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In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to the Earth´ s conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply, and regulations can vary considerably among countries. Most electrical systems connect one supply conductor to earth (ground). If a fault within an electrical device connects a "hot" (unearthed) supply conductor to an exposed conductive surface, anyone touching it while electrically connected to the earth (e.g., by standing on it, or touching an earthed sink) will complete a circuit back to the earthed supply conductor and receive an electric shock.

For an earth connection, a three-pin socket and plug are required. Due to the high current it draws, the earth pin is made thicker and larger than the other two pins. This ensures that the plug fits firmly into the socket, reducing the chances of sparking. The heat caused by sparking causes the terminals to wear off and damages the socket and the plug. Because it is larger, the earth connection is made first acting as a safety device.