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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