What are the differences between a disconnect switch and a circuit breaker? In what situations are they used respectively?

Definition and Core Functions

1. Disconnect Switch

Definition: Commonly known as a "knife switch," it is a switching device that creates a visible disconnect point with a safe insulating distance in a circuit.

Core Function: Isolating power supply.

Key Characteristics:

No arc-extinguishing device: It cannot effectively extinguish the powerful arc generated when cutting off current.

Visible disconnect point: The circuit's state of connection or disconnection is directly visible from the outside.

2. Circuit Breaker

Core Function: Control and protection.

Usage Situations

1. Disconnecting switches are used in the following situations:

* Isolating power supply during equipment maintenance: This is its primary and fundamental purpose. When it is necessary to maintain circuit breakers, transformers, lines, etc., the circuit breaker must be disconnected first, and then the disconnecting switches on both sides must be opened, forming a "double insurance" to ensure complete isolation between the equipment under maintenance and live parts.

* Performing circuit switching (switching operations): In complex power distribution systems, it is used to switch equipment or lines from one set of busbars to another.

* Switching small current circuits: Under strict adherence to operating procedures, it can switch unloaded transformers with only a very small excitation current, or very short unloaded lines. However, this has strict limitations and cannot be used to disconnect load current.

In summary: Disconnect switches are used to establish a safe working environment "after a power outage".

2. Circuit breakers are used in the following situations:

* Normal load switching: Automatically or manually connecting or disconnecting normal load current as needed. For example, power supply and de-energizing a distribution room.

* Automatic fault current interruption: When a line or equipment experiences a short circuit or severe overload, the circuit breaker's protective device (such as a trip unit) will trigger its automatic trip, quickly disconnecting the circuit and preventing the accident from escalating. This is its most important protective function.

* As the system's main switch: Typically used as the incoming switch for power supply or the outgoing switch for important loads.

Circuit breakers are used for control "during operation" and protection "during a fault".

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ZN85-40.5 Vacuum circuit breaker