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Residual current device (RCD). A RCD is an electronic switch that cuts the electricity supply to a device when a fault develops. They are also known as earth leakage circuit beakers (ELCB). In this case the RCD is connected to a hedge trimmer. Many devices, especially gardening equipment, have plastic casings and therefore no Earth wires. A lack of an Earth wire means there is no path for electricity to flow should the device become live, for example if the power cord is severed. The RCD works by continually measuring the Live and Neutral current flows, which should be equal, and as soon as a difference occurs the electricity is shut off in under 40 milliseconds. (KEYSTONE/SCIENCE PHOTO LIBRARY/SHEILA TERRY)