Table 2 – Minimum clearances in air – Voltage range II ( U m > 245 kV) Nominal voltage of system
Highest voltage for equipment c
Rated lightning impulse withstand voltage a
Voltage range U n
r.m.s.
kV
275
330
II
380
480
700
Rated switching impulse withstand voltage
Minimum phaseto-earth clearance
Rated switching impulse withstand voltage
Minimum phase-tophase clearance
Phase-toearth 250/ 2 500 µs (peak value)
Rod Conductor – – structure structure
Phase-tophase 250/ 2 500 µs (peak value)
Conductor Rod – – conductor conductor parallel
mm
kV
mm
r.m.s.
1,2/50 µ s (peak value)
kV
kV
kV
850/950
750
950/1 050
850
950/1 050
850
1 050/1 175
950
1 050/1 175
850
1 175/1 300
950
1 300/1 425
1 050
1 175/1 300
950
1 300/1 425
1 050
1 425/1 550
U m
N
1 600
1 900
1 125
2 300
2 600
2 400
1 275
2 600
3 100
2 400
1 275
2 600
3 100
2 900
1 425
3 100
3 600
2 400
1 360
2 900
3 400
2 900
1 425
3 100
3 600
3 400
1 575
3 600
4 200
2 900
1 615
3 700
4 300
2 600
3 400
1 680
3 900
4 600
1 175
3 100
4 100
1 763
4 200
5 000
1 675/1 800
1 300
3 600
4 800
2 210
6 100
7 400
1 800/1 950
1 425
4 200
5 600
2 423
7 200
9 000
1 950/2 100
1 550
4 900
6 400
2 480
7 600
9 400
1 700 b
300
362
420
525 (550) c
765 (800) c
1 800 1 900 b 1 800 1 900 b 2 200 1 900 2 200 b 2 200 2 400 b 2 600 2 200 2 400 b
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 U m = 550 kV (instead of 525 kV); 800 kV (instead of 765 kV); 1 200 kV, of a value between 765 kV and 1 200 kV, and of the associated rated withstand voltages, is under consideration.
5.4
Minimum clearances between parts under special conditions
5.4.1 The minimum clearances between parts of an installation which may be subject to phase opposition shall be 20 % higher than t he values given in table 1 and t able 2 and annex A.
Training Report Jan-2009 to Apr- 2009
ELECTRICAL CLEARANCES The clearances are mainly maintained to ensure satisfactory performance in service and ensure supply of personal inspection, operation and maintenance. Types of clearances are
Earth clearance
Phase clearance
Section clearance
Ground clearance
Bay clearance or Bay width
Earth Clearance The electrical clearance is defined as “The minimum distance required between live
parts and earth material”. Phase Clearance The difference between two or more live parts at difference potential then it is called
phase clearance. There are meant to prevent spark or flash over. There clearance for outdoor switchgear having voltage above 22KV are related to the impulse withstand load for which the equipment has been manufactured. Section Clearance This safety clearance to conductor is defined as “The clearance having minimum
difference between on unscreened live conductor and to the limits of a working clearance whenever the separation is related to the safety of operating personal. As per the standards “This is the minimum clearance from any point where a men may be required to stand to the nearest outdoor conductor in ai r”. The following table gives the difference clearance level for the various system voltages. This is up to 1000m from the sea level. GENERAL CLEARANCE:
HIGHEST
IMPULSE
SYSTEM
WITHSTANDING CLEARANCE
VOLTAGE
VOLTAGE (KV)
(KV) 12
60
MINIMUM
TO
MINIMUM
SECTION
CLEARANCE
CLEARANCE
EARTH BETWEEN
(m.m)
PHASE (m.m.)
90
90
(m.m)
2600 Page 24 of 178
Training Report Jan-2009 to Apr- 2009
75
120
120
2600
145
320
320
2750
170
630
630
3000
72.5
325
630
630
3500
123
450
900
900
3500
550
1000
1000
4000
450
900
900
3500
550
1100
1100
4000
650
1300
1300
4000
850
1600
1700
4500
950
1900
1900
4500
1050
2400
2400
5000
36
145
245
Table-2
The following table gives the normal practice of voltage:
RATED VOLTAGE
FOR
EQUIPMENT
RIGID
CONDUCTOR
& PHASE
TO
PHASE
FOR
IN ISOLATOR & SWITCHES
ARE PHASE
TO PHASE
TO ISOLATOR
SWITCHES WITH
PHASE
EARTH
(mm)
(mm)
(mm)
(mm)
7000
3500
7000
-
230
3350
1700
3350
3962
132
2800
1500
1500
-
110
1700
1000
2135
3050
66
1220
750
1525
2135
33
915
610
765
1220
22
610
460
765
1070
11
560
3005
6010
910
0.4
380
305
-
-
ARCING
HORN
400
Page 25 of 178
Training Report Jan-2009 to Apr- 2009
Table-3
Ground Clearance VOLTAGE RATING
MINIMUM
(kV)
TO GROUND(mm)
CLEARANCE(mm)
400
8000
5500
230
5500
4270
132
4600
3500/4000
110
4600
3360
66
4600
3050
33
3650
2750
22
3650
2750
11
3650
2600
CLEARANCE SECTION
Table-4
Bay Clearance as Per Standard It is the minimum distance between the two centre points of the tower.
Rated voltage(KV)
Bay width (m)
400
27
230
16
132
11
110
9.5 TO 10
66
7
33
4.6
22
3.8
11
3.5 Table-5
ISOLATOR / DISCONNECTORS Definition “A mechanical switching device which provides, in the open position, an isolating
distance in accordance with specified requirements” this is according to the IS 9921-1. A disconnector is capable of opening and closing a circuit when either negligible current is broken or made, or when no significant change in the voltage across the terminals of each of Page 26 of 178
