Collection of standards in electronic format (PDF) 1. Copyright

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1 Collection of standards in electronic format (PDF) 1. Copyright This standard is available to staff members of companies that have subscribed to the complete collection of SANS standards in accordance with a formal copyright agreement. The document may reside on a CENTRAL FILESERVER or INTRANET SYSTEM only. Unless specific permission has been granted, this document MAY NOT be sent or given to staff members from other companies. Doing so would constitute a VIOLATION of SABS copyright rules. 2. Indemnity Standards South Africa accepts no liability for any damage whatsoever that may result from the use of this material or the information contained therein, irrespective of the cause and quantum thererof. Click on one of the two boxes I agree with the above I do not agree

2 ICS ISBN SOUTH AFRICAN NATIONAL STANDARD The selection, handling and installation of electric power cables of rating not exceeding 33 kv Part 2: Selection of cable type and methods of installation Published by Standards South Africa 1 dr lategan road groenkloof private bag x191 pretoria 0001 tel: fax: international code Standards South Africa

3 Table of changes Change No. Date Scope Abstract Deals with factors to be taken into account when an electrical distribution system is being designed. It covers criteria to be considered when an electric power cable is being selected, and gives a general introduction to the methods available for the laying of power cables. Keywords cable types, electric cables, installation, power cables, selection. Acknowledgement Standards South Africa wishes to acknowledge the valuable assistance received from the Association of Electric Cable Manufacturers of South Africa.

4 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 :1988). SANS 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

5 Contents Page Abstract Keywords Acknowledgement Foreword Scope Normative reference Definitions Selection of cable General Sustained current rating Allowable voltage drop Short-circuit capacity Mechanical and environmental protection Interference with telecommunication cables Method of installation General provisions Outdoor installations Indoor installations Installation in covered cableways and service tunnels Installation in vertical or inclined shafts Examples Annex A (informative) Induction effects of earth faults Bibliography

6 The selection, handling and installation of electric power cables of rating not exceeding 33 kv Part 2: Selection of cable type and methods of installation 1 Scope This part of SANS covers criteria to be considered when an electric power cable is being selected, and gives a general introduction to the methods available for the laying of power cables. It also covers cables that comply with the requirements of SANS 97, SANS to SANS , SANS 1576, SANS 1339, SANS and SANS The induction effects of earth faults in power cables running parallel to telecommunication circuits are given in annex A. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this part of SANS 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 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 , The wiring of premises - Part 1: Low voltage installations. SANS , The selection, handling and installation of electric power cables of rating not exceeding 33kV - Part 1: Definitions and statutory requirements. 3 Definitions For the purposes of this part of SANS the definitions given in SANS apply. 4 Selection of cable 4.1 General An electric power cable has to perform two basic functions: it has to carry a specified current and it has to withstand the voltage and fault conditions of the system into which it is connected. These and other criteria to be considered when a cable is being chosen are detailed in 4.2 to

7 4.2 Sustained current rating The following factors might affect the sustained current rating and are considered in more detail in SANS : a) ambient air temperature; b) soil temperature; c) thermal resistivity of the soil; d) depth of burial; and e) layout and grouping of cables. 4.3 Allowable voltage drop Except for extremely long cable runs, the problem of voltage drop is largely confined to 600/1 000 V cables. Provisions covering voltage drop are laid down in SANS in respect of a supply within premises and in the Electricity Act, 1987 (Act No. 41 of 1987), in respect of supply authorities supplying consumers. In the case of industrial and general distribution networks, choose the size of cable with due regard to the maximum voltage drop that can be tolerated The maximum length of cable that can carry the rated current of the circuit before the allowable voltage drop is exceeded increases with conductor size. The voltage drop in 600/1 000 V cables used to connect squirrel cage induction motors to direct on-line starters is normally limited to 2,5 % at the normal full load current of the motor. Figures 1 and 2 show the relationship between cable length and conductor size when the permitted voltage drop along the cable is limited to 2,5 % Example: with reference to figure 1, the smallest copper conductor cable capable of carrying a current of 10 A over a length of 100 m, without exceeding a voltage drop of 2,5 %, is 4 mm Short-circuit capacity Under fault conditions, a cable is required to carry currents many times greater than its normal full load current. The time taken by the cable's protection system to clear the fault might vary from a few milliseconds to 3 s. Short-circuit current ratings are given in SANS , and are calculated on the assumption that the heat resulting from a short-circuit is stored in the conductor and causes an increase in temperature from the normal maximum operating temperature to the allowable maximum short time value for the installation. The short-circuit current rating of a cable can be further limited by the allowable temperatures of lead-sheathed or armoured wires, and that of a large size multicore paper-insulated/lead-sheathed cable can be limited by the current above which bursting can occur. These limits are indicated on the short-circuit rating graphs given in SANS

8 NOTE 1 600/1000 V multicore PVC SWA PVC. NOTE 2 Maximum conductor temperature 70 C. NOTE 3 Maximum voltage drop 2,5 % at 400 V. NOTE 4 Installation in air at 30 C. Figure 1 Cable current rating limited by voltage drop (copper conductors) 5

9 NOTE 1 600/1000 V multicore PVC SWA PVC. NOTE 2 Maximum conductor temperature 70 C. NOTE 3 Maximum voltage drop 2,5 % at 400 V. NOTE 4 Installation in air at 30 C. Figure 2 Cable current rating limited by voltage drop (aluminium conductors) 6

10 4.4.2 Example: consider a 2 MVA 11 kv-380 V transformer which is to be supplied by a paperinsulated cable from a circuit-breaker connected to a system of 500 MVA fault level and having a clearance time of 0,5 s. The transformer has a normal full load current of 105 A at 11 kv, and reference to the standard rating of SANS shows that under standard conditions this would be adequately catered for by 35 mm 2 paper-insulated cable of belted construction. However, the 500 MVA fault level demands a short-circuit rating and graphs of SANS show that a 150 mm 2 paper-insulated cable would have to be used. 4.5 Mechanical and environmental protection Cable sheath Extruded solid dielectric-insulated low voltage cables Extruded solid dielectric-insulated low voltage cables have an extruded sheath which acts as a bedding for armour when so required Paper-insulated cables Paper-insulated cables need an impervious metal sheath to prevent the ingress of moisture. This metal is normally either lead, lead alloy or aluminium. Pure lead is sensitive to intercrystalline fatigue fracture caused by vibration and, under certain conditions, by the effects of thermal cycling. Consequently, if a lead-sheathed cable is to be subjected to moderate vibration, for example near a road, a railway or heavy machinery, or is to be transported over long distance prior to installation, then a lead alloy E sheath should be specified. Where severe vibration is expected, specify a lead alloy B sheath with single-wire armour. Where movement due to load cycling can occur and a leadsheathed cable is required, give consideration to specifying alloy E or alloy B Cross-linked polyethylene (XLPE) insulated cables XLPE-insulated cables have a foil core screen over which is an extruded sheath that acts as a bedding for armour when so required Armour Single-core cables When armouring is required on a single-core cable, ensure that the armouring is non-magnetic. A single layer of aluminium wire is normally used Multicore cables One or two layers of galvanized steel wire are normally used as mechanical protection for multicore cables, especially in mine shafts. A double layer of steel tape is sometimes used as mechanical protection. Although steel tape provides some protection against penetration by hand excavating tools, it has insufficient longitudinal strength to withstand subsidence of the ground or excessive pulling during installation. NOTE The conductivity of the earth return path provided by armour wires and lead sheathing, if present, can be increased by replacing a number of steel wires with tinned copper wires. Conventionally, sufficient copper is used to bring the conductance of the earth path to a value of at least half that of the largest conductor in the cable. 7

11 4.5.3 Outer protection The outer protection provided for most types of cable is an extruded PVC sheath. Polyethylene shall be used when the cable is to be laid by direct burial where water is present, but do not use it indoors or in ducts where fire hazards may arise. Consider the use of flame-retardant nonhalogenated compounds when cables in air are subject to abnormal fire hazards, such as when groups of cables are to be run in long vertical cableways. NOTE 1 All plastics are affected by ultraviolet (UV) radiation. The effect of UV radiation can be minimized by the use of carbon-black loading in the compound. NOTE 2 PVC can become work-hardened. It should therefore be ensured that PVC-sheathed cables in an aerial installation are supported on a catenary wire. NOTE 3 Excessive clamping pressure should be avoided when PVC-sheathed cables are cleated. NOTE4 PVC is adversely affected by oils and petrol and is attacked by a number of chemicals, particularly the long-chain fatty acids (e.g. those produced by the decomposition of meat and found in abattoirs). 4.6 Interference with telecommunication cables When an earth fault occurs in a power cable that runs parallel to a telecommunication cable for some distance, an induced voltage (which might be of sufficient magnitude to endanger human life or even cause damage to telecommunication equipment or to the telecommunication cable) will appear across the terminals of the telecommunication cable. The induced voltage will depend on whether telephone transformers or voltage diverters are installed and on the time taken for the power system protection to clear the fault. In the case of Telkom telecommunication cables, the maximum allowable induced voltages are specified by Telkom. A method of calculating the magnitude of an induced voltage is given in annex A. Induced voltages can be reduced by increasing the separation, and by reducing or eliminating any parallelism between the power circuit and the telecommunication circuit. When details of a proposed power cable installation are submitted to Telkom (see SANS ), include the following information: a) a map or plan showing all power cable routes, except those within buildings; NOTE Each cable route should be numbered for reference purposes. b) a full description of each cable, e.g. 6,35/11 kv three-core 95 mm 2 copper conductor, paperinsulated, screened, lead sheathed, single-wire-armoured and served; c) for each cable, the maximum earth fault current that could occur and the total fault clearance time (i.e. the relay operating time plus the circuit-breaker clearance time); and d) for each cable, details of earthing arrangements at the supply end and at the load end of the cable. 5 Method of installation 5.1 General provisions Install cables in such a way as to a) minimize the likelihood of damage and consequent failure of the distribution system, 8

12 b) ensure as far as possible the safety of personnel working in the area in which the cables are installed, and c) keep the overall cost of the installation to a minimum Where power cables are laid parallel to or across telecommunication cables, comply with the requirements of Telkom as stated in letters of approval and detailed on marked-up approval plans. Where alternative methods or installation practices are proposed, obtain approval for each method or practice from Telkom before work is begun When a cable heats up and cools down due to cyclic changes in load, it tends to expand and contract. If the cable is so restrained that this expansion and contraction is prevented (i.e. the installation is a fully restrained system), thermo-mechanical forces will occur which, if not contained, might cause considerable damage to the cable especially at joint positions. If expansion and contraction are allowed to occur, for example by snaking the cable during installation or by installing it in cleats or on suitable hangers and allowing it to sag between cleats and hangers (i.e. the installation is an unrestrained system), damage can be avoided. Whenever possible, avoid a partly restrained, partly unrestrained system When a short-circuit occurs in a distribution system, all cables feeding the fault will, as a result of the short-circuit currents, be subjected to electromagnetic forces that will tend to separate the cores. In a three-core or four-core cable these forces shall be contained by sheath and armour. Where three single-core cables are installed in air, the cables will tend to fly apart. Ensure that these forces are contained by using trefoil cleats to anchor the cables, and by using restraining bands or straps, usually of stainless steel, between the anchors. Well-compacted backfill in direct buried installations is normally sufficient to contain such forces. 5.2 Outdoor installations Direct burial When a cable is to be installed outdoors, bury it directly in the ground wherever possible. Lay the cable in a trench on a bed of selected sand or sifted soil of known thermal characteristics and cover it with the same material. Ensure that this backfill material immediately surrounding the cable is well compacted to reduce its thermal resistance. If necessary, protect the material with concrete or suitable cover tiles. As an additional or alternative precaution, a brightly coloured plastic warning tape shall be laid above the cable at a depth of at least 200 mm below the ground surface. At the ends of a direct-buried section and on either side of a joint position, the cable should be snaked to minimize the effects of thermomechanical forces and ground subsidence In pipes Where a cable route crosses a road or railway, the cable shall be laid in a pipe to facilitate its replacement at a later date without disturbing the road surface or railway track. Pipes shall be of any material that will not collapse in service, and should ideally be set in concrete. Ferrous pipes can be used for multicore cables but not for individual single-core a.c. cables In air Where it is impracticable to install a cable by direct burial (because of rocky terrain, likely subsidence, or the possibility of damage being caused by the later installation of other cables or services), it shall be installed in air (above ground) provided that a) there is adequate support for the cable, 9

13 b) the cable is protected from damage that might be caused by road vehicles, mobile cranes, vandals, etc., c) there is free air circulation around the cable, and d) the cable is shielded from the direct rays of the sun. Where such provisions are not possible, considerable derating shall have to be applied. Alongside a railway line or a canal where public access is normally not permitted, a cable shall be installed in cleats or in J-hangers fixed to suitably spaced posts or to a wall. Install the cable with enough sag between fixed positions to allow for free cyclic expansion and contraction. Fix single-core cables in trefoil cleats and strap them at intermediate positions to contain short-circuit forces. A guide to cleat spacing and strapping is given in SANS Details of installation procedures for aerial bundled conductor SANS (ABC) cables are given in 5.3 Indoor installations On cable trays Suitable mechanical protection for cables shall be provided by running groups of cables on purpose made cable trays or "ladder racks" installed in tiers as necessary. Supports for the trays shall be free-standing or fixed to walls; alternatively, roof trusses or joists shall be used as supports. Do not fix such supports or the trays to structural steelwork by means of drilling, as this might weaken the structure. Ensure that cable trays or ladder racks are adequately supported, a typical spacing for supports being 2 m. Fix multicore cables neatly to the tray by means of clips or straps. Fix single-core cables in trefoil cleats and strap them at intermediate positions to contain short-circuit forces In cleats Individual cables or groups of cables shall be cleated to walls or to building steelwork In conduit or trunking Install small single-core cables supplying lighting, power points, etc., in conduit or trunking in accordance with the provisions of SANS Installation in covered cableways and service tunnels Covered cableways In covered cableways that are too small for free access of personnel, cleat cables to the walls, install them in hangers or lay them on the floor of the cableway. When cables are laid on the floor, snake them to allow for expansion and contraction Service tunnels Service tunnels that are provided in power stations and in large industrial complexes may, because of their size, be equipped with cable trays. Install the cables as in Make provision at regular intervals (e.g. every 50 m) for cables to cross over from one side of the tunnel to the other, and provide adequate drainage for the tunnel. 10

14 5.5 Installation in vertical or inclined shafts Ensure that cables installed in vertical or inclined shafts are adequately supported. If the cables are unarmoured, or are armoured other than with steel wire, provide support at vertical intervals not exceeding 2 m. If steel-wire-armoured cables are used, the vertical support interval shall be increased to not more than 5 m. Ensure that the devices used to support the cable are carefully selected and so installed that sufficient force is applied to grip the outer sheath of the cable firmly, but without any undue crushing of the cable. 6 Examples 6.1 General The selection of cables for a typical industrial installation is discussed in examples 6.2 to 6.4 inclusive (see figure 3) ,35/11 kv cables The fault level on the 11 kv side of the main supply transformer is 250 MVA. The 11 kv cable "A" supplying the main switchboard shall be capable of carrying a full load current of A. For this rating, a number of single core cables in parallel per phase will be required and the short-circuit current of 13,1 ka will not be a problem. The radial feed "B" to the 10 MVA transformer will require a cable or cables of rating 525 A, and here again the only limitation that has to be considered is the current rating. The radial feed "C" to the switchboard supplying the three smaller transformers shall be capable of carrying 184 A. If it is assumed that this cable is to be direct-buried and standard conditions of installation apply, a three core PILC belted cable with a 70 mm 2 copper or 120 mm 2 aluminium conductor appears to be suitable. If, however, account is taken of the fault level and clearance time of circuit-breaker 1/3 (250 MVA for 1,5 s), a 150 mm 2 copper or a 240 mm 2 aluminium conductor is required. A similar reasoning applies to cable "D" supplying the 2 MVA transformer. The normal full load current of 105 A could be carried by a PILC belted cable with a 35 mm 2 copper or a 50 mm 2 aluminium conductor. The fault level at switchboard 3 will be slightly below 250 MVA because of the impedance of the supply cable but the reduction will be small unless the length of the cable is appreciable. If the clearance time of circuit-breaker 3/1 is 0,5 s, the minimum conductor size will be 95 mm 2 copper or 120 mm 2 aluminium. Cables "E" and "F" supplying the 1 MVA and 0,5 MVA transformers are fuse-protected and any fault on the load side of the fuses will be cleared by the fuses in a few milliseconds. The I 2 t let-through will result in a temperature rise of only a few degrees and can be ignored. These cables shall therefore be rated according to normal full load current requirements, and the smallest cable available, a 16 mm 2 copper or aluminium equivalent, will be adequate even for a ducted installation. NOTE All of the 6,35/11 kv cables considered above could equally have been XLPE or one of the elastomeric cables. Problems of terminating such cables in equipment terminal boxes designed for PILC cables might, for the moment, restrict their use in the smaller sizes. 11

15 6.3 1,9/3,3 kv cables Paper-insulated, PVC-insulated and XLPE insulated cables are available at this voltage. Where the choice is not governed by the cost of the cable, PVC insulated or XLPE-insulated cables are generally preferred because of ease of termination, but a metal sheathed cable might be preferred when installation is to be by direct burial. The fault level at the bars of switchboard 2 is 150 MVA at 3,3 kv (26,2 ka). The 12 MVA (2,10 ka) and 10 MVA (1,750 ka) connections "G" and "H" to the switchboard should ideally be busbars but where this is impracticable, use single-core cables (two or more in parallel per phase). Paper-insulated or XLPE-insulated cables with their higher current-carrying capacity than PVC-insulated cables will enable three instead of four cables in parallel to be used for the 12 MVA connection and two instead of three cables in parallel for the 10 MVA connection. The fault rating is not a problem for any of these, but consider the fault rating for the outgoing radial feeds from the switchboard. Cable "J", for example, which is fed from circuit-breaker 2/1 and is required to withstand 150 MVA for 0,5 s, should have a minimum conductor size of 185 mm 2 copper for PILC or 300 mm 2 copper for PVC. If the load on cable "J" were 2 MVA maximum (350 A) and a fused contactor instead of a circuit-breaker were used so that short-circuit current could be ignored, a 185 mm 2 conductor PVC-insulated cable could be used. The cables ("K", for example) supplying high tension motors are fuse-protected and are selected on a current rating basis only. (Voltage drop is not a problem in this instance) /1 000 V cables 600/1 000 V cables are used in all the low voltage systems currently in operation in the Republic, the most common being 380 V or 400 V. 500 V or 525 V systems are also found in the mining, paper and steel industries. At any of these voltages, consider and check the voltage drop before deciding on cable conductor sizes. Consider also short-circuit current capacity as the fault currents at these voltages are generally much higher than at high voltage. Consider, for example, the 2 MVA transformer in figure 3. The short-circuit capacity on the LV side will be 31 MVA (45 ka at 400 V). A single multicore cable or three single-core PILC cables could not withstand this current, but as three or more single-core cables per phase will be required to carry the full load current (2,887 ka) and it can be assumed that the fault current will divide equally between them, the PILC cables shall be used. Transformers of this size are normally connected by busbars to the circuit-breaker in the main low voltage distribution board but, where this is impracticable, single-core PVC or PILC cables shall be used. NOTE 1 Where the HV side of the transformer is fuse-protected as in the case of the 1 MVA or 0,5 MVA transformer in this example, the cables should be rated simply on a full load current basis. NOTE 2 Voltage drop is unlikely to be a problem in the cables connecting the transformer terminals to the main low voltage switchboard, but conducted heat from connected equipment might influence the choice of conductor size. The A feed "L" from the main distribution board of the 2 MVA transformer is required to withstand system fault current for the time taken for the fault to be cleared. Most moulded case circuit-breakers and air circuit-breakers with direct acting trips will clear a full short-circuit in 1 to 3 cycles and will provide close overcurrent protection. 12

16 Figure 3 Typical industrial installation 13

17 Three single-core 630 mm 2 copper conductor PILC unarmoured cables can carry A in air but under earth fault conditions would suffer from sheath overheating in under 40 ms. In such a case XLPE-insulated cables would be preferred. Other cables of lower rating (such as "M" in figure 3) that are supplied through fuses or MCB's shall be selected on a full load current and voltage drop basis only. If the full load current of the 400 V motor in the example is, for example, 22 A and the length of cable "N" is 90 m then, with reference to figure 1, a 10 mm 2 copper conductor cable is required. 14

18 Annex A (informative) Induction effects of earth faults A.1 Voltages induced in telecommunication circuits as a result of earth faults in power cables running along parallel routes A.1.1 Earth faults in power cables Consider an earth fault occurring at some distance along a power cable as shown in figure A.1. For simplicity, only the faulty phase is represented. Figure A.1 Earth fault currents The fault current will return to the supply neutral point via one or more of the following paths: a) Current i 1 directly back to the supply neutral via the armour/sheath resistance R A1. b) Current i 2 to the gland at the load end of the cable via the armour/sheath resistance R A2, the equipment earth R G2E and back to the supply neutral via R NE. c) Current i 3 directly to earth via the fault-to-earth resistance R FE and back to the supply neutral via R NE. The paths taken by these currents are shown in figure A.2. Figure A.2 Fault current paths 15

19 Currents i 2 and i 3 will return to the supply neutral via the paths of least resistance, which shall not follow the route taken by the cable. The voltage induced in any parallel telecommunication cable will be the result of the "go" and "return" paths of i 2 and i 3 having different mutual coupling with the telecommunication cable. If the "return" paths of i 2 and i 3 are ignored (they partly cancel the effect of their "go" paths) and i 3 is assumed to be very small (i 3 will be negligible provided that the cable has some form of metal sheath or armour), a circuit can be drawn representing the fault paths as lumped resistances, as shown in figure A.3. Figure A.3 Fault path resistance circuit 16

20 Thus, net currents (I-i 1 ) and i 2 flow along the cable. These will be equal if i 3 is zero and will be in the normal direction of current flow. In the worst case, RF is assumed to be zero, then R G2E and R NE are lumped together and called R E, the resistance of the earth path. (The simplified arrangement is shown in figure A.4.) Figure A.4 Simplified fault path resistance circuit The net current causing interference i 2 = (I-i 1 ) will be a maximum when the fault occurs at or near the load end of the cable. R C1 will be a maximum with the fault in this position but, as R C1 will in most cases be much less than R E, the above still holds. The circuit can then be further simplified as shown in figure A.5. From figure A.5: Figure A.5 Further simplified fault path resistance circuit R' = R C + R A R E R + R A E where R C is the resistance of the conductor; 17

21 R A is the resistance of the armour; R E is the resistance of the earth path. Let R A R C be α (ratio of armour to conductor resistances), and let Then R E R C be β (ratio of earth to conductor resistances). R' = R C (1 + αβ ) α + β and i 1 i2 = β α I = i 2 (1 + α ) β but I = U o 2 2 ( XL ) + (R' ) where U o is the phase to neutral voltage. Therefore i 2 = U o β 2 2 αβ 2 (1 + ) X L + R C (1 + ) α α + β In the above expression, U U o = 3 and X L = U o 2 S where U S is the phase to phase voltage; is the system fault level, MVA. 18

22 A.1.2 Voltage induced in parallel circuit If two wires run parallel for a length L m at a distance d m apart, their mutual inductance M is given by 2 2 L L d d M = 2 L log e d d 2 L 2 L The r.m.s. voltage V induced in one wire by an r.m.s. current i flowing in the other is given by where V = 2 π f.m.i f is the supply frequency, Hz. The voltage induced per ampere per kilometre route length is given for separations d from 0,2 m to m in figure A.6. A.1.3 Induced voltages The r.m.s. a.c. voltage that a normal healthy person can safely touch decreases with increasing duration of contact and is given in table A.1. (See also IEC ) Table A.1 Recommended maximum voltages for human safety 1 2 Duration of contact s Voltage r.m.s. a.c 0, , , , ,5 90 1, As a conventional oil circuit-breaker has a tripping time of 0,07 s and it will normally be tripped by operation of an earth fault relay, the total fault clearance time is unlikely to be much less than 0,1 s and may well be 0,3 s or even longer. 19

23 A.1.4 Information required for calculation of induced voltage The following information is required for the calculation of induced voltage: Cable length, m... Conductor resistance (length L 1 ) at working temperature, Ω... Sheath/armour resistance (length L 1 ) at working temperature, Ω... Earth path resistance - far end gland to supply neutral, Ω... System fault level (MVA)... Phase to neutral r.m.s. voltage, V... Length of power cable running parallel to telecommunication cable, m... Distance between cables, m... Reactance of supply, Ω... L 1 R C R A R E S U U o = 3 L 2 d XL A.1.5 Calculation Calculate α = R A R C where α is a constant for the cable; and β = R E R C XL = 3 U o 2 S Ω where U o is expressed in kilovolts; calculate i 2 = where (1 + β ) X α 2 L U o 2 + R (1 + αβ 2 ) C α + β U o is expressed in volts; calculate induced voltage V 20

24 V L 2 = 0,5 i k where k is a function of the distance between cables d as given in table A.2. Table A.2 Relationship of factor k to distance between cables 1 2 Distance between cables d m 1 0,82 2 0,74 5 0, , , , , , , ,058 k Figure A.6(a) Distance between cables 0,2 m to 10 m 21

25 Figure A.6(b) Distance between cables 10 m to m Figure A.6 Induced voltages against distance between cables 22

26 A.1.6 Example Consider a three core 95 mm 2 aluminium conductor paper/lead single-wire-armoured 11 V power cable that complies with the requirements given in table L of SANS 97 (SABS 97:2001): System fault level 250 MVA at supply terminals Earth path resistance Cable length L 1 Length in parallel L 2 Distance between cables d 5 Ω m 740 m 10 m R C = 0,365 R A = 0,343 α = 0,941 β = 11, = 0,438 Ω = 0,412 Ω XL = i 2 = 3 (6,35) 2 = 0,484 Ω (1 + 12,14) 0, ,192 = 508,1 A (1 + 0,869) 2 Induced voltage V = 0,5 508, ,54 = 101,5 V NOTE If the above calculation is repeated for values of R E between 2 Ω and 10 Ω, the curve of induced voltage against R E so obtained is shown in figure A.7. 23

27 Figure A.7 Induced voltage VS earth path resistance It appears from this case that an earth path resistance of at least 5 Ω or more is desirable from an "interference with telecommunications" point of view. 24

28 Bibliography IEC , Electrical installations of buildings - Part 4: Protection for safety - Chapter 41: Protection against electric shock. SANS 97 (SABS 97), Electric cables - Impregnated paper-insulated metal-sheathed cables for rated voltages 3,3/3,3 kv to 19/33 kv (excluding pressure assisted cables). SANS (SABS ), Electric cables with extruded solid dielectric insulation for fixed installations (300/500 V to 1900/3 300 V) - Part 1: General. SANS (SABS ), Electric cables with extruded solid dielectric insulation for fixed installations (300/500 V to 1900/3 300 V) - Part 2: Wiring cables. SANS (SABS ), Electric cables with extruded solid dielectric insulation for fixed installations (300/500 V to 1900/3 300 V) - Part 3: PVC Distribution cables. SANS (SABS ), Electric cables with extruded solid dielectric insulation for fixed installations (300/500 V to 1900/3 300 V) - Part 4: XLPE Distribution cables. SANS (SABS ), Electric cables with extruded solid dielectric insulation for fixed installations (300/500 V to 1900/3 300 V) - Part 5: Halogen-free distribution cables. SANS (SABS ), Electric cables with extruded solid dielectric insulation for fixed installations (300/500 V to 1900/3 300 V) - Part 6: Service cables. SANS (SABS ), The selection, handling and installation of electric power cables of rating not exceeding 33 kv - Part 4: Current ratings. SANS (SABS ), The selection, handling and installation of electric power cables of rating not exceeding 33 kv - Part 8: Cable laying and installation. SANS (SABS ), The selection, handling and installation of electric power cables of rating not exceeding 33 kv - Part 13: Testing, commissioning and fault location. SANS (SABS ), The selection, handling and installation of electric power cables of rating not exceeding 33 kv - Part 14: Installation of aerial bundled conductor (ABC) cables. Standards South Africa 25