How to Choose a Suitable Circuit Breaker

The circuit breaker is a device for controlling the current on and off, which is mainly used for the protection of lines and equipment. In case of overload, short circuit, undervoltage and other faults in the circuit, the circuit breaker can quickly cut off the power supply to protect the safety of lines, loads and related equipment. Circuit breakers are widely used in electromechanical equipment and circuits. However, with the different ability of the protected object to withstand the overload current, the protection characteristics of the selected circuit breaker are different, so how to select the appropriate circuit breaker becomes very important.

Classification and selection of circuit breakers

According to their use, circuit breakers can be divided into distribution circuit breakers, motor protection circuit breakers, household protection circuit breakers, leakage circuit breakers, etc. According to their different protection characteristics, this paper introduces how to select suitable circuit breakers, so that the selection of circuit breakers is the basis.

1. Selection of circuit breaker for power distribution. Circuit breakers for power distribution are generally used in low-voltage power grids to distribute electric energy, including main power circuit breakers and load branch circuit breakers. When selecting this type of circuit breaker, pay special attention to the following selection principles:

1) The allowable current carrying capacity of the line shall not be less than the setting value of the long delay action current of the circuit breaker. If wires and cables are used, the setting value of the long delay action current of the circuit breaker can be 80% of the allowable current carrying capacity of wires and cables. 2) The starting time of the motor with starting current in the line shall not be greater than the returnable time of 3 times the setting value of the long delay action current. 3) Instantaneous current setting value I1: I1=1.1 (Ijx+klkIedm). Wherein, kl is the impact coefficient of motor starting current, generally taking kl=1.7~2; Iedm is the rated current of * large motor.

2. Selection of motor protective circuit breaker. The motor has two characteristics: first, the starting current is usually several times of the rated current; Second, it has a certain overload capacity. Therefore, when selecting the circuit breaker to protect the motor, we must pay attention to these two characteristics of the motor. In order to ensure the reliable operation of the motor, the following points should be noted when selecting the circuit breaker:

1) The setting value of the long delay action current of the circuit breaker is determined by the rated current of the motor. 2) The returnable time of the setting value of 6 times long delay action current of the circuit breaker is greater than the actual starting time of the motor. 3) Setting value of instantaneous action current of circuit breaker: cage motor shall be 8-15 times rated current of release; The wound motor shall be 3-6 times the rated current of the release.

3 Selection of household protective circuit breaker. The circuit breaker is usually used as the main power supply protection switch or branch line protection switch in household power supply. In case of short circuit or overload of lines or household appliances, the circuit breaker can automatically trip and cut off the power supply, so as to effectively protect these equipment from damage and reduce the accident to a small range.

Single pole (1P) is generally used for branch protection in families; Two pole (2P) circuit breaker is used for main power protection. It is important to select the rated capacity current of the circuit breaker in the family, because if the rated current of the circuit breaker is too large, the circuit breaker cannot automatically trip and cut off the power supply when the circuit or household appliances are short circuited or overloaded. If the selection is too small, the circuit breaker is easy to trip frequently, causing unnecessary power failure, affecting normal life and causing unnecessary trouble.

Generally, the specifications of miniature circuit breakers are distinguished by rated current, mainly 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, etc; General families should pay attention to the following points when selecting or checking the total value of the total load current:

1) Value of each branch current

① The nameplate of the pure resistive load appliance indicates that the power is directly divided by the voltage,

Formula I=power/200v;

For example, for a 200w bulb, the branch current I=200W/220=0.9A

Resistive load electrical appliances include bulbs, electric heaters, electric fans, electric frying pans, electric heaters, electric irons, electric blankets, electric rice cookers, vacuum cleaners, electric water heaters, air conditioners, etc.

② The calculation of inductive load is slightly complicated. The consumed power and power factor should be considered in the specific calculation. In order to facilitate the estimation, I give a simple calculation method. For general inductive load, the power calculated according to the indicated load can be twice. For example, the branch current of 200W fluorescent lamp I=200W/220v=0.9A, doubled to 0.9 * 2=1.8A (0.3A more than the accurate calculated value of 1.5A).

Inductive electrical appliances include TV, washing machine, fluorescent lamp, refrigerator, fluorescent lamp, etc.

2) The total load current is the sum of all branch currents

Knowing the branch current and total current, you can select the specifications of the branch circuit breaker and the main circuit breaker, or check whether the specifications of these designed electrical components meet the safety requirements; In order to ensure safety and reliability, the rated working current of electrical components should generally be more than twice the required * heavy load current; In addition, when selecting electrical components, the possibility of increasing power load in the future shall be taken into account to reserve margin for future demand.

4 Selection of residual current circuit breaker

The residual current circuit breaker is generally divided into two pole, three pole and four pole, which are respectively applied in different lines. Only the correct selection and use can play its due role, so the following five points should be paid attention to when selecting the residual current circuit breaker:

1) Rated current of overload release ≥ * heavy load current of line; 2) Limit making and breaking capacity of circuit breaker ≥ circuit * large short circuit current; 3) The leakage current that the line should protect should be ≤ the specified leakage protection current of the circuit breaker; 4) Normal working voltage and current of line equipment ≤ rated voltage and current of circuit breaker; 5) It has short breaking reaction time and can protect lines and equipment.

Sometimes it is difficult to judge whether to select a four pole circuit breaker in application. Here we propose the principles to be followed when selecting a four pole circuit breaker. Under the following circumstances, a four pole circuit breaker should be selected:

1) According to IEC465.1.5, the transfer circuit breaker between the normal power supply and the standby generator; 2) The double power conversion circuit breaker with leakage protection shall be a four pole circuit breaker. Two upper level circuit breakers are equipped with leakage protection, and the lower level power conversion circuit breaker; 3) Power switching circuit breaker between two different grounding systems; 4) Incoming power circuit breaker of TT system; 5) The circuit breaker used in IT system when the neutral line is led out.

Four pole circuit breakers are generally not required or prohibited under the following circumstances:

1) TN-S and TN-C-S systems generally do not need to be equipped with four pole circuit breakers; 2) The use of four pole circuit breakers in TN-C system is strictly prohibited.

5 Precautions during the use of circuit breaker after selection

1) After the selection of the circuit breaker, the following contents shall be paid attention to in the use process: the leakage, overload and short circuit protection characteristics of the circuit breaker are set by the manufacturer, and cannot be adjusted at will in the use process, so as not to affect the performance;

2) After the circuit is connected, check whether the wiring is correct. It can be checked by the test button. After the circuit breaker is disconnected due to short circuit, the contact shall be checked. If the main contact is seriously burnt or has pits, it shall be repaired; If the circuit breaker cannot be disconnected, it indicates that the circuit breaker or circuit is faulty and needs maintenance.

3) After the leakage protector is put into operation, the user shall check whether the circuit breaker operates normally through the test button after a period of time; The load wiring of the residual current circuit breaker must pass through the load end of the circuit breaker, and it is not allowed that any phase line or zero line of the load does not pass through the residual current circuit breaker, otherwise it will cause artificial "leakage" and cause the circuit breaker to fail to close, resulting in "maloperation".

4) The four pole residual current circuit breaker must be connected to the neutral line to make the electronic circuit work normally; The inspection button is used to check the operation status of the circuit breaker when it is closed and energized after it is newly installed or operates for a certain period of time. Press the test button, the circuit breaker can be disconnected, indicating that it operates normally and can be used continuously;

5) In order to protect lines and equipment more effectively, leakage circuit breakers can be used together with fuses. If the circuit breaker breaks due to the failure of the protected circuit, the operating handle is in the tripping position. After the cause is found out and the fault is eliminated, pull the operating handle downward to make the operating mechanism "re buckle" before the closing operation.