How to Choose Between Type A, B, C, and D Miniature Circuit Breakers?

1. Table of Contents
   • Type A, B, C, and D are common tripping characteristics of circuit breakers. But how should we make our selection?
   • Applicability of Type C Breakers in Motor Circuits The difference between Type C and D Miniature Circuit Breakers (MCBs) is as follows:
   • Differences and Applications of Miniature Circuit Breakers
     • As for MCBs, 1P+N, 1P, 2P are generally used as the control for single-phase electrical appliances.
   • Three-phase circuits generally use three types of circuit breakers, namely 3P, 3PN, and 4P.
   Type A, B, C, and D are common tripping characteristics of circuit breakers. But how should we make our selection?

(1) Type A circuit breaker

With a nominal current twice the rated current, it is seldom used, generally for semiconductor protection (fuses are typically employed in most cases).

This so-called “multiples of current” refers to the impact current that the switch can withstand for a certain duration without tripping; its characteristic is to avoid the impact current.

Choosing the type of low-voltage circuit breaker trip unit: The types of trip units for circuit breakers include overcurrent trip units, under-voltage trip units, shunt trip units, and more.

Overcurrent trip units can further be classified into overload trip units and short-circuit current trip units, and can be long-delayed, short-delayed, or instantaneous, with overcurrent trip units being the most commonly used.

The set value of the action current of the overcurrent trip unit can either be fixed or adjustable. It is usually adjusted by rotating or using an adjustment lever. Both fixed and adjustable types are available for electromagnetic overcurrent trip units, whereas electronic overcurrent trip units are generally adjustable.

The breaking capacity of a circuit breaker refers to its ability to withstand the maximum short-circuit current. Therefore, the breaking capacity of a rotary circuit breaker must be greater than the short-circuit current of the protected equipment.

Overcurrent trip units can also be divided into fixed installation or modular installation according to the installation method. Fixed installation means that the trip unit and circuit breaker are integrated during manufacturing. Once the product leaves the factory, the rated current of its trip unit cannot be adjusted. Meanwhile, a modularly installed trip unit, as an installation module of the circuit breaker, can be adjusted at any time, offering great flexibility.

Instantaneous type: 0.02S, used for short-circuit protection; Short-delay type: 0.1-0.4S, used for short-circuit and overload protection; Long-delay type: less than 10S, used for overload protection;

Circuit breakers serve to protect wiring and prevent fire outbreaks. Therefore, the selection should be based on the size of the wiring, not the power of the electrical system. If the chosen breaker is too large, it won’t serve its purpose of protecting the wires. If the wires overload and the breaker doesn’t trip, it could pose a risk to household safety.

For air switches used for motor loads, a type D characteristic should be selected to withstand the high startup current, which is 5-8 times the normal, during motor startup.

Type B circuit breaker:

2-3 times the rated current, generally used for purely resistive loads and low voltage lighting circuits, commonly used in household distribution boxes to protect household appliances and personal safety, currently less used.

Type C circuit breaker:

5-10 times the rated current, it needs to trip within 0.1 seconds. This type of breaker is most commonly used, often used to protect distribution lines and lighting circuits with higher inrush currents.

Type D circuit breaker

10-20 times the rated current, mainly used in environments with large instantaneous currents. It is less used in regular households, suitable for systems with high inductive loads and larger inrush currents, often used to protect equipment with high impact currents.

Applicability of Type C Breakers in Motor Circuits The difference between Type C and D Miniature Circuit Breakers (MCBs) is as follows:

Type C MCB: Features overload and short-circuit protection, the short-circuit trip level is 5-10 times the rated current;

Type D MCB: Features overload and short-circuit protection, the short-circuit trip level is 10-20 times the rated current;

Both share the same overload protection, the difference lies solely in the short-circuit trip range. Generally, normal loads have no startup current, i.e., the startup current is the rated current. However, the startup current for a motor is about 7-10 times the rated current.

For instance, a 4kW three-phase motor with a rated current of 9A and a startup current calculated at 10 times, i.e., 90A;

Usually, a Type D 16A MCB is chosen as the protective device. Calculated at 10 times the actuating current, the short-circuit protective action current is 160A, which can bypass the motor’s startup current.

However, if a Type C 16A MCB is chosen, calculated at 5 times the actuating current, the short-circuit protective action current is 80A, unable to bypass the motor’s startup current. Does this mean we can never select a Type C breaker?

If a Type C 25A MCB is chosen as the protective device, calculated at 5 times the actuating current, the short-circuit protective action current is 125A, which can bypass the motor’s startup current. Technically, there’s no issue here.

Typically, we choose breakers based on their rated current being higher than the load current, then select Type C or D based on the nature of the load. Type D is designed by manufacturers specifically for motor loads.