Switches and disconnectors
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In power systems, what is the difference between switches and disconnectors? Do they have the same tasks? Are there any regulations regarding the sequence of operation switching?
The function of a knife switch is to serve as a distinct breaking point in an electrical circuit, but it cannot interrupt current under load and lacks the capability for overload or short-circuit protection； The switch features long-delay overload protection, instantaneous protection, and short-circuit protection, and is capable of breaking under load.
One can carry load and break circuits, while the other cannot
A knife switch serves an isolation function, while a circuit breaker is used to disconnect the load.
A knife switch serves an isolation function, while a circuit breaker is used to disconnect the load.
The circuit breaker you mentioned can be considered similar to a disconnect switch; it serves merely as a point for disconnecting the circuit, without any functions for protecting the wires or loads. It is used as a clear point of disconnection during maintenance, and it cannot be switched off while there is current flowing through it, as this would cause arcing and lead to accidents. The switch you’re referring to is of the circuit breaker type; it can monitor parameters such as voltage and current in the circuit, issue alerts, and take action. It serves not only as a switching point but also provides basic protection functions against under-voltage, over-current, short circuits, and ground faults. Thanks to its arc-quenching device and internal structure, it is also able to open the circuit while carrying a load. In power supply systems, the isolator is generally located above the circuit breaker; therefore, the sequence of operation is as follows: when closing the circuit, the isolator is closed first, followed by the circuit breaker ； When shutting down, first disconnect the circuit breaker, then disconnect the isolator.
A knife switch belongs to the category of isolating switches; it is not designed to operate under load and can only serve a separating function. A circuit breaker, on the other hand, can operate under load and provides protection against overcurrent and overload conditions. Generally, isolating switches and circuit breakers are used together – during power supply, the isolating switch must be closed first, followed by the circuit breaker; during power disconnection, the circuit breaker must be turned off first, after which the isolating switch can be turned off