to be shocked with over DC, since AC goes by zero volts 50-60 times per second and gives you a chance to get away from it. DC will grab you and keep you until power is disconnected.

220-230VAC is more power efficient, but what do you do when the entire country is set up to run 110-120VAC. We would need to change the entire infrastructure of this power grid we now have. That's just not terribly likely to happen any time soon.


If Hitler had his way, we probably would have done it, but since he did not win we get to keep our 110-120VAC.

Of course it's not preferable to be shocked at any time. In my line of work, it's bound to happen. I usually let go a few expletives, and go on with my work.

I have been hit by 770VAC 3 phase, since I had followed at least the basic safety precautions, all I got was knocked away from the current source. I got a slight burn to my left hand, since I was not earthed. I likely avoided more serious injury. I did lose a perfectly good screwdriver in the deal.

I know the body does not respond that quickly. I am pointing out the mere fact that the voltage is actually zero 50-60 times per second gives a better chance to got off the live line. Not to mention the RMS AC voltage is actually lower than the same DC voltage level would be. I know, given the same resistance this means the current would be higher. The chance to get away would still be greater.

The path of current flow through the body would be a big determining factor as to how fatal the shock would be, as well. My workspace is a ESD rubber mat. My work bench is also ESD rubber, my lift table (work bench) is metal and is grounded. All areas of potential contact are covered. I also work with an isolation transformer/variac that has a GFCI circuit inside it.
When I am working with a "live" set, I work with only one hand, keeping the other in my pocket. I also wear rubber soled shoes. If I am touching a potentially live circuit, I always touch it with the back of my hand (the muscles cannot contract far enough back to clamp down). It is always wise to use commonly held safety practices with electricity and electronics. Generally, I just do not take any chances, and just use my DVM to check for voltages being present.

Let's face it, NOBODY wants to get shocked, let alone electrocuted. The point is in theory only. I obviously would not attempt to apply voltage from my right hip area, to my left shoulder area (the best way to stop/start your heart). At that point, even a 9 volt battery can be lethal, in the right conditions.

Answer 2

All else being equal - the higher the voltage, the greater the current across a resistance. 230 volts is inherently more dangerous than 120, but it is also more cost effective. Less amperage means the same power (wattage) can be transmitted for less cost in the associated wiring and circuitry.

Answer 3

Both can kill. According to NFPA 70E any voltage in excess of 50V is potentially fatal.

120Vac can easily kill if you are not careful. Only 100mA through the heart is required to put your heart into defib. At that level who cares if the source is breakered at 15A or 7A.

the bodies natural resistance is anywhere between 10,000 ohms and 200ohms dependant on the condition, with 1000-1500ohms being a typical minimum.

Lets assume an opitimistic level. 5000 ohms. And you were unfortunate engouh to hold on to a grounded piece of metal while gripping a 120V wire.

120V/5000 = 24mA.

Not fatal yet. However, The "let-go" threshold is ~15mA. So if you are holding the source shock in your hands.. you will not be able to let go. You totally loose control of your muscles in the path of the current. You will grip the item with all of your muscle power. Reducing the bodies natural resistance even farther.

120V/2500 = 48mA. Lets assume you are unable to pull away from the shock. The tightning of your muscles improves your contact with the source of the shock. Possibly reducing your resistance .. lets say by half (thus 2500ohms). At this point, you will have difficulty breathing and will be in enormous discomfort. Fear and the exertion of your muscles will cause you to perspire.

120V/1000ohms = 120mA

There you go, your sweat has greatly reduced your natural resistance. Your heart rythm is now disrupted and now you are in extreme danger. You have a handful of seconds to avoid heart defib.

The differencefor 230Vac in the above scenarios. typically 230Vac will induce fatal currents faster. So there is less margin for error. 120V may take longer to kill, but it certainly can.

Edward B. I have been shocked by 240V, 50Hz. I was only able to release myself by using my legs and running away from the source of the shock. When I reached the end of the wire my momentum was enough to pull the wire from my hands. Otherwise I was toast.

While there is a zero crossing 60 times a second it doesnt matter. You have current passing through you, your muscles are completely out of your control. That reprieve of 50/60 times a second means nothing. Do you see the light flash at 50/60 hz. No, your eyes don;t correct that quickly to detect it. It the same with your muscles.. they dont correct fast enough to reduce the strentgh of your grip.

When exposed to enough current, AC or DC your muscles are contracting as hard as you can, even ignoring your normal pain threshold to do it.

Lets put it another way. AC requires only 60mA.. or more likely 100mA to discrupt the heart. DC is usually higher..at 300mA. Ac is more effective at causeing a heart defib.

Answer 4

The higher voltage presents more of a shock hazard; the higher current capability of the lower voltage presents more of a fire hazard for the same power capability. It's a tradeoff.

Answer 5

lower voltage is safer, but only 1/4 ampere that passes your heart will kill you.

Answer 6

voltage = resistance X amps

120v = 100,000 ohms ( body's resistance) X 1.2 milliamps

240v = 100,000 ohms X 2.4 milliamps

240v is more likely to electrocute you because of the higher voltage

UK is 50 hertz and USA is 60 hertz.... i think the 60 hertz is the more deadly frequency

http://en.wikipedia.org/wiki/Electric_shock

Answer 7

120 won't kill you, but it gives you a good jolt.