1. 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.

Tripping Characteristics

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.

AC MCB

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.

AC Miniature Circuit Breaker type

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.

AC MCB1

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.

(1) Differences: 1P—Single-pole circuit breaker: Has thermal trip function, can only control live wire (phase wire), module size 18mm; 1P+N—Single-pole + N circuit breaker: Controls both live and neutral wires, but only the live wire has the thermal trip function; module size is also 18mm; 2P—Single-phase 2-pole circuit breaker: Controls both live and neutral wires, and both have thermal trip functions, module size is 2*18mm = 36mm;

(2) Applications:

  1. To reduce costs, a 1P can be used, but the upstream circuit breaker must have a leakage trip function, and to prevent accidents due to confusion between live and neutral wires during maintenance, the upstream power supply must be cut off;
  2. To avoid the problem with 1 during maintenance, a 1P+N can be used;
  3. The reason to use a 2P: For the same 18mm module size circuit breaker housing, the internal installation of a 1P and 1P+N is different. The former’s “ultimate breaking capacity” under a short-circuit accident is definitely higher than the latter, as space is a crucial factor affecting the breaking capacity. Thus, for essential circuits that require frequent maintenance and operation and are prone to faults, it’s best to use a 2P (although it’s more costly);
  4. Using a 1P requires that the lighting distribution box must have a leakage trip function, at least the incoming line (or the one above the outgoing line) should use a residual current circuit breaker;
  5. Ordinary socket circuits can fully utilize a 1P+N, but it won’t work if you want to add leakage protection, as a 1P+N circuit breaker cannot be assembled with leakage protection accessories and other electrical accessories.

AC Miniature Circuit Breaker

Three-phase circuits generally use three types of circuit breakers, namely 3P, 3PN, and 4P.

3P: With three terminals, it is used for purely three-phase electrical appliances. It will trip in the event of a short circuit either between phases or to the ground. It can’t handle single-phase loads; otherwise, if there is a load on any phase to neutral wire, the returning current in the neutral wire will act as a leakage current in the breaker.

3PN: With four terminals (L1L2L3N) all passing through the transformer coil, it can handle both three-phase and single-phase electricity. The leakage circuit breaker does not operate regardless of whether the three-phase load is balanced; it only operates when there is leakage, that is, during a single-phase grounding or phase-to-phase short circuit.

4P: With four terminals (L1L2L3), it operates like the 3PN, with the only difference being that the 4P interrupts the neutral wire, while the 3P does not.

(2) Division:

Four-pole circuit breakers are divided into A, B, C, and D types:

A: The neutral (N) pole does not install an overcurrent trip device and is always on, not joining the other three poles in operation.

B: The neutral (N) pole does not install an overcurrent trip device but operates together with the other three poles.

C: The neutral (N) pole installs an overcurrent trip device and operates together with the other three poles.

D: The neutral (N) pole installs an overcurrent trip device but is always on, not joining the other three poles in operation.

(3) Applications:

In the case of using four-pole devices, it is necessary to specify which type is being selected from the product range. Even though they are all four-pole devices, the function and purpose are different depending on whether an overcurrent trip device is installed on the neutral wire.

A circuit breaker with an overcurrent trip device installed on the neutral wire can be used in lines where the single-phase load is dominant in three-phase four-wire power distribution, or in non-linear loads generating a large amount of harmonics such as gas discharge lamps, silicon-controlled rectifier dimming/speed control circuits, or other cases with special requirements. General equipment circuits can use circuit breakers that do not install an overcurrent trip device on the neutral wire.

In reality, although types A and D are called four-pole circuit breakers, their neutral pole is always on and does not operate together with the other three poles. Hence, these types of MCCBs are commonly known as “pseudo four-pole” or 3P+N and have no essential difference from three-pole MCCBs. The only advantage of these over the three-pole types is that they may provide convenience for line entry and exit in switchgear assemblies.

Therefore, these types of circuit breakers can only be suitable for applications in three-phase load scenarios but also have a small amount of single-phase load (such as packaged systems using 220V control power).

If the wrong type is selected, not only will it not provide protection, but it could also lead to significant problems. This is currently one of the most chaotic issues in design and usage and should receive attention.