Quick Answer
Electrical safety is the practice of keeping current out of your body and out of anything that can catch fire. Current does the harm; voltage is what drives it through you, and mains supplies far more than a person survives. Safe work means isolating the supply, proving the circuit dead, and assuming any one precaution can fail.
Intuition
What actually hurts you
What injures a person is current passing through the body, and above all current that crosses the chest. Voltage matters because it decides how much of that current gets in. The wattage printed on the label is no measure of the danger at all.
Dry skin is a reasonably good insulator, which is why you can hold both terminals of a torch battery and feel nothing. At mains levels that protection disappears: the skin breaks down, and what carries the current from then on is the wet, salty tissue underneath, which resists very little.
The amounts involved are small. A current you can only just feel is roughly a thousandth of an ampere. Ten times that is enough for your own muscles to clamp your hand shut around whatever you are holding, so that you cannot let go, and not far above that a current across the chest can throw the heart into a rhythm that does not pump blood. That last one is the mechanism by which mains electricity kills, and all of it happens well below the current an ordinary desk lamp draws.
Contact does not have to be dramatic to be complete. A fingertip on a live terminal, with your other hand resting on an earthed metal case, is already a path through your chest.
Shock is only one of the two hazards, in any case. An electrical fault that never touches a person can heat a cable, ignite its insulation and burn a building down.
Electronics is still perfectly safe to learn. A battery-powered breadboard project cannot hurt you, and most of this lesson is about the small number of situations that can. The reason to build the habits now, while the stakes are low, is so that they are already automatic by the time the stakes are not.
Practitioner
The working rules
For alternating current at mains frequency on a path through the body, perception starts around 1 mA. The let-go threshold — above which involuntary muscle contraction stops you releasing the conductor — is of the order of 10 mA. Above roughly 30 mA, ventricular fibrillation becomes a real risk, and that is the level the domestic residual-current device is set to catch.
The current that actually flows through a person is set by the same relationship that governs any other conductive path:
Worked example — What mains can push through a person
Take a hand-to-hand contact across 230 V mains, with a total body impedance for that path of about 1 kΩ.
Dividing voltage by resistance gives the current that would flow: 230 mA. That is 23 times the let-go threshold and 7.7 times the level at which fibrillation becomes likely.
The same body path across a 12 V bench supply gives 12 mA instead, which is perceptible and unpleasant if the contact is good but sits in a different category altogether. The path is identical in both cases; only the voltage differs.
The body impedance used there is an order-of-magnitude figure rather than a specification. Real impedance depends on the current path, contact area, moisture, contact pressure and the voltage itself, and it varies by more than a factor of ten between a dry fingertip and a sweating palm gripping a pipe. Design your habits for the bad case.
The procedure for working on equipment that may be live
The sequence below matters as much as the individual steps, and it is the sequence that gets abandoned first.
- Decide whether you need it powered at all. Most work — inspection, resoldering, replacing a part, continuity checks — is done dead. Only measurements that require operation justify a live circuit.
- Isolate. Switch off at the source and unplug. For fixed installations, open the breaker.
- Lock off and label. Fit a lock or a tag to the isolator so that nobody restores power while you are inside the equipment, yourself included when you come back to it tomorrow.
- Discharge stored energy. Bulk capacitors in power supplies, motor drives and flash circuits hold a dangerous charge after disconnection. Discharge through a suitable resistor, never by shorting with a screwdriver.
- Prove dead — with a tester you have proved live. Check your tester against a known live source, test the circuit, then check the tester again. A meter that has failed silently reads zero on everything.
- Work with one hand where you can. Keep the other hand out of the equipment and away from earthed metal, so that no path exists across the chest.
- Stand on something insulating and dry. Do not work on live equipment while touching a radiator, a sink, a metal bench or damp concrete.
- Use rated equipment. Meters and leads carry a CAT rating that describes the transient energy they are built to survive at a given point in an installation. A lead with damaged insulation is scrap and should be thrown out rather than kept for occasional use.
- Never put a meter in series with mains. A meter on a current range is close to a short circuit. Use a clamp meter for mains current — see multimeter.
- Restore deliberately. Refit covers and earth connections before restoring power, remove the lock, and stand clear when you switch on.
- Know what to do if it goes wrong. Do not touch a person who is still in contact with a live conductor — isolate first, or push them clear with something insulating. Then call for help. Any shock across the chest, however brief, is a reason to be seen by a doctor, because the arrhythmia can be delayed.
Safety
This lesson is an introduction to habits, not a qualification. Mains installation work, and any work on equipment connected to the supply, is regulated in most countries and should be done by, or under the supervision of, a competent person.
Stored energy survives isolation. A bulk capacitor can hold a lethal charge for minutes after the plug is out, and camera flash and motor-drive circuits are worse. An earth connection is a safety component, not an optional wire; equipment with a broken earth may work perfectly right up to the moment its metalwork becomes live. Isolation is not the same as switching off, because a switch in the neutral, a soft-power button or a standby mode all leave live parts inside the case.
Do not work alone on mains-connected equipment, do not defeat an interlock, and do not fit a larger fuse to stop something blowing. If you are unsure whether a circuit is safe to touch, treat it as though it is not.
Fire is the other half of the subject, and it has its own set of causes: overloaded cable, a loose terminal, a failing joint, a fuse rated too high. All of them end in heat. Protective devices are chosen to open before the wiring is damaged — see fuses — and current limiting is a design responsibility in its own right, covered in current limiting.
A low voltage rules out shock and nothing else. A bench supply or a large lithium battery at a few volts cannot shock you meaningfully, but it can deliver an enormous fault current: enough to melt a ring off a finger, vaporise a tool and start a fire. Remove metal jewellery and watch straps before working on any battery of consequence.
Going deeper
The protection systems, and what each one does not do
Earthing works by giving fault current a path that is not you. In a Class I appliance the exposed metalwork is bonded to protective earth, so a live conductor touching the case produces a large fault current that trips the protective device instead of energising the case. The whole scheme depends on that earth connection being intact and low-impedance, which is why earth continuity is the first thing tested on portable appliance inspection.
A Class II appliance takes the opposite route and has no exposed earthed metal at all, relying instead on two independent layers of insulation; the double-square symbol on a plastic-cased drill or charger is what marks it out. It still has a failure mode of its own, which is why a cracked case on one of those is a serious defect rather than a cosmetic one.
What does a residual-current device actually measure? It compares the current going out with the current coming back, and if the two differ by more than its rated residual current, some of it is leaving the circuit by another route, so the device disconnects within a fraction of a second. That makes it very effective against a person becoming part of the path to earth. A hand-to-hand shock that never involves earth is invisible to it, and so is a straightforward overload, which is the breaker's job; the two functions live in the same device often but not always.
An isolation transformer breaks the reference rather than the energy. With no connection to earth on the secondary, touching one conductor does not complete a circuit. Touching both still does, and a second undetected fault turns a safe supply back into a hazardous one. They make bench work on mains-referenced equipment safer, though not safe.
Insulation coordination answers an electrical question with a mechanical measurement. Creepage and clearance distances, specified against working voltage, pollution degree and material group, decide the physical geometry of any mains-connected board, and bridging one of those barriers with a probe, a cable tie or a smear of flux defeats a protection that was designed in before layout began. Capacitors that deliberately bridge such a barrier form a specified safety class of their own — see safety capacitors.
Arcing is a hazard in its own right, quite separate from shock. In a high-fault-current installation a short circuit produces an arc whose radiated heat and pressure wave injure at a distance, with no contact at all. Work near switchgear therefore has its own equipment and its own rules, and a meter's CAT rating is a statement about surviving a transient rather than about reading accurately.
Frequency and waveform move all of these thresholds. The figures in Layer 2 describe alternating current at mains frequency, which happens to sit close to the most hazardous case for the heart. Direct current of the same magnitude behaves differently, higher frequencies are less arrhythmogenic but burn, and the duration of contact matters as much as the magnitude. IEC 60479-1 sets all of this out as time-current zones rather than as single thresholds, and the single numbers used here are the conventional summary of those zones.
Static electricity inverts the whole picture. ESD involves thousands of volts, and it is harmless to people while being lethal to components; mains involves hundreds of volts and does the reverse. An intuition built on one is no guide to the other.
Common mistakes
- Thinking voltage alone decides danger — current through the body does the damage. Voltage matters because it determines how much current gets through the skin.
- Trusting a switch as an isolator — switches can be fitted in the neutral, and standby modes leave live parts inside. Unplug, and prove dead.
- Assuming a disconnected supply is discharged — bulk capacitors hold their charge. Discharge deliberately through a resistor and verify with a meter.
- Proving dead with an unproven tester — check the tester on a known live source before and after. A failed tester reads zero everywhere.
- Working two-handed inside live equipment — that puts a path across the chest. Keep one hand in and the other well clear of everything.
- Believing an RCD covers everything — it detects current leaving to earth. A hand-to-hand shock and a straightforward overload are both invisible to it.
- Fitting a bigger fuse — the fuse is sized to protect the wiring. A larger one moves the failure from a cheap component to the cable inside the wall.
Frequently asked questions
Is it voltage or current that kills?
Current through the body causes the injury, but voltage is what drives that current through your skin and tissue. Both matter; safety limits are written in volts because voltage is what you can control.
How much current is dangerous?
Far less than most people expect: perception begins around a milliampere, muscular lock-on around ten, and the risk of a fatal heart rhythm around thirty, all well below what a small lamp draws.
Why is one-handed working recommended?
Because it prevents a current path from one hand to the other, which passes directly across the chest and the heart. Keeping the second hand clear removes the most dangerous route.
Does an RCD make mains work safe?
No. It disconnects quickly when current leaks to earth, which covers many shock scenarios, but it cannot see a hand-to-hand shock and does not protect against overload or fire from a loose joint.
Is a low-voltage battery circuit safe?
Usually safe from shock, but not from fire or burns: a large battery shorted by a tool or a ring can deliver hundreds of amperes, melt metal and ignite what is nearby.
What should I do if someone is being shocked?
Do not touch them while they are in contact. Isolate the supply, or move them clear with something insulating, then call emergency services. Any shock across the chest needs medical assessment even if the person seems fine.
Knowledge check
A hand-to-hand path across 230 V mains presents about 1 kΩ. What current flows, and how does it compare with the fibrillation threshold? (Show answer)
The same body path is placed across a 12 V bench supply. What changes? (Show answer)
You have switched a device off at its front-panel button. Is it safe to open? (Show answer)
What does an RCD detect, and what is it blind to? (Show answer)
Why remove rings and metal watch straps before working on a large battery? (Show answer)
References
- IEC 60479-1, Effects of current on human beings and livestock — Part 1: General aspects — the source of the perception, let-go and ventricular-fibrillation thresholds quoted here, which the standard defines as time-current zones rather than single values.
- IEC 60664-1, Insulation coordination for equipment within low-voltage supply systems — creepage and clearance distances as a function of working voltage, pollution degree and material group.