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RCD protection: someone touches a live wire

Bench build · Power, transformers and three-phase · Intermediate · about 30 minutes

A water-heater circuit behind a 30 mA RCD, with a switch that lets a "person" (about 1 kΩ, hand to wet feet) touch the live wire. A breaker trips on overload and short circuits: tens or hundreds of amps. A person touching 220 V and standing on a wet floor passes about 0.2 A to earth, enough to stop the heart and far too little for any breaker. The current goes out on L and returns through the earth, not on N. An RCD compares L and N and trips on a 30 mA difference within 40 ms.

RCD protection: someone touches a live wire: the finished build running on the simulator's bench

Build it in the labGuided build, test sheet and fault finding, free in your browser.

Parts list

Test procedure

Each test has an expected value and a pass range, like a factory test sheet:

  1. Normal use: the 3.5 kW heater runs, the RCD stays on
  2. A person touches the live wire: the RCD trips
  3. After the trip, no current through the person
  4. Without the RCD (bypassed): current through the person
  5. Without the RCD: the 20 A breaker never notices
  6. Test button

How it works

What it does: A water-heater circuit behind a 30 mA RCD, with a switch that lets a "person" (about 1 kΩ, hand to wet feet) touch the live wire.

Build it (guided or ready-made), then run its test procedure below. Every test has an expected value and a pass range, just like a factory test sheet.

Background

RCDs (also RCCB, GFCI, ELCB) are required on bathroom, outdoor and kitchen circuits in most wiring codes. Press the T button monthly: it leaks a test current inside the device and should trip it at once.

More builds: power, transformers and three-phase