Electricity does its harm by flowing through you. A live wire touched by a hand, with feet on the ground, makes a path: in at the hand, through the chest, out at the feet. The flow along that path is what locks muscles, stops breathing and stops hearts. Mains electricity in Britain pushes at 230 volts.
Voltsthe push
↓→
Ohmsthe resistance
↓→
Currentpush divided by resistance
Three words. Volts are the push. Ohms are how much the path resists. Current is what actually flows, and it is the push divided by the resistance. The body's resistance is mostly in the skin, and skin's resistance falls hard when it is wet. The push is fixed at 230. What varies is you.
230 voltsfixed
++
Your skindry or wet
The wire is 230 volts whether your hand is wet or dry. What decides how much current flows through you?
How long you hold it.
Not yet. Time decides how much damage the current does, and matters a lot. It does not decide how much flows, which is push divided by resistance.
Only the volts. More volts, more harm.
Not yet. Volts set the push, and the push is fixed here. With the same 230 volts, a dry fingertip and a wet hand pass a hundred times different currents.
The resistance of the path, which is mostly your skin.
Right. Push divided by resistance. The push is the same wire for everyone; the resistance is the skin at the moment it touches, and wet skin resists far less.
Half a milliampjust felt
↓→
10 milliampscannot let go
↓→
40 to 50heart at risk, after a second
↓→
A kettle13,000
Rough thresholds for mains current.
Current is measured in milliamps, thousandths of an amp. About half a milliamp can just be felt. At about 10, the muscles of the hand lock and the person cannot let go. Above that, breathing gets hard. From about 40 to 50 milliamps for a second or more, the heart can lose its rhythm. A kettle draws about 13,000.
Move the control to change the skin touching a 230-volt wire. Current is volts divided by ohms. The two hand figures come from the standard safety engineers use; the dry fingertip and the wet case are rough.
Current through the body2.3 mA
Enough to stop a heart, roughly50 mA
Dry fingertipDry fingertip: about 100,000 ohms. 230 volts divided by 100,000 ohms is about 2.3 milliamps through the body: a light tingle.
Current through the body170.4 mA
Enough to stop a heart, roughly50 mA
Dry hand, grippingDry hand, gripping: about 1,350 ohms. 230 volts divided by 1,350 ohms is about 170 milliamps through the body: muscles lock; the heart is at risk.
Current through the body230 mA
Enough to stop a heart, roughly50 mA
Sweaty handSweaty hand: about 1,000 ohms. 230 volts divided by 1,000 ohms is about 230 milliamps through the body: well past what can stop a heart.
Current through the body460 mA
Enough to stop a heart, roughly50 mA
Wet, standing in waterWet, standing in water: about 500 ohms. 230 volts divided by 500 ohms is about 460 milliamps through the body: far past it.
A dry fingertip passes about 2 milliamps. A sweaty hand gripping the same wire passes how much?
About 4 milliamps, roughly double.
Not yet. Sweat does not halve the skin's resistance; it cuts it by about a hundred times. The current goes up by the same hundred.
About 230 milliamps, roughly a hundred times more, and several times what can stop a heart.
Right. A dry fingertip is about 100,000 ohms; a sweaty palm about 1,000. The push did not change. The hundred-fold drop in resistance is the whole difference between a tingle and a death.
The same 2 milliamps. It is the same wire.
Not yet. The same wire gives the same push. The current depends on the path, and a sweaty hand gripping is a very different path from a dry fingertip.
Move the control.
230 volts
↓→
1,000 ohms
↓→
milliamps?
A hand at 1,000 ohms touches a 230-volt wire. Current is volts divided by ohms. How many milliamps flow?
mA
Divide, then turn amps into milliamps by multiplying by a thousand.
Right. A quarter of an amp, 230 milliamps: about five times the current that can stop a heart. Household voltage is enough to kill with a damp hand, which is why the protections exist.
melts at 13,000 mA; saves the wireFuse
vsvs
trips at 30 mA; saves the personRCD
Two devices, and they protect different things. A fuse melts when the current is too high: 13 amps for a kettle, 13,000 milliamps. It stops the wiring catching fire. It does nothing for a person, who is dead at 50. An RCD, a trip switch, cuts off at 30 milliamps in a fraction of a second. That one protects you.
Each card is a protection against electricity. What does it protect?
One card at a time. Tap the pile it belongs to.
Card 1 of 5
The 13-amp fuse in a kettle plug.
The RCD, or trip switch, in the fuse box.
The plastic insulation on a cable.
The circuit breaker that trips when too many heaters are plugged in.
A rule that nothing mains-powered is used near the sink.
The 13-amp fuse in a kettle plug. → The wiring
Not yet. The wiring. Thirteen amps is 13,000 milliamps, two hundred times what can stop a heart. The fuse blows for a fault that would melt the cable, long after a person would be gone.
The RCD, or trip switch, in the fuse box. → The person
Not yet. The person. It compares what goes out with what comes back, and 30 milliamps missing, flowing through someone, trips it in a fraction of a second.
The plastic insulation on a cable. → The person
Not yet. The person. The insulation is what keeps a hand from being part of the circuit; the wire inside works just as well bare.
The circuit breaker that trips when too many heaters are plugged in. → The wiring
Not yet. The wiring. An overload breaker trips at tens of amps, to stop the cable in the wall overheating. It is a fuse that can be reset.
A rule that nothing mains-powered is used near the sink. → The person
Not yet. The person. Water is what turns a 100,000-ohm fingertip into a 500-ohm path, and the rule keeps the two apart.
Wiringthousands of mA
vsvs
Persontens of mA
Right. If it acts at thousands of milliamps, it is there for the cable. If it acts at tens, or stops the body being part of the path at all, it is there for the person. Most homes and workplaces need both, and they are not substitutes.
Cut cable230 volts
++
Wet grass
++
13-amp fuseno RCD
A gardener uses a mains hedge trimmer on wet grass and cuts through its cable. The plug has a 13-amp fuse and there is no RCD. What does the fuse do for the gardener?
It blows and saves them. That is what a fuse is for.
Not yet. A fuse is for the cable. It blows at 13,000 milliamps; the shock that stops a heart is about 50. The fuse sits there, intact, while the current flows through the person.
Nothing. The current through a person is a fraction of an amp, and the fuse needs 13 to blow.
Right. The gardener's body passes perhaps a quarter of an amp. The fuse is built for 13 and never notices. An RCD would have tripped at 30 milliamps, before the heart knew.
It halves the shock.
Not yet. A fuse does nothing until it melts, and it does not melt at the currents a body passes. It is all or nothing, and here it is nothing.
Lesson complete
A shock is current through the body; wet skin lets more through, and a fuse protects the wire, not you.