Earthing of electrical networks and installations is important to ensure correct operation and serves a vital role. These facts are amply reinforced in legislation and codes of practice, as you will find out later in the course. By earthing, we generally mean an electrical connection to the general mass of earth, the latter being a volume of soil/rock etc., whose dimensions are very large in comparison to the electricity system being considered.

Functions of an Earthing System

The most often quoted reasons for having an earthed system is:

 1   To provide sufficiently low impedance to facilitate satisfactory protection operation under fault conditions.
 2   To ensure that living beings in the vicinity of substations are not exposed to unsafe potentials under steady state or fault conditions.
 3 To retain system voltages within reasonable limits under fault conditions (such as lightning, switching surges or inadvertent contact with higher voltage systems), and ensure that insulation breakdown voltages are not exceeded, i.e. insulation co-ordination.
 4 To limit the voltage to earth on conductive materials which enclose electrical conductors or equipment.
 5 Graded insulation can be used in power transformers.

Less often quoted reasons include:

 6   To stabilise the phase to earth voltages on electricity lines under steady state conditions, e.g. by dissipating electrostatic charges which have built up due to clouds, dust, sleet, etc.
 7  A means of monitoring the insulation of the power delivery system.
 8  To eliminate persistent arcing ground faults.
 9 To ensure that a fault which develops between the high and low voltage windings of a transformer can be detected by primary protection.
 10 To provide an alternative path for induced current and thereby minimise the electrical noise' in communication and signalling cables.
 11 Provide an equipotential platform on which electronic equipment can operate.

To perform adequately in fulfilling any of the above functions, the earthing system must generally have low impedance such that in dispersing or collecting current to/from the ground, an excessive voltage rise does not occur.

Within installations an earth connection is also necessary to ensure the correct operation of some equipment - for example electronic devices, where an earthed shield may be required.  It is essential to consider the earthing within a whole installation as one complete system and for this to be designed and installed accordingly.

The earthing system is normally designed to provide two safety functions...

The first, to prevent a shock due to different potentials on exposed metalwork, is achieved by bonding.  Most electrical equipment is housed inside metal enclosures and if a live conductor comes into contact with this, the enclosure will temporarily also become live. Any parts of exposed conductive metalwork, which can be touched, are connected together via bonding conductors.  This ensures that, should such an electrical fault develop, the potential on all exposed conductive metalwork is virtually the same.  An equipotential 'platform' is created and a person who is in contact simultaneously with two different pieces of exposed metalwork should not receive a shock.

The same principle applies within large electricity substations, railway installations, factories and houses. For example, within factories, bonding of exposed metalwork would normally ensure that an electrical fault to the frame of one machine did not create a potential difference between that and earthed metalwork on an adjacent machine.

The second function of the earthing system is to ensure that, in the event of an earth fault, any fault current occurring can return to source in a controlled manner. By a controlled manner, we mean that the return path is predetermined such that damage to equipment or injury to individuals is avoided. The impedance of the earthing system should be low enough that sufficient earth fault current can flow to operate protective devices correctly, which will in turn initiate the operation of circuit breakers or fuses to successfully interrupt the flow of current.