Types of Earthing Systems


Purpose of earthing within an electric power system is to limit, with regards to the general mass of earth, the probable of current carrying conductors, which are part of the equipment, and non- current holding metal works, from the equipment, equipment and appliances connected to the system.

Type of Earthing Systems

There are four main type of earthing systems. These include

TN systems

Point right to ground, the exposed conductive parts connected up to now installing the coverage of conductors. Two types of TN system are approved for new installations in South Africa. These are defined in line with the arrangement of natural and protective conductors, as follows


TN-S system: where throughout the system, a separate protecting conductor can be used;


TN-C-S system: in which the neutral and protective functions combined in one command string of system.

TT system

Which one point is directly earthed, the exposed-conductive-parts of the set up being connected to earth electrodes electrically in addition to the earth electrodes of the power system. The TT system is not allowed for new installations in South Africa, but nonetheless exists in lots of the elderly areas.

IT system

Which has all live parts isolated from earth, or one point linked to earth through an impedance, the exposed-conductive-parts of the electrical power set up being earthed independently or collectively or even to the earthing of the system. The IT system is not used for the general public distribution of electricity, but is used in private installations, mainly for reasons of continuity of service. Three stages IT systems with a sent out neutral can using circumstances result in dangerous conditions. This latter kind of IT system is usually to be deprecated.

The problems of voltage variations between period and neutral that can derive from a break in the neutral, are common to all or any types of low voltage distribution systems. The increasing consumption of the TN-C-S system has however released a further dimensions to the results of a loss of the neutral conductor.

TN-C-S system

Mainly for financial reasons, throughout the world, the TN-C-S electricity system is becoming more and more trusted for the distribution of low voltage electrical power. The concept of combining the natural and defensive functions within a conductor immediately resulted in the elimination of one of the five allocated conductors that are necessary for a three period TN-S system (three stages, one natural and one defensive conductor).

In a TN-C-S system, the independent protecting conductor (PE) for every single consumer, is created simply by tapping off a second parallel conductor from the PEN conductor at a location that is near or at the idea of supply to specific consumers. The parallel conductor in the set up then becomes the neutral conductor.

The main downside of the TN-C-S distribution system is this unprotected unsafe condition that can arise in the event of an escape in the Pencil conductor. This threat continues to exist in addition to the occurrence of any safeguard devices which may be installed. In case of a break in the PEN conductor, all uncovered conductive parts will remain "live" regardless of the operating state of the main switching or isolating device - whether it's ON (finished) or OFF (open up).

Current practice of bonding in HK

Types of low voltage system as described by the IEC and IEE are identified by the cable connections to globe of the source (system earth) and the uncovered conductive elements of the electrical installation (equipment earth). If these earth terminals aren't boned together by a metallic conductor the machine is labeled as TT, and the road for earth fault currents is then through the actual earth or ground, .

If the terminals are bonded as well as a trusted and low impedance metallic interconnection the return course will never be through the earth and the machine is classified as TN-S.

The terms on which the CLP Power Hong Kong Ltd. and the Hong Kong Electric Co. , Ltd. will give electricity are subject to the Electricity Source Ordinance, and the regulations and conditions laid down in the normal set of Source Rules released by both companies. By virture of his software for electricity source or his obtaining electricity from the Company, a consumer is destined by and shall follow the Supply Rules and other terms and conditions.

Supply Rule 407. 1 requires that for each and every electrical installation the consumer must definitely provide his own earth electrode system by which the exposed conductive parts of his installation are connected to earth. That's, all revealed conductive elements of the installation shall be connected by protecting conductors to the primary earthing terminal of the set up which terminal will be connected to earth electrode(s) via an earthing conductor. Which means that every assembly is linked to be part of a TT system.

Rule 407. 3. f. (iii) further expresses that where the supply is used direct from the business's transformer or via underground cable tv, the business may permit the consumer to give a bonding conductor between his main earthing terminal and the Company's transformer earth or metallic sheaths of the service cable tv. Therefore, the set up will be run within a TN-S system. However, this rule also states that in the dimension of the planet earth fault loop impedance or evaluating the procedure of protective devices, the said bonding conductor must be disconnected, i. e. the look and commissioning of the unit installation must expect a TT system.

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