How to select the appropriate circuit breaker for the Small Power Distribution Unit (SPDU)?

The circuit breaker is a device for controlling the on-off of current, 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 breakers are different. Therefore, how to select the appropriate circuit breaker becomes very important.

2 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 as to be the basis for selecting circuit breakers in the Small Power Distribution Unit (SPDU).

2.1 selection of circuit breaker in Small Power Distribution Unit (SPDU). The circuit breaker for distribution is generally used in low-voltage power grid to distribute electric energy, including the main power circuit breaker and load branch circuit breaker. When selecting this type of circuit breaker, special attention shall be paid to the following selection principles:

1) The allowable current carrying capacity of the line is not less than the setting value of the long-time action current of the circuit breaker. If wires and cables are used, the setting value of the long-time delay action current of the circuit breaker can be 80% of the allowable ampacity of wires and cables. 2) The starting time of the motor with large starting current in the line shall not be greater than the returnable time of 3 times the setting value of the long-time delay action current. 3) Instantaneous current setting value i1:i1=1.1 (ijx+klkiedm). Where: KL is the impact coefficient of motor starting current, generally taken as kl=1.7 ~ 2; IEDM is the rated current of a large motor.

2.2 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 a 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, we should pay attention to the following points when selecting a circuit breaker:

1) The setting value of the long time 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 of the long delay action current of the circuit breaker > the actual starting time of the motor. 3) The instantaneous action current setting value of the circuit breaker: the cage motor should be 8 ~ 15 times the rated current of the release; Wound motor shall be 3 ~ 6 times the rated current of release.

2.3 selection of domestic protective circuit breaker. In the household power supply, the circuit breaker is usually used as the main power supply protection switch or branch line protection switch. 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 extent.

Monopole (1P) is generally used as branch protection in the family; Two pole (i.e. 2P) circuit breaker is used as the main power protection. It is very 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 in case of short circuit or overload of the line or household appliances. If the rated current is too small, the circuit breaker is easy to trip frequently, causing unnecessary power cuts, affecting the normal life and causing unnecessary trouble.

Generally, the specifications of miniature circuit breakers are distinguished by the rated current, mainly including 6a, 10a, 16a, 20a, 25A, 32a, 40a, 50a, 63A, 80A, 100A, etc; The following points shall be paid attention to when selecting or checking the total value of the total load current in general households:

1) Current value of each branch

① It is indicated on the nameplate of pure resistive load that the power is divided by the voltage directly,

Formula i= power /200v;

For example, for 200W bulb, branch current i=200w/220=0.9a

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

② The calculation of inductive load is slightly complicated. The consumed power should be considered, and the power factor should also 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 double. For example, the branch current i=200w/220v=0.9a, doubled to 0.9*2=1.8a (0.3A more than the accurate calculated value of 1.5A).

Perceptual electrical appliances include TV sets, washing machines, fluorescent lamps, refrigerators, fluorescent lamps, etc.

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

Knowing the branch current and total current, you can choose 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 shall generally be greater than 2 times the required * large load current; In addition, when selecting electrical components, the possibility of future power load increase should be taken into account to leave margin for future demand.

2.4 selection of leakage circuit breaker in Small Power Distribution Unit (SPDU)

Leakage circuit breakers are generally divided into two poles, three poles and four poles, which are respectively used in different lines. Only correct selection and use can play its due role, so the following five points should be paid attention to when selecting leakage circuit breaker:

1) The rated current of the overload release is ≥ the * large load current of the line; 2) The limit making and breaking capacity of the circuit breaker ≥ circuit * large short-circuit current; 3) The leakage current of the line shall 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, the principles to be followed for selecting a four pole circuit breaker are put forward. The four pole circuit breaker should be selected in the following cases:

1) Transfer circuit breaker between normal power supply and standby generator according to iec465.1.5; 2) The double power conversion circuit breaker with leakage protection shall be a four pole circuit breaker. Two upper circuit breakers are provided with leakage protection, and the lower power conversion circuit breaker; 3) Switching the circuit breaker between two different grounding systems; 4) Power incoming circuit breaker of TT system; 5) Circuit breaker used in it system when 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) It is forbidden to use four pole circuit breaker in TN-C system.

2.5 precautions during use after selecting the circuit breaker in the Small Power Distribution Unit (SPDU)

After the circuit breaker is selected, the following contents should be paid attention to in the process of use: 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 process of use, so as not to affect the performance; 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 to be repaired; After the leakage protector is put into operation, the user should 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. It is not allowed that any phase line or zero line of the load does not pass through the residual current circuit breaker. Otherwise, artificial "leakage" will occur, which will cause the circuit breaker to fail to close and cause "misoperation". The four pole leakage circuit breaker must be connected to the zero line to make the electronic circuit work normally; The function of the check button is to check the operation state of the circuit breaker when it is closed and energized after it is newly installed or operated for a certain period of time. Press the test button, the circuit breaker can be disconnected, indicating that the operation is normal and can continue to be used; In order to protect lines and equipment more effectively, the leakage circuit breaker can be used together with the fuse. If the circuit breaker breaks due to the fault of the protected circuit, the operating handle is in the tripping position. After finding out the cause and troubleshooting, pull the operating handle down first to make the operating mechanism "re buckle", and then the closing operation can be carried out.