Clearances required for substation up to 800 kV
( According to CBIP )
Minimum clearance $ safety
clearances
Highest Lightning Switching
(mm)
system impulse impulse voltage
Voltage ( kV ) voltage ( kVp ) ( kVp ) Phase -to-
Phase -to- Phase
earth ( mm ) ( mm )
36 170 - 320 320 2800
72.5 325 - 630 630 3100
123 450 - 900 900 3400
550 - 1100 1100 3700
145 550 - 1100 1100 3700
650 - 1300 1300 3800
245 950 - 1900 1900 4300
1050 - 2100 2100 4600
420 1425 1050 (Ph- E) 3400 * - 6400
1575 (Ph-Ph) - 4200 **
800 2100 1550 (Ph-E) 6400 * 9400 ** 10300
2550 (Ph-Ph)
* Based on Rod - structure air gap
** Based on Rod - conductor air gap
$ This value of air clearances are the minimum values dictated by electrical consideration and do not include
any addition for construction tolerances, effect of short circuits, wind effects and safety of personnel, etc.
Minimum Electrical clearance for outdoor switchgear
According to IS : 10118 ( part III )
Voltage rating Minimum
Impulse
(Highest Min clearance clearance Sectional
Withstand
System to Earth between clearance
Level
Voltage) phases
kV (rms) kV (peak) mm mm mm
12 60 90 90 2600
75 120 120 2600
36 145 - 270 2750
170 320 320 3000
72.5 325 630 630 3500
123 450 900 900 3500
550 1100 1100 4000
145 450 900 900 3500
550 1100 1100 4000
650 1300 1300 4000
245 650 1300 1300 4000
750 1500 1500 -
850 1600 1700 4500
950 1900 1900 4500
1050 2400 2100 5000
CORRELATIONS BETWEEN INSULATION LEVELS AND
Minimum clearances in air- Voltage range I ( 1kV < Um ≤ 245 kV )
( Table 1 of IEC 61936-1 )
Rated short
Nominal Highest duration power- Rated lightning
Minimum phase-to-earth and phase-
voltage of voltage for frequency impulse withstand
to-phase clearance,N c
system equipment withstand voltage a
voltage
Voltage
range
Un Um 1.2/50 μs Indoor Outdoor
r.m.s
r.m.s r.m.s ( peak value ) installations installations
kV kV kV kV mm mm
20 60 120
3 3.6 10
40 60 120
40 60 120
6 7.2 20
60 90 120
60 90 150
10 12 28 75 120 150
95 160 160
75 120 160
15 17.5 38
95 160 160
95 160
20 24 50 125 220
145 270
145 270
30 36 70
170 320
I 45 52 95 250 480
66 72.5 140 325 630
185 b 450 900
110 123
230 550 1100
185 b 450 b 900
132 145 230 550 1100
275 650 1300
230 b 550 b 1100
150 170 275 650 1300
325 750 1500
275 b 650 b 1300
325 b 750 b 1500
220 245 360 850 1700
395 950 1900
460 1050 2100
a. The rated lightning impulse is applicable to phase- to-phase and phase- to earth.
b. If values are considered insufficient to prove that the required phase-to-phase withstand voltages are met,
additional phase-to-phase withstand tests are needed.
c. For rod-structure.
Minimum clearances in air- Voltage range II ( Um> 245 kV )
( Table 2 of IEC 61936-1 )
Rated
Rated lightning
Nominal Highest Rated switching switching
impulse Minimum phase-to-phase
Voltage of Voltage for impulse withstand Minimum phase-to-earth clearence impulse
withstand clearence
System equipment c voltage withstand
voltage a
voltage
Voltage
range Conducto Rod
Rod Phase-to-
Phase-to-earth Conductor r -
Un Um 1.2/50 μs - phase
250/2500 μs (peak - - conductor
r.m.s r.m.s (peak value) structure 250/2500 μs
value) structure conductor
N (peak value)
parallel
kV kV kV kV mm kV mm
1600
850/950 750 1900 1125 2300 2600
275 300 1700 b
1800
950/1050 850 2400 1275 2600 3100
1900 b
1800
950/1050 850 2400 1275 2600 3100
330 362 1900 b
1050/1175 950 2200 2900 1425 3100 3600
1900
1050/1175 850 2400 1360 2900 3400
2200 b
II
380 420 2200
1175/1300 950 2900 1425 3100 3600
2400 b
1300/1425 1050 2600 3400 1575 3600 4200
2200
525 1175/1300 950 2900 1615 3700 4300
2400 b
480
(550) c 1300/1425 1050 2600 3400 1680 3900 4600
1425/1550 1175 3100 4100 1763 4200 5000
1675/1800 1300 3600 4800 2210 6100 7400
765
700 1800/1950 1425 4200 5600 2423 7200 9000
(800) c
1950/2100 1550 4900 6400 2480 7600 9400
a. The rated lightning impulse is applicable phase- to-phase and phase- to earth.
b. Minimum clearance required for upper value of rated lightning impulse withstand voltage.
c. The introduction of Um = 550 kV (instead of 525 kV): 800 kV(Instead of 765 kV): 1200 kV, of a value between 765 kV
and 1200kV, and of the associated rated withstand voltages, is under consideration.
Clearences practice used within UK
Section 5 of BS 7354 : 1990
Safety working
Basic Electrical Safety Working Insulation Height
Nominal system clearence Phase- to-phase
clearence clearence (pedestrian access)
voltage/BIL/SIL (horizontal) clearence
(phase-to-earth) (Vertical) (see note 2)
(see note 1)
kV m m m m m
6.6/75 0.5 2) 2.9 2.3 2.1 0.25
11/95 0.5 2) 2.9 2.3 2.1 0.25
33/170 0.5 2) 2.9 2.3 2.1 0.43
66/325 0.7 3.1 2.5 2.1 0.78
132/550/650 1.1 3.5 2.9 2.1 1.4
275/1050/850 2.1 4.8 3.9 2.4 2.4
400/1425/1050 2.8 5.5 4.6 2.4 3.6
Note: 1. The safety working clearences(horizontal) in column 4 are regarded as minimum clearences.
For systems where the predominant volatges are higher (275 kV and 400 kV) it is practice in the UK that the
horizontal and vertical clearences are equal, conforming to the minimum figures in column 3.
Note: 2. The insulation height(pedestrain access)figures in column 5 are shown in respect of the appropriate system
voltage as follows:
(a) for systems predominantly involving distribution voltages,i.e. upto and including 132 kV,2.1 m is the recommended
minimum requirement:
(b) for systems where the predominant voltages are higher,i.e. 275 kV and 400kV, 2.4 m is the recommended
minimum requirement.
The above figures are regarded as minimum clearences. Users and operators may wish to allow further clearence greater
than that shown for the voltage level in column 5, particularly for equipment at the higher distribution voltages situated in
a single compound with higher voltage equipment present in a compound subject to a greater clearence. In these
circumstances, it might be convenient to design and operate to the clearence for the highest voltage equipment.
Clearances required for different voltage levels as per TCE standard
Highest Basic Switching Minimum clearance Sectional Ground
system insulation level insulation level clearance clearance (
Voltage ( kV ) ( kVp ) ( kVp ) ( mm ) mm ) *
Phase - Phase -Phase
Earth ( mm ) ( mm )
36 170 - 320 320 2800 3700
72.5 325 - 630 630 3000 4000
123 450 - 900 900 3500 4600
550 - 1100 1100 4000 4600
145 550 - 1100 1100 4000 4600
650 - 1300 1300 4000 4600
245 950 - 1900 1900 4500 5500
1050 - 2100 2100 5000 5500
420 1425 1050 (Ph- E) 3400 ** - 6500 8000
1575 (Ph-Ph) - 4200 ***
* This is defined by IE rules
** Based on Rod - structure air gap
*** Based on Rod - conductor air gap
As per IE rules ( Rule 64)
Highest system voltage (kV) Safety working clearance (m)
12 2.6
36 2.8
72.5 3.1
145 3.7
245 4.3
420 6.4
800 10.3
