Types of electrical power distribution systemsElectrical power is distribution either three wires or four wires (3 wire for phases and 1 wire for Neutral). Voltage between phase to phase called Line Voltage and voltage between phase and neutral is calledPhase Voltage. This forth wire may or may not be distributed in distribution system and same way this neutral may or may not be earthed. Depending of this neutral condition (earthed-not earthed-access-not access) there are various type of earthing systems. The neutral may be directly connected to earth or connected through a resistor or a reactor. This system is called Directly earthed or Earthed system. When a connection has not been made between the neutral point and earth, we say that theneutral is unearthed. In a network, the earthing system plays a very important role. When an insulation fault occurs or a phase is accidentally earthed, the values taken by the fault currents, the touch voltages and over voltages are closely linked to the type of neutral earthing connection. A directly earthed neutral strongly limits over voltages but it causes very high fault currents, here as an unearthed neutral limits fault currents to very low values but encourages the occurrence of high over voltages. In any installation, service continuity in the event of an insulation fault is also directly related to the earthing system. An unearthed neutral permits service continuity during an insulation fault. Contrary to this, a directly earthed neutral, or low impedance-earthed neutral, causes tripping as soon as the first insulation fault occurs. The choice of earthing system in both low voltage and medium voltage networks depends on the type of installation as well as the type of network. It is also influenced by the type of loads and service continuity required. The Main objectives of an earthing system are Provide an alternative path for the fault current to flow so that it will not endanger the user, ensure that all exposed conductive parts do not reach a dangerous potential, maintain the voltage at any part of an electrical system at a known value and prevent over current or excessive voltage on the appliances or equipment. Different earthing systems are capable of carrying different amounts of over current. Since the amount of over current produced in different types of installation differs from each other, required type of earthing will also differ according to the type of installation. so in order to ensure that the installation goes with the existing earthing system or else to do any modification accordingly, we need to have a proper idea of the present earthing system. It would enhance the safety as well as the reliability. As per IEC 60364-3 There are two types of systems: Advantages . T = directly earthed exposed conductive parts 2. If the neutral is not accessible.249))) System characteristics 1. N = exposed conductive parts directly connected to the neutral conductor Unearthed System 1. I = unearthed or high impedance-earthed neutral (e. Expanded by 0.000 Ω)The second letter defines the exposed conductive parts of the electrical installation in relation to earth: 1. It runs off external over voltages. 10 pt Font color: Custom Color(RGB(77. Second Letter T = Frame parts of the loads are interconnected and earthed at Load Side Formatted: Font: (Default) Helvetica. The first letter defines the neutral point in relation to earth: 1. 3. IT system unearthed (high impedance earthed neutral) First Letter I = the neutral is unearthed at Transformer or Generator side. 2.g. the overvoltage limiter is installed between a phase and earth. It is compulsory to install an over voltage limiter between the MV/LV transformer neutral point and earth. ( 2) E AR THE D SY STE M: TT TN (TN-S.249.77.77)). T = directly earthed neutral (from the French word Terre) 2. transmitted by the transformer. TN-C. to the earth and protects the low voltage network from a voltage increase due to flashover between the transformer’s medium voltage and low voltage windings. 2.( 1) UNE AR THE D SY S TE M: IT System.1 pt. TN-C-S). Pattern: Clear (Custom Color(RGB(249. which is caused by the system leakage capacitance if an earth fault occurs. When an insulation fault with reference to the earth occurs. Disadvantages 1. 10 pt Font color: Custom Color(RGB(77. 7. 9. Overvoltage limiters must be installed. 3. If this is not possible RCDs must be installed. 8. 3.77. the voltage of the two healthy phases in relation to the earth take on the value of the phase-to-phase voltage So when Select Size of equipments it is need to higher insulation level of the Equipments. 2. Locating faults is difficult in widespread networks. Protection against direct and indirect contact is not guaranteed. Second letter T = the exposed conductive parts of the loads are interconnected and earthed. Better accident prevention the fault current is limited by the body impedance. 6. 2. The compulsory insulation monitoring. Short-circuit and earth fault currents may cause fires and destroy parts of the plant. Requires all the installation’s exposed conductive parts to be Same Voltage level. 10.77)). When an insulation fault occurs. The risk of high internal over voltages making it advisable to reinforce the equipment insulation. 10. The frames of the electrical loads are also connected to an earth connection. the short-circuit current is very low. Higher operational safety only a capacitive current flows. with visual and audible indication of the first fault if tripping is not triggered until the second fault occurs. System providing the best service continuity during use. 4. 5. Top Earthed System (1) TT system directly earthed neutral First letter T = the neutral is directly earthed. Pattern: Clear (Custom Color(RGB(249. The transformer neutral is earthed. 4. Expanded by 0.1 pt. earthing resistance and the high impedance of the earth fault loop. Requires a good level of network insulation (High leakage current must be supplied by insulating transformers). Requires presence of maintenance personnel to monitor and locate the first fault during use.249))) . Formatted: Font: (Default) Helvetica.249.1. 6. 2. Reduces the risk of over voltages occurring. 4. These systems are costly. 3. implement. 3. Requires the use of differential protection devices so that the fault clearance time is not long. Authorizes the use of equipment with a normal phase to earth insulating level. Use of an RCD on each outgoing feeder to obtain total selectivity.System characteristics 1. Second Letter N = the Frames of Electrical loads are connected to the neutral Conductor. Induce Potential gradient. 8. Very high fault currents leading to maximum damage and disturbance in telecommunication networks. 9. 7. Switching upon occurrence of the first insulation fault. Low earth fault current 3. Disadvantages 1. The risk for personnel is high while the fault lasts. the touch voltages which develop being high. 7. High earth fault loop impedance 2. Special measures must be taken for the loads or parts of the installation causing high leakage currents during normal operation in order to avoid spurious tripping (feed the loads by insulating transformers or use high threshold RCDs. Higher touch voltage. 5. the short-circuit current is small. TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing 4. 8. compatible with the exposed conductive part earth resistance). Individual earth system needs higher investment. The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. Save earth wires 2. Does not have the risk of a broken neutral. Utility company need not to provide earth for consumer Advantages 1. Upon occurrence of an insulation fault. There are two types of TN systems. 9. The simplest system to design. depending on whether the neutral conductor and Earth conductor are combined or not: . 5. 6. Easily find location of faults. (2) TN System: Neutral-connected exposed conductive part First Letter T = the neutral is directly earthed at Transformer. monitor and use. High demand of E/F relays. 10. the exposed metallic part may have float voltage. 10 pt Font color: Custom Color(RGB(77.(2a) TN-C In TNC System (the third letter C = combined neutral and narth conductor). The TNC system may be less costly upon installation (elimination of one switchgear pole Disadvantages 1. Advantages 1. 3. 4. 2. High earth fault level. 3. as well as downstream of a TNS system (As per IEC 60364-5). Impedance of earth fault loop could be predicted. 2. Low earth fault loop impedance. 5. 5.249))) System Characteristics 1. Pattern: Clear (Custom Color(RGB(249.249. More than one earth fault loops. voltage detected type could be employed. Neutral never have float voltage. allow of multi-point earth.77)). Formatted: Font: (Default) Helvetica. current operated type device is not appropriated. In this system earthing connections must be evenly placed along the length of the neutral(earth) conductor to avoid potential rises in the exposed conductive parts at Load Side if a fault occurs. Expanded by 0. and the neutral earth broken. Better earthing continuity. .1 pt. 4. If not multi-point earthed. intervene the operation of earth fault protective device. No earth wire required.77. This combined ceutral-earth wire is than distributed to load side. High earth fault current. Third and multiples of third harmonics circulate in the protective conductor (TNC system). 3. This system must not be used for copper cross-sections of less than 10 mm² and aluminum cross-sections of less than 16 mm². 2. and one conductor). the neutral and Earth conductors are combined in a single conductor and earthed at source end. Disadvantages 1. Pattern: Clear (Custom Color(RGB(249. A tripping check on occurrence of the insulation fault should be carried out. Low earth fault loop impedance Advantages 2. if possible. 10 pt Font color: Custom Color(RGB(77. Expanded by 0. 4. it cannot be used in places presenting a fire risk (TNC system). generally near to the Source.77.6. 3. Use of over current protective devices to ensure protection against indirect contact. 2. this check is the only guarantee that the system operates both on commissioning and during operation. minimize the use of earth fault relay because of low earth fault loop impedance. when the network is being designed using calculations.249))) System characteristics 1. In This System Earthing connections must be evenly placed along the length of the Neutral(Earth) conductor to avoid potential rises in the exposed conductive parts at Load Side if a fault occurs. The neutral and Earth conductors are separately distributed to load. moreover. Requires earthing connections to be evenly placed in the installation so that the protective conductor remains at the same potential as the earth.77)).249. extension). better earthing continuity. as well as after any kind of work on the network (modification. 5. (2b) TN-S: In TN-S system (the third letter S = Separate Neutral and Earth Conductor) neutral of the source of energy is connected with earth at one point only. . Switching on occurrence of the first insulation fault. The TNC system involves the use of fixed and rigid trunkings 3. 2. This system must not be used upstream of a TNC system. Passage of the protective conductor in the same trunkings as the live conductors of the corresponding circuits.1 pt. High earth fault current 1. Allow multi point earth. Earth fault protection device operates faster. Formatted: Font: (Default) Helvetica. and must be performed during commissioning using measurements. The fire risk is higher and. 249))) Advantages Cost for core cable is cheaper than a 3 core. At the property.249. . with the earth terminal usually being on the side of the cutout. Throughout the supply network. high earth fault level under earth fault condition.1 pt.77.77)). On occurrence of an insulation fault.6. Typically this will be a concentric cable. This happens as the earth and neutral are connected in the cutout. This multiple earthing is why a TNCS supply is often called PME (Protective Multiple Earthing). . there are no problems with corrosion. 7. the live and neutral are linked. particularly where some properties have metal water pipes and others have plastic. 9. Expanded by 0. 10 pt Font color: Custom Color(RGB(77. Pattern: Clear (Custom Color(RGB(249. Requires extra equal potential bonding. and there is no direct connection to the ground other than in the supply network. and a ring of wires around this for the combined neutral and earth. low power factor (high inductance of long cable) 8. either buried underground or at the poles for overhead supplies. the current flowing in the customer’s earthing conductors can be much greater that for a TNS system. Disadvantages When the combined earth/neutral conductor is broken. which can be a shock risk. Formatted: Font: (Default) Helvetica. 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