Why are there disconnect switches installed both upstream and downstream of circuit breakers in substations?

Disconnect switches are installed at both ends of a circuit breaker to create a "visually verifiable, safe electrical isolation point." This ensures absolute personnel and equipment safety during maintenance of the circuit breaker, power lines, or other equipment.

Core Reason: Creating Safe Maintenance Conditions

A circuit breaker is a switching device designed to connect and disconnect both normal and fault currents. However, it is an enclosed unit with internal contacts, making it impossible to visually confirm whether it is truly open from the outside.

The primary function of a disconnect switch (knife switch) is not to interrupt current flow. Instead, after the circuit has been opened by the circuit breaker, it creates an air insulation gap, providing a visually verifiable, definite point of disconnection.

Specific Functions and Application Scenarios of Two Disconnectors

Suppose we need to maintain the circuit breaker shown in the diagram. The standard safety procedure is as follows:

Step 1: Disconnect the Circuit Breaker

First, ensure the circuit breaker has interrupted the current in the circuit. It is the only device capable of safely interrupting load current or fault current.

Step 2: Disconnect the downstream isolating switch (DS1, line-side switch)

Purpose: Isolate potential backfeed from the load side (e.g., the line).

Scenario: If the line is connected to other power sources (such as distributed PV systems or backfeed from other substations), disconnecting this switch ensures no electricity flows back from the load side during breaker maintenance.

If mishandled: Operating the switch under load while the circuit breaker remains closed will generate a massive arc, causing severe personal injury and equipment damage (commonly known as "switching under load").

Step 3: Disconnect the upstream isolating switch (UDS, bus-side switch)

Purpose: Isolate current from the power source side (bus).

Scenario: After disconnecting this switch, the circuit breaker under maintenance is completely isolated from the energized busbar, creating a safe working area.

In case of incorrect operation: Similarly, operation under load is strictly prohibited.

After completing the above steps, maintenance personnel will also attach grounding wires on both sides of the circuit breaker and perform voltage verification, ultimately establishing a "triple protection" safety working environment.

Therefore, the presence of disconnect switches at both ends of the circuit breaker is a critical design feature in electrical systems to ensure "five-prevention" safety (particularly preventing switching operations under load and preventing entry into energized compartments). It achieves reliable electrical isolation, providing the most fundamental safety assurance for equipment maintenance and servicing.

VS1-24 630A 25KA Vacuum Circuit Breaker

VS1-24 630A 25KA Vacuum Circuit Breaker produced by Shaanxi West Power Tongzhong Electrical Co., Ltd.is an indoor equipment for rated voltage 24kV, three-phase AC 50Hz. It is suitable for protecting and controlling electrical equipment in industrial and mining enterprises, power plants, and substations. This type of circuit breaker conforms to national standards and IEC standards, with performance levels reaching M2-E2-C2.

The circuit breaker features composite insulated cylinder, spring energy storage operation, and screw propulsion. It is characterized by its compact size and reliable operation. Additionally, it can be easily adapted to switchgear, meeting the "Five Prevention" requirements of switchgear.

VS1-24 630A 25KA Vacuum circuit breaker