Differences between Circuit Breaker and Isolator Switch?

Figure 1: A circuit breaker in operation.

A circuit breaker refers to a switching device that can close, carry and break current under normal circuit conditions and can do the same under abnormal circuit conditions within a specified time.
The isolator switch, also called isolating switch or disconnector, is a switching device mainly used for isolating power supply, switching off operation, connecting and cutting off small current circuits, and without arc extinguishing function.
Both the two devices can control the on and off of the circuit, but they are different devices with different working principles and functions, which will be elaborated as follows.

Different Working Principles

The working principle of the circuit breaker: when a short circuit occurs, the magnetic field generated by the large current overcomes the reaction force spring; the release pulls the operating mechanism to act, and the switch trips instantaneously.
When overloaded, the current becomes larger and the heat generation increases; the bimetallic sheet deforms to a certain extent to promote the action of the mechanism; the larger the current, the shorter the action time.

The isolator switch needs a manual operating mechanism or other electric (or pneumatic) operating mechanisms; the two contacts of the isolator switch are opened or closed to provide circuit conditions for the circuit power supply.
The isolator switch has no arc extinguishing ability, so it can only be used as the isolation element of the circuit, and is installed at the head of the circuit or in front of the breaking element that needs to be repaired.

Figure 2: Harok rotary isolator switch.

When the circuit needs power failure maintenance, the breaking element first disconnects the circuit power supply, and then the isolator switch is opened to ensure that the circuit has an obvious disconnection point to ensure personal safety.

Different Functions

Circuit breakers can be used to distribute electrical energy, start asynchronous motors infrequently, and protect power lines and motors. When serious overload or short-circuit and under-voltage faults occur, they can automatically cut off the circuit. Its function is equivalent to the combination of fuse switch and overheating relay. Moreover, the parts of the circuit breaker generally do not need to be changed after breaking the fault current.

Figure 3: Operating a circuit breaker.

After opening, the function of the isolator switch is to establish a reliable insulation gap, and separate the equipment or line that needs to be repaired from the power supply with an obvious disconnection point to ensure the safety of maintenance personnel and equipment.
It can also divide and combine small currents in the line, such as the charging current of bushings, busbars, connectors, and short cables, the capacitive current of grading capacitors, the circulating current when double busbars are switched, and the exciting current of voltage transformers, etc.

In general, the circuit breaker and the isolator switch work together, with the former just downstream of the latter. But the roles of the two are different. The function of the circuit breaker is to carry and break the fault current. The isolator switch is disconnected to cut off the downstream circuit breaker in the event of a circuit breaker failure, such as for maintenance. In addition, the isolator switch does not have the function of breaking the fault current, and it is opened or closed by human will.

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