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.
A protective earth, known as an equipment grounding conductor in the US National Electrical Code, avoids this hazard by keeping the exposed conductive surfaces of a device at earth potential. To avoid possible voltage drop no current is allowed to flow in this conductor under normal circumstances, but fault currents will usually trip or blow the fuse or circuit breaker protecting the circuit. A high impedance line-to-ground fault insufficient to trip the overcurrent protection may still trip a residual-current device (ground fault circuit interrupter or GFCI in North America) if one is present.
In contrast, a functional earth connection serves a purpose other than shock protection, and may normally carry current. Examples of devices that use functional earth connections include surge suppressors and electromagnetic interference filters, certain antennas and measurement instruments. But the most important example of a functional earth is the neutral in an electrical supply system. It is a current-carrying conductor connected to earth, often but not always at only one point to avoid earth currents.
The NEC calls it a groundED supply conductor to distinguish it from the equipment groundING conductor. In most developed countries, 220/230/240V sockets with earthed contacts were introduced either just before or soon after WW2, though with considerable national variation in popularity. Until the mid 1990s, US 110V power outlets generally lacked protective earth terminals. In much of the developing world, the situation is stil unclear, and earthed outlets may or may not be provided, and where they are these may not always be reliably connected.
In the absence of a supply earth, devices needing an earth connection often used the supply neutral. Some used dedicated ground rods. Many 110V appliances have polarized plugs to maintain a distinction between “live” and “neutral”, but using the supply neutral for equipment earthing can be highly problematical. “Live” and “neutral” might be accidentally reversed in the outlet or plug, or the neutral-to-earth connection might fail or be improperly installed. Even normal load currents in the neutral might generate hazardous voltage drops. For these reasons, most countries have now mandated dedicated protective earth connections that are now almost universal.