# Why is it written 'High voltage' in danger boards if current is the one which actually causes the shock?

After reading the answers given to this question I could understand that the amount of shock is dependent on the current and not majorly on the voltage even-though in some cases it depends.

But, in that case if current is the actual villain why do I see danger boards on which it is written '10,000 V-Danger' etc rather than showing the amount of current that can pass on two somebody's body.

• "It is the current that kills" or "the shock depends on the current" is a godawful thing to tell a layman as it leaves them thinking they know something when they just don't get it. You measure the current to determine the risk or limit the current to control the risk, but current and potential difference are intimately linked. And duration matters. And frequency matters. There simply isn't a one-sentence version that is correct without the hearer having internalized a great deal of background information first. – dmckee Nov 8 '13 at 19:45
• The phrase "current kills, not voltage kills" is truthful in only the most anal interpretation imaginable, and for all intents and purposes can be considered false. A AA-cell battery is capable of putting out several hundred times as much current as is required to stop a human heart, but they're perfectly safe to touch. See "lionelbrits" answer for an explanation of this apparent paradox, and why voltage really does matter. – DumpsterDoofus Nov 8 '13 at 21:25
• Because DANGER HIGH VOLTAGE scares people, and "HIGH CURRENT" does not. Much as inflammable materials are labeled "FLAMMABLE". The former protects people lacking physics knowledge; the latter protect people with a poor vocabulary. – JEB Feb 4 '18 at 2:55

## 3 Answers

Because the current will only flow through you in such amounts if there is a sufficient potential difference across your body. For an Ohmic conductor,

$I = \frac{V}{R}$

Your body has a large resistance, so for small voltages, the current is also small. This is why a $20\,V$ welding machine is not going to fry you, but will melt steel - the steel has very small resistance, so the current is enormous.

The power flowing through a circuit element is given by $P = IV$. Now, if the element is resistive (i.e., obeys Ohm's law), like your body, or a piece of steel, then

$P = \frac{V}{R} \times V = \frac{V^2}{R}$.

The power that a resistor uses is consumed as heat, so for a fixed resistance, you can see that it depends on the square of the voltage (it also depends on the square of the current).

This has all to do with melting steel and burning you. If all you are concerned about is not stopping your heart, then it becomes much more complicated, because your heart and nervous system is very complicated and even small amounts of current can be dangerous.

• Plus extremely high voltages ionize the conductor and drop resistance to near zero. – Brandon Enright Nov 8 '13 at 19:43
• Yes, as @BrandonEnright points out, the Ohmic approximation is just an approximation :) – lionelbrits Nov 9 '13 at 11:58

To be precise: there is no current behind the sign, but there is voltage, and it is known to be high (>1000 V AC, >1500V DC). When someone gets in there, it's unknown what the current will be--there are many factors. What matters is that the power company will supply it as needed, and it will be fatal. While the current kills you, it is the voltage that is the hazard.

Consider a sign that says "DANGER CLIFF". Cliffs are the hazard (or at least your gravitational potential energy is), but it's the ground at the bottom that kills you--or more precisely, your kinetic energy when you hit it.

50 volts at 1mA, you will feel it. 230 volts at 1mA, you feel it more. 2000 volts at 1mA, it hurts. 100,000 volts at 1mA, you most likely get fried like a chip.

• This misses the point. You can't just vary voltage without changing current- the two are intrinsically linked. – Chris Feb 4 '18 at 0:55
• you use a variable transformer with a regular 2 coil transformer with a resistor of some kind. – slushed12345 Feb 6 '18 at 22:09
• No. Your body has some (frequency-dependent, possibly amplitude dependent) impedance. If there is $100~\rm V$ across your body at some frequency, the current is set by this impedance. – Chris Feb 6 '18 at 22:31
• it worked for me... thing is if you just use might resistance resistors they could generate 100's of watts of power which would not only burn the resistor but the solder/metal joints – slushed12345 Feb 8 '18 at 19:31