What Electricity Does to a Body

This course begins with the mechanism, because everything else in it follows from one fact that is widely misunderstood.

Most people believe voltage is what hurts you. It is not.

Current is what injures you

Current is what does the damage. Voltage only determines how much current gets pushed through you.

The relationship is simple arithmetic. Current equals voltage divided by resistance. The voltage is decided by the circuit you touched. The resistance is mostly you — your skin, how much of it is in contact, and how wet it is.

So the same wire can give one person a sharp unpleasant jolt and kill the next, with no change in the voltage at all.

Current is measured in amps, and the amounts that matter here are small enough that we use milliamps — thousandths of an amp:

• 1 mA — the threshold of perception. A tingle
• 5 mA — a definite shock. Startling, not usually harmful
• 10 to 16 mA — the let-go threshold. Muscles contract and will not release
• 30 mA and up — the muscles that drive breathing can be paralysed
• 50 to 100 mA and up — the heart can be thrown into fibrillation

For perspective: a single 100-watt light bulb draws well over 800 mA. The current that can kill you is a small fraction of what an ordinary household circuit carries without noticing.

The circuit breaker in a panel is not there to protect you. It protects the wiring from overheating. A current far below what trips a 15-amp breaker is more than enough to stop your heart. This surprises people, and it is worth sitting with.

What decides how much current flows

Since the voltage is fixed by the circuit, everything you can influence is on the resistance side.

Skin moisture. Dry, intact skin offers substantial resistance. Damp skin offers a small fraction of it. Sweat counts. Rain counts. So does working in a hot ceiling void.

Contact area. A fingertip brushing a terminal is a small contact. A palm wrapped around a conduit is a large one, and a large contact means lower resistance.

Broken skin. A cut or a graze bypasses the most resistant layer you have.

What you are standing on. Bare feet on a wet concrete slab is close to an ideal connection to earth. Dry rubber soles on a dry timber floor is a poor one. This is a real difference, though it is never a reason to treat something energised as safe.

Whether you are holding something metal. A metal ladder, a conduit, a length of strut — all of them enlarge your contact and can complete a path you never intended.

The path matters more than the amount

Where the current travels through you decides the outcome, sometimes more than how much of it there is.

Hand to hand is the worst. It runs straight across the chest — through the heart and the muscles that drive breathing. This is precisely why electricians are taught to work with one hand where there is any possibility of contact, and to keep the other out of the way rather than resting it on something earthed.

Hand to foot is similarly serious, for the same reason: the chest is on the route.

Contained in one limb — finger to thumb, say — still burns, and can be a severe injury, but it keeps current away from the organs that decide whether you live.

Two consequences follow, and they are habits rather than knowledge:

• Work one-handed near anything that might be live
• Do not lean on earthed metalwork while your other hand is working

Duration

The third factor is time, and it interacts with everything else.

A brief contact at a given current may do nothing lasting. The same current sustained for a second or two is a different injury altogether. The heart is most vulnerable during a specific part of its cycle, and a longer shock guarantees it is caught there.

This is why the let-go threshold matters so much. A shock the victim can break away from lasts a fraction of a second. A shock at 15 mA does not let them break away — the hand grips harder, the contact holds, and a survivable current becomes a fatal one because it will not stop.

Nobody plans to hold on. The point is that the choice is taken away from you.

Burns, and the injuries that come later

Shock is not the only outcome.

Current generates heat wherever it meets resistance, and inside a body the resistance is highest at the skin. Entry and exit wounds can look deceptively small while the tissue along the path between them is badly damaged.

Internal injury is easy to underestimate. Muscle and nerve damage along the current path may not be visible at all. Kidney injury can follow from the products of damaged muscle.

Effects can be delayed. Cardiac rhythm disturbances can appear hours after the event, which is why anybody who has taken a meaningful shock needs medical assessment even if they feel fine and would rather get on with the job.

And there is the fall. A shock at height causes a flinch, and the flinch causes a fall. On many sites the fall is the injury that actually puts somebody in hospital.

What this means on site

Five things follow directly from the mechanism, and they are worth memorising as consequences rather than rules:

1. Treat every conductor as live until you have proved otherwise — proved, with an instrument, not assumed
2. Work one-handed where contact is conceivable
3. Keep dry. Wet hands, wet clothing and standing water all raise the current a given voltage will drive through you
4. Never rely on being able to let go. You will not be able to
5. Report every shock, however small it felt. Two reasons: you may be injured in ways you cannot feel, and something on that site is faulty and will find the next person

The rest of this course is mostly about the first of those. Everything else is detail.