How shock hurts
It is the current through the body, not the voltage alone, that harms. The current flows when a person completes a circuit between two points at different voltages, for example a live wire and the ground. At mains frequency (50 or 60 Hz) the effects grow with current and with how long it flows:
| Current | Typical effect (50/60 Hz, a few seconds) |
|---|---|
| Under 1 mA | Perceptible tingle at most |
| 1 to 10 mA | Painful; muscles still obey |
| 10 to 30 mA | Cannot let go of the conductor: muscles clamp the hand on; breathing difficult |
| 30 to 100 mA | Breathing may stop; severe pain; heart rhythm disturbed |
| Above 100 mA | Ventricular fibrillation (heart quivers instead of pumping); burns; death if not treated |
Body resistance and current
The current is I = V ÷ R, and the body’s resistance depends a lot on the skin. Dry skin has a high resistance (tens to hundreds of kilo-ohms); wet or sweaty skin is a hundred times lower; broken skin or an immersed hand lower still.
| Condition | Resistance (typical) | Current at 120 V | Current at 230 V |
|---|---|---|---|
| Dry skin, hand to hand | 100 000 Ω | 1.2 mA | 2.3 mA |
| Damp skin or sweaty hands | 10 000 Ω | 12.0 mA | 23.0 mA |
| Wet hands, standing on wet concrete | 1 000 Ω | 120.0 mA | 230.0 mA |
| Internal body path only (skin broken, immersed) | 500 Ω | 240.0 mA | 460.0 mA |
Worked example: why wet matters
- With dry skin (100 kΩ) 120 V gives 1.2 mA: a tingle. With wet hands on a wet floor (1 kΩ) the same 120 V gives 120 mA: 12 times the let-go threshold and in the fibrillation range.
- At 230 V and 1 kΩ the current is 230 mA. Wet conditions turn a household supply into a lethal one.
- The fibrillation threshold depends on time (Dalziel’s formula, I = 116 ÷ √t in mA for a 50 kg person): 116 mA for 1 s, 67 mA for 3 s, 367 mA for 0.1 s. At 120 mA a shock longer than about one second can cause fibrillation; one cut off in 40 ms is below the threshold of 580 mA.
- The time is under your control only if something cuts the supply quickly: that is the job of an RCD.
RCDs and GFCIs
A residual-current device (RCD), called a ground-fault circuit interrupter (GFCI) in North America, compares the current going out on the live wire with the current coming back on the neutral. If they differ by more than about 30 mA (the 30 mA type; 5 to 6 mA for GFCIs), the difference is leaking somewhere, perhaps through a person, and it cuts the supply in about 40 ms.
Worked example: what an RCD does
- A 120 mA body current (a wet-hand shock at 120 V) lasting 3 s through 1 000 Ω dissipates I² R t = 0.014 × 1 000 × 3 = 43 J and is above the fibrillation threshold.
- With an RCD tripping in 40 ms the energy is 0.58 J, 75 times less, and the current for 40 ms is far below the 580 mA fibrillation limit. The person feels a jolt and lets go.
- An RCD does not protect against touching both live and neutral (all the current returns through the neutral so the device sees nothing): it protects against a leak to earth. It does not replace care.
- Press the test button monthly. A device that does not trip is not protecting you.
- Use RCD/GFCI protection on every shop socket, on extension leads for outdoors and on portable tools; a plug-in type is available when the circuit has none.
Earthing and fault loops
The earth (ground) wire gives fault current an easy path back to the supply so that the breaker trips quickly instead of leaving the metal case of a tool live. The breaker trips fast only if the fault current is high enough, which depends on the resistance (impedance) of the loop: live conductor, fault and earth return.
Worked example: will the breaker trip?
- A 16 A type B breaker trips magnetically at 5 × 16 = 80 A. At 230 V this needs a loop impedance below 230 ÷ 80 = 2.9 Ω.
- A loop impedance of 0.8 Ω gives a fault current of 288 A (18 times the rating) and a trip in a few tenths of a second or less.
- A bad earth connection raises the loop impedance to 5 Ω: the fault current is only 46 A, below the trip level. The breaker may not trip at all, and if half an ohm of the return is in the tool’s earth wire the metal case rises to 23 V above earth.
- An RCD solves this because it trips on a small leak, not a large fault current. It is the reason an RCD is essential where earthing may be poor, such as garages and outbuildings.
- A 20 Ω earth rod alone carries only 11 A and raises the case to 224 V in a fault: earth rods do not trip breakers; use the supply earth and an RCD.
Cords, plugs and sockets
Worked example: a thin cord under load
A 20 m cord of 1.0 mm² conductors (resistance 18.1 Ω/km) carrying 10 A dissipates 2 × 20 × 0.018 × 10² = 72 W as heat along its length. Coiled on a reel the heat cannot escape and the insulation melts. A heater of 2 000 W draws 16.7 A at 120 V (8.7 A at 230 V): too much for a light cord on the 120 V supply. See Power, Lighting and Compressed Air for the cord table.
- Inspect cords and plugs every time: cuts, kinks, exposed conductors, a loose plug, a burnt smell. Replace; do not tape.
- Never carry a tool by its cord or pull the plug out by the cord.
- Use cords rated for the load, fully unreeled, and kept out of water and away from the cut.
- Three-pin plugs stay three-pin. Do not cut off the earth pin or use a cheater adapter to defeat the earth.
- Do not overload sockets or chain adapters; a socket carries about 15 A in total.
- Double-insulated tools (the square-in-square symbol) have no earth but need an RCD in the wet.
- Keep sockets above the floor and away from water; use covers outdoors.
Isolating equipment and lockout
Before you open a machine, change a blade, clear a jam or work on any wiring, make sure it cannot start. For a plug-in tool that means unplugging it and keeping the plug in your hand or pocket. For a fixed machine it means isolating the supply at the switch or breaker, locking it off with your own padlock and tag (lockout and tagout), and proving it dead.
| Step | Action |
|---|---|
| 1. Prepare | Know what energy the machine has (electrical, stored in capacitors, pneumatic, hydraulic, spring, gravity) |
| 2. Shut down | Stop it normally |
| 3. Isolate | Open the isolator or pull the plug |
| 4. Lock and tag | Your own lock and a tag with your name; keep the key |
| 5. Release stored energy | Wait for capacitors, bleed pressure, block raised parts |
| 6. Prove it dead | Try to start it; test the wiring with a voltage tester you have checked on a known live source |
| 7. Work | Only then |
| 8. Restore | Remove tools, replace guards, clear people, remove your lock, test the machine |
Stored energy: capacitors and batteries
Worked example: a capacitor in a drive or a power supply
- A 470 µF capacitor charged to 340 V (the peak of a 240 V supply) stores ½ C V² = ½ × 470 × 10⁻⁶ F × 340² = 27 J: enough to burn or kill if it discharges through a person.
- It discharges through a 100 kΩ bleed resistor with a time constant R C = 100 000 × 470 × 10⁻⁶ = 47 s. To fall to a safe 50 V takes R C ln(340 ÷ 50) = 90 s. If the bleeder has failed the capacitor stays charged for hours.
- Many variable-speed drives and power supplies say wait five minutes (300 s) after isolation, and still require a measurement.
- Batteries cannot be isolated by a switch: disconnect the pack, cover the terminals, and remember that a 12 V lead-acid battery can supply hundreds of amps into a short (see Fire, Fuels and Explosion).
- See Electronics Fundamentals for work on circuits.
Working rules
- Treat every conductor as live until you have proved it dead with a tester you have checked.
- Do not work on mains wiring unless you are qualified, and follow the local code (a permit and inspection are often required).
- Work with one hand when you cannot isolate: keep the other away from earthed metal.
- Do not work alone on anything live, and know where the main switch is and how to use it.
- Keep the floor dry and stand on an insulating mat where the shock risk is high.
- Use tools with insulated handles rated for the voltage, and remove rings and watches.
- Repairs to a machine’s mains wiring and drive belong to a qualified person (see System Integration).
After a shock
- Do not touch the person until the supply is off (or move them with a dry non-conductive object such as a wooden broom handle). Turn off the power at the switch or the breaker, or pull the plug.
- Call emergency services for any shock with a loss of consciousness, difficulty in breathing, burns or a shock above about 30 mA, and for any shock that crossed the chest.
- If they are not breathing, start CPR and use a defibrillator if one is available (see First Aid, Emergencies and Safe Habits).
- See a doctor even if the person feels fine: heart rhythm problems can start hours later, and burns are often deeper than they look.
- Treat burns with cool running water for 20 minutes and cover them loosely.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| RCD trips with nothing wrong | Moisture, a leaky appliance, too many items | Unplug appliances one at a time; dry the outlets; replace the faulty one |
| Tingle from a tool case | Leak to earth, no earth, damaged insulation | Stop; unplug; have it tested and repaired |
| Breaker trips on start-up | Surge current of a motor | See Power, Lighting and Compressed Air |
| Cord or plug is warm | Overload, loose contact | Unplug; reduce the load; replace the cord or plug |
| Smell of burning from an outlet | Loose connection, overload | Switch off at the breaker; get an electrician; do not use |
| Light flickers when a machine starts | Voltage dip from surge current | Separate circuits; check the wiring and the connections |
Related
- Workshop Safety: the overview of this guide.
- Fire, Fuels and Explosion: electrical fires and batteries.
- First Aid, Emergencies and Safe Habits: CPR and first aid.
- Power, Lighting and Compressed Air and Grounding, Shielding and EMC (the industrial view of earthing).
FAQ
Do I need an RCD or GFCI in my garage workshop?
Yes. It is the most effective protection against fatal shock; fit one on every shop circuit or use plug-in ones, and test them monthly.
How much current is dangerous?
About 10 mA you cannot let go; 30 mA breathing is affected; above about 100 mA for a second or more the heart can fibrillate.
Why is a wet shop more dangerous?
Wet skin lowers the body resistance from 100 kΩ to about 1 kΩ, so the current from a 120 V supply rises from about 1 mA to about 120 mA.
Can I fix mains wiring myself?
Only if you are qualified and the local code allows it; use a licensed electrician for new circuits, the panel and repairs to fixed wiring.
References
- IEC 60479-1, Effects of current on human beings and livestock; Dalziel, C. F., Electric Shock Hazard (the 116 ÷ √t formula).
- NFPA 70 (National Electrical Code), NFPA 70E (electrical safety in the workplace); IEC 60364 and BS 7671 (wiring regulations).
- OSHA 29 CFR 1910.147 (control of hazardous energy: lockout and tagout) and 1910.304 (wiring design and protection).
Last reviewed: October 2026. Currents and thresholds are typical for adults: individuals differ. Treat any shock as serious.