Varieties and calculation of three-phase circuit breakers

To prevent a short circuit and overload of the electrical network, a three-phase circuit breaker is used. The switching device can be used for DC and AC line. The construction of the standard model is represented by extenders with switching depending on the circuit frequency.

Which machine is suitable for 15 kW

The purpose of the three-phase circuit breaker is overload protection

The purpose of the 3-phase circuit breaker is protection against overcurrents and overloads. The 15 kW modification works in a 380 V network, that is, a 25A device is needed for the input. When choosing, it should be borne in mind that in conditions of short circuits, the current increases and can cause a fire in the wiring.

When choosing a 15 kW machine model for a three-phase load, you will need to take into account the parameters of the permissible voltage and current during a short circuit. It is worth focusing on the calculated indicators of the current of the cable with a minimum cross-section that protects the switch and the rated current of the receiver.

When calculating the input switching machine according to the power parameters in the 380 V network, take into account:

  • electric power - actual and additional;
  • cable loading intensity;
  • availability of free capacity in the design indicator of a residential building;
  • remoteness of outbuildings and non-residential premises from the point of cable entry.

The cost of a machine for 25A for 380V

In a network of 15 kilowatts with additional power, an ASU device is installed.

Functions of three-phase machines

A three-phase device simultaneously serves several single-phase zones of the circuit

Before choosing an automatic switch, you should understand its functionality. Users are often mistaken in thinking that the device protects home appliances. The machine does not react to its electrical indicators, operating exclusively in the event of a short circuit or overload. The functions of a three-phase unit include:

  • simultaneous service of several single-phase zones of the circuit;
  • preventing the formation of overcurrents on the line;
  • joint work with AC rectifiers;
  • protection of high-power equipment;
  • increased power due to the installation of a special converter;
  • fast 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 purpose of the circuit breaker

Circuit breaker characteristics

The three-phase circuit breaker is activated by an electromagnetic splitter in the event of a line fault. The principle of operation of the element consists in heating 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 higher than the calculated ones to affect the wiring. Without it, the cable cores are heated to the melting temperature, which leads to the ignition of the insulating layer.For this reason, it is important to know if the network can withstand the voltage.

Compliance of wires with load

The problem is typical for old buildings, in which new machines, a meter, and an RCD are installed on the existing line. The machines are matched to the total power of the equipment, but sometimes they do not work - the cable smokes or burns.

For example, the veins of an old cable with a cross-section of 1.5 mm2 have a current limit of 19 A. When the equipment is switched on at a time with a total current of 22.7 A, only a modification of 25 Amperes will provide protection.

The wires will heat up, but the switch will remain on until the insulation melts. A complete replacement of the wiring with a copper cable with a cross section of 2.5 mm2 can prevent a fire.

Protection of the weakest section of the cable wiring

Based on clause 3.1.4 of the PUE, the task of the automatic device is to prevent overloading at the weakest link in the electrical circuit. Its rated current is matched to the current of the connected household appliances.

If the machine is not selected correctly, the unprotected area will cause a fire.

Principles of calculating an automatic machine for a cable cross-section

The calculations of a 3-phase difavtomat are carried out based on the cable cross-section. For the 25 A model, you will need to refer to the table.

Wire section, mm2Permissible load current for cable material

The 25 Ampere version can be used to protect the wiring or installed on the input.

For example, a copper wire with a cross-section of 1.5 mm2 with an allowable load current of 19 A is used for wiring. To prevent the cable from heating up, you will need to select a lower value - 16 A.

The cost of machines for 16A

Determination of the dependence of power on the section by the formula

Selection table for cable cross-section depending on power

If the cable cross-section is unknown, you can use the formula:

Icalc = P / Unomwhere:

  • Icalculation - rated current,
  • P - power of devices,
  • Unom - voltage rating.

As an example, you can calculate an automatic machine that will need to be installed on a boiler with a load of 3 kW and a network voltage of 220 V:

  1. Convert 3 kW to Watts - 3x1000 = 3000.
  2. Divide by voltage: 3000/220 = 13.636.
  3. Round off the rated current to 14 A.

Depending on the environmental conditions and the method of laying the cable, it is necessary 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 Icalculation= 14 + 5 = 19 A. Further, according to the PUE table, the cross-section of the copper wire is selected.

Section, mm2Load current, A
Single core cableTwo-core cableThree-core cable
Single wire2 wires together3 wires together4 wires togetherSingle stylingSingle styling

Selection of automatic switch by power

Power table of electrical appliances in the kitchen

Calculating the total power of household appliances will help you choose a protective switch. 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 W.

Summing up the indicators, we get 6500 W or 6.5 kilowatts. Next, you need to refer to the table of machines, depending on the connection power.

Single-phase connection 220 VThree-phase connectionMachine power
Triangle circuit 380 VStar circuit, 220 V
3.5 kW18.2 kW10.6 kW16 A
4.4 kW22.8 kW13.2 kW20 A
5.5kw28.5 kW16.5 kW25 A
7 kW36.5 kW21.1 kW32 A
8.8 kW45.6 kW26.4 kW40 A

Based on the table for standard voltage wiring, you can select a 32 A device that is suitable for a total power of 7 kW.

The cost of the machine for 32A

If you plan to connect additional equipment, the increase factor is used. The average value of 1.5 is multiplied by the calculated power. The 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.

The choice of the machine depending on the power of the load

For apartments and houses with new electrical wiring, the choice of the machine is made based on the calculated load current.

It is possible to calculate a three-phase type device by the rated load current or by the speed of operation in conditions of excess current value. For calculations, it is required to add up the power of all consumers and calculate the current passing through the line. The work is carried out according to the formula:

I = P / Uwhere:

  • R - the total capacity of all household appliances;
  • U - mains voltage.

For example, the power is 7.2 kW, calculated according to the formula 7200/220 = 32.72 A. The table shows the ratings of 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 more than the nominal, we multiply: 32x1.13 = 36.1 A. The table shows that it is better to put the model at 40 A.

Methods for selecting a difavtomat

The nominal value of the difavtomat and its time-current characteristic

For example, consider a kitchen where a large amount of equipment is connected. First, you need to set the total power rating for a room with a refrigerator (500 W), a microwave oven (1000 W), a kettle (1500 W) and a cooker hood (100 W). The total power indicator is 3.1 kW. On its basis, various methods of choosing a machine for 3 phases are used.

Tabular method

Based on the table of devices, according to the connection power, a single-phase or three-phase device is selected. But the value in the calculations may not coincide with the tabular data. For a 3.1 kW network section, you will need a 16 A model - the closest in value is 3.5 kW.

Graphical method

The selection technology does not differ from the tabular one - you will need to find the schedule on the Internet. In the figure, the switches with their current load are standard horizontally, and the power consumption in one section of the circuit is vertically.

To establish the power of the device, you will need to draw a line horizontally to the point with the rated current. The total network load of 3.1 kW corresponds to a 16 A switch.

Selection criteria for a three-phase switch

Before buying, it is worth considering all the parameters that the input device will have.

Phase and voltage

Single-phase 220 V models are connected to one terminal, three-phase 380 V models - to three.

Leakage current

On the body 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, a boiler - 10 mA.

Varieties of current

The machine has indices A (operation with DC leakage) and AC (operation with AC leakage).

Number of poles

Single-pole circuit breaker is used for one phase

Depending on the number of poles, a three-phase switch can be purchased:

  • single-pole type of devices for the protection of one cable and one phase;
  • two-pole, represented by two devices with a common switch - shutdown occurs when the permissible value of one of them is exceeded, the neutral and the phase in a single-phase network are simultaneously cut off;
  • three-pole device providing break and protection of the phase circuit - are three devices with a common activation / deactivation handle;
  • a four-pole device, which is mounted only on the input of a three-phase switchgear, breaks all three phases and a working zero. A break in the protection ground is not allowed.

Regardless of the number of poles, the disconnection time of the device should not exceed 0.3 seconds.

Place of installation

For domestic use, an electric machine for 3 phases is intended with marking C for 25 A. In this case, it is better to install products C50, C65, C85, C95 at the input. 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 be triggered when the current is 5-10 times the nominal value.

Nuances to consider

Table of power consumption of various electrical appliances

Nobody can know exactly what household appliances will be in the house or apartment. For this reason, it follows:

  • increase the total design power of a three-phase difavtomat by 50%, or apply an increase factor of 1.5;
  • the reduction factor is taken into account when there are not enough outlets in the room for the simultaneous connection of equipment;
  • for simplicity of calculations, 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 the voltage;
  • wiring is the main factor to be guided by when choosing an automatic 3-phase switch; old aluminum wires can withstand 10 A, but if you take them for 16 A outlets, 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 take into account 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.
Add a comment

  1. Oleg

    Well, if the excess is in one phase out of three, will the AB work or not?

  2. Engineer

    Complete heresy. For one outlet, the load of kettles is 7 kW, current is 32A. Let the author stick a 4mm2 copper wire into the terminal of an ordinary 16A socket. I'll see how he does it