How to Selection Miniature Circuit Breaker

Miniature circuit breakers have a fixed trip setting; changing the operating current value requires changing the whole circuit breaker. Larger circuit breakers can have adjustable trip settings, allowing standardized elements to be applied but with a setting intended to improve protection. For example, a circuit breaker with a 400 ampere “frame size” might have its overcurrent detection set to operate at only 300 amperes, to protect a feeder cable.

IEC 60898-1 and  EN 60898-1, define the rated current I_n of a circuit breaker for low voltage distribution applications as the maximum current that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C). The commonly available preferred values for the rated current are1A,2A,4A, 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A, and 125 A  The circuit breaker is labeled with the rated current in amperes, but excluding the unit symbol, A. Instead, the ampere figure is preceded by a letter, B, C, or D, which indicates the instantaneous tripping current — that is, the minimum value of current that causes the circuit breaker to trip without intentional time delay

MCB Breaking Capacity (AIC)

Circuit breakers are rated both by the normal current that they are expected to carry, and the maximum short-circuit current that they can safely interrupt. This latter figure is the ampere interrupting capacity (AIC) of the breaker.

Typical domestic panel circuit breakers are rated to interrupt 10 kA (10000 A) short-circuit current and 6KA(60000A)

MCB Internal structure

1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped).

2. Actuator mechanism – forces the contacts together or apart.

3. Contacts – allow current when touching and break the current when moved apart.

4. Terminals

5. Bimetallic strip – separates contacts in response to smaller, longer-term overcurrents

6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly.

7. Solenoid – separates contacts rapidly in response to high overcurrents

8. Arc divider/extinguisher

9. 

MCB Pole number and application

Miniature circuit breakers 1P, 1P+N, 2P, 3P, 3P+N, 4P!

1P, 2P, 3P, 4P are pole numbers, indicating that several lines can be segmented. Now +N, all N lines are broken, but there is no trip device on the N line. Only when the phase line is broken, the N line is cut off. For example, for a 4P 16A circuit breaker, if the N-line current is greater than 16A, or greater than 20A, in theory, the circuit breaker will trip. But the 3P+N circuit breaker, N line current 160A, will not jump. The premise is that the phase current does not exceed the rated value.

1P, 2P single-phase equipment, lighting can use 1P, socket to 2P; 3P used in three-phase equipment, and there is a three-phase electric household.

For miniature circuit breakers, 1P+N, 1P, and 2P are generally used as on-off control for single-phase electrical appliances, but the effects are different.

1P—— single-pole circuit breaker with thermal magnetic tripping function, only control the fire line (phase line), the modular wide is 18mm;

1P+N—-1Pole+N circuit breaker, which controls the live and neutral lines at the same time, but only the 1P fire wire has the function of thermal magnetic tripping; the modulus is also 18mm;

2P—— Single-phase 2-pole circuit breaker, which controls both the live and neutral lines, and both have a thermal magnetic tripping function, and the modulus is 2*18mm=36mm.

Therefore, the following conclusions can be drawn:

1. In order to reduce the cost, 1P can be used, but the upper circuit breaker must have the leakage trip function. In order to prevent accidents caused by fire and zero disorder during maintenance, the upper power supply must be cut off;

2. For avoiding one problem during maintenance, 1P+N (ie DPN) can be used;

3, the reason for using 2P: For the circuit breaker case which is also 18mm modulus, there is a difference between the internal 1P and the 1P+N. The former “limit breaking capacity” in the short-circuit accident state is definitely higher than that. After all, space is an important factor affecting the ability to break. Therefore, for power circuits that are more important, frequently repaired and operated, and prone to failure, it is best to use 2P (higher cost).