Imagine the situation: a short circuit has occurred in the electrical network of your home or industrial facility. At this moment, the current increases to colossal values in a fraction of a second. It is at this critical moment that the circuit breaker comes into play, and its characteristics determine whether the arc will go out safely or an explosion will occur.
The key parameter determining the reliability of protection is ultimate breaking capacity. This is the maximum short circuit current that the device can break without collapsing itself or damaging the surrounding infrastructure. If you ignore this parameter when designing a switchboard, the consequences can be fatal to the equipment.
Many people mistakenly believe that it is enough to select a machine based on the rated load current, forgetting about short-circuit currents. However IEC 60898 and GOST R 50345 clearly regulate the requirements for switching resistance. Understanding the physics of the arc formation process and the principles of arc extinction is necessary for every electrical engineer and competent installer.
The physical essence of the PKS parameter
When the contacts open under load, an electric arc inevitably occurs between them. At short circuit currents, this arc has enormous temperature and energy. Ultimate breaking capacity (UCC) - this is the threshold current value at which the circuit breaker is guaranteed to extinguish the arc in a special arc-extinguishing chamber.
If the short-circuit current exceeds the PKS value, the device may not have time to break the circuit before the contacts weld or melt. In the worst case, the device body will be destroyed by the pressure of the gases generated by the arc, which will lead to a fire. Therefore The PCS must always be higher than or equal to the rated short-circuit current at the installation point.
Modern modular automatic machines are equipped with complex gas-cushion and slot arc extinguishing systems. The design of the chamber, the speed of operation of the release and the material of the contacts directly affect this parameter. Manufacturers test equipment in laboratories, simulating extreme conditions.
⚠️ Attention: Installing a machine with insufficient PCS in a network where short-circuit currents are high is equivalent to installing a paper partition before an explosion. The device may work once, but if it fails again, it will become a source of danger.
It is important to distinguish between operating and ultimate breaking capacity. The operating characterizes the number of cycles of switching on and off under load, and the limit characterizes the ability to withstand a single exposure to overcurrent. For domestic needs, standard values are usually sufficient, but in industry the requirements are stricter.
Standard values and device markings
On the body of each high-quality circuit breaker you will find indications of its breaking capacity. This is usually a number in a box, such as 4500, 6000 or 10000. These numbers indicate the current in amperes that is capable of breaking that particular specimen.
According to international standards, there is a gradation of current limiting classes and breaking capacity. For household series ABB S200, Legrand DX3 or Schneider Electric Acti9 the most common values are 4.5 kA and 6 kA. Industrial series are often labeled 10 kA, 15 kA and even 25 kA.
The choice of a specific value depends on the distance from the transformer substation. The closer the switchboard is to the energy source, the higher the short-circuit currents and the higher the PKS should be. In old houses with long lines, short-circuit currents can be small, and in new buildings with powerful transformers they can reach tens of kiloamperes.
The marking is applied with indelible paint and must be readable even after installation. If you cannot find the PKS value on the case, most likely you are looking at a counterfeit product or a low-quality device, which it is better to refrain from using.
Calculation of short circuit currents at the installation point
Before choosing a machine, it is necessary to theoretically calculate the short circuit current at the point of its installation. This is a complex engineering problem that requires taking into account transformer resistance, cable length and cross-section, and contact resistances.
To put it simply, we can say that the short-circuit current depends on the power of the source and the resistance of the phase-zero loop. The lower the line resistance, the greater the current. For accurate calculations, specialized software systems are used, such as ETAP or TKZ calculation.
However, empirical data can be used for a preliminary assessment. In apartments remote from the substation, the short-circuit current rarely exceeds 3-4 kA. In private houses with their own transformer or in industrial workshops, it can reach 10-15 kA or more.
Engineers often use the equivalent equivalent circuit method. All network elements are represented as resistances, which are summed up. The resulting impedance is divided by the network voltage. The result gives the maximum current that can flow during a metallic fault.
Influence of current limiting class on safety
Circuit breakers are divided into current limiting classes, which are indicated by a number in the square under the PKS value. Class 3 is the fastest, followed by class 2 and class 1. The higher the class, the faster the contacts open when a short circuit occurs.
A high current limiting class allows you to reduce dynamic and thermal loads on wiring and equipment. Even if the PKS of the machine is large, but the current limiting class is low, the wiring may not withstand thermal shock. Therefore, these parameters need to be considered in conjunction.
- 🚀 Class 3: opening time less than 1/6 half cycle (less than 3 ms), provides maximum protection.
- ⚡ Class 2: opening time from 1/6 to 1/2 half cycle, standard for most household solutions.
- 🐢 Class 1: opening time is more than 1/2 half cycle; such machines are practically not produced for modular design.
The use of class 3 current limiting circuit breakers is especially important in networks with long cable runs, where it is important not only to turn off the current, but also to do this before the current reaches its maximum value. This prevents the insulation from melting and causing fire.
⚠️ Attention: When replacing old machines with new ones, be sure to check the current limiting class. Installing a high-speed circuit breaker into old wiring with damaged insulation can lead to false alarms due to high inrush currents.
Comparative table of machine characteristics
For ease of choice, we provide a comparison of popular series of circuit breakers and their characteristics. The data is averaged and may vary depending on the specific modification and year of manufacture.
| Series/Brand | Rated current (A) | PKS (kA) | Current limit class | Application |
|---|---|---|---|---|
| ABB S201 | 6 - 63 | 6 | 3 | Residential buildings, offices |
| Legrand DX3 | 0.5 - 63 | 6 / 10 / 15 | 3 | Industry, housing and communal services |
| IEK BA47-29 | 6 - 63 | 4.5 / 6 | Not standardized | Budget sector |
| Schneider Acti9 | 1 - 63 | 6 / 10 | 3 | Commercial real estate |
The table shows that even within the same brand there is a division in breaking capacity. More expensive series usually have a higher PKS and a better current limiting class. Saving on these parameters in critical units is unacceptable.
Installation rules and common mistakes
Choosing the right machine is only half the battle. The second half is high-quality editing. Poor contact at the connection point can lead to heating and changes in the characteristics of the release, which will affect the actual breaking capacity.
A common mistake is to install machines with different PCS in the same chain without taking into account selectivity. If there is a machine with a 6 kA PKS at the input line, and 4.5 kA at the outgoing line, then if there is a short circuit between them, both devices may fail, although the downstream one should have worked first.
It is also important to observe operating temperature conditions. At high temperatures, the rated current of the circuit breaker is reduced, which can lead to premature operation. In hot climates or when tightly installed in a panel, correction factors should be applied.
Never use machines with visible mechanical damage to the body. Cracks in the plastic can break the seal of the arc chute, and gases will escape during a short circuit, causing a fire. The integrity of the housing is the key to safe arc extinguishing.
Questions and answers (FAQ)
What happens if you install a circuit breaker with a 4.5 kA PKS where the short-circuit current is 10 kA?
In the event of a short circuit, such a machine will most likely not be able to break the arc. The contacts will weld, the housing will melt or explode, which will lead to continued arcing and a possible fire. This is a critical violation of electrical safety regulations.
Is it possible to replace a 4.5 kA circuit breaker with a 6 kA circuit breaker in a home panel?
Yes, it is possible and even necessary if your budget allows. A circuit breaker with a larger breaking capacity has a greater safety margin. It is guaranteed to turn off the current, even if the energy supply organization replaces the transformer with a more powerful one.
Does PKS depend on the number of poles of the machine?
Yes, it depends. As a rule, single-pole circuit breakers have a higher breaking capacity compared to multi-pole circuit breakers of the same series at the same voltage. In three-phase networks, the PKS is indicated for each pole separately.
How often should circuit breakers be replaced?
The service life of the machines is 10-15 years, but after each operation with a short-circuit current, their service life decreases. If the circuit breaker has tripped several times due to a short circuit, it is recommended to replace it, even if visually it is intact, since the characteristics of the arc chute may have deteriorated.