An isolator is an electrical switching device that disconnects the equipment or electrical lines. It is generally an offline disconnector. It can be operated manually or automatically. Isolate generally means to disconnect so its name is an isolator. It ensures the total disconnection of the circuit such that no chance of any energizes part in the system. It is operated after the use of circuit breaker CB. There is always an ambiguity between isolator and CB. This article discusses the types of isolators depending on usage and construction, it’s applications for high and low voltages, for industries and substations and also differences between Circuit Breaker and Isolator.
What is an Isolator
Definition: Isolators are also called as disconnect switches which operate under no-load condition. They are not manufactured for any making or breaking current capacity. They don’t have an arc quenching system within it. Its main use in the electrical system is to disconnect a particular part after the current interruption from that particular part. They ensure there is no current in the circuit until everything is in order. They are placed on each side of the circuit breaker to provide isolation. Generally, an automatic type is preferred.
These switches are generally used for repair or replacement of a circuit breaker. It acts as a switch which disconnects the required part from the circuit where the repair has to be done. It does not have any current ratings as compared to CB as these are operated with current being made zero in the circuit. These switches cannot be opened unless CB is open and CB cannot be closed unless the isolator is closed.
Types of Electrical Isolators
They are mainly classified according to the construction, line voltage and position they are placed.
Construction and Isolation: depending on their construction and isolation they are classified as
- Double Break Isolator: This type has three stacks as shown in the figure, first is central or flat male contact which is the movable and the second one is female contact or fixed one which is fitted at both sides of the central one. The rotating one comes itself as fixed one to close the circuit and movement of the movable contact out of the fixed one disconnects the system.
- Single Break Isolator: The contact arm is divided into male and female contact and it moves due to the rotation of post insulator. Rotation in the opposite direction closes the circuit and counter-rotation of it opens the circuit. As shown in the figure.
- Pantograph Type Isolator: It has a supporting structure and a contact on the supporting structure as shown in the figure, which almost looks similar to the pantograph on the electric locomotive which is used as a current collecting device. It also has a fixed and movable contact, the movable contact connects or disconnects the circuit.
2). Depending on their Position
- Bus Side Isolator: These are connected to the main buses in the substation. Whenever there is a need for repair or replacement of any bus, they are operated.
- Lineside Isolator: These are connected between line and any feeder. A feeder is a line that carries power from Substation to the distribution system.
- Transfer Bus Side Isolator: In an electrical system for reliability main or transfer bus is used. In case the main bus side isolator is unavailable due to some fault/maintenance transfer bus isolator is used. These are used in combination with a circuit breaker.
3). Depending on Line Voltage
- Low Voltage: These isolators are designed to withstand voltage up to 11KV
- Medium Voltage: Isolators for voltages between 11KV to 66KV comes under this type.
- High Voltage: Isolators for voltages between 66KV to 330KV come under this category and they have a high withstanding capacity.
Isolator in Substation
Depending on the voltage in the substation, isolators of that voltage level is used. They are used to disconnect the transformers, circuit breakers and bus bars for repair and maintenance. As mentioned before, they are operated under no current conditions (no load) only since they don’t have rated making or breaking current capacity.
Isolator in Microwave
The main use in a microwave is to isolate the reflected wave from the output side to the input side. It just sends the energy signals in one direction that is from the input side to the output side, or it acts as a unilateral device. A mechanism with vertical and horizontal polarization is used such that energy cannot be transferred back to the input side. It should have a non-reciprocal material and at radio frequency ferrites are the best ones with a combination of static magnetic fields.
The main purpose is to disconnect or de-energize the circuit completely and for this purpose they are used in substations, power generating stations and signal isolation. It provides more safety for the operator as it ensures the total disconnection of the electrical system. It is provided before and after the circuit breaker in high voltage systems. First, the circuit breaker operates then the this operates such that the equipment, busbars, and transformers are safe.
Difference between Isolator and Circuit Breaker:
The main difference between Isolator and Circuit Breaker is, an isolator is offload switching whereas CB is on-load switching. Circuit Breaker is designed to operate under load conditions as they have the arc quenching mechanism when an energized part is disconnected arc is produced, and it should be quenched otherwise it will damage the equipment.
Always isolators are operated after the CBs are operated. While connecting, first the circuit breaker is connected then the isolator is closed. CB is operated after getting the fault signal from the relay to disconnect the faulty part from the healthy one with high current passing through that part, and then the isolator operates either manually or automatically depending on the availability.