The primary objective of earthing (or grounding as referred to in North American standards) is safety, to reduce risks to both human life and installations. The principles of electrical earthing are agreed upon internationally, though in practice, there are differences as to how these principles are best achieved.
Electrical systems were not always earthed. The first systems were unearthed ones with no earth reference at all. Even though such systems still exist in specific areas, they are the exceptions rather than the rule and by and large, some form of earthing is adopted for all power systems.
Electrical earthing requirement could be expounded into these five main purposes:
- To reduce the risk of personnel shock
Under normal or fault conditions on a plant, the resistance of return paths must be sufficiently low so as not to produce unsafe voltages. Fault currents may be calculated with a target earth loop impedance to prove personnel safety aspects are satisfied.
There must likewise be concerns for low voltage installation considering that the heat generated from low-voltage systems at high currents can cause injury in the form of burns.
- To operate electrical protective devices
Occasionally an electrical fault will occur inside a piece of equipment such as a switchboard or motor that causes a conductor to touch the casing or frame. In most power systems this type of fault would cause a much larger than normal current to flow in the conductors. This current would flow through the casing or frame and in so doing would usually cause serious damage to the conductors, their insulation and casing metalwork due to sparking or arcing. The damage will usually increase with time.
Effective earthing provides suffcient faults current to operate electrical protection relays reducing further damage to the electrical system.
- To guard against lightning surges
Lightning generate a vast static charge that build up to an extremely high voltage. Lightning protection conductors are installed not to handle the vast discharge current flow but to conduct substantial but smaller currents to prevent higher voltages from developing preventing insulation breakdown.
- To control electrostatic discharge
Static electrical charge is caused by the forced separation of molecules of non-conducting materials. Movement of the material or friction against the material can cause the charging effect. When wrenched apart, a surface charge builds up a high potential difference between the separated materials. The size of the charge depends on the violence with which this separation occurs. The non-conducting materials act as a dielectric, and allow the accumulation and storage of the charge as in a capacitor.
Earthing prevents static build up which discharge can produce ignition, explosion or fires.
- To minimize electrical interference
Electrical noise can be generated by non-linear loads, likewise by parasitic capacitances of non-shielded electrical equipments.
Earthing will reduce the coupling capacitance effect and conduct stray currents back to the source of supply.