How to choose miniature circuit breaker?

How to choose miniature circuit breaker?

According to online resources, when choosing a miniature circuit breaker (MCB), one should consider factors like the current rating, tripping curve, breaking capacity, pole types, and voltage rating. Each of these specifications plays a crucial role in ensuring the MCB meets the required electrical safety standards and performance characteristics.

The miniature circuit breaker is essential for protecting electrical lines against overload and short circuits, specifically for circuits operating at a rated voltage of 400V and below and rated current of 125A.

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Miniature circuit breakers have gained widespread use across various sectors including industry, commerce, high-rise buildings, and residential areas, thanks to their compact design, modular functionalities, aesthetic appeal, and safety features.

Main Parameters of Miniature Circuit Breaker

1. Rated Voltage: This voltage value indicates the operational voltage at which the miniature circuit breaker functions. It is crucial that the rated voltage of the MCB is not less than the circuit voltage.

2. Rated Current: Referring to the maximum sustained current, this parameter indicates the normal operating point of the MCB. When the load current rises above this threshold, the thermal trip mechanism activates, causing the MCB to turn off.

3. Shell Rating Current: This refers to the rated current that the MCB's housing can support, which is different from the MCB's rated current. Identical frame sizes can accommodate varied ratings, impacting the overall dimensions of the circuit breaker.

4. Rated Breaking Capacity and Ultimate Breaking Capacity:

(1) The rated limit short-circuit breaking capacity is the maximum short-circuit current the MCB can withstand before tripping permanently.

(2) The rated operation short-circuit breaking capacity is the MCB's ability to trip due to short-circuit current during normal operation, which allows re-use after tripping.

5. Circuit breakers are classified based on their tripping characteristics: B, C, D types.

Type B has a tripping current between 3 to 5 times the rated current, suitable for household appliances and personal safety. Type C, with a tripping current between 5-10 times the rated current, is ideal for lines with high inrush current. Type D, having a tripping current of 10 to 20 times, is maximized for protection against equipment with high impulse currents.

Selection Rules for Miniature Circuit Breaker

General Selection Rules

1. The rated working voltage of the MCB must match or exceed the rated voltage of the circuit.

2. The rated current of the MCB should equal or exceed the calculated load current of the circuit.

3. The rated short-circuit breaking capacity must meet or exceed the maximum possible short-circuit current in the circuit.

4. The instantaneous release setting should be lower than 0.8 times the single-phase ground short-circuit current at the end of the line.

5. Take into account short-time delay features and coordination between protection phases.

6. Choose the number of poles for the MCB based on the lines that need protection, ensuring proper coverage for single-phase and multi-phase systems.

NDB1-125 Series

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NDB1-125 Series wiring diagram

7. Determine the amperage of the miniature circuit breaker based on the load; for instance, with a line carrying capacity of 20A and an appliance rating of 10A, the circuit breaker should be greater than 10A but less than 20A.

8. The type of instantaneous protection is based on the appliance. Types B, C, and D have corresponding protection capabilities for specific current needs.

Selection for Conductor Protection

Adjust the long delay current to be less than the line load current, and set the instantaneous action between 6-20 times the calculated load current.

Selection for Motor Protection

Long delay current should match the rated current of the motor, whereas the instantaneous action should be 8-15 times for squirrel cage motors and 3-6 times for wound motors.

Choosing Miniature Circuit Breakers Using Online Tools

On Quisure.com, it’s easy to select the required miniature circuit breaker model based on specific parameters and make a purchase.

Find the right miniature circuit breaker at Quisure.