RCD
The Residual Current Device, or RCD, is one of the most important electrical safety devices in both the home and commercial environments.
In short, an RCD protects against injury and death caused by electric shock by detecting an imbalance between the outgoing and incoming total current of a given number of circuits. It works on the very simple principle that if there is a difference in the electricity coming in against that which is going out, then it must have gone somewhere.
This phenomena is commonly known as 'earth leakage' - (or the 'Residual Current' in the RCD's title). It can be caused by a number of problems including poor electrical wiring and DIY accidents such as drilling through a cable when mounting a picture hook or cutting through a cable with the lawn mower. In this type of situation the electricity must go somewhere and choosing the easiest route can travel through the lawnmower or drill to the human holding them to create a circuit and cause electric shock.