Modern systems for protecting electrical wiring from burnout and ignition involve the use of circuit breakers and are divided according to the type of network into single-phase and three-phase. In the private sector, in most cases, devices of the second type are used, so the correct one becomes relevant. calculation of an automatic machine for power for 380 volts, ensuring the reliability and durability of the use of electrical networks.
Content
- Purpose and work
- Protection module design
- Operating principle
- Device characteristics
- Power selection
- Nuances when calculating
Purpose and work
The first automatic device designed to protect an electrical circuit from overcurrents was invented by an American scientist studying electromagnetism, Charles Grafton Page, in 1836. But only 40 years later, a similar design was described by Edison. The modern type of protective devices was patented in 1924. Corporation Brown, Boveri & Cie from Switzerland.
The design innovation has become the reusability due to the ability to turn on the module when it is triggered by pressing one button. The advantages compared to fuses were undeniable, while the accuracy of the machine was much better. When using the device in a 380 volt network, all phases are disconnected at once. This approach avoids skewing signal levels and overvoltage.
The direct purpose of a three-phase circuit breaker is to disconnect the line when a short circuit occurs in it or when the power consumption is exceeded by the devices. Protection modules belong to the group of switching equipment and due to their simple design, convenience use and reliability, they are widely used in both household and industrial energy networks. Typically, the device assumes manual control., but some types are equipped with an electromagnetic or electric motor drive, which makes it possible to control them remotely.
Some users mistakenly assume that the machine protects the devices connected to it, but in reality this is not the case. It does not react in any way to the types and types of devices connected to it, and the only reason for its operation is overload and the appearance of an overcurrent. At the same time, if the machine does not disconnect the line, the electrical wiring will begin to heat up, which will damage it or even ignite.
The choice of an automatic protection module is associated with the ability of the electric line to withstand a current of a certain magnitude, which is directly related to the cable material and its cross-section. In other words, when choosing a module, the main parameter is the power or the maximum current, which leads to the operation of the machine.
Protection module design
Despite the wide range of products offered by different manufacturers, the designs of the circuit breakers are similar to each other. The body of the device is made of a dielectric, resistant to temperatures, and does not support combustion. On the front panel there is a manual control lever, as well as the main technical characteristics.
Structurally, the body consists of two halves, bolted together. In the middle of it are the following elements:
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Connection terminals - designed to provide a reliable connection to the incoming and outgoing electrical lines. - Moving and fixed power output - these contacts are used to close or open the load circuit with the power one.
- Spark extinguishing chamber - when the contacts are suddenly opened, an arc of sufficiently high power is formed between them, which can lead to damage to the elements of the module. Therefore, to extinguish it, a special chamber is used, consisting of vertical plates installed in a checkerboard pattern. The spark, passing through them, loses its power, and then is completely extinguished.
- Thermal and electromagnetic release - it is their reaction to changes in the parameters of the electric line that leads to the operation of the protection device.
- Lever switch - a manual lever is used and cocking closes the inlet and outlet lines.
- Adjusting screw - sets the threshold of the module. Factory configurable.
- Gas outlet channel - when the spark is extinguished, the thermal energy is converted into gas, which is removed from the device through a specially designed labyrinth.
It is the designs of the releases that ensure the almost instantaneous tripping of the circuit breaker. An electromechanical contact reacts to the occurrence of a current in the protected circuit, the parameters of which exceed the nominal value. The design of the release includes an inductor with a core, the position of which is fixed by a spring, and it is already connected to a moving power contact. The solenoid windings are connected in series with the load. The thermal release is a compressed strip of two metals with different thermal conductivity (bimetallic plate).
Operating principle
After connecting the power and load electrical lines to the three-phase machine, it is turned on by moving the lever to the upper position. As a result, the lever engages through the latch with the closing contact. The formed connection is ensured by displacement of the movable contact group relative to their holder.
In a normal situation, the current passes through the contact of the power and movable contact. Then it goes to the bimetallic plate and the solenoid winding, and from it it already gets to the terminal and the load connected to the machine.
If a current begins to flow through the switch with a value exceeding the permissible value, then the bimetallic plate begins to heat up. Due to the different thermal expansion of metals, it bends, eventually breaking the contact. The strength of the current at which the connection breaks depends on the thickness of the plate. The thermomagnetic release is characterized by slow operation, although it can detect even small changes in the current value. Its adjustment is carried out at the factory by changing the distance between the plate and the moving contact. For this, an adjusting screw is used.
But for a current that instantly increases its value, the reaction rate of the bimetallic plate will be extremely low, therefore, a solenoid is also used with it. In the normal state, the core is pushed out by the spring and closes the contact of the machine. With an abnormal value of the signal in the turns of the coil, the magnetic field rapidly increases, the fluxes of which pull the core inward, overcoming the action of the spring, and this leads to a circuit break.
The electromagnetic release operates in a fraction of a second, while it does not react to currents slightly exceeding the rated ones. Simultaneously with the disconnection of the entire three-phase line, the lever is also lowered, which again needs to be moved to the upper position to connect the load to the network.
Device characteristics
The correct selection of a 3-phase machine is not only in determining the conditions for its operation, but also in terms of power and type of load that will be connected to it. Incorrectly selected module power leads to a deterioration in the protection of the electric wire, while such a device itself can become a source of an emergency.
But still, no matter how important it is to choose the right power, automatic devices are also characterized by other technical parameters that affect their operation. The main ones include:
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operating voltage - determines the value at which the circuit breaker works without degrading its parameters (usually a drop in the range of 15% is allowed); - rated current - a parameter directly related to power, denotes the limit value of the current at which the protection module is triggered;
- power consumption - automatic devices are low-power devices;
- durability - indicates the number of guaranteed turn-on and turn-off cycles of the machine;
- minimum and maximum operating temperature - the range in which the technical parameters of the protective module do not change;
- rated breaking capacity - the highest load value at which the circuit breaker can break the line while maintaining its operability;
- response time - determines the interval during which the load is disconnected from the power line;
- time-current characteristic - divided into classes, each of which corresponds to the instantaneous tripping current (for example, type C is used for a current exceeding the rated value 5-10 times).
In addition to technical parameters, automatic devices are also characterized by quality indicators. The most common ones include the type of drive, the method of connecting external conductors, the cut-off design, and others.
Power selection
There are two ways to determine the required power for a 3-phase circuit breaker. At the same time, one complements the other, and does not exclude it. The first method is associated with finding the total value of the consumed energy and load, and the second - with the cross-section of the wiring.
Based on the definition that the machine does not protect the equipment, but the wiring, you need to select the power, focusing on the parameters of the latter. This is true, but only until the network upgrade is planned. For example, the existing wiring in a house is 1.5 square meters. According to the technical specifications, copper wiring of this diameter will be able to withstand a long-term current of no more than 10 amperes. Accordingly, the maximum simultaneous energy consumption by devices connected to the output of the machine should not exceed 3.8 kW. This value is obtained from a simple formula for finding the power - P = U * I, where:
- P is the maximum allowable power consumption, W;
- U - voltage of a three-phase network, 380 volts;
- I is the maximum current withstood by the wiring, A.
The resulting number indicates that the total load connected to the line at the same time should not exceed this value, i.e. when you turn on a 2 kW boiler, nothing terrible will happen. But if an electric furnace of 3 kW is connected to this line, then the wiring will not withstand and will light up, therefore, for to prevent an accident, it is necessary to install an automatic machine for 10 A, which allows the line to be loaded only up to 2.2 kW.
The advantage of using a three-phase machine is that three lines can be connected to it at the same time, while the value of the rated current will be determined by summing the powers of all phases. Thus, for a 380 volt machine, it will be 6.6 kW, and in the case of connecting a load of the "triangle" type - 11.4 kW. That is, for the given example, if it is not possible to separate the line to different phase outputs of the protection device, you will need to purchase a 6 A.
If you plan to upgrade the wiring or use a thick cable, then the calculation can be made based on the power consumption of the load. For example, if the load of each phase does not exceed 4 kW, then the rated current is calculated as the sum of the powers plus 15–20% of the stock (I = 4 * 3 = 12 A + stock = 14 A), so the most suitable device in this case would be a machine for 16 A.
Nuances when calculating
To simplify the finding of power, it is customary to use not a percentage as a margin, but multiplication by a factor. This additional number is considered to be equal to 1.52.
In practice, however, it is rarely possible to load all three phases equally, therefore, when one of the lines consumes a lot of energy, the calculation of the rating of the circuit breaker is carried out according to the power this phase. In this case, the largest value of the consumed energy is taken into account and multiplied by a factor of 4.55, and then it will be possible to do without using tables.
Thus, when calculating the power, first of all, the parameters of the electrical wiring are taken into account, and then the energy consumed by the protected automatic machine of electrical equipment. Here, the correct remark from the rules for the installation of electrical installations (PUE) is taken into account, indicating that the installed circuit breaker must protect the weakest section of the chain.
