Why does high current kill?

You've heard it before: current kills. Image source: YouTube.

As an electrical engineer, you’re likely familiar with the safety tip that goes, “It’s not the voltage that kills you, it’s the current.” Of course there’s truth to this element, but there’s also more to understand about shock hazard than the phrase lets on. Simply put, if voltage didn’t present any danger, the familiar signs that scream out, “DANGER – HIGH VOLTAGE” wouldn’t exist.

Essentially, “current kills” is correct, because after all, it’s the electric current that burns tissue, freezes muscles, and fibrillates hearts. But electric current doesn’t occur on its own — there must be voltage available to motivate electrons to flow through a victim. What also must be taken into account is that a human body presents resistance to current.

Let’s take a look at Ohm’s Law. For voltage, current, and resistance, and expressing it in terms of current for a given voltage and resistance, we get the following equation:

The amount of current through a body is equal to the amount of voltage applied between two points on that body, divided by the electrical resistance offered by the body between those two points. Of course, the more voltage available to cause electrons to flow, the easier they’ll move through any given amount of resistance. This is what brings about the danger of high voltage, which means potential for large amounts of current through the body, which can injure or kill. But the more resistance a body offers to current, the slower the electrons will flow for any given amount of voltage. Basically, just how much voltage is dangerous depends on how much total resistance is in the circuit to oppose the flow of electrons.

Body resistance is not a fixed quantity — it varies from person to person. It also varies depending on how contact is made with the skin. For example, is it from hand-to-hand, hand-to-foot, foot-to-foot, hand-to-elbow, etc.? Sweat, being rich in salts and minerals, is an excellent conductor of electricity. So is blood, which has a similar content of conductive chemicals. Contact with a wire made by a sweaty hand or open wound will offer much less resistance to current than contact made by dry skin.

Offhand it might seem that a shock of 10,000 volts would be more deadly than 100 volts, but that isn’t necessarily so. Individuals have been electrocuted by appliances using ordinary house currents of 110 volts and by electrical apparatus in industry using as little as 42 volts direct current. The true measure of a shock's intensity is within the amount of current forced though the body, not the voltage. Meaning, any electrical device used on a house wiring circuit is able to, under certain conditions, transmit a fatal current.

While any amount of current over 10 mA is capable of producing painful to severe shock, currents between 100 and 200 mA are considered lethal. Currents above 200 mA, though they can produce severe burns and unconsciousness, do not normally cause death if the victim is given immediate attention. Resuscitation, consisting of artificial respiration, will usually revive the victim.

Overall, electricity's effect on the body depends on the specific path the current takes through the body, and on the characteristics of the individual's body. A large amount of current can kill a person by cooking the insides. A smaller amount of current can kill a person if it flows through the heart or central nervous system.

The best protection against shock from a live circuit is resistance, which can be added to the body through the use of insulated tools, gloves, boots, and similar gear.

Sources: allaboutcircuits.com, physics.ohio-state.edu