Circuit breakers provide protection for consumers connected to the network in the event of a short circuit and in other emergency situations. The breaking capacity (OS) of the machine is the most important parameter that determines the preservation of functionality at increased current loads. The corresponding value must be taken into account when selecting the components of the power supply system.
What is the breaking capacity of a circuit breaker
The machine is installed in the power supply circuit. With an excessive increase in power consumption, the bimetallic element heats up. At a certain temperature level, a significant change in its shape breaks the contact of the conductor line.
Another protective device breaks the circuit when a high current is applied. In addition to a short circuit, a similar reaction is caused by the connection of a reactive load that is too powerful, for example, a welding machine. In a hazardous situation, the solenoid coil moves the circuit breaker operating mechanism.
The breaking capacity of a circuit breaker is a complex parameter. It characterizes the guaranteed performance of the main functions by the technique in the event of emergency situations.
Which OS to choose for the machine
The value of this parameter is indicated by a special color marking and a number (kA) at the bottom of the front panel. In the last century, the relatively low consumption of electricity by domestic households meant the possibility of using protective devices of 3.5 kA or less. However, these days, experienced experts recommend choosing machines as follows:
- 4.5 kA - separate groups of consumers;
- 6 kA - input of apartment power supply;
- 10 kA - outgoing lines of distributors of an apartment building.
Corrections are made taking into account the specifics of a specific project. The increased breaking capacity of the circuit breaker (AB) will come in handy at a short distance to the local substation, industrial plants.
Rated breaking capacity AB
The current value is normalized at which the machine will break the circuit and retain sufficient functionality to perform similar actions in the normal mode. It should be emphasized that the thematic standards mean the periodic component of the short-circuit current. The following designations for the ratings of the breaking capacity of the circuit breaker are used for different groups of devices based on their intended purpose:
- industrial machines - Icu (limit);
- household models - Icn (operational).
The rated breaking capacity of a circuit breaker is a basic parameter that determines the reliability of a protective device. Technological checks during production and certification tests are carried out taking into account the speed of the power circuit breaking in the corresponding categories:
- A - instantly;
- B - with a set delay.
The tests are carried out according to the standard program:
- simulate a short circuit with the subsequent shutdown of the machine;
- check the performance;
- repeat the procedure for different values of cos ϕ.
At the final stage, the compliance of the basic technical parameters with the manufacturer's passport data is specified.
In addition to the safety of the insulation, they control the reliability and speed of separation of contact groups, the absence of mechanical damage.
Limiting switching capacity of the circuit breaker
The current standards establish the procedure for conducting special tests. In particular, they check the preservation of performance after multiple short circuits. It should be understood that when the current and voltage vectors coincide, the circuit is broken at a lower energy potential. In the opposite situation (cos ϕ = 0), the risk of equipment damage increases. If cos ϕ = 0.5, it is recommended to select the ultimate switching capacity of the circuit breaker with Icu in the range of 6-10 kA.
Working largest OS
The probability of the most unfavorable situation is extremely small. Usually, in the event of emergency situations, the short-circuit currents are significantly lower than the breaking capacity of the circuit breaker (Icu). This explains the long service life of the protective devices under real operating conditions.
However, it is impossible to exclude the possibility of a repeated occurrence of a short circuit after a short period of time after turning on the power. To increase the safety margin, the additional Ics parameter is normalized in industrial models. The corresponding value is indicated in the accompanying product documentation, as a% of Icu according to standard gradation:
- 25;
- 50;
- 75;
- 100.
Type tests check the retention of the switching capacity of the circuit breaker after 3 cycles with an open circuit after a short circuit. After the completion of the procedure, the compliance of the shutdown speed and other technical parameters with the manufacturer's passport data is specified.
You will have to pay more for a high-quality machine from a well-known brand. However, such products are created according to the rule of equality of Icu and Ics (100%).
By additional tests, the actual value of the maximum current (peak level) - Icm is established. In this case, in addition to the signal amplitude, the rate of change of the energy parameters is also important. In the calculation formulas, an appropriate correction factor is used, which in turn depends on cos ϕ.
Features of AB that determine the OS
A study of the current market offers confirms a noticeable increase in cost as the breaking capacity of the machine increases. Which model to choose will become clear after a comprehensive project assessment.
A device designed for a lower rating will not perform its functions. In the worst situation, even with a destroyed case, electrical contact will remain. The development of an emergency situation provokes additional breakdowns and costs.
Design features
In practice, the definition of "limiting switching resistance" is used. According to this indicator, the resistance of the machine to maximum loads is determined. If a one-time PCS is specified, then the protection will be triggered only once. Increase the resource of equipment by modernizing functional blocks. In particular, heat dissipation is improved to maintain the integrity of the structure in the short circuit mode and to reduce the negative impact on the contact groups.
It is recommended to pay attention to design features that simplify installation and inspection. In some models, special holes are provided for operational visual control.Be sure to consider the proximity of transformers and other potential sources of hazardous voltage surges. The ultimate breaking capacity of the circuit breaker is selected with a margin.
The connected loads are checked in the modes of maximum consumption.
