TC 5 Earthing and Surge Protection Devices PDF

TC 5 EARTHING AND SURGE PROTECTIO DEVICES इ रसेट IRISET TC 5 EARTHING AND SURGE PROTECTION DEVICES The Material Presented in this IRISET Notes is for guidance only. It does not over rule or alter any of the Provisions contained in Manuals or Railway Board’s directives INDIAN RAILWAY INSTITUTE OF SIGNAL ENGINEERING AND TELECOMMUNICATIONS SECUNDERABAD - 500017 February 2020 TC 5 EARTHING AND SURGE PROTECTION DEVICES INDEX Page S.No. Chapter No. 1 Surges and their effects on S&T Installations 1.0 Introduction 1 1.1 What are Surges? 1.2 How does Lightning take place? 1.3 Physical Effects of Lightning 1.4 Concept of Lightning Protection Zones 2 Fundamentals of Earthing 2.1 Earthing De-mystified 7 2.2 Earth Resistance 2.3 Pipe Electrode 2.4 Plate Electrode 2.5 Methods of Reducing Earth Resistance 2.6 Ring Earth System 3 Surge Protection Standard IEC 62305 3.0 IEC 62305 - Lightning protection standard 10 3.1 Structure of BS EN/IEC 62305 3.2 BS EN/IEC 62305-1 General principles 3.3 Lightning Protection Levels (LPL) 3.4 BS EN/IEC 62305-2 Risk Management 3.5 Non-conventional air termination systems 3.6 Earth termination system 3.7 External zones 3.8 Internal zones 3.9 Surge Protection Measures (SPM) 3.10 Coordinated SPDs 3.11 Enhanced SPDs 3.12 Conclusion 4 RDSO Specification for Earthing System for Signal and Telecom Installations 4.1 Introduction 28 4.2 The Objective of Earthing System 4.3 TERMINOLOGY 4.4 Soil Resistivity 4.5 Location of Earth Pit 4.6 Earth Electrodes 4.7 Measurement of Earth Resistance 4.8 Sphere of influence of earth electrode 5 Code of Practice of Earthing and Bonding System for S&T Installations 5.1 Scope 38 5.2 References for Earthing & Bonding Practices 5.3 Characteristics of Good Earthing System 5.4 Acceptable Earth Resistance value 5.5 Components of Earthing & Bonding system 5.6 Design of Earthing & Bonding system 5.7 Construction of Unit Earth Pit 5.8 Construction of Loop Earth by Providing Multiple Earth Pits 5.9 Measurement of Earth Resistance 5.10 Inspection Chamber 5.11 Equipotential Earth Busbars 5.12 Bonding Connections 5.13 Drawing of Earthing & Bonding System 6 Surge Protection Devices for Telecom Equipments 46 7 Annexure-I (IEC standards for SPDs) & Annexure-II (Introduction, 50 Classification, Parameters & Protection of SPDs, Calculations to select SPDs, Some of the surge protection devices) 8 Question Bank 60 9 Abbreviations/Acronyms 64 10 Glossary 65 Prepared by : Jayarajan D, ILP2 Reviewed by : B. B. K. Murthy, Prof. Tele Approved by : C. Chandrasekhara Sastry, Sr.Prof.-Tele DTP and Drawings : K. Srinivas, JE (D) Version No. : 1.0, February 2020 No. of Pages : 69 No. of Sheets : 35 © IRISET “This is the intellectual property for exclusive use of Indian Railways. No part of this publication may be stored in a retrieval system, transmitted or reproduced in any way, including but not limited to photocopy, photograph, magnetic, optical or other record without the prior agreement and written permission of IRISET, Secunderabad, India” http://www.iriset.indianrailways.gov.in Surges and their Effects on S&T Installations CHAPTER-1 SURGES AND THEIR EFFECTS ON S&T INSTALLATIONS Fig. 1.1 Lightning flash 1.0 Introduction: Signal and Telecommunication systems such as Electronic Interlocking, Digital Axle Counters, Track Circuits, ISDN Exchanges, OFC Communication, Data Network, Control Communication, etc. are functioning round the clock for the safe and smooth running of trains over Indian Railways. These systems consist of sophisticated devices such as ICs, Microprocessors and Microcontrollers and they are prone to transient surge voltage and currents. Hence, it is required to safeguard these systems from surges so as to ensure uninterrupted service rendered by them and also to avoid replacement cost due to damages. 1.1 What are surges? Surges are transient phenomena involving build-up of potentials and flow of currents of magnitudes several times higher than the normal working voltages and currents, resulting in partial or complete damage of equipment or reduction in life of components / equipment. Surges are caused by: Lightning discharges Switching on/off of inductive loads (for example transformers, relay coils, & motors) Ignition and interruption of electric arcs (for example welding process ) Tripping of fuses and circuit breakers. Power transitions in other large equipments on the same power line. Malfunctioning caused by the power supply systems. Short circuits. IRISET 1 TC5 - Earthing and Surge Protection Devices Surges and their Effects on S&T Installations 1.1.1 Result of surges: Interruption of the service rendered by the equipment. Replacement cost of the circuit or the equipment. A study conducted by IEC (International Electrotechnical Commission) in several countries revealed that the loss (cost of damages) due to lightning is very high and is next only to loss due to negligence in handling equipment (Please refer table 1.1). Cause % of Total Loss Negligence 36.1% Surges 27.4% Burglaries 12.9% Floods/Storms 6.9% Others 16.7% Table 1.1 Losses (Cost of Damages) due to various reasons 1.2 How does Lightning take place? Lightning takes place due to accumulation of electric charges in cloud mass in atmospheric conditions that prevail in thunderstorms. Thunderstorms and the resultant lightning have different mechanisms in tropical regions and temperate regions. In tropical regions, thunderstorms are known as heat storms. These are caused by warm air drifting up pushing down the cold air. This causes formation of single or many ‘cloud cells’ spanning several kilometers. In temperate regions, thunderstorms are known as frontal storms. Cloud frontal waves push-up the warm air. This causes the formation of large number of ‘cloud cells’, each spanning many kilometers and successively spaced. Violent up-draught of air through centre of cloud cell causes the following: 1. Ice crystals, which are positively charged 2. Water droplets, which are negatively charged 3. Positive charge centre lies in the upper part of the atmosphere (about 10000 m) & negative charge centre in lower part (about 5000 m) in case of heat storms in tropical regions 4. Positive charge centre lies in the upper part of the atmosphere (about 6000 m) & negative charge centre in lower part (about 2500 m) in case of frontal storms in temperate region. The lower negatively charged part of the cloud electro-statically induces positive charge on the ground directly below it. More concentration of positive charge takes place on raised structures such as trees and buildings. On building up of potential, the negative charge will be accelerated towards the ground and it is called as the ‘Stepped leader’. When lightning begins, a step leader comes from the cloud to the ground. The step leader is not very bright but it propagates for a short distance and stops for a while, then proceeds in different direction and stops again. This IRISET 2 TC5 - Earthing and Surge Protection Devices Surges and their Effects on S&T Installations process repeats many times, making a zigzag path filled with negative charge. These high speed electrons ionize air molecules thus providing a conducting path for the stroke. When the step leader comes close to the ground a strong electric field is created which drives the positive charge on the ground to neutralize the negative charge in the path. This is called the returning stroke which is also called as the ‘Streamer’. This returning stroke is much brighter than the step leader. Hence lightning is the flow of positive ions, mostly from raised structures on earth to the clouds above. What we see as lightning is a discharge which actually goes from the ground to the cloud. The returning stroke is the origin of intense light, heat and sound in lightning. Lightning also takes place within the same cloud and between different clouds. But our concern is the lightning between cloud and ground which may ruin the S&T equipments partially or completely. Fig.1.2. Types of Lightning IRISET 3 TC5 - Earthing and Surge Protection Devices Surges and their Effects on S&T Installations Fig. 1.3 Cloud to Ground Lightning Summarizing the lightning process, the following points are noteworthy: Negative electrical charges build up within clouds Electric field intensification Positive charges gather on ground Air breakdown leads to stepped Leader Further electric field intensification Generation of strong upward positive streamer Positive upward streamer meets the downward step leader conducting path forms Visible lightning flash 1.3 Physical Effects of Lightning Lightning has the following physical effects: 1. Heating of air up to 30,0000 K (297270 C) 2. Creation of pressure shock wave 3. Flow of current of magnitude 10 kA to 200 kA 4. Heavy potential difference of the order of 1 to 10 Million Volts Fortunately, the above effects are transients (short lived) and are to be discharged through suitable protection mechanisms to safeguard electrical and electronic installations. Surge current due to lightning is expressed in 3 parameters: 1. Surge amplitude 2. Time taken by the surge to reach its maximum value 3. Time taken by the surge to fall to its half max. value IRISET 4 TC5 - Earthing and Surge Protection Devices Surges and their Effects on S&T Installations Example: 10 kA, 8/20 micro sec. means surge of 10 KA amplitude, taking 8 micro sec to reach peak value and 20 micro sec. to fall to half peak value (of 5 kA) Fig. 1.4 Parameters of surge current Normally, we encounter surges of 200kA 10/350 micro sec., 50 kA 10/350 micro sec., 15 kA 8/20 micro sec., as per severity of lightning. Area under the curve gives the damaging energy of lightning surge. 1.4 Concept of Lightning Protection Zones (LPZs) Lightning strike can be – 1. Direct strike on equipment room 2. Indirect strike (despite use of lightning arrester connected to earth through down conductor) Galvanic coupling , through metallic conductor coming in contact with surge Capacitance coupling due to capacitive effect caused by parallel surface Inductive coupling due to conductors, parallel to surge movement / discharge path Hence, the concept of protection zones is to be understood for any electrical installation. Please refer to fig. 1.5 marking the protection zones and correlate with the description in table Fig. 1.5 Lightning Protection Zones IRISET 5 TC5 - Earthing and Surge Protection Devices Surges and their Effects on S&T Installations Zone Details LPZ 0A Direct strikes - Full lightning current - Full magnetic field No direct strikes - Partial lightning or induced current - full magnetic LPZ 0B field No direct strikes - Partial lightning or induced current - Damped LPZ1 magnetic field LPZ2 No direct strikes - Induced current - Further damped magnetic field Table 1.2 Interpretations of Lightning Protection Zones Having identified the lightning protection zones, what needs to be done is – 1. Provision of equipotential bonding system, comprising of external earthing system joined to equipotential busbar inside 2. Provision of surge arresters as per specifications, connected to the earthing system To filter and clamp the transients at power entry point To ensure stage-wise reduction of surge voltage to harmless level Earthing system is discussed in chapter-2, surge arrestors in chapter-3 and RDSO specifications regarding earthing are presented in chapter-4. IRISET 6 TC5 - Earthing and Surge Protection Devices Fundamentals of Earthing CHAPTER-2 FUNDAMENTALS OF EARTHING 2.1 Earthing De-Mystified: The very first question that crops up on earthing is “Is Earth a good conductor?” Earth, as per material phenomenon, is a bad conductor. Table 2.1 confirms the same. Material Resistivity Copper 1.7 X 10 -8 Ohm. Meter GI 10 -7 Ohm. Meter Wet soil 10 Ohm. Meter Moist soil 100 Ohm. Meter Dry soil 1000 Ohm. Meter Bedrock 10000 Ohm. Meter Table 2.1 Resistivity of various materials Then, why at all earthing is done? We resort to earthing, not because it is a good conductor, but because the earth is ideal equipotential surface. A very large charge is required to raise potential of earth everywhere. In other words, effect of injected fault current is felt only locally. Fault current returns to the source in case of Generation/Transmission systems and surges bypass equipment in other cases. Having understood material property and equipotential surface phenomenon of earth, let us turn our attention to earth electrodes, i.e. the conductors used for earthing, their shapes, sizes, etc. 2.2 Earth Resistance 2.2.1 What determines Earth Resistance? When we talk about earth resistance, two components are to be considered: Electrode resistance and electrode to earth resistance. Electrode resistance is the resistance of material used (i.e. in the shape and size of electrode). Obviously, we have to use metallic bodies like GI/Copper electrodes, to keep electrode resistance less than 1 Ohm. The resistance offered by earth to spread electric field is called ‘Electrode to earth resistance’. It is important because current injected into earth by the electrode causes electric field set-up which causes potential difference. ‘Electrode to earth resistance’, which depends on soil resistivity and geometry and size of electrode, is also required to be low to make earth resistance (electrode resistance plus electrode to earth resistance ) less than 1 Ohm. Electrode to earth resistance depends on Geometry and size of electrode Soil resistivity 2.3. Pipe Electrode In case of pipe electrode, electrode to earth resistance is given by: R = (ρ / 2πL) [ln {(8L / D) - 1}] IRISET 7 TC5 - Earthing and Surge Protection Devices Fundamentals of Earthing Where L is the length of the electrode in metre, D is the diameter of electrode in metre and ρ is soil resistivity From the above formula, it can be easily inferred that electrode material is not of concern in deciding the value of electrode to earth resistance. If the length of electrode is more, electrode to earth resistance is less. However, electrode has to be metallic (GI/Copper) to keep electrode resistance very low, as already discussed in section 2.2.1. A very important point to be kept in mind is two parallel electrodes not necessarily reduce electrode to earth resistance to half. If the length of electrode is L, we have to consider soil in the hemisphere of radius L as the deciding portion of soil for electrode to earth resistance. This implies each electrode requires exclusive soil in a hemi-sphere of radius L. So, if electrode to earth resistance with one electrode is R, it can be R/2 with two electrodes only if the electrodes have separation of 2L. 2.4. Plate Electrode In case of Plate electrode, electrode to earth resistance is given by: R = ρ/2πL [ln(8L/T) + ln(L/h)-2+(2h/L)-(h/L)2] L is length, h is the depth of laying and T is thickness As can be seen, L has a major influence, T has minor influence. For several years, electrical substations were being provided with earthing plates of large area, which could not solve problems of high ‘electrode to earth’ resistance. Finally, having understood the implications of the above formula, electrical engineers started providing electrode-grids for the sub-stations. 2.5. Methods of Reducing Earth Resistance: As Electrode to earth resistance depends on soil resistivity, it has to be reduced. The simplest way to do so is By adding salt, charcoal and sand mixture to the pit By adding earth enhancement material. By using a bigger grounding electrode By burying the ground electrode as deep as possible By having parallel ground electrodes with a distance of 10m between them Resistivity of different Soils Type of Soil Resistivity in Ohm-m Surface soil, loam, etc. 1 - 50 Clay 2 - 100 Sand and gravel 150 - 1000 Surface limestone 100 - 10000 Shale 5 - 100 Sandstone 20 - 2000 Granites, basalts, etc. 1000 Decomposed gneisses 50 - 500 Slates, etc. 10 - 100 Table 2.2 IRISET 8 TC5 - Earthing and Surge Protection Devices Fundamentals of Earthing When large number of earth systems is to be taken care of, it is difficult to keep track of re- treatment. Hence, provision of maintenance free earth is recommended. It essentially consists of filling the augured earth pit by earth-enhancement material like bentonite clay. For one pit associated with each pipe electrode, it is recommended to use at least 20 kg of bentonite clay. If less amount of bentonite clay is used, we may get low value of resistivity initially, but with the passage of time, the resistivity increases. 2.6. Ring Earth System: Ring earth system comprises of: 1. Equipotential bonding of earth electrodes planted externally (outside the building) as a ring. 2. Provision of equipotential busbar or ring inside equipment room. 3. Joining the external and internal rings. IRISET 9 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 CHAPTER - 3 SURGE PROTECTION STANDARD IEC 62305 3.0 IEC 62305 - Lightning protection standard International Electrotechnical Commission (IEC) headquartered in Geneva, Switzerland provided the BS EN/IEC 62305 standard recognised for lightning protection. Key to BS EN/IEC 62305 is that all considerations for lightning protection are driven by a comprehensive and complex risk assessment and this assessment not only takes into account the structure to be protected, but also the services to which the structure is connected. In essence, structural lightning protection can no longer be considered in isolation, protection against transient overvoltages or electrical surges is integral to BS EN/IEC 62305. 3.1 Structure of BS EN/IEC 62305 The BS EN/IEC 62305 series consists of four parts, all of which need to be taken into consideration. These four parts are: Part 1: General principles, Part 2: Risk management, Part 3: Physical damage to structures and life hazard, and Part 4: Electrical and electronic systems within structures 3.2 BS EN/IEC 62305-1 General principles This opening part of the BS EN/IEC 62305 suite of standards serves as an introduction to the further parts of the standard. It classifies the sources and types of damage to be evaluated and introduces the risks or types of loss to be anticipated as a result of lightning. Furthermore, it defines the relationships between damage and loss that form the basis for the risk assessment calculations in part 2 of the standard. Lightning current parameters are defined. These are used as the basis for the selection and implementation of the appropriate protection measures detailed in parts 3 and 4 of the standard. Part 1 of the standard also introduces new concepts for consideration when preparing a lightning protection scheme, such as Lightning Protection Zones (LPZs) and separation distance. 3.2.1 Damage and loss: BS EN/IEC 62305 identifies four main sources of damage: S1 Flashes to the structure S2 Flashes near to the structure S3 Flashes to a service S4 Flashes near to a service IRISET 10 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 Each source of damage may result in one or more of three types of damage: D1 Injury of living beings due to step and touch voltages D2 Physical damage (fire, explosion, mechanical destruction, chemical release) due to lightning current effects including sparking D3 Failure of internal systems due to Lightning Electromagnetic Impulse (LEMP) The following types of loss may result from damage due to lightning: L1 Loss of human life L2 Loss of service to the public L3 Loss of cultural heritage L4 Loss of economic value Source of Type of Type of Point of strike damage damage loss L1, L4 D1 L1, L2, L3, Structure S1 D2 L4 D3 L1, L2, L4 Near a structure S2 D3 L1, L2, L4 L1, L4 D1 Service connected L1, L2, L3, S3 D2 to the structure L4 D3 L1, L2, L4 Near a service S4 D3 L1, L2, L4 3.2.2 Scheme design criteria The ideal lightning protection for a structure and its connected services would be to enclose the structure within an earthed and perfectly conducting metallic shield (box), and in addition provide adequate bonding of any connected services at the entrance point into the shield. This in essence would prevent the penetration of the lightning current and the induced electromagnetic field into the structure. However, in practice it is not possible or indeed cost effective to go to such lengths. This standard thus sets out a defined set of lightning current parameters where protection measures, adopted in accordance with its recommendations, will reduce any damage and consequential loss as a result of a lightning strike. This reduction in damage and consequential loss is valid provided the lightning strike parameters fall within defined limits, established as Lightning Protection Levels (LPL). 3.3 Lightning Protection Levels (LPL) Four protection levels have been determined and each level has a fixed set of maximum and minimum lightning current parameters. These parameters are shown in table below. IRISET 11 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 The maximum values have been used in the design of products such as lightning protection components and Surge Protective Devices (SPDs). The minimum values of lightning current have been used to derive the rolling sphere radius for each level. LPL I II III IV Maximum Current (kA) 200 150 100 100 Minimum Current (kA) 3 5 10 16 Fig. 3.1 Types of damage and loss resulting from a lightning strike on or near a structure 3.3.1 Lightning Protection Zones (LPZ) The concept of Lightning Protection Zones (LPZ) was introduced within BS EN/IEC 62305 particularly to assist in determining the protection measures required to establish protection measures to counter Lightning Electromagnetic Impulse (LEMP) within a structure. The general principle is that the equipment requiring protection should be located in an LPZ whose electromagnetic characteristics are compatible with the equipment stress withstand or immunity capability. IRISET 12 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 The concept caters for external zones, with risk of direct lightning stroke (LPZ 0A), or risk of partial lightning current occurring (LPZ 0B), and levels of protection within internal zones (LPZ 1 & LPZ 2). In general, the higher the number of the zone (LPZ 2, LPZ 3 etc), the lower the electromagnetic effects expected. Typically, any sensitive electronic equipment should be located in higher numbered LPZs and be protected against LEMP by relevant Surge Protection Measures (SPM). Figure below highlights the LPZ concept as applied to the structure and to SPM. The concept is expanded upon in BS EN/IEC 62305-3 and BS EN/IEC 62305-4. Selection of the most suitable SPM is made using the risk assessment in accordance with BS EN/IEC 62305-2. Fig. 3.2 Lightning Protection Zone (LPZ) Concept 3.4 BS EN/IEC 62305-2 Risk Management: BS EN/IEC 62305-2 is key to the correct implementation of BS EN/IEC 62305-3 and BS EN/IEC 62305-4. The assessment and management of risk is significantly more in depth and extensive. IRISET 13 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 BS EN/IEC 62305-2 specifically deals with making a risk assessment, the results of which define the level of Lightning Protection System (LPS) required. The first stage of the risk assessment is to identify which of the four types of loss (as identified in BS EN/IEC 62305-1) the structure and its contents can incur. The ultimate aim of the risk assessment is to quantify and if necessary reduce the relevant primary risks i.e.: R1: risk of loss of human life R2: risk of loss of service to the public R3: risk of loss of cultural heritage R4: risk of loss of economic value For each of the first three primary risks, a tolerable risk (RT) is set. Each primary risk (Rn) is determined through a long series of calculations as defined within the standard. If the actual risk (Rn) is less than or equal to the tolerable risk (RT), then no protection measures are needed. If the actual risk (Rn) is greater than its corresponding tolerable risk (RT), then protection measures must be instigated. The above process is repeated (using new values that relate to the chosen protection measures) until Rn is less than or equal to its corresponding RT. It is this reiterative process that decides the choice or indeed Lightning Protection Level (LPL) of Lightning Protection System (LPS) and Surge Protective Measures (SPM) to counter Lightning Electromagnetic impulse (LEMP). 3.4.1 BS EN/IEC 62305-3 Physical damage to structures and life hazard This part of the suite of standards deals with protection measures in and around a structure. The main body of this part of the standard gives guidance on the design of an external Lightning Protection System (LPS), internal LPS and maintenance and inspection programmes. 3.4.2 Lightning Protection System (LPS) BS EN/IEC 62305-1 has defined four Lightning Protection Levels (LPLs) based on probable minimum and maximum lightning currents. These LPLs equate directly to classes of Lightning Protection System (LPS). The greater the LPL, the higher class of LPS is required. LPL CLASS OF LPS I I II II III III IV IV Table: Relation between Lightning Protection Level (LPL) and Class of LPS The class of LPS to be installed is governed by the result of the risk assessment calculation highlighted in BS EN/IEC 62305-2. 3.4.3 External LPS design considerations The lightning protection designer must initially consider the thermal and explosive effects caused at the point of a lightning strike and the consequences to the structure under consideration. Depending upon the consequences the designer may choose either of the following types of external LPS: a) Isolated b) Non-isolated IRISET 14 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 An Isolated LPS is typically chosen when the structure is constructed of combustible materials or presents a risk of explosion. Conversely a non-isolated system may be fitted where no such danger exists. An external LPS consists of: a) Air termination system b) Down conductor system c) Earth termination system These individual elements of an LPS should be connected together using appropriate lightning protection components (LPC) complying (in the case of BS EN 62305) with BS EN 50164 series (note this BS EN series is due to be superseded by the BS EN/IEC 62561 series). This will ensure that in the event of a lightning current discharge to the structure, the correct design and choice of components will minimize any potential damage. a) Air termination system The role of an air termination system is to capture the lightning discharge current and dissipate it harmlessly to earth via the down conductor and earth termination system. Therefore it is vitally important to use a correctly designed air termination system. BS EN/IEC 62305-3 advocates the following, in any combination, for the design of the air termination: i) Air rods (or finials) whether they are free standing masts or linked with conductors to form a mesh on the roof ii) Catenary (or suspended) conductors, whether they are supported by free standing masts or linked with conductors to form a mesh on the roof iii) Meshed conductor network that may lie in direct contact with the roof or be suspended above it (in the event that it is of paramount importance that the roof is not exposed to a direct lightning discharge) The standard makes it quite clear that all types of air termination systems that are used shall meet the positioning requirements laid down in the body of the standard. It highlights that the air termination components should be installed on corners, exposed points and edges of the structure. The three basic methods recommended for determining the position of the air termination systems are: i) The rolling sphere method ii) The protective angle method iii) The mesh method These methods are detailed over the following pages. i) The rolling sphere method The rolling sphere method is a simple means of identifying areas of a structure that need protection, taking into account the possibility of side strikes to the structure. The basic concept of applying the rolling sphere to a structure is illustrated in Figure. IRISET 15 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 Fig. 3.3 Application of Rolling Sphere Method The rolling sphere method was used in BS 6651, the only difference being that in BS EN/IEC 62305 there are different radii of the rolling sphere that correspond to the relevant class of LPS (see Table). Class of Rolling Sphere LPS Radius (m) I 20 II 30 III 45 IV 60 Maximum values of rolling sphere radius corresponding to the Class of LPS This method is suitable for defining zones of protection for all types of structures, particularly those of complex geometry. ii) The protective angle method The protective angle method is a mathematical simplification of the rolling sphere method. The protective angle (α) is the angle created between the tip (A) of the vertical rod and a line projected down to the surface on which the rod sits (see Figure 3.4). The protective angle afforded by an air rod is clearly a three dimensional concept whereby the rod is assigned a cone of protection by sweeping the line AC at the angle of protection a full 360º around the air rod. The protective angle differs with varying height of the air rod and class of LPS. The protective angle afforded by an air rod is determined from Table of BS EN/IEC 62305-3 (see Figure 3.4). Varying the protection angle is a change to the simple 45º zone of protection afforded in most cases in BS 6651. Furthermore the new standard uses the height of the air termination system above the reference plane, whether that be ground or roof level (See Figure). IRISET 16 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 Fig. 3.4 The protective angle method for a single air rod Varying the protection angle is a change to the simple 45º zone of protection afforded in most cases in BS 6651. Furthermore the new standard uses the height of the air termination system above the reference plane, whether that be ground or roof level (See Figure). Fig.3.5 Determination of the protective angle Note 1: Not applicable beyond the values marked with only rolling sphere and mesh methods apply in these cases. Note 2: h is the height of air-termination above the reference plane of the area to be protected. Note 3: The angle will not change for values of h below 2m. The protective angle method is suitable for simple shaped buildings. However this method is only valid up to a height equal to the rolling sphere radius of the appropriate LPL. Fig. 3.6 Effect of the height of the reference plane on the protection angle iii) The mesh method This is the method that was most commonly used under the recommendations of BS 6651. Again, within BS EN/IEC 62305 four different air termination mesh sizes are defined and correspond to the relevant class of LPS (see Table). IRISET 17 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 Class of LPS Mesh size (m) I 5X5 II 10 X 10 III 15 X 15 IV 20 X 20 Maximum values of mesh size corresponding to the Class of LPS This method is suitable where plain surfaces require protection if the following conditions are met: a) Air termination conductors must be positioned at roof edges, roof overhangs and on the ridges of the roof with a pitch in excess of 1 in 10 (5.7º) b) No metal installation protrudes above the air termination system Modern research on lightning inflicted damage has shown that the edges and corners of roofs are most susceptible to damage. So, on all structures particularly with flat roofs, perimeter conductors should be installed as close to the outer edges of the roof as is practicable. Fig. 3.7 Concealed air termination network As in BS 6651, the current standard permits the use of conductors (whether they be fortuitous metalwork or dedicated LP conductors) under the roof. Vertical air rods (finials) or strike plates should be mounted above the roof and connected to the conductor system beneath. The air rods should be spaced not more than 10 m apart and if strike plates are used as an alternative, these should be strategically placed over the roof area not more than 5 m apart. 3.5 Non-conventional air termination systems A lot of technical (and commercial) debate has raged over the years regarding the validity of the claims made by the proponents of such systems. This topic was discussed extensively within the technical working groups that compiled BS EN/IEC 62305. The outcome was to remain with the information housed within this standard. BS EN/IEC 62305 states unequivocally that the volume or zone of protection afforded by the air termination system (e.g. air rod) shall be determined only by the real physical dimension of the air termination system. IRISET 18 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 This statement is reinforced within the 2011 version of BS EN 62305, by being incorporated in the body of the standard, rather than forming part of an Annex (Annex A of BS EN/IEC 62305- 3:2006). Typically if the air rod is 5 m tall then the only claim for the zone of protection afforded by this air rod would be based on 5 m and the relevant class of LPS and not any enhanced dimension claimed by some non-conventional air rods. There is no other standard being contemplated to run in parallel with this standard BS EN/IEC 62305. 3.5.1 Natural components When metallic roofs are being considered as a natural air termination arrangement, then BS 6651 gave guidance on the minimum thickness and type of material under consideration. BS EN/IEC 62305-3 gives similar guidance as well as additional information if the roof has to be considered puncture proof from a lightning discharge (see Table). Class of LPS Material Thickness Thickness (1) in mm (2) in mm Lead - 2.0 Steel (stainless, galvanised) 4 0.5 Titanium 4 0.5 I to IV Copper 5 0.5 Aluminium 7 0.65 Zinc - 0.7 Minimum thickness of metal sheets or metal pipes in air termination systems (1) Thickness ‘t’ prevents puncture, hot spot or ignition. (2) Thickness ‘t’ only for metal sheets if it is not important to prevent puncture, hot spot or ignition problems. 3.5.2 Down conductors: Down conductors should within the bounds of practical constraints take the most direct route from the air termination system to the earth termination system. The greater the number of down conductors the better the lightning current is shared between them. This is enhanced further by equipotential bonding to the conductive parts of the structure. Lateral connections sometimes referred to as coronal bands or ring conductors provided either by fortuitous metal work or external conductors at regular intervals are also encouraged. The down conductor spacing should correspond with the relevant class of LPS (see Table below). Class of LPS Typical distances (m) I 10 II 10 III 15 IV 20 Typical values of the distance between down conductors according to the Class of LPS IRISET 19 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 There should always be a minimum of two down conductors distributed around the perimeter of the structure. Down conductors should wherever possible be installed at each exposed corner of the structure as research has shown these to carry the major part of the lightning current. 3.5.3 Natural components: BS EN/IEC 62305, like BS 6651, encourages the use of fortuitous metal parts on or within the structure to be incorporated into the LPS. Where BS 6651 encouraged an electrical continuity when using reinforcing bars located in concrete structures, so too does BS EN/IEC 62305-3. Additionally, it states that reinforcing bars are welded, clamped with suitable connection components or overlapped a minimum of 20 times the rebar diameter. This is to ensure that those reinforcing bars likely to carry lightning currents have secure connections from one length to the next. When internal reinforcing bars are required to be connected to external down conductors or earthing network either of the arrangements shown in Figure 20 is suitable. If the connection from the bonding conductor to the rebar is to be encased in concrete, then the standard recommends that two clamps are used, one connected to one length of rebar and the other to a different length of rebar. The joints should then be encased by a moisture inhibiting compound such as Denso tape. If the reinforcing bars (or structural steel frames) are to be used as down conductors then electrical continuity should be ascertained from the air termination system to the earthing system. For new build structures this can be decided at the early construction stage by using dedicated reinforcing bars or alternatively to run a dedicated copper conductor from the top of the structure to the foundation prior to the pouring of the concrete. This dedicated copper conductor should be bonded to the adjoining/adjacent reinforcing bars periodically. If there is doubt as to the route and continuity of the reinforcing bars within existing structures then an external down conductor system should be installed. These should ideally be bonded into the reinforcing network of the structures at the top and bottom of the structure. Fig. 3.8 Typical methods of bonding to steel reinforcement within concrete IRISET 20 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 3.6 Earth termination system: The earth termination system is vital for the dispersion of lightning current safely and effectively into the ground. In line with BS 6651, the new standard recommends a single integrated earth termination system for a structure, combining lightning protection, power and telecommunication systems. The agreement of the operating authority or owner of the relevant systems should be obtained prior to any bonding taking place. A good earth connection should possess the following characteristics: 1. Low electrical resistance between the electrode and the earth. The lower the earth electrode resistance the more likely the lightning current will choose to flow down that path in preference to any other, allowing the current to be conducted safely to and dissipated in the earth 2. Good corrosion resistance. The choice of material for the earth electrode and its connections is of vital importance. It will be buried in soil for many years so has to be totally dependable The standard advocates a low earthing resistance requirement and points out that it can be achieved with an overall earth termination system of 10 ohms or less. Three basic earth electrode arrangements are used. a) Type A arrangement b) Type B arrangement c) Foundation earth electrodes a) Type A arrangement: This consists of horizontal or vertical earth electrodes, connected to each down conductor fixed on the outside of the structure. This is in essence the earthing system used in BS 6651, where each down conductor has an earth electrode (rod) connected to it. b) Type B arrangement: This arrangement is essentially a fully connected ring earth electrode that is sited around the periphery of the structure and is in contact with the surrounding soil for a minimum of 80% of its total length (i.e. 20% of its overall length may be housed in say the basement of the structure and not in direct contact with the earth). c) Foundation earth electrodes: This is essentially a type B earthing arrangement. It comprises conductors that are installed in the concrete foundation of the structure. If any additional lengths of electrodes are required they need to meet the same criteria as those for type B arrangement. Foundation earth electrodes can be used to augment the steel reinforcing foundation mesh. IRISET 21 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 Fig. 3.9 A sample of high quality earthing components 3.6.1 Separation (isolation) distance of the external LPS A separation distance (i.e. the electrical insulation) between the external LPS and the structural metal parts is essentially required. This will minimize any chance of partial lightning current being introduced internally in the structure. This can be achieved by placing lightning conductors sufficiently far away from any conductive parts that have routes leading into the structure. So, if the lightning discharge strikes the lightning conductor, it cannot `bridge the gap’ and flash over to the adjacent metalwork. BS EN/IEC 62305 recommends a single integrated earth termination system for a structure, combining lightning protection, power and telecommunication systems. 3.6.2 Internal LPS design considerations: The fundamental role of the internal LPS is to ensure the avoidance of dangerous sparking occurring within the structure to be protected. This could be due, following a lightning discharge, to lightning current flowing in the external LPS or indeed other conductive parts of the structure and attempting to flash or spark over to internal metallic installations. Carrying out appropriate equipotential bonding measures or ensuring there is a sufficient electrical insulation distance between the metallic parts can avoid dangerous sparking between different metallic parts. 3.6.3 Lightning equipotential bonding: Equipotential bonding is simply the electrical interconnection of all appropriate metallic installations/parts, such that in the event of lightning currents flowing, no metallic part is at a different voltage potential with respect to one another. If the metallic parts are essentially at the same potential then the risk of sparking or flashover is nullified. This electrical interconnection can be achieved by natural/fortuitous bonding or by using specific bonding conductors that are sized according to Tables of BS EN/IEC 62305-3. Bonding can also be accomplished by the use of surge protective devices (SPDs) where the direct connection with bonding conductors is not suitable. Figure (which is based on BS EN/IEC 62305-3 fig E.43) shows a typical example of an equipotential bonding arrangement. The gas, water and central heating system are all bonded IRISET 22 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 directly to the equipotential bonding bar located inside but close to an outer wall near ground level. The power cable is bonded via a suitable SPD, upstream from the electric meter, to the equipotential bonding bar. This bonding bar should be located close to the main distribution board (MDB) and also closely connected to the earth termination system with short length conductors. In larger or extended structures several bonding bars may be required but they should all be interconnected with each other. The screen of any antenna cable along with any shielded power supply to electronic appliances being routed into the structure should also be bonded at the equipotential bar. Further guidance relating to equipotential bonding, meshed interconnection earthing systems and SPD selection can be found in the Furse guidebook. Fig. 3.10: Example of main equipotential bonding 3.6.4 BS EN/IEC 62305-4 Electrical and electronic systems within structures: Electronic systems now pervade almost every aspect of our lives, from the work environment, through filling the car with petrol and even shopping at the local supermarket. As a society, we are now heavily reliant on the continuous and efficient running of such systems. The use of computers, electronic process controls and telecommunications has exploded during the last two decades. Not only are there more systems in existence, the physical size of the electronics involved has reduced considerably (smaller size means less energy required to damage circuits). BS EN/IEC 62305 accepts that we now live in the electronic age, making LEMP (Lightning Electromagnetic Impulse) protection for electronic and electrical systems integral to the standard through part 4. LEMP is the term given to the overall electromagnetic effects of lightning, including conducted surges (transient overvoltages and currents) and radiated electromagnetic field effects. LEMP damage is so prevalent that it is identified as one of the specific types (D3) to be protected against and that LEMP damage can occur from ALL strike points to the structure or connected services - direct or indirect. This extended approach also takes into account the danger of fire or explosion associated with services connected to the structure, e.g. power, telecoms and other metallic lines. IRISET 23 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 Lightning is not the only threat… Transient over voltages caused by electrical switching events are very common and can be a source of considerable interference. Current flowing through a conductor creates a magnetic field in which energy is stored. When the current is interrupted or switched off, the energy in the magnetic field is suddenly released. In an attempt to dissipate itself it becomes a high voltage transient. The more stored energy, the larger the resulting transient. Higher currents and longer lengths of conductor both contribute to more energy stored and also released! This is why inductive loads such as motors, transformers and electrical drives are all common causes of switching transients. Fig. 3.11 Motors create switching events Significance of BS EN/IEC 62305-4 Previously transient overvoltage or surge protection was included as an advisory annex in the BS 6651 standard, with a separate risk assessment. As a result protection was often fitted after equipment damage was suffered, often through obligation to insurance companies. However, the single risk assessment in BS EN/IEC 62305 dictates whether structural and/or LEMP protection is required hence structural lightning protection cannot now be considered in isolation from transient overvoltage protection - known as Surge Protective Devices (SPDs) within this new standard. This in itself is a significant deviation from that of BS 6651. Indeed, as per BS EN/IEC 62305-3, an LPS system can no longer be fitted without lightning current or equipotential bonding SPDs to incoming metallic services that have “live cores” - such as power and telecoms cables - which cannot be directly bonded to earth. Such SPDs are required to protect against the risk of loss of human life by preventing dangerous sparking that could present fire or electric shock hazards. Lightning current or equipotential bonding SPDs are also used on overhead service lines feeding the structure that are at risk from a direct strike. However, the use of these SPDs alone “provides no effective protection against failure of sensitive electrical or electronic systems”, to quote BS EN/IEC 62305 part 4, which is specifically dedicated to the protection of electrical and electronic systems within structures. Lightning current SPDs form one part of a coordinated set of SPDs that include overvoltage SPDs - which are needed in total to effectively protect sensitive electrical and electronic systems from both lightning and switching transients. IRISET 24 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 3.6.5 Lightning Protection Zones (LPZs): Whilst BS 6651 recognised a concept of zoning in Annex C (Location Categories A, B and C), BS EN/IEC 62305-4 defines the concept of Lightning Protection Zones (LPZs). Figure 12 illustrates the basic LPZ concept defined by protection measures against LEMP as detailed within part 4. SPD 0/1 - Lightning Current Protection SPD 1/2 - Overvoltage Protection Connected service directly bonded Fig. 3.12 Basic LPZ concept - BS EN/IEC 62305-4 Within a structure a series of LPZs are created to have, or identified as already having, successively less exposure to the effects of lightning. Successive zones use a combination of bonding, shielding and coordinated SPDs to achieve a significant reduction in LEMP severity, from conducted surge currents and transient overvoltages, as well as radiated magnetic field effects. Designers coordinate these levels so that the more sensitive equipment is sited in the more protected zones. The LPZs can be split into two categories - 2 external zones (LPZ 0A, LPZ 0B) and usually 2 internal zones (LPZ 1, 2) although further zones can be introduced for a further reduction of the electromagnetic field and lightning current if required. 3.7 External zones LPZ 0A is the area subject to direct lightning strokes and therefore may have to carry up to the full lightning current. This is typically the roof area of a structure. The full electromagnetic field occurs here. LPZ 0B is the area not subject to direct lightning strokes and is typically the sidewalls of a structure. However the full electromagnetic field still occurs here and conducted partial lightning currents and switching surges can occur here. 3.8 Internal zones LPZ 1 is the internal area that is subject to partial lightning currents. The conducted lightning currents and/or switching surges are reduced compared with the external zones LPZ 0A, LPZ 0B. IRISET 25 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 This is typically the area where services enter the structure or where the main power switchboard is located. LPZ 2 is an internal area that is further located inside the structure where the remnants of lightning impulse currents and/or switching surges are reduced compared with LPZ 1. This is typically a screened room or, for mains power, at the sub-distribution board area. Protection levels within a zone must be coordinated with the immunity characteristics of the equipment to be protected, i.e., the more sensitive the equipment, the more protected the zone required. The existing fabric and layout of a building may make readily apparent zones, or LPZ techniques may have to be applied to create the required zones. 3.9 Surge Protection Measures (SPM) Some areas of a structure, such as a screened room, are naturally better protected from lightning than others and it is possible to extend the more protected zones by careful design of the LPS, earth bonding of metallic services such as water and gas, and cabling techniques. However it is the correct installation of coordinated Surge Protective Devices (SPDs) that protect equipment from damage as well as ensuring continuity of its operation - critical for eliminating downtime. These measures in total are referred to as Surge Protection Measures (SPM) (formerly LEMP Protection Measures System (LPMS)). When applying bonding, shielding and SPDs, technical excellence must be balanced with economic necessity. For new builds, bonding and screening measures can be integrally designed to form part of the complete SPM. However, for an existing structure, retrofitting a set of coordinated SPDs is likely to be the easiest and most cost-effective solution. 3.10 Coordinated SPDs BS EN/IEC 62305-4 emphasizes the use of coordinated SPDs for the protection of equipment within their environment. This simply means a series of SPDs whose locations and LEMP handling attributes are coordinated in such a way as to protect the equipment in their environment by reducing the LEMP effects to a safe level. So there may be a heavy duty lightning current SPD at the service entrance to handle the majority of the surge energy (partial lightning current from an LPS and/or overhead lines) with the respective transient overvoltage controlled to safe levels by coordinated plus downstream overvoltage SPDs to protect terminal equipment including potential damage by switching sources, e.g. large inductive motors. Appropriate SPDs should be fitted wherever services cross from one LPZ to another. Coordinated SPDs have to effectively operate together as a cascaded system to protect equipment in their environment. For example the lightning current SPD at the service entrance should handle the majority of surge energy, sufficiently relieving the downstream overvoltage SPDs to control the overvoltage. Poor coordination could mean that the overvoltage SPDs are subject to too much surge energy putting both itself and potentially equipment at risk from damage. Furthermore, voltage protection levels or let-through voltages of installed SPDs must be coordinated with the insulating withstand voltage of the parts of the installation and the immunity withstand voltage of electronic equipment. Note: Appropriate SPDs should be fitted wherever services cross from one LPZ to another. IRISET 26 TC5 - Earthing and Surge Protection Devices Surge Protection Standard IEC 62305 3.11 Enhanced SPDs Whilst outright damage to equipment is not desirable, the need to minimize downtime as a result of loss of operation or malfunction of equipment can also be critical. This is particularly important for industries that serve the public, be they hospitals, financial institutions, manufacturing plants or commercial businesses, where the inability to provide their service due to the loss of operation of equipment would result in significant health and safety and/or financial consequences. Standard SPDs may only protect against common mode surges (between live conductors and earth), providing effective protection against outright damage but not against downtime due to system disruption. BS EN 62305 therefore considers the use of enhanced SPDs (SPD*) that further reduce the risk of damage and malfunction to critical equipment where continuous operation is required. Installers will therefore need to be much more aware of the application and installation requirements of SPDs than perhaps they may have been previously. Superior or enhanced SPDs provide lower (better) let-through voltage protection against surges in both common mode and differential mode (between live conductors) and therefore also provide additional protection over bonding and shielding measures. Such enhanced SPDs can even offer up to mains Type 1+2+3 or data/telecom Test Cat D+C+B protection within one unit. As terminal equipment, e.g. computers, tends to be more vulnerable to differential mode surges, this additional protection can be a vital consideration. Furthermore, the capacity to protect against common and differential mode surges permits equipment to remain in continued operation during surge activity - offering considerable benefit to commercial, industrial and public service organizations alike. All SPDs offer enhanced SPD performance with industry leading low let-through voltages (voltage protection level, Up), as this is the best choice to achieve cost-effective, maintenance- free repeated protection in addition to preventing costly system downtime. Low let-through voltage protection in all common and differential modes means fewer units are required to provide protection, which saves on unit and installation costs, as well as installation time. 3.12 Conclusion Lightning poses a clear threat to a structure but a growing threat to the systems within the structure due to the increased use and reliance of electrical and electronic equipment. The BS EN/IEC 62305 series of standards clearly acknowledge this. Structural lightning protection can no longer be in isolation from transient overvoltage or surge protection of equipment. The use of enhanced SPDs provides a practical cost-effective means of protection allowing continuous operation of critical systems during LEMP activity. IRISET 27 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations CHAPTER - 4 RDSO SPECIFICATIONS FOR EARTHING SYSTEM FOR SIGNAL AND TELECOM INSTALLATIONS 4.1 Introduction RDSO Spec. RDSO/SPN/197/2008 covers the specifications of earthing system for S&T installation. These are presented in this chapter. These specifications are based on Indian Standard code of practice for earthing vide IS 3043- 1987. 4.2 The Objective of Earthing System The main aim of earthing is to maintain a zero potential or zero voltage of all non-current carrying parts of electrical system which has the probability of electrified by some fault. If by some means the non-current carrying parts are electrified which is not earthed, the person touching it will receive a lethal shock. Also sometimes earth is used for carrying current from one place to another in a circuit such as telegraph systems etc. Thus the purpose of the earth may be one or more of the following: 1. To afford safety to personnel against shock by earthing the casing or other exposed parts. 2. To provide a return path as, for example, in block instruments, unbalanced HF serial circuits, unbalanced three phase power supply system, etc. 3. To protect equipment against build up of unduly high voltages by earthing protective devices like surge arrestors and lightning dischargers. 4. To ensure safe and reliable operation of equipment by eliminating/limiting voltage and currents due to EMI and RFI by earthing of metallic sheathing and armoring of cables. 5. To provide path for heavy fault currents to ensure effective and quick operation of protective devices, as in power supply induced systems. 4.2.1 System Earthing & Equipment Earthing Earthing associated with current-carrying conductor is normally essential to the security of the system and is generally known as system earthing, while earthing of non-current carrying metal work and conductor is essential to the safety of human life, animals and property, and is generally known as equipment earthing. 4.3 TERMINOLOGY For the purpose of this document, the following definitions shall apply. 1. Bonding Conductor - A protective conductor providing equipotential bonding. 2. Earth - The conductive mass of the earth, whose electric potential at any point is conventionally taken as zero. 3. Earth Electrode - A conductor or group of conductors in intimate contact with and providing an electrical connection to earth. 4. Earthing Conductor - A protective conductor connecting the main earthing terminal (or the equipotential bonding conductor of an installation when there is no earth bus) to an earth electrode or to other means of earthing. IRISET 28 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations 5. Earth Grid - A system of grounding electrodes consisting of inter-connected conductors buried in the earth to provide a common ground for electrical devices and metallic structures. (Note: The term ‘earth grid’ does not include earth mat’). 6. Earth Mat - A grounding system formed by a grid of horizontally buried conductors and which serves to dissipate the fault current to earth and also as an equipotential bonding conductor system. 7. Electrically Independent Earth Electrodes - Earth electrodes located at such a distance from one another so that the maximum current likely to flow through one of them does not significantly affect the potential of the other(s). 8. Equipotential Bonding - Electrical connection keeping various exposed conductive parts and extraneous conductive parts at a substantially equal potential. 9. Equipotential Line or Contour - The locus of points having the same potential at a given time. 10. Exposed conductive part - A conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions. 11. Functional Earthing - Connection to earth necessary for proper functioning of electrical equipment. 12. Main Earthing Terminal - The terminal or bar (which is the equipotential bonding conductor) provided for the connection of protective conductors and the conductors of functional earthing, if any, to the means of earthing. 13. Mutual Resistance of Grounding Electrodes - Equal to the voltage change in one of them produced by a change of one ampere of direct current in the other and is expressed in ohms. 14. Potential Gradient (at a point) - The potential difference per unit length measured in the direction in which it is maximum. 15. Protective Conductor - A conductor used as a measure of protection against electric shock and intended for connecting any of the following parts:- a. Exposed conductive parts, b. Extraneous conductive parts, c. Main earthing terminal, and d. Earthed point of the source or an artificial neutral. 16. Resistance Area (For an Earth Electrode only) - The surface area of ground (around an earth electrode) on which a significant voltage gradient may exist. 17. Step Voltage - The potential difference between two points on the earth’s surface, separated by a distance of one pace that will be assumed to be one metre in the direction of maximum potential gradient. 18. Touch Voltage - The potential difference between grounded metallic structure and a point on the earth’s surface separated by a distance equal to the normal maximum horizontal reach, approximately one meter. IRISET 29 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations 4.4 Soil Resistivity: Soil resistivity can be defined as the resistance of a cube of soil of 1 m size measured between any two opposite faces. The unit in which it is usually expressed is ohmmeter. 4.4.1 Effect of nature of soil on Soil Resistivity The resistivity of soil depends upon the moisture content, chemical composition of the soil and concentration of salts dissolved in the contained moisture. Grain size, mode of distribution and closeness of packing also affect the resistivity as these factors control the manner in which the moisture is held in soil. Many of these factors vary locally and some seasonally, and as such soil resistivity varies not only from location to location but also from season to season. Besides, the areas where the soil is stratified, the effective resistivity also depends upon the underlying geological formation. 4.4.2 Effect of moisture content on Soil Resistivity Moisture content is one of the controlling factors in earth resistivity. Above about 20 percent moisture, the resistivity is very little affected; while below 20 percent the resistivity increases very abruptly with decrease in the moisture content. A difference of a few percent moisture will therefore, make a very marked difference in the effectiveness of earth connection if the moisture content falls below 20 percent. The normal moisture content of soil ranges from 10 percent in dry seasons to 35 percent in wet seasons, and an approximate average may be perhaps 16 to 18 percent. 4.4.3 Effect of Temperature on Soil Resistivity Temperature also affects the resistivity of the soil. However, it is of consequence only around and below the freezing point, which means that earth electrodes should be installed at depths where frost cannot penetrate. The temperature coefficient of resistivity for soil is negative, but is negligible for temperatures above freezing point. At about 200C, the resistivity change is about 9 percent per degree Celsius. Below 00C the water in the soil begins to freeze and introduces a tremendous increase in the temperature coefficient, so that as the temperature becomes lower the resistivity rises enormously. It is, therefore, recommended that in areas where the temperature is expected to be quite low, the earth electrodes should be installed well below the frost line. Where winter seasons are severe, this may be about 2 meters below the surface, whereas in mild climates the frost may penetrate only a few centimeters or perhaps the ground may not freeze at all. Earth electrodes which are not driven below the frost depth may have a very great variation in resistance throughout the seasons of the year. Even when driven below the frost line, there is some variation, because the upper soil, when frozen, presents a decided increase in soil resistivity and has the effect of shortening the active length of electrode in contact with soil of normal resistivity. 4.5 Location of Earth Pit: Where there is an option, site should be chosen in one of the following types of soil in the order of preference given:- a) Wet marshy ground; b) Clay, loamy soil, arable land, clayey soil or loam mixed with small quantities of sand; c) Clay and loam mixed with varying proportions of sand, gravel and stones; d) Damp and wet sand, peat. IRISET 30 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations Dry sand, gravel chalk, limestone, granite and very stony ground should be avoided, and also all locations where virgin rock is very close to the surface. A site should be chosen that is not naturally well-drained. A water-logged situation is not, however, essential, unless the soil is sand or gravel, as in general no advantage results from an increase in moisture content above about 15 to 20 percent. Care should be taken to avoid a site kept moist by water flowing over it (for example, the bed of stream) as the beneficial salts may be entirely removed from the soil in such situations. 4.6 Earth Electrodes: 4.6.1 Effect of shape on Electrode Resistance: With all electrodes other than extended systems, the greater part of the fall in potential occurs in the soil within a few feet of the electrode surface, since the current density is highest. To obtain a low overall resistance, the current density should be as low as possible in the medium adjacent to the electrode, which should be so designed as to cause the current density to decrease rapidly with distance from the electrode. This requirement is met by making the dimensions in one direction large compared with those in the other two, thus a pipe, rod or strip has a much lower resistance than a plate of equal surface area. The resistance is not, however, inversely proportional to the surface area of the electrode. 4.6.2 Resistance of common types of Earth Electrodes: Plate Electrode: In case of Plate electrode, electrode to earth resistance is given by: R = ρ/2πL [ln(8L/T) + ln(L/h)-2+(2h/L)-(h/L)2] L is length of electrode, h is the depth of laying and T is thickness As can be seen, L has major influence, T has minor influence. For several years, electrical substations were being provided with earthing plates of large area, which could not solve problems of high ‘electrode to earth’ resistance. Finally, having understood the implications of the above formula, electrical engineers started providing electrode-grids for the sub-stations. Strip or Conductor Electrodes: These have special advantages where high resistivity soil underlies shallow surface layers of low resistivity. The resistance R is given by: R= [(100/2 п l) loge (2l2 / w t)] Ohms Where ρ = resistivity of the soil (in Ω.m) (assumed uniform) l = length of the strip in cm; w = depth of burial of the electrode in cm; and t = width (in the case of strip) or twice the diameter (for conductors) in cm. Care should be taken in positioning these electrodes, especially to avoid damage by agricultural operations. Earth resistance decreases first sharply and then after slowly with increase in electrode length. The effect of conductor size and depth over the range normally used is very small. IRISET 31 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations If several strip electrodes are required for connection in parallel in order to reduce the resistance, they may be installed in parallel lines or they may radiate from a point. In the former case, the resistance of two strips at a separation of 2.4m is less than 65 percent of the individual resistance of either of them. 4.6.3 Selection of metals for Earth-Electrodes: Although electrode material does not affect initial earth resistance, care should be taken to select a material that is resistant to corrosion in the type of soil in which it will be used. Tests in a wide variety of soils have shown that copper, whether tinned or not, is entirely satisfactory (subject to the precautions given in this sub clause), the average loss in weight of specimens 150mm x 25mm x 3mm buried for 12 years in no case exceed 0.2 percent per year. Corresponding average losses for unprotected ferrous specimens (for example, cast iron, wrought iron or mild steel) used in the tests were as high as 2.2 percent per year. Considerable and apparently permanent protection appears to be given to mild steel by galvanizing. The test showing galvanized mild steel to be little inferior to copper with an average loss not greater than 0.5 percent per year. Only in a few cases, there was any indication in all these tests that corrosion was accelerating and in these cases the indications were not very significant. 4.6.4 Current Density at the surface of an Earth Electrode: An earth electrode should be designed to have a loading capacity adequate for the system of which it forms a part, i.e, it should be capable of dissipating the energy in the earth path at the point at which it is installed under any condition of operation on the system without failure. Failure is fundamentally due to excessive temperature rise at the surface of the electrode and is thus a function of current density and duration as well as electrical and thermal properties of the soil. In general, soils have a negative temperature coefficient of resistance, so that sustained current loading results in an initial decrease in electrode resistance and consequent rise in the earth fault current for a given applied voltage. As soil moisture is driven away from the soil-electrode interface, however, the resistance increases and will ultimately become infinite if the temperature-rise is sufficient. Three conditions of operation require consideration, that is, long-duration loading as with normal system operation; short-time overloading under fault conditions in directly earthed system, and long-time over loading under fault conditions in systems protected by arc-suppression coils. The little experimental work which has been done on this subject by experts at the international level has been confined to model tests with spherical electrodes in clay or loam of low resistivity and has led to the following conclusions: a) Long-duration loading due to normal unbalance of the system will not cause failure of earth- electrodes, provided that the current density at the electrode surface does not exceed 40A/m2. Limitation to values below this would generally be imposed by the necessity to secure a low-resistance earth. b) Time to failure on short-time overload is inversely proportional to the specific loading, which is given by i2, where i is the current density at the electrode surface. For the soils investigated, the maximum permissible current density, i is given by i = 7.57 x 103 / √ (ρ t) A/m2 IRISET 32 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations Where t = duration of the earth fault (in s) and ρ = resistivity of the soil (in Ω.m) Experience indicates that this formula is appropriate for plate electrodes. 4.6.5 Cross Sectional Area Of Protective Conductor: The cross sectional area of every protective conductor which does not form part of the supply cable or cable enclosure shall be, in any case, not less than: a) 2.5 mm2, if mechanical protection is provided and b) 4 mm2, if mechanical protection is not provided. 4.7 Measurement of Earth Resistance: Earth resistance is measured using Earth tester. Earth tester is a four terminal instrument with a voltage source and a meter to show resistance value in ohms. It gives soil resistance and electrode resistance but earth electrode resistance can be neglected as it is negligible. It also consists of accessories like spikes or rods, connecting wires, etc. Fig. 4.1. Earth testers 4.7.1 Practical methods of measuring earth resistance: Following are the methods used in measuring earth resistance Fall of potential method which is a three terminal method Dead earth method which is a two terminal method Clamp on test method 4.7.2 Principle of earth testing: Earth resistivity is measured using four terminal instruments. Four small sized electrodes are driven to the same depth and equal distance apart in a straight line as in Fig 4.2. The terminals C1 & C2 are called current reference electrodes, P1 & P2 are called potential reference electrodes. Four separate lead wires connect the electrodes to the four terminals of the instrument. Hence, the name, four terminal method. IRISET 33 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations Fig. 4.2. Principle of Earth testing 4.7.3 Fall of Potential (3-Terminal) method of measuring Earth Resistance: In the three terminal method, C1 and P1 are shorted. If the distance between actual earth electrode (C1) and current reference electrode C2 is 100 feet then the distance between C1 and P2 should be 62 feet. The resistance ‘R’ is given by R=V / I, V is the reading of voltmeter in volts and I is the reading of the ammeter in amperes. Fig. 4.3. Fall of potential or three terminal method Fig. 4.4. Three terminal method of earth resistance method IRISET 34 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations 4.8 Sphere of influence of earth electrode: The volume of earth mass surrounding the earth electrode in which spreading of electrical charges takes place is called the sphere of influence and the current radiates in all directions from the earth electrode. The earth mass surrounding the earth can be imagined to be made up of shells of equal thickness. The shell closest to the electrode has the smallest area, hence it has the highest earth resistance and as the shells distance increase from the electrode, the surface area also increases with decreasing earth resistance. Fig. 4.5. Sphere of influence of earth electrode Hence care shall be taken while measuring earth resistance that placing of earth rods should be outside the influence of one another. And even while providing multiple earth pits for reducing the earth resistance value, the same rule should be followed. Fig. 4.6. Wrong placing of C Fig. 4.7. Correct placing of C 4.8.1 Earth resistance with two electrodes When two earth electrodes are to be provided, then the separation distance between the electrodes must be equal to twice the length of the electrode or greater than that. Fig. 4.8. Correct placing of earth electrodes IRISET 35 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations 4.8.2 Dead earth method of measuring earth resistance: (Two terminal method) This is the simplest method of measuring earth resistance in which water pipe is used as the second terminal. In this method C1 is shorted with P1 and P2 with C2. Earth electrode is connected to C1P1 and water tap is connected to C2P2. With this method, resistance of two electrodes in series is measured - the driven rod and the water system. 4.8.3 Precautions to be taken in Dead Earth method are: 1. The water pipe system must be extensive enough to have a negligible resistance. 2. The water pipe system must be metallic throughout, without any insulating couplings or flanges. 3. The earth electrode under test must be far enough away from the water-pipe system and to be outside its sphere of influence. Fig. 4.9. Earth resistance by Dead Earth method Note – In most cases, there will be stray currents flowing in the soil and unless some steps are taken to eliminate their effect, they may produce serious errors in the measured value. If the testing current is of the same frequency as the stray current, this elimination becomes very difficult and it is better to use an earth tester incorporating a hand-driven generator. These earth testers usually generate direct current, and have rotary current-reverser and synchronous rectifier mounted on the generator shaft so that alternating current is supplied to the test circuit and the resulting potentials are rectified for measurement by a direct reading moving-coil ohm- meter. The presence of stray currents in the soil is indicated by wandering of the instrument pointer, but an increase or decrease of generator handle speed will cause this to disappear. The source of current shall be isolated from the supply by a double wound transformer. At the time of test, where possible, the test electrode shall be separated from the earthing system. IRISET 36 TC5 - Earthing and Surge Protection Devices RDSO Specifications for Earthing system for S&T Installations The auxiliary electrodes usually consist of 12.5mm diameter mild steel rod driven up to 1 m into the ground. All the test electrodes and the current electrodes shall be so placed that they are independent of the resistance area of each other. If the test electrode is in the form of rod, pipe or plate, the auxiliary current electrode C shall be placed at least 30m away from it and the auxiliary potential electrode B midway between them. 4.8.4 Earth resistance measurement by Clamp-on method: The fall of potential method of earth testing so far discussed is extremely reliable and highly accurate. But it has its own drawbacks and they are: It is time consuming and labour intensive. Individual ground electrodes must be disconnected from the system to be measured. There are situations where disconnection is not possible. The Clamp-on test method performs a stake less test, i.e. no earth probes (spikes) are used. This is done without disconnecting the earth conductor from the equipment. In this method a known voltage is induced in a loop circuit and measures the resultant current flow and calculate the loop resistance of the circuit. Clamp on earth testing is employed in large electrical and electronic installations, where earth resistance of large number of earth locations have to be measured with minimum labour and without consuming much time. Fig. 4.10. Clamp-on earth tester IRISET 37 TC5 - Earthing and Surge Protection Devices Code of Practice for Earthing & Bonding System for S&T Equipments CHAPTER - 5 CODE OF PRACTICE FOR EARTHING AND BONDING SYSTEM FOR S&T EQUIPMENTS (RDSO SPECIFICATION NO. RDSO/SPN/197/2008) 5.1 Scope This document covers earthing & bonding system to be adopted for signalling equipments with solid state components which are more susceptible to damage due to surges, transients and over voltages being encountered in the system due to lightning, substation switching, etc. These signalling equipments include Electronic Interlocking, Integrated Power supply equipment, Digital Axle counter, Data logger, etc. 5.2 References for Earthing & Bonding Practices IS 3043 Code of practice for earthing ANSI/UL 467 Grounding & bonding equipment IEEE 80 IEEE guide for Safety in AC substation grounding Standard for qualifying permanent connections used in IEEE 837 substation grounding IEC 62305 Protection against lightning Table 5.1 References for Earthing & Bonding Practices 5.3 Characteristics of Good Earthing System (a) Excellent Electrical Conductivity 1. Low resistance and electrical impedance. 2. Conductors of sufficient dimensions capable of withstanding high fault currents with no evidence of fusing or mechanical deterioration. 3. Lower earth resistance ensures that energy is dissipated into the ground in the safest possible manner. 4. Lower the earth circuit impedance, the more likely that high frequency lightning impulses will flow through the ground electrode path, in preference to any other path. (b) High corrosion resistance The choice of the material for grounding conductors, electrodes and connections is vital as most of the grounding system will be buried in the earth's mass for many years. Copper is the most common material used. In addition to its inherent high conductivity, copper is usually universal with respect to other metals in association with grounding sites, which means that it is less likely to corrode in most environments. (c) Mechanically Robust and Reliable. 5.4 Acceptable Earth Resistance value The acceptable Earth Resistance at earth busbar shall not be more than 1 ohm. IRISET 38 TC5 - Earthing and Surge Protection Devices Code of Practice for Earthing & Bonding System for S&T Equipments 5.5 Components of Earthing & Bonding system The components of Earthing & Bonding system are: (a) Earth electrode (b) Earth enhancement material (c) Earth pit (d) Equipotential earth busbar (e) connecting cable & tape/strip and (f) All other associated accessories. 5.6 Design of Earthing & Bonding system 5.6.1 Earth Electrode Fig. 5.1. Earth electrode (a) The earth electrode shall be made of high tensile, low carbon, steel circular rods, molecularly bonded with copper on outer surface to meet the requirements of Underwriters Laboratories (UL) 467-2007 or latest. Such copper bonded steel rod is preferred due to its overall combination of strength, corrosion resistance, low resistance path to earth and cost effectiveness. (b) The earth electrode shall be UL listed and of minimum 17.0mm diameter and minimum 3.0mtrs. long. (c) The minimum copper bonding thickness shall be of 250 microns. (d) Marking: UL marking, Manufacturer’s name or trade name, length, diameter, catalogue number must be punched on every earth electrode. (e) Earth electrode can be visually inspected, checked for dimensions and thickness of copper coating using micron gauge. The supplier shall arrange for such inspection at the time of supply, if so desired. 5.6.2 Earth Enhancement Material Fig. 5.2. Earth enhancement Material Earth enhancement material is a superior conductive material that improves earthing effectiveness, especially in areas of poor conductivity (rocky ground, areas with moisture variation, sandy soils, etc.). It improves conductivity of the earth electrode and ground contact area. It shall have the following characteristics: IRISET 39 TC5 - Earthing and Surge Protection Devices Code of Practice for Earthing & Bonding System for S&T Equipments (a) Shall mainly consist of Graphite and Portland cement. Bentonite content shall be negligible. (b) Shall have high conductivity, improves earth’s absorbing power and humidity retention capability. (c) Shall be non-corrosive in nature, having low water solubility but highly hygroscopic. (d) Shall have resistivity of less than 0.2 ohm-meters. Resistivity shall be tested by making a 20cm cube of the material and checking resistance of the cube at the ends. The supplier shall arrange for such testing at the time of supply, if so desired. Necessary certificate from National/ International lab for the resistivity shall also be submitted. (e) Shall be suitable for installation in dry form or in a slurry form. (f) Shall not depend on the continuous presence of water to maintain its conductivity. (g) Shall be permanent & maintenance free and in its “set form”, maintains constant earth resistance with time. (h) Shall be thermally stable between -100 C to +600 C ambient temperatures. (i) Shall not dissolve, decompose or leach out with time. (j) Shall not require periodic charging treatment nor replacement and maintenance. (k) Shall be suitable for any kind of electrode and all kinds of soils of different resistivity. (l) Shall not pollute the soil or local water table and meets environmental friendly requirements for landfill. (m) Shall not be explosive. (n) Shall not cause burns, irritation to eyes, skin, etc. (o) Marking: The Earth enhancement material shall be supplied in sealed, moisture proof bags. These bags shall be marked with Manufacturer’s name or trade name, quantity, etc. 5.6.3 Backfill Material The excavated soil is suitable as a backfill but should be sieved to remove any large stones and placed around the electrode taking care to ensure that it is well compacted. Material like sand, salt, coke breeze, cinders and ash shall not be used because of its acidic and corrosive nature. 5.7 Construction of Unit Earth Pit: Refer typical installation drawing no. SDO/RDSO/E&B/001. (a) A hole of 100mm to 125mm dia shall be augured /dug to a depth of about 2.8 meters. (b) The earth electrode shall be placed into this hole. (c) It will be penetrated into the soil by gently driving on the top of the rod. Here natural soil is assumed to be available at the bottom of the electrode so that min. 150 mm of the electrode shall be inserted in the natural soil. (d) Earth enhancement material (minimum approx. 30-35 kg) shall be filled into the augured/dug hole in slurry form and allowed to set. After the material gets set, the diameter of the composite structure (earth electrode + earth enhancement material) shall be of minimum 100mm dia covering the entire length of the hole. (e) Remaining portion of the hole shall be covered by backfill soil, which is taken out during auguring /digging. (f) A copper strip of 150mmX25mmX6mm shall be exothermically welded to the main earth electrode for taking the connection to the main equipotential earth busbar in the equipment room and to other earth pits, if any. (g) Exothermic weld material shall be UL listed and tested as per provisions of IEEE 837 by NABL/ ILAC member labs. (h) The main earth pit shall be located as near to the main equipotential earth busbar in the equipment room as possible. IRISET 40 TC5 - Earthing and Surge Protection Devices Code of Practice for Earthing & Bonding System for S&T Equipments 5.8 Construction of Loop Earth by Providing Multiple Earth Pits (a) At certain locations, it may not be possible to achieve earth resistance of ≤1 ohm with one earth electrode /pit due to higher soil resistivity. In such cases, provision of loop earth consisting of more than one earth pit shall be done. The number of pits required shall be decided based on the resistance achieved for the earth pits already installed. The procedure mentioned above for one earth pit shall be repeated for other earth pits. (b) The distance between two successive earth electrodes shall be min. 3mtrs. and max. upto twice the length of the earth electrode i.e. 6 mtrs. approx. (c) These earth pits shall then be inter linked using 25X2 mm. copper tape to form a loop using exothermic welding technique. (d) The interconnecting tape shall be buried at a depth not less than 500mm below the ground level. This interconnecting tape shall also be covered with earth enhancing compound. Fig. 5.3. Exothermic welded earth terminal 5.9 Measurement of Earth Resistance The earth resistance shall be measured at the Main Equipotential Earth Busbar (MEEB) with all the earth pits interconnected using Fall of Potential method as per para 37 of IS: 3043 shown below. 37. MEASUREMENT OF EARTH ELECTRODE RESISTANCE (As per IS: 3043 – 1987) 37.1 Fall of Potential Method - In this method two auxiliary earth electrodes, besides the test electrode, are placed at suitable distances from the test electrode. A measured current is passed between the electrode A to be tested and an auxiliary current electrode C and the potential difference between the electrode A and the auxiliary potential electrode B is measured. The resistance of the test electrode A is then given by: R = V/I, where R = resistance of the test electrode in ohms, V = reading of the voltmeter in volts, and I = reading of the ammeter in amperes. IRISET 41 TC5 - Earthing and Surge Protection Devices Code of Practice for Earthing & Bonding System for S&T Equipments 37.1.1 If the test is made at power frequency, that is, 50 c/s, the resistance of the voltmeter should be high compared to that of the auxiliary potential electrode B and in no case

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