What is Isolator? Types of Isolator and their operation?
Table of Contents
What is function of Isolator?
Construction of Isolator
Isolator Operation
Types of Isolator
o Horizontal Break ( normally Double break) Isolator
o Vertical Break (normally single break) Isolator
o Pantograph type Isolator
Application of Isolator
What is function of Isolator?
Isolator may be defined as a mechanical switch which isolates a part of the circuit from the system
as and when required.
Electrical isolators separate a part of the system from rest for safe maintenance works. Isolators
are used to open a circuit under no load.
Its main purpose is to isolate one portion of the circuit from the other and is not intended to be
opened while current is flowing in the line (on load).
Isolators are generally used on both ends of the breaker so that repair or replacement of circuit
breaker can be done without any danger.
In high-voltage power systems, isolators are use in conjunction with circuit breakers to provide an
extra layer of safety.
Construction of Isolator
Isolators used for power systems are three pole isolators.
All the poles are identical and operated by a single mechanism and mechanical interlocking.
Each pole has a fabricated support on which two or three insulator posts are mounted.
The live part which is used for isolation is supported on the insulator posts
The live part has a conducting copper or aluminium rod with fixed or moving contacts
When the moving part is operated, the conducting rods move apart to achieve isolation.
Isolator Operation
Isolators must be opened after circuit breaker is open, and these must be closed before circuit
breaker is closed.
The isolator can be operated by hand locally as well as by motorized mechanism from a remote
position.
Motorized operation arrangement costs more compared to hand operation; hence decision must be
taken before choosing an isolator for the system whether hand operated or motor operated
economically optimum for the system.
For voltages up to 145 KV system hand operated isolators are used whereas for higher voltage
systems like 245 KV or 420 KV and above motorized isolators are used.
Types of Isolator
Horizontal Break ( normally Double break)
Vertical Break (normally single break)
Pantograph type
Horizontal Break ( normally Double break) Isolator
Construction of Horizontal Break (normally Double break)
Fig. shows the schematic diagram of a horizontal double Break Isolators. It consists of three insulator stacks
per pole. Two stacks on each side are fixed. The stack at the center is rotating one. The central stack can
swing about its vertical axis through about 90 degrees.
The central post insulator carries a tubular or flat moving contact which can be rotated horizontally with a
rotation of central post insulator. The fixed type contacts are fixed on the top of the other two post insulators.
The fixed contacts are generally in the form of spring-loaded figure contacts.
The rotational movement of moving contact causes to come itself into fixed contacts and isolators becomes
closed. The rotation of moving contact in the opposite direction make it to come out from fixed contacts
and isolators become open.
Operation of Horizontal Break (normally Double break)
Fixed contacts of metal are supported on stack of insulator. Moving contact rod is connected to central
insulating support which can be moved to have horizontal movement for opening or closing into the fixed
contacts.
The movement of the rod and its support may be done manually, electrically or pneumatically by operators.
The operating mechanism disengages or engages the rod into fixed contacts for closing and opening of
isolator.
The moving contact’s movement is horizontal and hence this type of isolator is named as horizontal break
isolator. The rod contact can be moved through 90 degrees.
Applications: Used in various power systems where horizontal space is more available.
Vertical Break (normally single break) Isolator
The vertical break type isolators consists of a fixed contact and a rotating switch blade( moving contact).
The rotating switch is supported by middle or the central post, whereas the fixed contact is mounted on
the top of the right most post.
During opening and closing, the switch blade vertically moves in or out of the fixed contact. The fixed
contacts are silver surfaced and lock the blade in place by increasing pressure on the switch blade during
momentary current flow.
Constant pressure is maintained by insulated stainless steel back up springs. The arcing horns divert the
current from primary contact and eliminates contact pitting and erosion.
Applications: Commonly used in substations and high-voltage applications where space is limited.
Pantograph type Isolator
Pantograph is a mechanical linkage connected in a manner based on parallelograms so that the movement
of one pen, in tracing an image, produces identical movements in a second pen. If a line drawing is traced
by the first point, an identical enlarged, or miniaturized copy will be drawn by a pen fixed to the other.
Construction of Pantograph type Isolator
The Pantograph isolators are single break and vertical reach type . The pantograph mechanism is installed
on the support insulator and transfers the movement of the operating rod insulator to the bars.
The movement mechanism is housed in an Aluminium enclosure, protecting it from pollution and ice.
Pantograph bars are built using light tubes made of aluminium magnesium alloy. It has high mechanical
strength and very good conductivity.
Due to the high elasticity of the material, pantograph construction is flexible. The upper pantograph bars
are fitted with silver plated contact strips. The lower bars are connected to the bottom aluminium housing
fitted on the supporting insulator post.
The current transfer takes place from the top bus bar to the bottom bus bar through through four arms .
Pantograph isolators cover less floor area. These are used at ultra high voltage substations.
Operation of Pantograph type Isolator
In the closed position, the upper two arms of the pantograph close on the overhead bus bar giving a grip.
The conducting pantograph bars carry the current from the upper bus bar to the lower bus bar.
To open the contacts the rotary insulator is rotated about its axis. The pantograph arms collapse in vertical
plane and the vertical isolation is achieved between the line terminal and the pantograph upper terminal.
By rotating the insulator column in the opposite direction, the linkage of the pantograph are brought
nearer while closing it again.
Applications: Commonly found in railways and high-voltage transmission lines, where they are needed
to provide isolation for maintenance and safety
