Types of Earthing Systems

Types of Earthing SystemsIntroductionPurpose of earthing in an electric power organisation is to limit, with respect to the general bulk of earth, the potential of current carrying conductors, which atomic number 18 part of the equipment, and non- current carrying metal works, associated with the equipment, apparatus and appliances machine-accessible to the system.Type of Earthing SystemsThere are four main fictitious character of earthing systems. These includeTN systemsPoint stationly to ground, the exposed conductive parts connected to this take aim the instalment of the protection of conductors. Two types of TN system are canonical for sassy inst anyations in South Africa. These are defined according to the arrangement of neutral and protective conductors, as fol ruggedsTN-S system in which throughout the system, a let on protective conductor is usedTN-C-S system in which the neutral and protective functions combined in a single command chain of system.TT systemWhich o ne point is directly earthed, the exposed-conductive-parts of the installation being connected to earth electrodes electrically independent of the earth electrodes of the power system. The TT system is not permitted for new installations in South Africa, but still exists in many of the older areas.IT systemWhich has all live parts isolated from earth, or one point connected to earth through an impedance, the exposed-conductive-parts of the electrical installation being earthed independently or collectively or to the earthing of the system. The IT system is not used for the open distribution of electricity, but is used in private installations, mainly for reasons of continuity of service. Three phases IT systems with a distributed neutral can in certain circumstances resultant in raging conditions. This latter type of IT system is to be deprecated.The problems of emf variations between phase and neutral that can result from a break in the neutral, are common to all types of low voltage distribution systems. The increasing usage of the TN-C-S system has however introduced a further dimension to the consequences of a privation of the neutral conductor.TN-C-S systemMainly for economic reasons, across the globe, the TN-C-S power system is becoming more and more widely used for the distribution of low voltage electrical power. The concept of combining the neutral and protective functions in a single conductor immediately resulted in the elimination of one of the five distributed conductors that are required for a three phase TN-S system (three phases, one neutral and one protective conductor).In a TN-C-S system, the separate protective conductor (PE) for severally consumer, is created by simply tapping off a second parallel conductor from the PEN conductor at a location that is close to or at the point of supply to individual consumers. The parallel conductor in the installation then becomes the neutral conductor.The main disadvantage of the TN-C-S distributi on system is this unprotected hazardous condition that could arise in the event of a break in the PEN conductor. This hazard continues to exist independent of the presence of any protection devices that may be installed. In the event of a break in the PEN conductor, all exposed conductive parts will remain live disregarding of the operating state of the main switching or isolating device whether it is ON (closed) or OFF (open).Current practice of bonding in HKTypes of low voltage system as defined by the IEC and IEE are identified by the connections to earth of the source (system earth) and the exposed conductive parts of the electrical installation (equipment earth). If these earth terminals are not boned together by a metallic conductor the system is classified as TT, and the racecourse for earth duty period currents is then through the actual earth or ground,.If the terminals are bonded together with a reliable and low impedance metallic connection the return path will not b e through the earth and the system is classified as TN-S.The terms on which the CLP Power Hong Kong Ltd. and the Hong Kong Electric Co., Ltd. will supply electricity are subject to the Electricity Supply Ordinance, and the regulations and conditions laid down in the common set of Supply Rules published by both companies. By virture of his applications programme for electricity supply or his obtaining electricity from the Company, a consumer is bound by and shall abide by the Supply Rules and other terms and conditions.Supply Rule 407.1 requires that for every(prenominal) electrical installation the consumer must provide his own earth electrode system by which the exposed conductive parts of his installation are connected to earth. That is, all exposed conductive parts of the installation shall be connected by protective conductors to the main earthing terminal of the installation and that terminal shall be connected to earth electrode(s) via an earthing conductor. This means that e very installation is connected to be part of a TT system.Rule 407.3.f. (iii) further states that where the supply is taken direct from the Companys transformer or via underground cable, the Company may allow the consumer to provide a bonding conductor between his main earthing terminal and the Companys transformer earth or metallic sheaths of the service cable. Therefore, the installation will be operated as part of a TN-S system. However, this rule also states that in the measurement of the earth fault lace impedance or testing the operation of protective devices, the said bonding conductor must be disconnected, i.e. the design and commissioning of the installation must gain a TT system.