Purposes of Earthing

Earthing serves several distinct purposes that are often confused with one another. Being clear about which purpose a particular conductor serves is the beginning of understanding an earthing system.

System earthing

Connecting a point of the power system — normally the star point of a transformer or generator — to earth. It fixes the voltage of the live conductors relative to earth, limits the insulation stress, and provides a return path so that earth fault current flows and protection can detect it. Without a system earth connection, an earth fault produces almost no current and cannot be detected by conventional overcurrent protection.

Protective earthing

Connecting the exposed conductive parts of equipment — motor frames, panel enclosures, cable armour, luminaire bodies — to earth, so that a fault to the enclosure produces sufficient current to operate protection quickly, and so that the enclosure does not rise to a dangerous voltage in the meantime.

Functional earthing

Earth connections required for equipment to function correctly: instrumentation reference earths, screen earths, and computer or control system earths. These often have specific requirements about where and how many times they are connected, because multiple connections can create earth loops that inject noise into signals.

Static and lightning earthing

Dissipating accumulated static charge and providing a path for lightning current. These are covered in later lessons but share the same physical earth system.

The two quantities that matter

Earth fault loop impedance determines how much fault current flows and therefore how fast protection operates. It must be low enough to operate the protective device within the required disconnection time.

Earth electrode resistance — the resistance between the earth electrode and the general mass of earth — matters for lightning, for static and for systems where the earth itself carries fault current back to the source. Note that in most installations the fault current returns through a metallic protective conductor rather than through soil, so a modest electrode resistance does not by itself mean protection will not operate.