CH 19 Section 11.pdf - Earthing for Outdoor Signalling Equipment - PDF

Section 11: Earthing for Outdoor Signalling Equipment 19.11.1 Types of Earths (a) Earths are of two types (i) Functional Earth: The earth return used for block instruments is a functional earth which is used to conduct the current through earth durin...

Section 11: Earthing for Outdoor Signalling Equipment 19.11.1 Types of Earths (a) Earths are of two types (i) Functional Earth: The earth return used for block instruments is a functional earth which is used to conduct the current through earth during normal operation/function of equipment. (ii) Protective Earth: This earth for dissipating surges, which comes in contact with equipment’s connections through Surge Protection Devices (SPD) during Lightning or surges so as to protect the equipment. (b) Perimeter Ring Earth (PRE) shall be provided around building housing signalling equipment. Chasis of all S&T equipment shall be bonded to BRC (Bonding Ring Conductor) provided inside the Power Equipment Room or Signal Equipment Room. BRC is to be connected to MEEB in Power room and from there to PRE as per extant guidelines. In case, no BRC exists, chassis of the equipment shall be connected directly to MEEB (Main Equi- potential Earth Busbar). (c) Earthing shall be provided as per approved specifications, drawings and code of practice issued by RDSO. (d) The maximum earth resistance shall be specified as per OEM & RDSO recommendations. 19.11.2 Purpose of Earthing Earthing of cables, equipment, buildings and structures is done for one or more of the following purposes: (a) Lightning & surge protection of equipment. (b) Earthing of metal screens of telecommunications cables and equipment for reducing Electromagnetic interference. (c) Human safety. 19.11.3 Earthing to be provided at (a) The lever frame and other metallic frames of the cabin shall be earthed. (b) The earthing shall be provided at every location box where cables terminate. (c) The earthing shall be provided at each signal. (d) Wherever possible, the common earthing system to be provided for closely located location boxes, signal posts, etc. Separate earth is required for equipment requiring functional earth only. Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 409 of 530  (e) Sheath & Armour of Main cables to be earthed. Armour of OFC shall be earthed at both ends. It is not necessary to earth the armouring of unscreened cables when they are used as a tail cables except in special cases where the length of the tail cable exceeds normal prescribed limits. (f) In case of signals falling within 2 meters from the electrified track, the protection screen shall be connected to an earth. (g) There shall not be any possibility of simultaneous human contact with metallic bodies connected to different earths, where it is not possible to provide suitable spacing or partition between various metallic objects referred to above, they shall be connected to a common earth. (h) Common/Equi-potential earth for modern electronic equipment such as EI, DAC, Datalogger etc. to be used in Relay room. Earth value shall not be more than one ohm and shall be measured annually during dry season. (i) Earthing wires from subsystems to earth terminal shall be of distinctive color. Green or Green Yellow (GNYE) color is recommended for quick identification of a loose or disconnected earth wire. (j) All earth wires shall be as straight as possible and shall never be coiled. Earth wires should be of adequate current carrying capacity and should not be less than 4 Square mm copper cross-section or its equivalent. (k) Earth resistance up to 10 ohm is normally permissible for protective earth except when specified otherwise. For electronic equipment, Earth resistance shall not be more than one ohm. (l) Earthing of approved type shall be provided for each block instruments and other signalling equipment at a station. Dedicated earthing arrangement to be provided for earth return circuits individually if any. (m) The resistance of earth for signalling circuits shall not exceed 10 ohm or as prescribed by OEM/RDSO. If the resistance is more than the required value, steps to reduce the earth resistance shall be taken. If it is still not possible to reduce the value below the required value, even with the adoption of these methods, additional earths may be provided in parallel. (n) Where more than one earth electrode is used, the distance between two earthing electrodes shall be as per approved design. 19.11.4 Earthing Leads (a) Earth wires shall be protected against mechanical damage and possibility of corrosion particularly at the point of connection of earth electrode. (b) The earthing lead shall be mild steel flat of size 40 mm x 6 mm or as per the approved earthing practice/drawing. (c) The earthing lead shall be soldered or crimped on a lug, which shall be bolted to the earth electrode or preferably exothermically welded. The nut & bolt to be painted with anti corrosive paint. Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 410 of 530 19.11.5 Selecting Site for Earthing (a) The site for earthing shall be chosen in the following order of preference: (i) Wet marshy ground and grounds containing refuse, such as ashes, cinders and brine waste. (ii) Clay soil or loam mixed with small quantities of sand. (iii) Clay and loam mixed with varying proportions of sand, gravel and stone; and (iv) Damp and wet sand and peat. (b) A site which is naturally well drained shall be chosen. A water logged situation, however, is not essential unless the soil be sand or gravel. Perennial wells may also be used as sites for earth electrodes with advantage where the bottom of the earth is rocky. (c) Electrodes shall preferably be situated in a soil which has a fine texture and which is packed by watering and ramming as tightly as possible. Where practicable, the soil shall be sifted and all lumps broken up and stones removed in the immediate vicinity of the electrodes. (d) Where soil conductivity is poor, the chemical treatment may be resorted to improve the same. Common salt together with charcoal in alternate layers is generally used for this purpose and the addition of less than one part by weight of salt to 200 parts of soil mass may reduce the resistivity by 80% but there is little advantage in increasing the salt content above 3%. Calcium chloride and sodium carbonate are also beneficial. (e) Use should be made where possible of natural salts in soil produced by bacteriological action on decaying plants. The resistivity of the soil on which plants are growing will be less than that of a similar soil in the absence of plants. (f) As far as possible, the earthing arrangement shall be located in the natural soil. The made-up soil which has not consolidated or is likely to be eroded by weather, shall be avoided. (g) The minimum clearance of equipment earths from system earths (e.g. earthing of AT/Transformer etc.) provided by the Electrical Department either of the Railways or of the other Administrations shall be 20 meter. (h) Asphalt or concrete cover of about 50 mm thickness around the Earth for a radius of 1 meter to retain the soil moisture is desirable. Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 411 of 530 19.11.6 Earth Resistance Earth Resistance of an 'earth' is the sum of three separate resistances, viz., (a) the resistance of the conductor joining the earth electrode to the installation. (b) The contact resistance between the surface of the earth electrode and the soil, and the resistance of the body of soil surrounding the earth electrodes. (c) Normally the first two resistances are negligibly small compared with the third; so, the resistance of an 'earth' is primarily determined by the nature of the soil and not by the electrode itself. (d) The material used for a standard electrode system should be corrosion resistant. Under ordinary soil conditions, use of galvanized iron or mild steel electrode is recomme recommended. nded. In cases where soil corrosion is likely to be excessive, it is preferable to use either copper or copper clad electrode. The electrodes shall be free from paint, enamel or grease. (e) Earth tester normally used for measurement of earth resistivity comp comprises of the current source and meters in a single instrument and directly read the resistance value. 19.11.7 Connection for Four Terminal Megger When using a megger as shown below, the resistivity may be evaluated from equation P = 2πSR Where, P = resistivity of soil in ohm meters, S = distance between successive electrodes in meters, R = megger readings in ohms. Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 412 of 530  (a) Test Procedure: At the selected test site, four electrodes are driven into the earth along a straight line in a chosen direction at equal intervals 'S'. (b) The depth of the electrodes in the ground shall be of the order of 10 to 15 cm. The megger is placed on a steady and approximately level base, the link between terminals P1 and C1 opened and the four electrodes connected to the instrument terminals as shown in the fig. An approximate range on the instrument is then selected to obtain clear readings, avoiding the two ends of the scale, as far as possible. The readings are taken while turning the crank at about 135 rev/min (in case of Magneto Generator Type) or pressing concerned button in some Other Type of Meggers. Pl see equipment Manufacturers manual and follow as stated there in. (c) Resistivity is calculated by substituting the values of 'R' in the equation P = 2πSR. (d) The earth resistance shall be measured using suitable method. For measurement of small earthing system like that of single electrode or ring earth, ‘Fall of Potential’ method can be used. The typical diagram and measurement method is given in Annexure: 19-A3. 19.11.8 Maintenance of outdoor earths (a) Watering: Conventional Earths shall be regularly watered. Earth enhancement material should be periodically added to Maintenance free earth to improve the earth resistance. Earth pit to be regularly cleaned. (b) Earth Connections: All Earth connections shall be carefully examined and kept intact and joints soldered. The wire between each earth and the connected equipment shall be electrically isolated. The exothermic welding termination on maintenance free earth rod shall be checked and cleaned. 19.11.9 Regular Checks/Upkeep of Earths (a) Block earths and their connections shall be examined at intervals of not more than one month by JE (Signals) and not more than three months by Sectional SSE(Signal)/Incharge. (b) Block earths shall be tested for resistance at intervals of not more than 12 months by Sectional JE/SSE (Signal). Where the resistance exceeds 10 ohms, action shall be taken to reduce the resistance by providing additional earths in parallel. (c) If routine testing indicates that existing earth electrode system is not satisfactory, a new earth electrode system (or part of a system to supplement the existing system) shall be provided. Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 413 of 530