The function of an isolating switch

Isolate the power supply to ensure safety (primary function):

This is the most essential and basic function of an isolation switch. When electrical equipment (such as Circuit Breakers, transformers, lines, etc.) needs to be repaired or maintained, it must be completely disconnected from the live power source first.

Isolation switches can provide a clear and visible break point. The staff can clearly see with the naked eye that the knife switch of the switch has been completely disconnected, forming a sufficiently safe physical isolation distance to ensure that the maintenance part is indeed not electrified, thereby ensuring the personal safety of the maintenance personnel.

It usually comes with a locking device that can be locked in the open position to prevent accidental closing due to misoperation.

Switching operation mode (switching circuit):

In some circuits, especially in systems with dual bus connections or bypass buses, isolation switches are used to change the connection mode of the circuit.

For example, a line or device can be switched from one set of busbars to another, or a bypass can be connected/disconnected.

Important note: This switching operation must be carried out without load current (or with only a small current generated by Voltage Transformers, lightning arresters, etc.). The isolation switch itself does not have arc extinguishing capability and cannot be operated with load.

Connecting and disconnecting low current circuits:

Under specific conditions, isolation switches can be used to connect and disconnect some small current circuits, such as:

• The current of voltage transformers (PT) and lightning arresters.
• The no-load charging current of the busbar and equipment is very small.
• Some no-load transformers (with very small capacity and strictly calculated and regulated).

But it must never be used to connect or disconnect load current or short-circuit current.

Summarize key points:

Core function: Provide a visible and reliable disconnection point for isolating power supply and ensuring maintenance safety.

Operating conditions: It is strictly prohibited to operate with load. The isolation switch (whether opening or closing) can only be operated when the circuit has been disconnected by the circuit breaker and there is no load current passing through. On the contrary, during power transmission, the isolation switch must be closed first, and then the circuit breaker must be closed.

No arc extinguishing capability: The isolation switch does not have a dedicated arc extinguishing device, and forced operation with load can cause strong arcs, burn equipment, and even cause personal injury.

Simply put, an isolation switch is like a "visible safety gate" in a circuit, with the main purpose of clearly "disconnecting" when needed to ensure safety, rather than being used to "turn on/off lights" (i.e., to turn on/off normal operating current). The task of making and breaking current is usually accomplished by a circuit breaker.