Electrical safety – shock current, earthing and RCD breakers

What it shows

Mains electricity drives a current through anyone who becomes part of the circuit. The simulation applies Ohm's law, I = U/R, where R is the body's resistance plus the contact resistance between the feet and the ground. Damp skin and bare feet on a wet floor make R small, so 220 V can push tens of milliamps through the body, enough to cause muscle cramp, breathing difficulty or ventricular fibrillation. Two protective measures are modelled: earthing a metal appliance casing lowers the touch voltage, and a 30 mA RCD breaker disconnects the supply within 0.1 s.

How to use

Start with the Situation Holding a metal-cased appliance with a leakage fault and the Protection measure None, press Touch the wire and read the current and danger level. Lower the Body resistance to imitate wet skin and repeat, then tick Wearing insulated sandals, standing on dry floor and compare. Finally try each protection measure in every situation, noting that earthing only helps when the fault is on an appliance casing, while the RCD breaker trips once the current exceeds 30 mA.

Parameters you can change

• Situation Touching a bare live wire, Holding a metal-cased appliance with a leakage fault, Repairing wiring without switching off the power, Kite string catching on a power line
• Body resistance 1000–100000 Ω
• Protection measure None, Earthed appliance casing, 30 mA RCD breaker, Earthing and RCD breaker
• Wearing insulated sandals, standing on dry floor

Questions to explore

1. Why does the same 220 V supply drive a much larger current through a person with wet skin than with dry skin?
2. When the appliance casing is earthed, how does the current through the body change, and why?
3. In which situations does earthing alone fail to protect you, and which measure makes them safe?