MCCB

November 03, 2018

The Circuit Breaker is a device that controls the current on and off. It is mainly used to protect the line and equipment. If there is a fault such as overload, short circuit or undervoltage in the circuit, the circuit breaker can cut off the power quickly and protect the safety of the line, load and related equipment. . Circuit breakers are widely used in electromechanical equipment and circuits. However, the protection characteristics of the selected circuit breakers are different depending on the ability of the protected object to withstand the overload current. Therefore, it is very important to choose the appropriate circuit breaker.

How to choose a circuit breaker

Circuit breakers can be divided into power distribution type circuit breakers, motor protection type circuit breakers, household protection type circuit breakers, earth leakage circuit breakers, etc. according to their use. According to their different protection characteristics, this paper describes how to choose a suitable circuit breaker for selection. The circuit breaker is used as a basis.

1. Selection of circuit breakers for distribution

Power distribution circuit breakers are generally used in circuit breakers dedicated to distributing electrical energy in low voltage power grids, including power supply main circuit breakers and load branch circuit breakers. When selecting this type of circuit breaker, special attention should be paid to the following selection principles:

1) The line allowable current carrying capacity is not less than the long delay action current setting value of the circuit breaker. In the case of wire and cable, the long-delay action current setting of the circuit breaker can take 80% of the allowable current capacity of the wire and cable. 2) The starting time of the motor with large starting current in the line is not more than 3 times the returnable time of the long-delay action current setting value. 3) Instantaneous current setting value I1: I1 = 1.1 (Ijx + klkIedm). Where: kl is the impact coefficient of the motor starting current, generally taking kl = 1.7 ~ 2; Iedm is * the rated current of a large motor.

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2. Selection of motor protection type circuit breaker

The motor has two characteristics: First, the starting current is usually several times 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, the following points should be noted when selecting the circuit breaker:

1) Determine the long delay action current setting value of the circuit breaker based on the rated current of the motor. 2) The return time of the 6 times long delay action current setting value of the circuit breaker" The actual starting time of the motor. 3) The instantaneous operating current setting value of the circuit breaker: the cage motor should be 8 to 15 times the rated current of the trip unit; the winding type motor should be 3 to 6 times the rated current of the trip unit.

How to choose the circuit breaker _ breaker selection principle _ circuit breaker selection considerations

3, the choice of household protection circuit breaker

In household power supply, the circuit breaker is usually used as a main power protection switch or a branch line protection switch. If the line or household appliance is short-circuited or overloaded, the circuit breaker can automatically trip and cut off the power supply, thus effectively protecting these devices from damage and reducing the accident to a small range.

Unipolar (1P) is generally used for branch protection in the home; two-pole (ie 2P) circuit breakers are used for total power protection. It is important to select the rated capacity current of the circuit breaker in the home. If the rated current of the circuit breaker is too large, the circuit breaker or the household appliance may be short-circuited or overloaded, the circuit breaker cannot automatically trip, and the power supply is cut off. If the selection is too small, Then the circuit breaker is easy to trip frequently, causing unnecessary power outages, affecting normal life and causing unnecessary trouble.

Generally, the rated current is divided into small circuit breaker specifications, mainly 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, etc.; the general family should pay attention to when selecting or checking the total value of the total load current. The following points:

1) The value of each branch current

1 On the nameplate of the electrical appliance with pure resistive load, indicate that the power is directly divided by the voltage.

Formula I = power / 200v;

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

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

2 Inductive load calculation is a little complicated, we must consider the power consumed, and the power factor should be considered in the specific calculation. For the estimation, I will give a simple calculation method. For the general inductive load, the calculated power according to the indicated load. It can be doubled. For example, the branch current of the 200W fluorescent lamp is I=200W/220v=0.9A, which is doubled to 0.9*2=1.8A (1.5A more than the exact calculation value, 0.3A more).

Sensitive appliances include televisions, washing machines, fluorescent lamps, refrigerators, fluorescent lamps, and the like.

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 breaker and the total breaker, or check whether the specifications of the designed electrical components meet the safety requirements; in order to ensure safety and reliability, the rated operating current of the electrical components should generally be More than 2 times the required * large load current; in addition, when selecting electrical components, it is also necessary to consider the possibility of increasing the electrical load in the future, leaving a margin for future demand.

How to choose the circuit breaker _ breaker selection principle _ circuit breaker selection considerations

4, the choice of Leakage Circuit Breaker

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

1) The rated current of the overload release ≥ * The large load current of the line; 2) The limit breaking capacity of the circuit breaker ≥ the circuit * the 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) The normal working voltage and current of the line equipment ≤ the rated voltage and current of the circuit breaker; 5) The shorter breaking reaction time can protect the line and equipment.

Sometimes it is difficult to make a judgment on whether to use a four-pole circuit breaker in the application. Here, the principle to be followed for the selection of a four-pole circuit breaker is proposed. The four-pole circuit breaker should be selected in the following cases:

1) According to IEC 465.1.5, the circuit breaker between the normal power supply and the standby generator; 2) The double-power circuit breaker with leakage protection shall adopt a four-pole circuit breaker. Two upper-level circuit breakers with leakage protection, their lower-level power conversion circuit breakers; 3) power switching circuit breakers between two different grounding systems; 4) TT system power supply line-breakers; 5) IT systems The circuit breaker used when the line is taken out.

It is generally not necessary to use a four-pole circuit breaker or to prohibit the use of a four-pole circuit breaker in the following cases:

1) TN-S and TN-CS systems generally do not need to be equipped with four-pole circuit breakers; 2) Four-pole circuit breakers are strictly prohibited for TN-C systems.

How to choose the circuit breaker _ breaker selection principle _ circuit breaker selection considerations

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