What are the interlocking devices, working principles, and structure of switchgear?

Switchgear interlocking, also known as "five-prevention interlocking," aims to prevent misoperation and ensure the mandatory, correct, and sequential nature of operating procedures, thereby avoiding:

Incorrect opening or closing of circuit breakers

Opening or closing disconnecting switches under load (one of the most dangerous operations, generating a strong electric arc)

Incorrectly closing grounding switches while energized (causing a three-phase short-circuit to ground fault)

Incorrectly closing and energizing the circuit with the grounding wire connected (grounding switch closed)

Incorrectly entering a energized compartment

I. Classification and Structure of Interlocking Devices

Switchgear interlocking is mainly divided into two categories: mechanical interlocking and electrical interlocking. Modern switchgear typically uses a combination of both.

1. Mechanical Interlocking
This is achieved through the physical interference of mechanical components (such as metal baffles, linkages, keyholes, keys, program locks, etc.), resulting in extremely high reliability.

♦Structural Components:

Linkage Mechanism: Metal linkages connecting the circuit breaker operating mechanism, disconnector (or truck), cabinet door, and grounding switch.

Baffle/Vehicle: Insulating baffles installed in front of the stationary contacts in the busbar compartment and cable compartment, automatically opening and closing with operation.

Program Lock/Key Box: A series of mechanical locks and matching keys; the operation of one key is a prerequisite for the release of the next key.

♦Common Interlocking Types:

Door and Grounding Switch Interlock: The cabinet door can only be opened after the grounding switch is closed; conversely, the grounding switch can only be opened after the cabinet door is closed and locked.

Trolley Position Interlock: The circuit breaker trolley can only be operated (closed/opened) in the "test position" or "operating position"; secondary connectors can only be plugged in or unplugged in the "test position".

Trolley and Grounding Switch Interlock: When the trolley is in the "operating position" (i.e., the system is energized), the grounding switch cannot be closed; when the grounding switch is closed, the trolley cannot be moved to the operating position.

Automatic Door Opening and Closing Mechanism: During the movement of the trolley from the test position to the operating position, the mechanical linkage will push open the insulating baffle (door) of the stationary contact, exposing the contact for connection; when the trolley exits, the door automatically closes, isolating the energized body.

Slider and Bolt: In truck-type switchgear, used to lock the truck position and chassis.

 

2. Electrical Interlocking

Logic control is achieved through auxiliary contacts (microswitches), electromagnetic locks, relays, and secondary circuits.

Structural Components:

Auxiliary Switches (S8, S9, etc.): Microswitches installed on circuit breakers, disconnectors, grounding switches, and handcart chassis to detect their position status (closed/open, working/testing).

Electromagnetic Locks: Installed on components requiring restricted operation (such as cabinet door handles, grounding switch operating holes), locking or releasing when energized or de-energized.

Secondary Circuits: The contacts of the above components are connected in series or parallel to form the control logic circuit.

 

Example of operating principle (preventing disconnection under load):

The normally closed auxiliary contact of the circuit breaker is connected in series in the operating power supply circuit of the disconnection switch.

Only when the circuit breaker is in the open state will its normally closed contact close, allowing the operating motor or electromagnetic lock of the disconnection switch to receive power and enabling operation.

If the circuit breaker is in the closed state (under load), its normally closed contact will open, and the disconnection switch will be electrically locked and cannot be operated.

 

II. Core Working Principle (Taking the common KYN28 centrally mounted switchgear as an example)

 

Let's understand how interlocking works collaboratively using a typical operating procedure:

Scenario: From maintenance to power supply

1.Initial State: The circuit breaker trolley is outside the cabinet, the grounding switch is closed, and the cabinet door is open (maintenance completed).

2.Door Closing Preparation: Close and lock the front and rear cabinet doors. At this time, the mechanical interlock unlocks the grounding switch operating shaft.

3.Opening the Grounding Switch: Operate the grounding switch to the open position. Its auxiliary contacts (electrical interlock) change state.

4.Pushing in the Trolley: Push the circuit breaker trolley into the cabinet to the "test position". The mechanical device automatically locks the chassis and triggers the "test position" auxiliary switch.

Critical Interlock: At this time, because the trolley is not in the working position, the mechanical linkage does not open the stationary contact valve. Even if the busbar is energized, the trolley contacts are isolated from the stationary contacts.

5.Secondary Plug Connection: In the test position, the trolley is locked, and the secondary control plug can be safely inserted.

6.Cranking the Handcart to the Working Position: Use the crank to crank the handcart from the test position to the working position.

Process Interlock: During cranking, the mechanical linkage opens the insulating valves on the busbar and cable sides, exposing the stationary contacts.

Position Locking: After reaching the working position, the mechanical device locks again. The "Working Position" auxiliary switch is activated.

Core Interlock Activation: When the handcart is in the working position, its mechanical structure blocks the operating hole of the grounding switch, preventing it from closing (mechanical interlocking). Simultaneously, the working position signal is sent to the electrical circuit.

7.Closing Operation:

Electrical Interlock Check: The closing circuit typically includes contacts connected in series such as "handcart in working/test position" and "grounding switch open." The closing circuit is only open when all conditions are met.

Pressing the closing button closes the circuit breaker and energizes the line.

 

Reverse Operation (Power Outage Maintenance)

The sequence is strictly reversed: Open the circuit breaker → Move the handcart to the test position (the valve automatically closes, isolating the live static contact) → Open and lock the circuit breaker → Close the grounding switch → Open the cabinet door. Any step in the sequence will be prevented by the interlocking device.

 

Summary

 

Features	Mechanical Interlock	Electrical Interlock
Core Principles	Physical component interference	Electrical logic control
Reliability	Extremely high, unaffected by circuitry	High, but dependent on power supply and components
Flexibility	Low, structurally fixed	High, complex logic can be achieved by modifying the circuit
Main Components	Links, baffles, program locks, keys	Auxiliary switches, electromagnetic locks, relays
Functional Focus	Achieve direct, critical operational mutual exclusion (e.g., door and floor knife)	Enable conditional judgment and remote/automatic control
Cost	Relatively low	Relatively high

 

Modern Trends: In intelligent switchgear, in addition to traditional mechanical and electrical interlocking, intelligent interlocking based on PLCs (Programmable Logic Controllers) and integrated protection devices is also integrated. The system can collect all position and status signals, perform more complex logical judgments, and provide operation prompts through HMIs (Human-Machine Interfaces), even enabling remote authorization and programmed operation, thus elevating error prevention to a new level.

In summary, switchgear interlocking is a multi-layered, three-dimensional safety protection system, and its design and application are indispensable cornerstones for ensuring the safe operation of power systems.

 

Contact Us

 

Shaanxi Huadian Intelligent Switchgear deeply integrates "mechanical-electrical-intelligent" triple interlocking protection, embedding "five-proof" safety into its product DNA. Our precision-forged linkages, baffles, and programmable lock mechanisms achieve forced interlocking through pure mechanical mechanics. Absolute mutual exclusion between the handcart and grounding switch, and between the cabinet door and operating permissions, eliminates the possibility of human error. Optional integrated PLC control and intelligent monitoring units enable full status perception, programmable logic, and remote programmed operation. Not only does it prevent misoperation, but it also provides predictive maintenance and digital management interfaces. Shaanxi Huadian Intelligent Switchgear—deep interlocking, intelligent protection, ensuring every operation is precise and error-free. Contact information:pannie@hdswitchgear.com.