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Isolators
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Isolators
It is a mechanical switching device capable of opening and closing a circuit when a negligible current is broken or made or when no significant voltage difference across the terminals of each pole exists
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Isolators
to facilitate maintenance work
Operates when the circuit is dead
Act as a disconnecting link capable of carrying only the rated current of the switchgear
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C onnected
one to each side of a circuit
breaker
Interlocks are provided to prevent opening of the isolators when the breaker is closed
Lines or cables are earthed through the isolator installed on one side, by fitting an earthing switch to it
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Isolator has two positions - ON and OFF
The line side isolator fitted with an earthing switch has three positions - ON, OFF and EARTH
A three-phase isolator has three similar units each of which is supported on a steel framework
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Each unit comprises two or three insulator pillars depending on the type of isolator
Fixed contacts mounted on insulator pillars and moving contacts attached to a blade which is also mounted on an insulator pillar
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The simultaneous making and breaking of contacts of all the three phases are achieved by an operating mechanism
Operating mechanism is coupled to the switch by a vertical shaft which provides the necessary rotational movement to the blade through built-in bearings, links and horizontal rods
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hen W hen
the isolator is operated, a toggle action results and it gets self-locked in position when when either fully open or closed
Isolator fitted with arcing horns, called quick-break, makes makes suitable for low current switching duties
The basic specification for isolators and earthing switches is BS5253
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Arcing
horns
Switches controlling transformers or Switches feeders should be capable of breaking their magnetising or charging current and hence should have arcing horns opper or copper clad horns should C opper generally be used for switche switches s Brass or GI rods may be used for horns when copper is not readily available
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Selection
of arcing horns
voltage(kV)
Size of copper Nearest size of rod (S W G) brass or GI W G) rods(mm)
110
2/0
9.52
66
0
7.94
33
1
6.35
22
2
6.35
11
2
6.35
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High pressure contact switches for outdoor service are generally with a contact pressure of ½ lb. per ampere of capacity and this may be used as a general guide in the design of switches
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The rated normal current of an isolator or earthing switch is selected from the following standard values: 200A, 400A, 630A, 800A, 1250A, 1600A, 2000A, 2500A, 3150A, 4000A, 5000A
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Br eaking
capacity
Breaking current in amperes I = KL/E where K= 0.06 when breaking a capacitor or unloaded cable K = 0.15 when breaking an unloaded transmission line K = 0.20 when breaking an unloaded transformer K = 0.40 when breaking load current
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L = C entre to centre phase distance in mm, not exceeding four times the flashover distance across an open pole E = Breaking voltage in kVrms
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Two
classes of Isolators
Distribution
class and Station class
Distribution
Rocking type isolators are generally used in distribution systems
isolators provide single phase or three phase sectionalizing Link switches
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Classes
of Isolators
Single break isolators and centre-rotating double break isolators are also in use Gang operated isolators are preferred for distribution systems
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Distribution
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class Isolators
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Distribution
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class Isolators
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Isolators
For sub-stations up to 110kV mechanically linked gang operated isolators are used
For voltages 220 kV and above single phase isolators are either mechanically, electrically or pneumatically coupled for three phase operation
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Station
class Isolators
Horizontal breaks and vertical breaks are available C entre-break type and centre rotating double break type are the normal horizontal type isolators Vertical opening single break isolators and pantograph type isolators are the general vertical models 2/1/2011
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Types
of isolators
Rocking or Vertical break type
Horizontal break type
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Vertical
br eak type
Advantage of closer spacing between poles Arcing horns are provided and arc runs vertically up the arcing horns while opening the isolator This is employed up to 33 kV but at higher voltages, design difficulties are experienced due to the inertia of the moving parts
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Horizontal
br eak type
The blade rotates in the same plane as the centre-post insulator to give two breaks in series
Manufactured in two varieties-two post and three post
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Horizontal
br eak type
Two post is economical but requires a more rigid base structure to avoid undue deflection of the insulator supporting steel framework under load
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The two insulator pillars of each phase assembly carry a half-length of blade and both pillars are provided with built-in bearings on the supporting frame
Each insulator pillar along with half-length of the blade rotates through 90 deg in the opposite direction
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If the insulator is installed on the line side and line earthing is required, a special part is built on to the framework of each phase to support the driving shaft of the moving contact arm of the earthswitch
The other contact is fitted to one of the half-blades of the isolator
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The operating mechanism is fitted under one phase of the isolator similar to that of the isolator proper
The coupling links between the vertical driving and horizontal transmission shaft are located in the framework
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W hen
the switch is not earthed, the contact rods lie up against the framework and the advantage is that they do not take any extra space in the installation
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W hen
number of isolators are on the same line, the adjacent arms rotate in different directions
The direction of opening is altered just by coupling the operating mechanisms differently
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The continuously increasing service voltage and system currents have resulted in more stringent requirements on isolator
Two column rotary isolators are now available with rated service voltage in the range 123 to 420 kV with rated normal current of 2500 to 4000 ampere
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Centr e-br eak
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type Isolator
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In these isolators, the contact design is improved in respect of robustness so that the contacts do not separate even during short periods of the passage of short-circuit current
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Here the current transfer takes place via 1.
Tulip-type contact fingers arranged around two copper bolts with the 2500 ampere version, and
2.
Tapered-roller contacts with the 4000 ampere version
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The current path consists of a welded aluminium structure and is equipped with a catch-hook which, in the case of short-circuit currents prevents the two halves from separating in longitudinal direction
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Single
column pantograph isolators
A new development in two-post isolator was introduced in the late seventies to meet the challenges of extra-high voltage and currents
in the range 170 to 765 kV and 2500 to 4000 A
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Pantograph Type
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Isolator
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suitable for outdoor installations with conductors arranged at two different levels, one above the other, requiring only a minimal ground space The three single poles necessarily need not to be in line as with the rotating pole type Each isolator pole is equipped with an earthing switch for earthing and shortcircuiting disconnected plant sections
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This is somewhat costlier but has several noteworthy advantages: Switching position is clearly recognizable from a large distance The catching range is wide Adjustment is easy because of stud-bolt arrangement Down-dropper connections are eliminated Foundations and supports are lighter and cheaper 2/1/2011
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Suspended contact could be arranged either transverse or parallel to the conductor
Maintenance is almost nil because of welded pantograph construction
Due
to dead centre interlock positive switching position is ensured
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Construction
The top intermediate piece arranged between the support insulator and the gear box with the pantograph serves as the mounting point for the bottom conductor
W here
earthing switches are required, the earthing contact also is attached to the top intermediate piece
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P antograph
is of welded aluminium
construction Together with the cast aluminium gear box, forms a mechanical unit to ensure protection against atmospheric influences To make universal connection possible, flat terminals are provided on all the four sides
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mounted between the pantograph joints dampens the vibrations in the pantograph caused by the short-circuit current The suspended contact situated above the isolator on the overhead line is grasped, when the pantograph is in closed position, with a high pressure, by a scissor action
Damper
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is transferred in pantograph joints and the gear box by way of flexible connections consisting of multilayered 0.1 mm silver-plated copper strips instead of multi-stranded conductors which are prone to corrosion due to their larger surface area
C urrent
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C orona
protection fittings attached to the pantograph arms serves as a catching device in case of vertical movement of the suspended contact
To prevent any accidental tendency for the pantograph isolators to open, the mechanism moves to a position beyond the dead centre of the linkage travel
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Two-post isolators in EHV ranges not favoured in our country but installed in one or two cases
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e-Post, Centr e Pillar Thr ee-
Rotating Type
Isolators Most popular Each phase has three insulator posts The outer ones carry the fixed contacts as well as the connections On the centre-post, a blade with moving contacts is mounted which rotates through 90 degrees on its own axis during connection or isolation
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The centre post moves through 70 deg
The last 20 deg is used to move the contacts at the end of the arm on their own axis
Turn until the maximum dimension of moving contacts engages with the fixed contacts to give the required pressure
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Principle
of fr ee entry
To allow a very high contact pressure to be used without the necessity of having a powerful operating mechanism
Disadvantage
in extreme cold climate due to the formation of ice
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The centre-rotating post horizontalbreak type isolator requires greater phase centres between units as compared to the vertical-break type
For installing in areas subject to snowstorms during winter, some form of ice-breaking contact should be provided
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In one design, a mechanical means to partly rotate the blade to break the ice before it is withdrawn from the contact
In another design, a hinged tip at the end of the blade gives a toggle action to break the ice on opening
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Metal enclosed
type
U sually
consists of a pair of contacts mounted in a gas-filled enclosure and operate as plain break device
May incorporate a quick break auxiliary contact, if needed, to interrupt the capacitive current of the associated busbars and connections of larger switching stations
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W hen
an isolator operates under these conditions, high frequency transient voltages may arise, typically of frequencies around 2 to 3 MHz, due to repetitive prestriking or restriking
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Earthing Switch
It is a mechanical switching device for earthing a part of the circuit and capable of withstanding short circuit current for a specified time
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Earthing
switches
Earthing switches are of the straight swinging arm type
Above 220 kV, semi-pantographic design is used, the advantages being
-Easy to operate due to light weight -C ounter-balancing problems are obviated 2/1/2011
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The three phases of 400 kV earthing switch are mechanically coupled
Manually operated by a horizontal wheel
A half blade design is adopted to accommodate electrical clearance
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Earth
switch
In open terminal substation, an earth switch may be a free-standing independent device or part of an isolator In metal enclosed switchgear, the earth switch may be one of the three types If it is a maintenance earth switch, its only duty is to close on to a dead circuit and the circuit will be earthed through the circuit breaker
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Earth
switch
If used to discharge high values of capacitance, it must close at high speed to minimize arcing
If used to short-circuit a fault in the event of an internal flashover, it must close at high speed against the rated fault current
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It is standard practice in the C EGB always to earth through a circuit breaker
The principal disadvantage of the other types being the danger of inadvertently tripping the switch on to a live system and losing all or part of a busbar
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p U
to 132 kV- manually operated
At 220 and 400 kV- power-operated
Operation speeds for 220 kV ± 3 secs and 400 kV ± 20 secs
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Interlocks
To ensure safety of equipment and operating personnel
To prevent unauthorized or inadvertent operation of equipment
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Types
Mechanical
Key
Electrical
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Mechanical - when the operating mechanisms are closed togethermaintenance purpose
Key - operated by inserting the correct key ± maintenance purpose
Electrical-operational purpose
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Key type ± used between circuit breakers and isolators
For 132 and 220 kV, electrical interlock is for switching operations such as onload busbar changeover and key interlock is for maintenance purposes such as circuit isolation and earthing
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Operating Mechanism of Isolators The operating mechanism of isolators is generally manual plus one of the following 1.
Electrical with motor mechanism
2.
P neumatic
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mechanism
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Operating Mechanism of Isolators
Manual operation alone will be provided, when the operation is infrequent and in minor substations
P ower operation is used when the
isolator is to be opened or closed from remote control rooms and the operation frequency is more
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Motor
operated isolators
In motor operated isolators suitable gears are used to reduce the speed of closing and opening of the switches
In the pneumatic mechanism,
compressed air provides reciprocating motion which is converted to rotary motion by rack and pinion gearing
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Inter locking and Isolator operations The following inter lockings are generally provided as applicable to the station bus bar arrangement (i)The bus isolating switch in a double bus arrangement can be opened only if both the circuit breaker and the other bus isolator of the circuit are open
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Interlocking (iii) In a combined isolator - earthing switch device, main isolator or earthing switch of a line can be operated only alternatively (iv) i.e., at any time one of the above shall be open, and this interlocking is provided by mechanical linkage
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Interlocking (iv)In a double bus arrangement, if one bus isolator is closed, the other bus isolator can be closed, only if the buses are coupled (v)Bus coupler isolators can be opened, only if the bus coupling circuit breaker is open
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Dimensions
The length of break of isolators, when in the fully open position, is at least 10 % in excess of the dry arcing distance at power frequency over the insulators
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Length of br eak Minimum
Nominal
voltage rating kV
No
of insulator units
metal-to-metal distances(mm/in)
Minimum
centerline-tocentr eline spacings of insulator(mm/in)
Single-
Double-
Single-
Double-
br eak
br eak
br eak
br eak
66
2
813(32)
559(22)
1070(42)
762(30)
110
3
1270(50)
813(32)
1520(60)
1070(42)
132
4
1520(60)
965(38)
1830(72)
1220(48)
220
5
2130(84)
1270(50)
2440(96)
1730(68)
6
2640(104)
1450(57)
2900(114)
1900(75)
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Nominal
voltage rating kV
No.
of insulator units
Minimum Vertical
br eak
spacing for insulators (mm/in) Horizontal
Horn-gap
type
66
2
type 1520(60)
110
3
2130(84)
2740(108)
3050(120)
132
4
2440(96)
3350(132)
3660(144)
220
5
3350(132)
4870(192)
4870(192)
6
3960(156)
5500(216)
5500(216)
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br eak type 1830(72)
2130(84)
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Basic
impulse level
The selection of insulation levels for airbreak isolators and bus supports is a function of -the degree of surge arrestor protection -the insulation level of associated equipment such as circuit-breakers and transformers -the degree of atmospheric contamination to be encountered, and -the amount of maintenance(insulator cleaning) anticipated
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Isolators are selected generally on the basis of impulse withstand voltage rather than nominal voltage rating
BIL should be equal to that of the associated circuit-breakers and transformers
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Testing
standar ds
Short-circuit tests are made to ensure that the isolator is capable of carrying its rated peak current and short time current
Isolators are rated in rms amperes based on a maximum temperature rise of 45 deg C above an ambient temperature of 40 deg C and the temperature rise shall not exceed this limit
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The isolator is located in a closed room and tested in a horizontal position
A conductor, at least 1.2 metre (4 ft) long is connected to each terminal and the ends into the test circuit
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The sizes of conductors used for tests for different current ratings Continuous current Conductor size, copper rating (amps) equipment of mm (in) 400
18 (3/4)
600
25 (1)
1200
50 (2)
1600
62 (2 ½)
2000
75 * 6 (3 * ¼) sq. tubing
3000
100 *10 (4 * 3/8) sq. tubing
4000
152 * 6 (6 * ¼) sq. tubing
5000
175 * 8 (7 * 5/16) sq. tubing95
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Dielectric tests
If a disruptive discharge does not occur during any of the three consecutive impulses when making the impulse test, the switch is considered to have met the test
If two of the three applied impulse waves cause disruptive discharges, the switch fails the test
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If only one of the three applied impulses causes disruptive discharge, three additional impulses are applied
If disruptive discharge does not occur on any of the three additional impulses, the switch is considered to have met the test
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The switch shall also withstand between live parts a test voltage which is 10 % in excess of the corresponding test voltage specified in table given
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Nominal
Number
voltage rating (kV)
of insulator units
Withstand Test voltage kV 60 hz kV rms Dry
Wet (10 secs) 145
Impulse 1.2 * 50 microsec kV cr est
66
2
(1 minute) 175
110
3
280
230
550
132
4
335
275
650
220
5
465
385
900
6
545
455
1050
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Ice testing
This test is stipulated for sites experiencing severe cold and snow storms
Difficulty
in switch operation is encountered with light coatings of ice formed at above ±10 deg C (15 deg F)
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Procedur e
The room temperature is lowered down to around 2 deg C
W ater
The temperature is further lowered down to between ±6.7 and ±3 deg C
is sprayed for a minimum period of one hour, maintaining the room temperature between 0.5 and 2.8 deg C
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spraying is continued until 18 mm thick ice is formed on the top surface of the test bar (25 mm * 600 mm long), mounted in a horizontal position near the isolator under test The spray is then discontinued but room temperature is maintained between ±6.7 and ±3 deg C for about 2 to 6 hours to equalize all parts of the switch to constant temperature
W ater
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The isolator is considered to have met the test, if it can be manually operated by a single operator without any damage either to it or operating mechanism
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Maintenance of Isolators
P eriodically the isolators and earthing switches shall be inspected and maintenance done
The main rotating bearings shall be greased with grease gun through the nipples provided 2/1/2011
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Maintenance of Isolators
Other moving parts in the tandem pipes and linkages shall be externally greased
The main contacts shall be thoroughly cleaned and white petroleum jelly applied
The auxiliary box shall be cleaned with a blower, terminal connections of wires tightened and fuse carriers checked 2/1/2011
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Maintenance of Isolators
The terminal connections from the main contacts shall be tightened wherever necessary
The flexible earth connection from the operating handle shall be checked and connection ensured
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Maintenance of Isolators
Y ard metalling for the desired thickness shall be invariably maintained at the places from where personnel operate the handles of main isolator and earthing switch
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BIS Specifications
9921(P1 to 5)- Alternating current isolators and earthing switches for voltages above 1000 V
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