In any medium or low voltage three-phase system there are three single-phase voltages which are measured between each phase and a common point called the "neutral point". In balanced operating conditions these three voltages are phase shifted by 120° and have the value:
The neutral may be connected to earth either directly or via a resistor or reactor. In the first case, we say that the neutral is solidly (or directly) earthed and, in the second case, we say that the neutral is impedance-earthed.
When there is no intentional connection between the neutral point and earth, we say that the neutral is isolated or unearthed.
The earthing system plays a very important role in a network. On occurrence of an insulation fault or a phase being accidentally earthed, the values taken by the fault currents, touch voltages and over-voltages are closely related to the type of neutral earthing connection.
A solidly earthed neutral helps to limit over-voltages; however, it generates very high fault currents. On the other hand, an isolated or unearthed neutral limits fault currents to very low values but encourages the occurrence of high over-voltages.
In any installation, service continuity in the presence of an insulation fault also depends on the earthing system. An unearthed neutral allows continuity of service in medium voltage, as long as the security of persons is respected. On the other hand, a solidly earthed neutral, or low impedance-earthed neutral, requires tripping to take place on occurrence of the first insulation fault.
The extent of the damage to some equipment, such as motors and generators having an internal insulation fault, also depends on the earthing system.
In a network with a solidly earthed neutral, a machine affected by an insulation fault suffers extensive damage due to the high fault currents.
On the other hand, in an unearthed network or high impedance-earthed network, the damage is reduced, but the equipment must have an insulation level compatible with the level of over voltages able to develop in this type of network.
The earthing system also has a considerable amount of influence on the nature and level of electromagnetic disturbances generated in an electrical installation.
Earthing systems which encourage high fault currents and their circulation in the metallic structures of buildings are highly disturbing.
On the other hand, earthing systems which tend to reduce these currents and which guarantee good equi-potential bonding of exposed conductive parts and metallic structures are not very disturbing.
The choice of earthing system, as much in low voltage as in medium voltage, depends both on the type of installation and network. It is also influenced by the type of loads, the service continuity required and the limitation of the level of disturbance applied to sensitive equipment.
