Residual-current circuit breakers are essential safety devices in our domestic mains supply installation, but sometimes they can switch off (trip) for no apparent reason. A professional earth-resistance tester (or megohmmeter) will help to locate the leak, but the easy-to-build earth-leakage tracer presented in this article proves to be a safe, affordable and useful alternative, using standard CR2032 lithium batteries for the high-voltage power supply.