The Functions of Air Circuit Breakers and Disconnect Switches
Air circuit breakers (circuit breakers) and disconnect switches are commonly used switching devices in the power system, and
their **functions and application scenarios have significant differences**. The following is a detailed analysis of them:
I. Air Circuit Breaker (Circuit Breaker, Circuit Breaker)
1. Core Functions
- **Overload Protection**: When the circuit current exceeds the rated value (such as excessive load), the internal bimetallic strip
heats up and deforms, triggering the tripping mechanism to trip.
- **Short-circuit Protection**: When a short circuit occurs in the circuit (a sudden increase in current), the electromagnetic trip
device acts quickly and trips instantly.
- **Arc Extinguishing Ability**: Use air as the medium to extinguish the arc, and it can open and close the circuit with a load.
2. Working Principle
- **Bimetallic Strip + Electromagnetic Trip Device**:
- Normal Current: The bimetallic strip does not act, and the switch is closed.
- Overload: The bimetallic strip bends due to excessive current heating, pushing the trip device to trip.
- Short-circuit: The electromagnetic trip device senses the large current and directly triggers the trip.
3. Main Functions
- **Residential Power Distribution**: Protect circuits such as lighting, sockets, and air conditioners to prevent fires caused by
overloads (such as type C/D circuit breakers in residential distribution boxes).
- **Industrial Control**: Serve as the power switches for motors and frequency converters, providing overload and short-circuit
protection.
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- **Automatic Reclosing**: Some circuit breakers support manual/automatic closing after the fault is eliminated (need to be used
in conjunction with a residual current operated protector).
II. Disconnect Switch (Disconnect Switch)
1. Core Functions
- **Electrical Isolation**: After disconnection, a clearly visible **physical break** is formed to ensure that the circuit is completely
separated from the power source during maintenance, ensuring the safety of personnel.
- **No Arc Extinguishing Ability**: **It is strictly prohibited to operate with a load**, and it can only be opened and closed when
there is no current or a small current in the circuit.
2. Working Principle
- **Mechanical Disconnection**: Separate the knife switch or contact blade from the stationary contact to form an air gap (the
break distance needs to meet the insulation requirements of the voltage level).
3. Main Functions
- **Safe Maintenance**: After the circuit breaker is opened, operate the disconnect switch to isolate the equipment from the
power source (such as when maintaining a transformer in a substation).
- **Circuit Switching**: Switch the circuit path in a low-voltage system (such as dual-power switching, but it needs to be used
in conjunction with a circuit breaker).
- **High-voltage System**: Used for lines of 10kV and above, and cooperate with the circuit breaker to achieve the operation
sequence of "open - break - close".
III. Key Difference Comparison
| Characteristics | Air Circuit Breaker (Circuit Breaker) | Disconnect Switch |
| Protection Function | Has overload and short-circuit protection | No protection function |
| Arc Extinguishing Ability | Strong (can open and close with a load) | None (operating with a load is strictly prohibited) |
| Visible Break | None (internal structure) | Yes (mechanical knife switch separation) |
| Operation Mode | Manual/automatic tripping | Manual operation |
| Application Scenarios | Protection and control of load circuits | Main power switch, maintenance isolation |
IV. Typical Application Scenarios
1. Residential Distribution Box
- **Main Switch**: **Disconnect Switch** (such as a 2P knife switch) + **Air Circuit Breaker** (main circuit breaker).
- Power-on Sequence: Close the disconnect switch → Close the main circuit breaker → Close the circuit breakers of each circuit.
- Power-off Sequence: Open the circuit breakers of each circuit → Open the main circuit breaker → Open the disconnect switch.
- **Branch Switches**: Use air circuit breakers (such as C16, C20) for the circuits of each room.
2. Industrial Power Distribution Cabinet
- **Main Circuit**: Disconnect switch (power inlet) + Circuit breaker (motor protection) + Contactor (start-stop control).
- **Maintenance Scenario**: First open the circuit breaker → Then open the disconnect switch → Hang the grounding
wire → Start the maintenance.
V. Selection and Precautions
1. Selection of Air Circuit Breakers
- **Rated Current**: ≥ The maximum working current of the line (for example, select C25 for the air conditioner circuit
and C10 for the lighting circuit).
- **Interrupting Capacity**: Select 6kA for residential use and above 10kA for industrial use.
- **Pole Number**: Use 1P/2P for single-phase and 3P/4P for three-phase.
2. Selection of Disconnect Switches
- **Rated Voltage/Current**: Match the system voltage (such as AC 220V/380V), and the current ≥ The rated current of the line.
- **Protection Level**: Select IP54 for outdoor use and IP20 for indoor use.
- **Operation Mode**: Manual lever type or electric operation (for high-voltage scenarios).
VI. Common Misunderstandings
- **Misunderstanding 1**: Using a disconnect switch instead of an air circuit breaker.
*Correction*: The disconnect switch has no protection function and cannot replace the circuit breaker.
- **Misunderstanding 2**: Directly closing the air circuit breaker after it trips.
*Correction*: First, check for faults (such as short circuits, overloads), and then close the circuit breaker after resetting.
Conclusion
- The **air circuit breaker** is a **"protector"**: It actively cuts off the fault current to ensure the safety of the circuit.
- The **disconnect switch** is a **"isolator"**: It passively provides a visible break to ensure the safety of maintenance.
The two are used **complementarily** and jointly form the "safety line" of the power system.
