ZENER 8000-V Installation Manual
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IM00165
Appendix A - Electrical distribution earthing systems
Electrical earthing
"Earthing" is the connection of the exposed-conductive parts of an electrical installation by means of protective
conductors to an electrode in contact with the earth's surface.
Earthing is provided for both safety (protective) and functional purposes
Protective earthing
Protective earthing avoids electric shock hazards by keeping the exposed conductive parts of electrical
equipment close to earth potential during a fault condition. In the event of a fault, a current is allowed to flow
to earth via the earthing system. The fault current is utilised in various ways to disconnect the electrical supply
thereby protecting the circuit and removing any fault-induced voltages from exposed conductive parts of the
installation in a timely manner. The details of the arrangements used for the detection of fault currents vary
according to both the electrical earthing system employed and special requirements of the industry/application.
Functional earthing
A functional earth connection is one provided for purposes other than safety. Applications that require
functional earth connections include surge suppressors and electromagnetic interference filters.
Electrical supply earthing arrangements
Some kind of earth reference point is required in the electrical supply system to enable a fault current to flow in
the event of an accidental (at the “fault”) connection between a live electrical conductor and objects connected
to earth in some way.
There are a number of different arrangements in use. Two systems in common use are:
TN system
In this system the neutral conductor of the supply is directly connected to an earth electrode. In the event of a
fault, either between the various power conductors or between a power conductor and ground, a high fault
current flows. This fault current directly operates fuses or circuit breakers to disconnect the electrical supply.
This is the system in common use in Australia for domestic and industrial installations operating on
240/415VAC.
IT system
In this system the neutral conductor of the supply (typically the star point of the secondary of the distribution
transformer) is either isolated completely from earth or a connection to earth provided by a relatively high
impedance component, typically a resistor. In both cases, the fault current flowing in the event of fault
between power conductors and earth will be quite small (or near non-existent in a fully isolated system), so
special relay devices are required to provide overall protection.
In general, there are two reasons for adopting an IT supply earthing arrangement:
(a)
An IT system with a fully isolated neutral offers the possibility to allow the system to continue to
operate in the presence of a single earth fault. This feature is desirable in some continuous process
applications. In this case, the earth fault detecting device simply provides an indication that there is a
fault in need of repair, rather than disconnecting the electrical supply immediately. A second earth
fault occurring while the first remains unrepaired would result in a high fault current and disconnection
of the electrical supply by means of over-current protective devices (fuses or circuit breakers).
