Selection principle and setting principle of low voltage circuit breaker

The general principles that need to be considered when designing a low-voltage circuit breaker are:

1 According to the load nature of the low-voltage distribution system, the fault category and the requirements for line protection, determine the type of circuit breaker selected and comply with the relevant national standards.

2 The rated voltage and rated frequency of the circuit breaker should be compatible with the nominal voltage and nominal frequency of the circuit in which it is located.

3 The rated current of the circuit breaker should not be less than the load calculation current of the circuit in which it is located.

4 The circuit breaker should be adapted to the environmental conditions of the location.

5 Circuit breakers should meet the requirements of dynamic stability and thermal stability under short-circuit conditions. When breaking the short-circuit current, the on-off capability under short-circuit conditions should be met.

Low-voltage circuit breakers should be selected according to different fault categories and specific engineering requirements. The principles of tuning generally include:

1 When the circuit breaker is in normal use and the electrical equipment is normally started, the installed protection should not be operated.

2 The most fundamental task of the circuit breaker is to play a protective role. It must effectively cut off the faulty circuit within the specified time to meet the most basic requirements of the specification.

3 The protection action characteristics of the circuit breakers of all levels of the low-voltage power distribution system shall be able to coordinate with each other. Selective actions shall be taken. In the event of a fault, the circuit breaker protection close to the fault point shall be cut off first, and close to the power supply side. The primary protection should not be operated to reduce the power failure range as much as possible.

In the low-voltage power distribution system, the main design task is to reasonably choose to protect the electrical appliances. According to the requirements of the circuit breaker's setting principles, various protection functions can be realized by correctly setting their parameters, but these tuning principles may contradict each other. For example, the rated current or setting current of the circuit breaker is limited by the first and second principles of the tuning principle, and the speed of the protection action is subject to the second and third principles of the tuning principle, so it must be accurately calculated and carefully Verification, coordination of conflicts between each other, the realization of the unity of the opposite, in order to meet the requirements of the specifications of the action characteristics, operating time and selective protection.

When designing and selecting low-voltage circuit breakers, firstly follow the national standards for electrical design such as GB 50054-1995 "Low-voltage power distribution design specifications" and manuals such as "Industrial and Civil Power Distribution Design Manual". As a manufacturer of low-voltage circuit breakers, Hagrid wants to talk about this topic from another angle.

The manufacture of low-voltage circuit breakers is based on the national standard GB14048-2 (equivalent to IEC 60947-2) "Low-voltage switchgear for low-voltage switchgear and control equipment" and GB10963 (equivalent to IEC 60898) "Overcurrent protection circuit breaker for household and similar places". According to the fact, when the designer chooses our products, it actually recognizes the above two national product standards. From this point of view, it is very necessary for designers to understand these two standards, and briefly introduce some problems that are easy to ignore:

1) Low-voltage circuit breakers manufactured by the above two product standards are commonly referred to as general-purpose circuit breakers. I apply the scope of clause 1.1 of the product standard: “For some specific applications (such as traction, rolling mills and marine) circuit breakers, some special regulations and supplementary requirements may be made” and Appendix E “Submitted to the manufacturer to negotiate with the user” The understanding of these two items is that for some special-purpose circuit breakers beyond the product standard, the user and the manufacturer are required to negotiate to confirm the performance and electrical parameters of the circuit breaker to meet the user's needs.

2) The most important function of the circuit breaker is to provide timely trip protection in the event of a line fault. The designer is very concerned about whether the circuit breaker can ensure that the fault line is properly cut off when the selected trip current value occurs. Therefore, it is necessary to understand the ambient temperature set by the tripping value of the circuit breaker manufactured according to the standard of Article 6.1.1 of GB 14048-1: “The ambient air temperature does not exceed +40 °C, and the average temperature value within 24 h does not exceed +35 °C", when the designer determines that the circuit breaker uses an ambient temperature higher than the above temperature, temperature coefficient adjustment is required. The circuit breaker manufacturer will provide a "temperature correction factor" in the product sample for reference in the designer's design.

3) According to the provisions of Article 6.1.2 of GB14048-1, the altitude requirement of the installation site is: “The installation location does not exceed 2000m”. If the installation location exceeds the 2000m altitude requirement, the manufacturer may specialize in manufacturing according to the agreement with the user or the designer may use derating according to the manuals such as the Manual of Industrial and Civil Power Distribution Design.

4) According to the requirements of national product standards, the manufacturer also provides the correction coefficient for each circuit breaker when multiple circuit breakers are installed in parallel in the product sample for reference.

5) The manufacturer does not provide the circuit breaker derating correction factor when multiple cables are laid side by side. However, the designer should consider the requirements for overheat protection when the circuit breaker protects multiple cables side by side, which is beyond the requirements of the standard for the manufacturer. In this regard, the correction factor for the derating of the circuit breaker should be used with reference to the Guidance Manual of the Industrial and Civil Power Distribution Design Manual or similar manual.

6) The agreed trip current value error also needs to be reminded by the designer. See clause 7.2.1.2.3 of GB 14048-2 for “accuracy of trip current value ±20% (simplified)”. This is why many design manuals and discussions have mentioned that two series-connected upper and lower circuit breakers pass the same load current, and the rated current In is selected: if they are all similar circuit breakers, the upper circuit breaker In should be at least larger than the lower circuit breaker 1.6 In. In order to have a selective trip. Engineers can understand that high precision means high cost in the industry. If high-precision circuit breakers are needed, the user and the manufacturer need to negotiate, and the price of the product may be greatly increased.

General principles for selection of low voltage circuit breakers

General principles for general low voltage circuit breaker selection:

1 The rated voltage of the low-voltage circuit breaker ≥ the rated voltage of the line.

2 The rated current of the low-voltage circuit breaker ≥ the calculated load current of the line.

3 The ultimate on-off capability of the low-voltage circuit breaker ≥ the maximum short-circuit current in the line.

4 line end single short-circuit current ≥ 1.25 times the low-voltage circuit breaker instantaneous (or short delay) trip setting current.

5 The rated current of the trip unit ≥ the calculated current of the line.

6 rated voltage of undervoltage release = rated voltage of the line.

7 Shunt release of the circuit breaker rated voltage = control power supply voltage.

8 rated working voltage of electric transmission to the mechanism = control power supply voltage. The principle of setting low voltage circuit breakers:

1) The circuit should not be cut off during normal operation and normal starting. Regarding the starting, the circuit breaker does not operate. Taking a representative cage motor as an example, calculation and verification are performed according to the following requirements. The long-delay tripping device of the circuit breaker sets the current Ir1, generally regardless of the influence of the motor starting. The short-delay tripping setting current Ir2 should avoid the starting current of the largest motor, and calculate by the following formula

Ir2≥1.1( IL+1.35 KIMm)

In the formula, IL calculates the load current for the line, A; K is the multiple of the starting current of the largest motor; IMm is the rated current of the largest motor, A.

The instantaneous tripping current Ir3 should escape the full starting current of the largest motor and be calculated by the following formula

Ir3≥1.1( IL+K1 KIMm)

Where, IL is the line to calculate the load current, A; K1 is the peak coefficient of the motor starting current, K1 = 1.7 ~ 2; K is the maximum starting current of one motor; IMm is the rated current of the largest motor, A.

2) When the line is faulty, the fault circuit should be reliably cut off.

3) When the line is faulty, the protection devices at all levels should selectively cut off the circuit.

These three requirements are often contradictory. The task of the power distribution system design is to reasonably choose to protect the electrical equipment and correctly set its parameters. For example, the rated current of the protective electrical appliance or the setting current is limited by items 1) and 2). The speed of the action time is subject to the constraints of items 2) and 3). It must be carefully calculated, verified, coordinated, and unified to achieve the requirements of the norm.