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ICS 29.240.20 ISBN 0-626-15488-X SOUTH AFRICAN NATIONAL STANDARD The selection, handling and installation of electric power cables of rating not exceeding 33 kv Part 3: Earthing systems General provisions Published by Standards South Africa 1 dr lategan road groenkloof private bag x191 pretoria 0001 tel: 012 428 7911 fax: 012 344 1568 international code + 27 12 www.stansa.co.za Standards South Africa 2004
Table of changes Change No. Date Scope Abstract Covers general provisions for the earthing of electric power cables and the apparatus in which the cables are terminated. Keywords earth conductors, earthing, electric cables, electric connectors, electrical protection equipment, installation, power cables. Acknowledgement Standards South Africa wishes to acknowledge the valuable assistance received from the Association of Electric Cable Manufacturers of South Africa.
Foreword This South African standard was approved by National Committee StanSA TC 66, Electric cables, in accordance with procedures of Standards South Africa, in compliance with annex 3 of the WTO/TBT agreement. This edition cancels and replaces the first edition (SABS 0198-3:1988). SANS 10198 consists of the following parts, under the general title The selection, handling and installation of electric power cables of rating not exceeding 33 kv: Part 1: Definitions and statutory requirements. Part 2: Selection of cable type and methods of installation. Part 3: Earthing systems - General provisions. Part 4: Current ratings. Part 5: Determination of thermal and electrical resistivity of soil. Part 6: Transportation and storage. Part 7: Safety precautions. Part 8: Cable laying and installation. Part 9: Jointing and termination of extruded solid dielectric-insulated cables up to 3,3 kv. Part 10: Jointing and termination of paper-insulated cables. Part 11: Jointing and termination of screened polymeric-insulated cables. Part 12: Installation of earthing system. Part 13: Testing, commissioning and fault location. Part 14: Installation of aerial bundled conductor (ABC) cables. NOTE The first five parts deal with factors to be taken into account when an electrical distribution system is being designed. The last nine parts deal with the practical aspects of handling and installing cables. Annex A is for information only. 1
Contents Page Abstract Keywords Acknowledgement Foreword...... 1 1 Scope...... 3 2 Normative references...... 3 3 Definitions...... 3 4 Earthing of transformer and switchgear cables...... 3 4.1 Distribution transformer cables...... 3 4.2 Earth fault paths...... 4 4.3 Earthing of 11/3,3 kv transformer cables...... 4 4.4 Earthing of 11/0,4 kv transformer cables...... 4 4.5 Earth connections...... 4 4.6 Resistance of cable glands, earthing leads, etc...... 5 Annex A (informative) Additional information on earth connections...... 9 Bibliography...... 11 2
The selection, handling and installation of electric power cables of rating not exceeding 33 kv Part 3: Earthing systems General provisions 1 Scope This part of SANS 10198 covers general provisions for the earthing of electric power cables and the apparatus in which the cables are terminated. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this part of SANS 10198. All standards are subject to revision and, since any reference to a standard is deemed to be a reference to the latest edition of that standard, parties to agreements based on this part of SANS 10198 are encouraged to take steps to ensure the use of the most recent editions of the standards indicated below. Information on currently valid national and international standards can be obtained from Standards South Africa. SANS 10198-1, The selection, handling and installation of electric power cables of rating not exceeding 33 kv - Part 1: Definitions and statutory requirements. SANS 10200 (SABS 0200), Neutral earthing in medium voltage industrial power systems. 3 Definitions For the purposes of this part of SANS 10198 the definitions given in SANS 10198-1 apply. 4 Earthing of transformer and switchgear cables 4.1 Distribution transformer cables A distribution transformer is normally connected in delta-star with the star winding supplying the load. The neutral point of the star winding is then earthed either directly or through a low impedance, as given in SANS 10200. Where a distribution transformer is so connected that no neutral is available (normally a transformer connected in star-delta with the delta winding supplying the load), an artificial neutral point is created as given in SANS 10200. 3
An earthing terminal is provided on the transformer tank. Separate earth electrodes are not normally required for switchboards, distribution boards or motor control centres, provided that an efficient earth connection can be achieved via the sheath or armour (or both) of the supply cable or via an earth continuity conductor. If the armour of a cable is used to provide earth continuity, ensure that effective contact is maintained between the armour and the earthing points. 4.2 Earth fault paths Ensure that the cable provides a path for earth fault currents to return to the neutral point of the supply transformer. Consider a typical industrial distribution system as shown schematically in figure 1. An earth fault, for example at switchboard "B", on the 11 kv winding of transformer "C" or transformer "G", or on any of the cables connecting "A" to "B", "B" to "C" or "B" to "G", shall return to the neutral point of transformer "A". An earth fault on the low voltage side of transformer "C" or transformer "G" will appear as an overcurrent on the high voltage side of the transformer. Parallel paths exist for the earth fault currents, one via the sheath wires or armour wires, or both, of the supply cables, the other through the main mass of earth. The earth fault currents often follow a combination of the two paths. An earth fault on the cable connecting switchboard "B" to transformer "G" will return to the neutral point of transformer "A" via a) the sheath/armour of cable "BG" to switchboard "B", the main earth bar of "B", and the sheath/armour of cable "AB", b) the sheath/armour of cable "BG" to transformer "G", the earth connection of "G", and through the main mass of earth to the earth connection of "A", and c) the earth or structural steelwork at the fault and the main mass of earth to the earth connection of "A". 4.3 Earthing of 11/3,3 kv transformer cables In the case of a transformer such as "C" in figure 1, which supplies only high voltage motors, neutral is frequently earthed via a low value impedance, usually a resistor. This limits the earth fault current and the voltage rise above earth at the fault position. Intermediate switchboards and motor control centres are earthed via the sheath/armour wires of the supply cables or via a separate earth continuity conductor, or both. It is good practice to provide an additional connection to structural steelwork at each high voltage motor by means of either a copper strip or an insulated lead. 4.4 Earthing of 11/0,4 kv transformer cables Ensure that a transformer such as "G" in the example, which supplies both a low voltage motor and a single-phase and a three-phase distribution system requiring a neutral connection, has its neutral solidly earthed as close as possible to the transformer terminals. NOTE Where an LV cable box is provided on a transformer, a separate neutral/earth cable gland is usually included. Subdistribution boards, motor control centres and LV motors usually rely on the sheath or armour (or both) of supply cables or separate earth continuity conductors for their earth connections. 4.5 Earth connections 4.5.1 General In a situation as shown in figure 2, make the following earth connections described in 4.5.2 to 4.5.6 inclusive. These are normally the responsibility of the team installing, jointing and terminating the main cables. 4
Make earth connections to structures and equipment at points specifically provided for this purpose. Do not make earth connections to bolts or clamps designed for mechanical support because these might be removed temporarily or permanently (for example during maintenance work or alterations to the structure). 4.5.2 Cable glands to substation earth bar To avoid having to break connections should equipment be changed at a later date, make the earth connections directly to the substation earth bar and not to equipment such as transformers. Ensure that the connections are as straight as possible in order to keep the inductance of the earth path to a minimum. 4.5.3 Transformer tank earthing terminal to substation earth bar Make the earth connection to the screwed boss provided on the transformer tank. 4.5.4 Switchboard earth bar to substation earth bar Make the earth connection to the switchboard earth bar which is normally a copper bar running the full length of the board near the base. 4.5.5 Structural steelwork to substation earth bar The earthing of the structural steelwork does not imply that the steelwork may be used as a conducting path for the earthing of apparatus. 4.5.6 Substation earth bar to substation earthing system Make the earth connections by means of either rectangular copper bar or stranded copper conductor, according to local practice. NOTE Recommended minimum conductor sizes are given in annex A. 4.6 Resistance of cable glands, earthing leads, etc. Ensure that the resistance of the earth fault path is as low as is practicable in order to keep the voltage to earth at the fault position to a minimum. In any system, a loose or corroded earthing strap might considerably increase the voltage to earth of the cable sheath or armour at the fault position. The earth fault current is limited by a) the system fault reactance, b) the resistance of the cable conductor up to the fault, and c) the resistance of the earth fault path. The reactance of an 11 kv 250 MVA system is 0,484 Ω and so, for practical purposes, the resistance of the cable conductor may be ignored. However, the reactance of, for example, a 400 V 31 MVA system is only 0,0052 Ω and the conductor resistance plays an important part in limiting fault current. Figure 3 shows how the voltage to earth of cable sheath or armour (or both) at the fault position varies with the resistance of the earth fault path for an 11 kv system of 250 MVA fault level. 5
Figure 1 Typical industrial distribution system 6
Figure 2 Typical layout of substation showing required earthing connections 7
NOTE 1 System voltage 6,35 kv. NOTE 2 System fault level 250 MVA. Figure 3 Graph showing increase in voltage at fault position with increase in resistance of earth fault path 8
Annex A (informative) Additional information on earth connections A.1 Recommended minimum sizes of copper conductor to be used for earth connections The minimum size of bare copper bar or stranded copper conductor, either bare or insulated, to be used for the earth connections detailed in 4.5, is governed by the maximum allowable temperature of the copper and the electromagnetic forces resulting from the short-circuit current. For an allowable final temperature T 2 of the earthing conductor, the current density is calculated from the expression where I = k' A t I is the short-circuit current, in amperes; A is the cross-sectional area, in millimetres squared; k' is a factor determined as follows: 1/2 234,5 + T2 k' = 226 log e 234,5 + T1 where T 1 and T 2 are the initial and final temperatures, in degrees Celsius; t is the clearance time, in seconds. NOTE This expression assumes that all the heat energy is stored in the conductor. Theoretically, a current density of 130 A/mm² will, within a clearance time of 3 s, cause a temperature rise of 450 C above an ambient temperature of 30 C. However, measurements made by the SABS have shown that, because of heat losses, a temperature rise of approximately 320 C occurs. Where bare copper bar is to be used, the recommended minimum sizes based on a current density of 130 A/mm² for a clearance time of 3 s are given in table A.1. Although a higher current density would be thermally permissible for shorter clearance times, tests have shown that the resultant electromagnetic forces, which are proportional to the square of the current, are difficult to contain. Where stranded copper conductor is to be used, the recommended minimum sizes are given in table A.2. If the stranded copper conductor is PVC-insulated, a final temperature of 150 C should not be exceeded and therefore a lower current density of 80 A/mm² has been employed in the calculation of the maximum current rating. Soldered lugs may be used on PVC-insulated earthing leads, provided that they have the appropriate maximum current rating (3 s). If crimped lugs are used on bare stranded copper conductor, a higher current density of 110 A/mm² can safely be used in calculations. Lugs intended to be crimped by means of a compression tool should be used. 9
A.2 Joints in copper earthing bar The results of tests carried out by the SABS show that joints in copper bar should be made by welding or brazing. Where this is not possible, a satisfactory joint can be made by tinning the ends of the bars to be jointed over a distance equal to twice the width of the bar and making a bolted overlap joint. Two bolts should be used on bars of width 25 mm and 31,5 mm, and four bolts on bars of width 40 mm and 50 mm. The bolts should be of brass to avoid galvanic corrosion, and should have an M6 thread and be of sufficient length to accept a brass nut and washer. Table A.1 Short-time current ratings of bare rectangular copper bar for use as earth connections on systems with solidly earthed neutral 1 2 3 4 Dimensions of copper bar, min. Cross-sectional area Maximum current mm rating (3 s) a Thickness Width mm² ka a 3,15 25,0 78,8 10,2 3,15 31,5 99,2 12,9 4,0 31,5 126 16,4 4,0 40,0 160 20,8 6,3 31,5 198 25,7 6,3 40,0 252 32,8 6,3 50,0 315 40,9 Although higher current ratings for periods of less than 3 s are thermally permissible, it is recommended that the ratings given above not be exceeded. Tests have shown that the electromagnetic forces resulting from such currents are difficult to contain. Table A.2 Short-time current ratings of PVC-insulated stranded copper conductor and bare stranded copper conductor 1 2 3 a b c Nominal cross-sectional area of conductor, min. PVC-insulated stranded copper Maximum current rating (3 s) a ka Bare stranded copper mm 2 conductor b conductor c 70 5,6 7,7 95 7,6 10,4 120 9,6 13,2 150 12,0 16,5 185 14,8 20,4 240 19,2 26,4 300 24,0 33,0 Soldered lugs may be used with PVC-insulated conductors, but crimped lugs shall be used on bare conductors when the ratings given in column 3 are adopted. See also footnote a to table A.1. Current density of 80 A/mm² giving a temperature rise of 120 C above an ambient air temperature of 30 C. Current density of 110 A/mm² giving a temperature rise of 270 C above an ambient air temperature of 30 C. 10
Bibliography SANS 10198-12 (SABS 0198-12), The selection, handling and installation of electric power cables of rating not exceeding 33 kv - Part 12: Installation of earthing system. SANS 10199 (SABS 0199), The design and installation of an earth electrode. Standards South Africa 11