To prevent a short circuit and overload of the mains, a three-phase circuit breaker is used. The switching device can be used for direct and alternating current lines. The design of the standard model is represented by extenders with switching depending on the frequency of the circuit.
Which machine is suitable for 15 kW
The purpose of the 3-phase circuit breaker is protection against overcurrents and overloads. The 15 kW modification works in a network with a voltage of 380 V, that is, a 25A device is required for input. When choosing, you need to take into account that in conditions of short circuits, the current increases and can cause a fire to the wiring.
Choosing a model of a 15 kW machine for a three-phase load, it will be necessary to take into account the parameters of the permissible voltage and current during a short circuit. It is worthwhile to focus on the calculated cable current indicators with a minimum cross section that protects the switch and the receiver's rated current.
When calculating the input switching automaton according to the power parameters in the 380 V network, take into account:
- electric power - actual and additional;
- cable loading rate;
- the availability of free power in the design indicator of a residential building;
- remoteness of farm buildings and non-residential premises from the cable entry point.
In a network of 15 kilowatts with additional power, an ASU device is installed.
Functions of three-phase machines
Before choosing an automatic switch, you should understand its functionality. Users are often mistaken in thinking that the device protects household appliances. The machine does not respond to its electrical indicators, working exclusively with a short circuit or overload. Three-phase functions include:
- simultaneous servicing of several single-phase zones of a circuit;
- prevention of the formation of overcurrents on the line;
- collaboration with AC rectifiers;
- protection of high-power equipment;
- increased power due to the installation of a special converter;
- quick response in short circuit mode on a line with a large number of consumers;
- the ability to disconnect in manual mode using a knife switch or switch;
- compatibility with additional protective terminals.
Without a difavtomat, the risk of cable fire increases.
The principle of operation and the purpose of the circuit breaker
A three-phase circuit breaker is activated in the event of a circuit fault by means of an electromagnetic splitter. The principle of operation of the element is to heat the bimetallic plate at the time of increasing the current rating and turning off the voltage.
The fuse does not allow short circuit and overcurrent with indicators above the calculated to affect the wiring. Without it, the cable cores are heated to the melting temperature, which leads to ignition of the insulating layer. For this reason, it is important to know if the network can withstand the voltage.
Conformity of wires to loading
The problem is typical for houses of the old buildings, in which new machines, meters, RCDs are put on the existing line. Automatic machines are selected for the total power of the equipment, but sometimes they do not work - the cable is smoking or burning.
For example, in the veins of an old cable with a cross section of 1.5 mm2, the current limit is 19 A.When turning on equipment with a total current of 22.7 A at a time, only 25 Ampere modification will provide protection.
The wires will heat up, but the switch will remain on until the insulation melts. To prevent a fire, a complete replacement of the wiring with a copper cable with a cross section of 2.5 mm2 can prevent it.
Protecting the weakest cable section
Based on clause 3.1.4 of the PUE, the objective of an automatic device is to prevent overload at the weakest link in the electrical circuit. Its rated current is selected by the current of connected household appliances.
If the machine is not selected correctly, an unprotected area will cause a fire.
Principles of calculating the machine for cable cross-section
Calculations of a 3-phase difavtomat are carried out on the basis of a section of a cable. For a 25 A model, you will need to refer to the table.
| Wire cross section, mm2 | Permissible load current for cable material | |
| Copper | Aluminum | |
| 0,75 | 11 | 8 |
| 1 | 15 | 11 |
| 1,5 | 17 | 13 |
| 2,5 | 25 | 19 |
| 4 | 35 | 28 |
Modification of 25 Amps can be used to protect the wiring or set to input.
For example, a copper wire with a cross section of 1.5 mm2 with a permissible load current of 19 A is used for wiring. To prevent the cable from heating up, you need to select a lower value - 16 A.
Determining the dependence of power on the cross section by the formula
If the cable cross section is unknown, you can use the formula:
Icalc = P / Unomwhere:
- Icalc - rated current
- P - power of devices,
- Un - voltage rating.
As an example, you can calculate an automatic machine that you need to put on a boiler with a load of 3 kW and a voltage of 220 V:
- Convert 3 kW to watts - 3x1000 = 3000.
- Divide the value by voltage: 3000/220 = 13.636.
- Round the rated current to 14 A.
Depending on the environmental conditions and the method of laying the cable, you need to take into account the correction factor for the 220 V network. The average value is 5 A. It will need to be added to the calculated current indicator Icalc= 14 + 5 = 19 A. Next, according to the PUE table, the cross section of the copper wire is selected.
| Section mm2 | Load current, A | |||||
| Single core cable | Twin core cable | Three core cable | ||||
| Single wire | 2 wires together | 3 wires together | 4 wires together | Single styling | Single styling | |
| 1 | 17 | 16 | 15 | 14 | 15 | 14 |
| 1,5 | 23 | 19 | 17 | 16 | 18 | 15 |
| 2,5 | 30 | 27 | 25 | 25 | 25 | 21 |
| 4 | 41 | 38 | 35 | 30 | 32 | 27 |
| 6 | 50 | 46 | 42 | 40 | 40 | 34 |
Selection of an automatic switch by power
The selection of a protective switch will help the calculation of the total power of household appliances. You will need to look at the value in the device passport. For example, in the kitchen, the outlet includes:
- coffee maker - 1000 W;
- electric oven - 2000 W;
- microwave oven - 2000 W;
- electric kettle - 1000 W;
- refrigerator - 500 watts.
Summing up the indicators, we get 6500 watts or 6.5 kilowatts. Next, you will need to refer to the table of machines depending on the power of the connection.
| Single-phase connection 220 V | Three phase connection | Machine power | |
| Triangle circuit 380 V | Star circuit, 220 V | ||
| 3,5 kW | 18.2 kW | 10.6 kW | 16 A |
| 4.4 kW | 22.8 kW | 13.2 kW | 20 A |
| 5.5 kW | 28.5 kW | 16.5 kW | 25 A |
| 7 kW | 36.5 kW | 21.1 kW | 32 A |
| 8.8 kW | 45.6 kW | 26.4 kW | 40 A |
Based on the table for wiring with standard voltage, you can choose a 32 A device that is suitable for a total power of 7 kW.
If you plan to connect additional equipment, the increase factor is used. The average value of 1.5 is multiplied by the power obtained in the calculations. A reduction factor is applied when it is impossible to operate several electrical appliances at the same time. It is equal to 1 or minus 1.
Machine selection depending on load power
For apartments and houses with new wiring, the choice of a machine is based on the calculated load current.
A three-phase type device can be calculated by the rated load current or by the response speed in case of exceeding the current value. For calculations, it is necessary to add the power of all consumers and calculate the current passing through the line. Work is performed according to the formula:
I = P / Uwhere:
- R - the total capacity of all household appliances;
- U - network voltage.
For example, the power is 7.2 kW, calculated by the formula 7200/220 = 32.72 A. The table shows the ratings 16, 20, 32, 25 and 40 A. The value of 32.72 A, taking into account the operation of the device at a value of 1 , 13 times the nominal value, multiply: 32x1.13 = 36.1 A. According to the table, it is clear that it is better to put the model at 40 A.
Methods for selecting a difavtomat
For example, consider the kitchen, where a large number of equipment is connected. First, you need to set the total power rating for a room with a refrigerator (500 W), a microwave (1000 W), a kettle (1500 W) and an extractor fan (100 W). The total power rating is 3.1 kW. On its basis, various methods of choosing an automaton for 3 phases are used.
Tabular method
Based on the table of devices, a single-phase or three-phase device is selected for the connection power. But the value in the calculations may not coincide with the tabular data. For a 3.1 kW network section, a 16 A model is needed - the closest value is 3.5 kW.
Graphical method
The selection technology does not differ from the table - you need to find a graph on the Internet. In the figure, standardly horizontally are switches with their current load, vertically - power consumption in one section of the circuit.
To establish the power of the device, you will need to draw a line horizontally to a point with a rated current. The total network load of 3.1 kW corresponds to a 16 A switch.
Three-Phase Switch Selection Criteria
Before buying, it is worth considering all the parameters that the input device will have.
Phase and voltage
Single-phase models at 220 V are connected to one terminal, three-phase models at 380 V - to three.
Leakage current
On the case there is a marking - the Greek letter "delta". The current leakage of a private house is about 350 mA, a separate group of devices - 30 mA, lamps and sockets - 30 mA, single links - 15 mA, boiler - 10 mA.
Varieties of current
On the machine there are indices A (operation in case of DC leakage) and AC (operation in case of AC leakage).
Number of poles
Depending on the number of poles, a three-phase switch can be purchased:
- a single-pole type of apparatus for protecting one cable and one phase;
- bipolar, represented by two devices with a common switch - shutdown occurs when the permissible value of one of them is exceeded, at the same time the neutral and phase in the single-phase network break;
- a three-pole device that provides rupture and protection of a phase circuit - are three devices with a common activation / deactivation handle;
- the four-pole device, which is mounted only on the input of a three-phase switchgear, breaks all three phases and the operating zero. An earth ground fault is not permitted.
Regardless of the number of poles, the device shutdown time should not exceed 0.3 seconds.
Installation location
For domestic use, an electric machine for 3 phases with a marking C for 25 A is intended. At the input, in this case, it is better to install products C50, C65, C85, C95. For sockets or other points - C 25 and C 15, for lighting - C 12 or C 17, for an electric stove - C 40. They will work when the current indicators are 5-10 times higher than the nominal value.
Nuances to consider
Nobody can know exactly what household appliances will be in the house or apartment. For this reason, you should:
- increase the total rated power of a three-phase difavtomat by 50%, or apply a coefficient of increase of 1.5;
- a reduction factor is taken into account when there are not enough sockets in the room for the simultaneous connection of equipment;
- for ease of calculation, the load should be divided into groups;
- powerful devices should be connected separately, taking into account the low-power load;
- to calculate a low-power load, the power will need to be divided by voltage;
- wiring is the main factor that is guided by when choosing a 3-phase circuit breaker; old aluminum wires can withstand 10 A, but if they are taken for 16 A sockets, they can melt;
- in domestic conditions, models with a current rating of 6, 16, 25, 32 and 40 A are most often used.
When buying a three-phase differential machine, you need to consider that the main markings are on the case or in the passport. Using formulas and tables will help you choose a model in accordance with the wiring in the apartment and the power of household appliances.
