Disconnect switches (also called isolation switches or isolators) are a control device without an arc-extinguishing mechanism. Their primary function is to isolate the power source to ensure safe maintenance of other electrical equipment. They must never be operated under load. Under specific conditions, they may make or break low-power circuits. They are the most frequently used devices in high-voltage switchgear.

Section 01 — Definition and Selection of Disconnect Switches

What Is a Disconnect Switch?

A disconnect switch opens a circuit carrying no load current, creating a clearly visible break point between the equipment under maintenance and the power source — ensuring physical safety of maintenance personnel. Since it has no arc-extinguishing capability, it cannot interrupt load current or short-circuit current, and must only be operated after the upstream circuit breaker has been opened.

Standard Switching Sequence

• Energizing: Close the disconnect switch first → then close the circuit breaker and load switches.

• De-energizing: Open the circuit breaker and load switches first → then open the disconnect switch.

Selection Criteria

Like all electrical equipment, disconnect switches must be selected based on the circuit's rated voltage, dynamic fault current, and thermal fault current. All parameters must comply with the application's requirements.

Qualifying isolation devices (with visible break indication) include: disconnect switches, fuse disconnectors, knife-blade disconnects, and plug-and-socket connector strips.

The disconnect switch must satisfy the circuit's rated voltage and calculated current, and be verified for short-time and peak withstand current. When switching capability is required, its breaking capacity must exceed the circuit's expected fault current. Important: when used purely as an isolation device, the handle cannot interrupt load current. Disconnect switches with load-breaking capability must use a fused knife-blade switch (arc shield type).

Section 02 — Purpose and Required Installation Locations

Why Install Disconnect Switches?

Whenever maintenance, testing, or repair work requires the power source to be disconnected, an isolation device must be provided. The purpose is to: (1) guarantee the personal safety of maintenance personnel, and (2) limit the scope of the affected circuit during an electrical system fault.

Required Installation Locations per GB 50052-2009

According to the Code for Design of Power Supply and Distribution Systems (GB 50052-2009), Clause 7.0.10:

"Distribution lines entering a building from outside shall be equipped with an isolation device at the indoor demarcation point, installed in a location convenient for operation and maintenance."

This applies to all building types — schools, individual villas, commercial buildings, etc. Any facility bringing a 220/380 V supply from outside to a main distribution board (MDB) must install a disconnect switch at the supply side of that MDB.

In addition, the following locations in high-rise buildings also require disconnect switch installation:

• Trunk-type (bus riser) floor distribution panels in high-rise buildings.

• Output circuit ends at elevator control panels within elevator distribution boards.