CHAPTER (5) EARTHING SYSTEMS IN EARTHING SYSTEMS IN DISTRIBUTION NETWORKS

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1 CHAPTER (5) EARTHING SYSTEMS IN DISTRIBUTION NETWORKS 5.1 Earthing Systems: In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has considerations for the safety and electromagnetic compatibility of the power supply. "Regulations for earthing (grounding) systems vary considerably between different countries". A protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It also ensures that in the case of an insulation fault, a high fault current flows, which will trigger an over current protection device (fuse, MCB) that disconnects the power supply. A functional earth connection serves a purpose other than providing protection against electrical shock. In contrast to a protective earth connection, a functional earth connection may carry a current during the normal operation of a device. Functional earth connections may be required by devices such as surge suppression and electromagneticcompatibility filters, some types of antennas and various measurement instruments. Generally the protective earth is also used as a functional earth, though this requires care in some situations. Design of Electrical Power System Distribution in Modern Buildings (267)

2 5.2 Definitions: Earth electrode: a conductor or group of conductors which provide an electrical connection with earth. Earth: the conductive mass of the earth whose electric potential is equal to zero. Earth electrode resistance: the contact resistance of an earth electrode with the earth. Earthing conductor: a protective conductor connects the main earthing terminals of an installation to an earth electrode. (TN systems) Exposed conductive part: a conductive part of equipment which can be touched &may become live under fault conditions. Protective conductor: a conductor used for protection against electric shock & connecting together any of the following parts: Exposed- conductive part, Earth electrode, Main earthing terminals. Bonding conductor: a protective conductor providing equipotential bonding. Main earthing terminal: the terminal or bar provided for the connection of protective conductors to the means of earthing. 5.3 IEC Nomenclature: International standard IEC distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT. The first letter indicates the connection between earth and the power-supply equipment (generator or transformer): T: direct connection of a point with earth I: no point is connected with earth (isolation), except via high impedance The second letter indicates the connection between earth and the electrical device being supplied: T: direct connection with earth, independent of any other earth connection in the supply system N: connection to earth via the supply network Earthing Schemes: The earthing schemes are classified as shown in the following diagram. Design of Electrical Power System Distribution in Modern Buildings (268)

3 EARTHING S T-T T-N I-T T-N-C T-N-S T-N-C-S TN Network: In a TN earthing system, one of the points in the generator or transformer is connected with earth," usually the star point in a three-phase system". The body of the electrical device is connected with earth via this earth connection at the transformer. The conductor that connects the exposed metallic parts of the consumer is called protective earth (PE). The conductor that connects to the star point in a threephase system, or that carries the return current in a single-phase system, is called neutral (N). Design of Electrical Power System Distribution in Modern Buildings (269)

4 Three Variants of TN Systems: TN-S: PE and N are separate conductors that are connected together only near the power source. TN-S: separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point. TN-C: A combined PEN conductor gets the functions of both a PE and an N conductor. TN-C: combined PE and N conductor all the way from the transformer to the consuming device. TN-C-S: Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, when the building separate PE&N conductors are used, this will reduce the risk of broken neutrals Design of Electrical Power System Distribution in Modern Buildings (270)

5 TN-C-S Earthing System: combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords. It is possible to have both TN-S and TN-C-S supplies from the same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where "bad earths" are found get converted to TN-C-S. TT Network: In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator. Design of Electrical Power System Distribution in Modern Buildings (271)

6 IT Network: In an IT network, the distribution system has no connection to earth at all; it has only a high impedance connection. In these systems, an insulation monitoring device used to monitor the impedance. Comparison of Earthing Systems: Design of Electrical Power System Distribution in Modern Buildings (272)

7 5.5 Choice Criteria: First Criterion: No earthing scheme is universal. To choose the earthing scheme, analyze every case separately based on constraints of the electrical installation, the needs of the user &rules by the power distribution utility. The best solution is that using of several different earthing schemes for different parts of the installation. Second Criterion: These solutions must satisfy the following criteria: Protection against electric shock. Protection against fire of electric origin. Power supply continuity. Protection against over voltages. Protection against electromagnetic disturbances. Third Criterion: Comparison of Earthing Schemes. The TT scheme is recommended for installations subject to modifications &that because it's the simplest scheme to implement in private or public distribution. The IT scheme is recommended if power supply continuity is necessary. TN-S scheme is recommended for installations that have a high level of installation not subject to modifications. TN-C& TN-C-S schemes are not recommended for use. They have the risk of fire and electromagnetic disturbances due to: 1- Voltage drops along PEN conductors. 2- High insulation fault currents. 3- UN eliminated impedant fault. Fourth Criterion: IT, TT&TN-S schemes are equally satisfactory in terms of over voltage withstand and electromagnetic disturbances. Design of Electrical Power System Distribution in Modern Buildings (273)

8 Fifth Criterion: When making an economic comparison, all costs must be taken into consideration such that costs related to: 1. Design. 2. Maintenance. 3. Modification. 4. Production losses. 5.6 Comparison Between Each Criterion: Criterion TT IT TN Provide equal protection against electric shock. Level of protection against electric shock. Protection against fire of electrical origin. Protection against over voltages. Protection against electromagnetic disturbances. In case of single fault, the insulation fault current is very low. Require lightening arrestors. If disturbances with frequencies greater than 1MHZ, the earthing scheme used is of no importance. Provide equal protection against electric shock. In case of single fault, the insulation fault current is very low. Protection against over voltages due to HV faults must be provided by an over voltage limiter. If disturbances with frequencies greater than 1MHZ, the earthing scheme used is of no Provide equal protection against electric shock. Protection against faults is insufficient unless residual current devices are included and incase of solid fault the insulation fault current is high and major damage can result. TNC provides a higher risk of fire. We should consider the required measures. For TN-S scheme a major disturbances are produced during an insulation fault, for TN-C or TN-C-S a Design of Electrical Power System Distribution in Modern Buildings (274)

9 importance. load imbalance current circulate continuously in PEN conductor, exposed conductive parts cable shielding. 5.7 Properties: Cost: TN networks save the cost of a low-impedance earth connection at the site of each consumer. TN-C networks save the cost of an additional conductor needed for separate N and PE connections. However, to mitigate the risk of broken neutrals, special cable types and lots of connections to earth are needed. Safety: In TN, an insulation fault is very likely to lead to a high short-circuit current that will trigger an over current circuit-breaker or fuse and disconnect the L conductors. In TN-S and TT systems (and in TN-C-S beyond the point of the split), a residual-current device can be used as an additional protection. In the absence of any insulation fault in the consumer device, the equation I L1 +I L2 +I L3 +I N = 0. In IT and TN-C networks, residual current devices are far less to detect an insulation fault. In single-ended single-phase systems where the Earth and neutral are combined (TN-C, and the part of TN-C-S systems which uses a combined neutral and earth core), if there is a contact problem in the PEN conductor, then all parts of the earthing system beyond the break will rise to the potential of the L conductor. In an unbalanced multi-phase system, the potential of the earthing system will move towards that of the most loaded live conductor. Therefore, TN-C connections must not go across plug/socket connections or flexible cables, where there is a higher probability of contact problems than with fixed wiring. Design of Electrical Power System Distribution in Modern Buildings (275)

10 Electromagnetic Compatibility: In TN-S and TT systems, the consumer has a low-noise connection to earth, which does not suffer from the voltage that appears on the N conductor as a result of the return currents and the impedance of that conductor. This is of particular importance with some types of telecommunication and measurement equipment. In TT systems, each consumer has its own high-quality connection with earth, and won t notice any currents that may be caused by other consumers on a shared PE line. 5.8 Regulations: For wiring less than 1000 V, the United States National Electrical Code and Canadian electrical code forbid the use of systems that combine the grounding conductor and neutral beyond the customer's disconnecting switch. Design of Electrical Power System Distribution in Modern Buildings (276)