Principles of Circuit Breakers, Isolating Switches, Contactors, Relays, and Changeover Switches

A circuit breaker, including high voltage and low voltage types, is an electrical protection device available in various forms, all featuring arc extinguishing devices. The air break switch, officially known as an air-type low voltage circuit breaker, is a specific type of circuit breaker mainly used in low voltage circuits. It utilizes air as the medium for arc extinction, hence it is referred to as an air-type low voltage circuit breaker or simply an air break switch, which is commonly used in household electrical distribution.

Dc Islators

Isolating switches are a type of high voltage switching equipment primarily used in high voltage circuits. They are switch devices without arc extinguishing capabilities, mainly used to disconnect circuits carrying no-load currents and to isolate power sources. In the open position, they provide a clear disconnection point to ensure the safe maintenance of other electrical equipment. In the closed position, they can reliably carry normal load currents and short-circuit fault currents. However, due to the lack of specialized arc extinguishing devices, they cannot disconnect load currents or short-circuit currents. Therefore, isolating switches should only be operated when the circuit has already been disconnected by a circuit breaker, and they must not be operated under load to avoid severe equipment damage and personal injury. Only voltage transformers, lightning arresters, and unloaded transformers with excitation currents not exceeding 2A, as well as unloaded lines with currents not exceeding 5A, can be directly operated using isolating switches. In power applications, circuit breakers and isolating switches are often used in conjunction, with circuit breakers for making and breaking load (fault) currents, and isolating switches for creating a clear disconnection point.


Circuit breakers and air break switches are collectively referred to as circuit breakers. Below are the differences between isolating switches and circuit breakers:

  1. Isolating Switch: Similar to a knife switch, it lacks protection against overcurrent and short circuits, and does not have an arc extinguishing device; it cannot disconnect load currents or short-circuit currents.
  2. Circuit Breaker: Features automatic trip functions for overcurrent and short circuits, equipped with de-magnetizing arc extinguishing devices, capable of connecting and disconnecting large currents.

Isolating switches serve only an isolating function; they cannot automatically disconnect a circuit in case of downstream faults but can be manually disconnected during maintenance to ensure the safety of personnel.

Circuit breakers not only serve an isolating function but also protect downstream lines and equipment. They contain thermomagnetic protective components and can be fitted with various accessories for remote control.

The term “air break switch” is commonly used to refer to circuit breakers. Isolating switches are typically used on the high voltage side to isolate circuits, mainly providing a clear separation point during maintenance and installation, and are often used in conjunction with short-circuiters. Circuit breakers are used in both high and low voltage applications and provide protection; some high voltage types feature arc extinguishing capabilities. Air break switches are mostly found on the low voltage side, like typical household switches and lighting control switches, and also include overcurrent and leakage protection functions.