Quite a common adage heard in HVAC schools or electrician training is that “Volts jolt, but current kills”. I would like to put forward my argument today that this is completely incorrect. Instead, I will argue that it is, in fact, the work done by a circuit element which causes the danger.

Voltage and Current

Any high school level student will be aware of the basic relation that the current in a circuit is proportional to the voltage across its terminals. The constant of proportionality is the reciprocal of the resistance, R. This gives us the basic relation of Ohm’s Law:

V = IR

Let us consider what happens if I were to hold two terminals in my hands, one which is grounded and one which is at +5V with respect to the other. By Ohm’s Law, a current flows through my body. However, also by Ohm’s Law, this current is proportional to voltage. Therefore, as the voltage across my body increases, so does the current through it. Given that this is true, does it make sense to say that either the voltage or the current could cause injury. No, of course not. In fact, without either the voltage or the current, the electrical circuit would not complete and no charges would flow at all.

As you can see, the essential truth which totally destroys this notion of either voltage or current being to blame for electrical injuries is that a high voltage causes a high current.

Well… sort of

Of course, this saying hasn’t been around forever without reason. The “wisdom” which it attempts to convey is that, if no current flows, simply being at a voltage shouldn’t necessarily cause you any harm. For instance, imagine in the first example if I were simply holding one terminal of the voltage source with one hand. Would any current flow? No, or at least not continuously. In this instance, assuming that you haven’t already been charged by static electricity somehow, your body will act like a capacitor as a current flows for a fraction of a microsecond and charges are dumped into your body but cannot flow anywhere. As more charges are added, the electromagnetic forces which act against the EMF increase and, eventually (well, actually extremely quickly), no more charges can be added and you are in a state of electromagnetic equilibrium.

This is the key reason why birds can sit on power lines without being killed. They essentially act like a high-value resistor in parallel with that section of the power line. Typical resistances of power line segments may be in the milliohms, which is at least six orders of magnitude lower than the minimum for that of a human body. By the principle of current division, we can say that at minimum 99.9999% of the current flows through the power line rather than the bird.

Then why do volts jolt?

The old saying may be referring to a temporary arc which could be created by a high voltage. As already stated, your body may act like a low-value capacitor in some instances. For the temporary transient created by the high voltage when charging up your body, some current I will flow, rapidly decaying capacitatively. However, this initial transient current could be rather high. This is the “jolt” you can feel.

The same effect is felt with static discharge. Your body has acted like a capacitor, storing charges and, therefore, being at a higher voltage than another conductor temporarily. When the static electricity discharges from you, a current on the order of amps may flow, but only for a millisecond or less.

What actually kills, then?

Well, the simple (but not quite correct) answer to this is that the energy deposited by the circuit into your body is what will kill you. The power dissipated by a component is equal to the voltage across it multiplied by the current through it. This means that, each second, the power source will do P joules of work on the component. As it turns out, this relies both upon the current and voltage. If you are wired in parallel with a 1 kiloohm resistor to the same 5V source as before, the majority of the current will take the kiloohm resistor path. However, the voltage across your body will still be the same. The difference in this case is that the current is much lower and therefore the power.

An additional factor to consider is that, when an electrical current flows through nerves, this can disrupt the closed-loop control of the human body, leading to break down of autonomous processes, such as heartbeat regulation and breathing, which just so happen to also be the most vital.