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Annex-A WEST PAKISTAN WATER AND POWER DEVELOPMENT AUTHORITY Office of the General Manager (Power) Dated, 30th October 1963 Standardization of Design Instruction No. 21 Minimum Clearances for Overhead Lines 1. Introduction. 1.1 The object or this instruction is to indicate minimum horizontal and vertical clearances between conductors at different voltages and ground, buildings, structures and other circuits. 1.2 The clearances set out below are meant to ensure that live conductors are not a source of danger to life. An attempt has been made to cover all cases that are likely to occur in practice. However cases may arise where this instruction is not applicable. In such 'cases this office should be referred for advice. 1.3 The clearances given are to be maintained under the worst conditions that can exist in practice. Clearances can be reduced by an increase of conductor sag due to creep or high temperature, due to conductor or insulator deflection under wind loads or due to other causes. During erection such factors are to be kept in mind. 1.4 Normally the worst conditions are encountered for maximum sag and maximum wind velocity. However special conditions may arise when this is not so. Vertical clearances should be measured on the basis of maximum sag occurring at 150' f and horizontal clearances should be measured when the line is at maximum deflection due to wind pressure which is taken as 12 Ibs per sq. ft. acting on i the area of conductors. For Railway installations the conditions stated in para 4.2 apply. The effect of conductor creepage is allowed for at the time of stringing by using 'initial' stringing charts. 1.5 The clearances given below are based on current departmental specifications or follow acceptable safe practices of other countries under conditions more or less identical to those prevailing in Pakistan. 2. Clearances for Cables. 2.1 No minimum clearances are specified for aerial insulated cables. However cables must be installed with clearances such that they are not opened to mechanical damage and do not cause obstructions. As a safe practice no cable should be installed at a height less than 9 fl. from ground level. Over roads and high ways the minimum clearance as for L. T. bare wires is to be maintained. 2.2 '.Covered.' conductors as distinct from insulated wires and cables are to be treated as bare conductors. Clearance for overhead lines from Ground and Buildings Minimum Clearances from Ground The minimum clearances which must be obtained at 150o F are given below. Where lines of more than one voltage are carried on the same pole the prescribed ground clearance for the circuit installed lowest on the pole must be maintained. In addition the specific clearance between each voltage circuit must be maintained. Sr. No. 3.1.1 1 2 3 3.1.2 4 5 6 3.1.3 7 8 9 10 11 Situation LV & MV (400 V) Lines Across a street or intersection Along a street Across private property, foot paths and places inaccessible to vehicular traffic High Voltage (11 kV & 33 kV) Lines Across a street or intersection Along a street Across private property and cross country E.H.V. (66 kV & above ) Lines 66 kV lines across country 132 kV lines across country 220 kV lines across country Over roads and highways Over rivers and major canals measured from highest flood level Clearance Ft-in 19-0 17-0 15-0 20-0 20-0 15-0 20-0 22-0 23-0 26-0 30-0 Minimum Clearances from Buildings Sr. No. 13.2.1 1 3.2.2 2 Situation Vertical Clearance Over roofs and over the highest point of a flat space such as a roof, open balcony or verandah roof Horizontal Clearances From any sloping or vertical roof, wall or part of a building 400 V lines ft-in 11 & 33 kV lines ft-in 66 kV lines ft-in 132 kV lines ft-in 220 kV lines ft-in 8-0 12-0 15-0 17-0 20-0 4-0 6-0 10-0 12-0 15-0 Minimum Clearance from Railway Installation Clearances between power lines and railway track have been agreed to between the PWR and the Electricity Department by a special agreement and must be adhered to in terms of that agreement For calculating the minimum clearances of conductors and wires above ground and rail level they shall be assumed to be at a temperature of 74o C (165o F) and 60o C (140o F) for ‘plains class’ and ‘valley class’ lines respectively. ‘Plains class’ lines are lines hat are situated at an altitude between zero and 2300 ft. above sea level. ‘Valley class’ lines are lines that are situated at an altitude between 3301 and 5000 ft. above sea level. The maximum wind pressure is to be taken as 18 lbs per sq. ft. acting on the full conductor area Minimum clearances between overhead lines and Railway Track Sr. No. 4.3.1 Situation For Railway Lines of 5’-6” Gauge Ft-in Ft-in Inside 31-6 33-0 station limits Outside 32-6 25-0 station limits For Railway Lines of 2’ – 6” Gauge Inside 28-6 30-0 station limits Outside 20-6 22-0 station limits 1 2 4.3.2 3 4 Ft-in Ft-in Ft-in Ft-in 34-0 35-0 36-0 37-0 26-0 27-0 28-0 29-0 31-0 32-0 33-0 34-0 23-0 24-0 25-0 26-0 Note: Clearances under column 3 (lines below 1000 V) also apply to guys, cables, grounded messengers and communication circuits crossing the railway track Minimum clearances between overhead lines and Railway Power Lines or other Railway Structures under the most adverse conditions Sr.No. 1 2 3 4 Clearance Ft-in 10-6 11-6 12-6 13-6 For voltage upto & including 66000v Above 66000 V upto & including 110000 V. Above 110000 V upto & including 165000 V. Above 165000 V 3. Minimum clearance between two lines at Crossing Points. It must invariably be ensured that in the case of two lines of different voltages the higher voltage line crosses over the lower voltage line. In the case of lines of the same voltage line crosses over the lower voltage line. In the case of lines of the same voltage the more important line should preferably cross over the other line. The minimum clearances are given below:- Sr. No. 1 2 3 4 5 Line being crossed Insulated services, guys, earthwires etc. Telegraph lines MV & LV (400 V) lines 11 kV lines. 33 kV lines Earths Guys ft-in. MV & LV ft-in. 11 kV 33 kV 66 kV 132 kV ft-in. ft-in. ft-in. ft-in. 220 kV ft-in. 2-0 2-0 6-0 6-0 9-0 11-0 13-0 2-0 2-0 4-0 4-0 4-0 2-0 - 6-0 6-0 4-0 - 6-0 6-0 6-0 6-0 9-0 7-0 7-0 7-0 11-0 9-0 9-0 9-0 13-0 11-0 11-0 11-0 6. Minimum clearance between different circuits mounted on the same pole. 6.1 The minimum clearances for lines upto 33 kV are given below. When circuits of different voltages are mounted on the same pole the minimum clearance to buildings for the higher voltage line must be ensured. The blanks in the table indicate that grouping of these voltages in that formation on the same pole is not allowed. Sr. No. 6.1.1 1 2 3 6.1.2 4 5 6 MV & LV Horizontal Clearances MV & LV (400 V) Lines 11 kV lines 33 kV lines Vertical Clearances MV & LV (400 V) lines 11 kV lines 33 kV lines ft-in. 0-9 - 11 KV lines ft-in. 4-0 - 33 KV lines ft-in. 4-0 4-0 4-0 4-0 4-0 4-0 4-0 4-0 4-0 4-0 6.2 For lines of 66 kV and above generally only two circuits will be mounted on the same pole and will be in vertical formation. At these voltages circuits of different voltages are not to be installed on the same pole. The horizontal clearances for circuits of the same voltage are given below. These are preferred design values for the conductors now used and spans of 900 ft, 1000 ft. and 1200 ft. for 66 K V, 132 K V and 220 K V lines respectively. For other spans or heavy conductors the clearances may have to be changed. Sr. No. Horizontal Clearance ft-in. 1 66 kV lines 14-0 2 132 kV lines 19-0 3 220 kV lines 29-0 7. Minimum clearances between conductors of the same circuit. 7.1 Minimum Clearances upto 33 kV are given below:Sr. No. 1 2 3 4 Between phases of LV & MV (400 V) lines Between phases of 6.6 & 11 kV lines Between phases of 33 kV lines Between 11 kV line conductor and teeoff conductor below. Horizontal Clearance ft-in. Vertical Clearance ft-in. 1-4 1-0 2-8 2-6 3-2 3-2 - 2-9 7.2 For lines of 66 kV and above the preferred design clearances are given below. These may be required to be changed in particular conditions as indicated in para 6.2. Sr. No. 1 2 3 4 Between phases of 66 kV lines. Between phases of 132 kV lines. Between phases of 220 kV lines. Between 11 kV line conductor and teeoff conductor below. Horizontal Clearance ft-in. Vertical Clearance ft-in. 11-0 8-0 16-0 8-0 26-0 16-0 - 2-9 8. Minimum clearances between lines running side-by-side. 8.1 For lines running side-by-side the clearance must be such that if one line falls over it will not foul with the other line. For lines on plane ground the horizontal clearance between the inner conductors, i. e. those adjacent to the lines, must be equal to the height of the poles or towers of the higher line plus five feet. If the lines run on undulating ground the clearance may have to be increased to ensure that the lines will not foul in falling. 9. Miscellaneous clearances between circuits, conductors or objects not covered above. ft-in. 9.1 Clearance between insulated service and open telegraph line. 2-0 9.2 Clearance between 11 kV line and open telegraph line. 6-0 9.3 Clearance between open telegraph line and cable communication circuit. 2-0 9.4 Minimum clearances between streetlight bracket & 400 V line or telegraph line. Note:- The height above ground of streetlight brackets is decided by lighting considerations. The above is only a safe minimum clearance. 9.5 Clearance between guy wire at pole attachment and:I) Open telegraph line or cable. 2) MV' & LV (400 V) lines. 3) 6.6 & II kV lines. 2.0 0.3 0.9 (R.A. SHEIKH) Director (Standards and Design) Encl: Dr. No. GW/TC-1