Electrical safety

A current of about 10 to 20 mA through the body can lock the hand onto a wire, and a person soaked with water may have only 10 kΩ of resistance.

Two kinds of danger: heat and shock

Electricity can hurt in two ways. A thermal hazard is too much power turning into heat where it should not, for example inside a wall, where it can start a fire. A shock hazard is current passing through a person. Both come straight out of the formulas in this course, P = V² ÷ R and I = V ÷ R, so you already know enough to see where the danger lies.

Short circuits

A short circuit is a path of very low resistance between the two sides of a supply. It happens, for example, when worn insulation lets the two wires in an appliance cord touch. Because the resistance r of the short is tiny, the power P = V² ÷ r is enormous. OpenStax takes 120 V across a short of 0.100 Ω: 120² ÷ 0.100 = 144,000 W, or 144 kW, far more than any household appliance uses. Heat arrives so fast that insulation melts or catches fire. In the lab the battery is ideal, so a short makes the current run away without limit and the lab flags it in red. A real battery has some internal resistance that limits the current, but it still gets hot quickly.

Overloads, fuses and circuit breakers

A quieter danger is overload: too much current in a wire for too long. The heat in the wire itself is P = I² R_w. A damaged cord with some strands broken might have 2.00 Ω instead of 0.100 Ω; with 10.0 A through it, it turns 200 W into heat inside the cord. Remember from the parallel unit that every appliance switched on adds its current to the supply wires, so plugging many high-power appliances into one outlet strip can overload it even if each one is fine on its own.

Fuses and circuit breakers guard against both problems. They sit in series with the circuit, so all of its current passes through them. A fuse has a metal strip with a low melting point; too much current melts it and breaks the circuit for good, so it must be replaced. A circuit breaker is an automatic switch that opens when the current stays above its limit, and it can be reset. Some devices draw a large current for a moment when they start, like the cold headlight in the Ohm's law unit, so fuses and breakers are built to allow brief surges and trip on sustained overloads.

What current does to the body

How badly a shock hurts depends mainly on the current, and also on the path it takes through the body, how long it lasts and the frequency of the supply. OpenStax gives typical effects for a 1 second shock through the trunk of the body from 60 Hz mains, listed in the table. Values for women are about 60 to 80 percent of those listed. The thresholds are low. The current that makes the hand clamp shut, so the person cannot let go, starts at about 10 to 20 mA, and the heart can go into fibrillation, a fatal irregular beat, from about 100 to 300 mA.

Ohm's law shows why water matters so much. Most of the body's resistance is in dry skin. OpenStax gives about 200 kΩ for a person with dry skin, so 120 V drives 120 ÷ 200,000 = 0.6 mA, which passes unfelt. Soaking wet, the same person may have only 10.0 kΩ, and the same 120 V now drives 12 mA, past the point where the hand cannot let go. At the 220 to 230 V used in Korea, Europe and many other places, the same two resistances give about 1.1 mA and 22 mA. Wet hands, bathrooms and anything electrical near water are dangerous for exactly this reason.

Earth wires, double insulation and leakage breakers

Modern wiring adds layers of protection. In the three-wire system, a third earth (ground) wire connects the metal case of an appliance to the ground, holding it near zero volts. If worn insulation lets the live wire touch the case, current flows down the earth wire instead of through the person holding it, and usually trips the breaker. Double-insulated appliances use a protective insulating system instead of an earth connection. They are identified by their marking; the number of plug pins alone is insufficient. A ground fault interrupter, called a residual current device or leakage breaker in many countries, compares the current going out on one wire with the current returning on the other. If they differ by more than a set amount, some current must be leaking somewhere, perhaps through a person, and it cuts the power in a fraction of a second. The set amount depends on the country. North American ground fault interrupters trip at about 5 mA, the largest harmless current in the table. The residual current devices used to protect people in Korea and Europe are usually rated at 30 mA, and Korean wiring rules ask for 15 mA on bathroom sockets.

• Keep electrical devices and sockets away from water, and dry your hands first.
• Do not run many high-power appliances from one outlet strip at once.
• Stop using cords with damaged insulation or plugs that get hot.
• If a fuse blows or a breaker trips again and again, something is wrong: find the cause instead of fitting a bigger fuse.
• Leave mains wiring and repairs inside appliances to a qualified electrician.