Long gone are the days of ceramic plugs that were screwed into home electrical panels. Currently, various types of circuit breakers that perform protective functions are widely used. These devices are very effective against short circuits and overloads. Many consumers have not yet fully mastered these devices, so the question often arises which machine should be installed at 15 kW. The reliable and durable operation of electrical networks, appliances and equipment in a house or apartment completely depends on the choice of machine.

Basic functions of machines

Before choosing an automatic protective device, you need to understand the principles of its operation and capabilities. Many people consider the main function of the machine to be the protection of household appliances. However, this judgment is absolutely wrong. The machine does not react in any way to devices connected to the network; it is triggered only during short circuits or overloads. These critical conditions lead to a sharp increase in current strength, causing overheating and even fire of cables.

A special increase in current strength is observed during a short circuit. At this moment, its value increases to several thousand and the cables are simply not able to withstand such a load, especially if its cross-section is 2.5 mm2. With such a cross-section, an instant fire occurs in the wire.

Therefore, a lot depends on the correct choice of machine. Accurate calculations, including by, make it possible to reliably protect the electrical network.

Machine calculation parameters

Each circuit breaker primarily protects the wiring connected downstream of it. Basic calculations of these devices are carried out based on the rated load current. Power calculations are carried out in the case when the entire length of the wire is designed to carry the load, in accordance with the rated current.

The final choice of rated current for the machine depends on the cross-section of the wire. Only after this can the load value be calculated. The maximum current allowed for a wire with a certain cross-section must be greater. Thus, when choosing a protective device, the minimum wire cross-section present in the electrical network is used.

When consumers have a question about which machine should be installed at 15 kW, the table also takes into account the three-phase electrical network. There is a methodology for such calculations. In these cases, the rated power of a three-phase circuit breaker is determined as the sum of the powers of all electrical appliances planned to be connected through the circuit breaker.

For example, if the load of each of the three phases is 5 kW, then the operating current is determined by multiplying the sum of the powers of all phases by a factor of 1.52. Thus, it turns out 5x3x1.52 = 22.8 amperes. The rated current of the machine must exceed the operating current. In this regard, the most suitable protective device would be rated 25 A. The most common ratings of circuit breakers are 6, 10, 16, 20, 25, 32, 40, 50, 63, 80 and 100 amperes. At the same time, the compliance of the cable cores with the declared loads is clarified.

This technique can be used only in cases where the load is the same on all three phases. If one of the phases consumes more power than all the others, then the rating of the circuit breaker is calculated based on the power of this particular phase. In this case, only the maximum power value is used, multiplied by a factor of 4.55. These calculations allow you to select a machine not only according to the table, but also according to the most accurate data obtained.

When assembling an electrical panel or connecting new large household appliances, the home master will definitely encounter such a problem as the need to select circuit breakers. They provide electrical and fire safety, so choosing the right machine is the key to the safety of you, your family and your property.

What is the machine used for?

A machine is installed in the power supply circuit to prevent overheating of the wiring. Any wiring is designed to carry a certain current. If the current passed exceeds this value, the conductor begins to heat up too much. If this situation persists for a sufficient period of time, the wiring begins to melt, resulting in a short circuit. A circuit breaker is installed to prevent this situation.

The second task of the circuit breaker is to turn off the power when a short circuit current (SC) occurs. When a short circuit occurs, the currents in the circuit increase many times over and can reach thousands of amperes. To prevent them from destroying the wiring and damaging the equipment included in the line, the circuit breaker must turn off the power as quickly as possible - as soon as the current exceeds a certain limit.

In order for the protective circuit breaker to properly perform its functions, it is necessary to correctly select the machine according to all parameters. There are not many of them - only three, but you need to deal with each one.

What types of circuit breakers are there?

To protect conductors of a single-phase 220 V network, there are single-pole and double-pole disconnecting devices. To single-pole wires, only one conductor is connected - phase, to double-pole wires, both phase and neutral. Single-pole circuit breakers are installed on 220 V indoor lighting circuits, on socket groups in rooms with normal operating conditions. They are also installed on some types of load in three-phase networks, connecting one of the phases.

For three-phase networks (380 V) there are three and four poles. These circuit breakers (the correct name is a circuit breaker) are installed on a three-phase load (ovens, hobs and other equipment that operate on a 380 V network).

In rooms with high humidity (bathroom, bathhouse, swimming pool, etc.) two-pole circuit breakers are installed. They are also recommended for installation on powerful appliances - washing machines, dishwashers, boilers, ovens, etc.

It’s just that in emergency situations - in the event of a short circuit or insulation breakdown - phase voltage can reach the neutral wire. If a single-pole device is installed on the power line, it will disconnect the phase wire, and the zero with dangerous voltage will remain connected. This means there is still a possibility of electric shock when touched. That is, the choice of machine is simple - single-pole switches are installed on some lines, and double-pole switches on others. The specific amount depends on the network condition.

For a three-phase network, there are three-pole circuit breakers. Such a machine is installed at the entrance and at consumers, to which all three phases are supplied - an electric stove, a three-phase hob, an oven, etc. The remaining consumers are equipped with two-pole circuit breakers. They must disconnect both phase and neutral.

Example of three-phase network wiring - types of circuit breakers

The choice of circuit breaker rating does not depend on the number of wires connected to it.

Deciding on the denomination

Actually, from the functions of the circuit breaker, the rule for determining the rating of the circuit breaker follows: it must operate until the current exceeds the capabilities of the wiring. This means that the current rating of the machine must be less than the maximum current that the wiring can withstand.

Based on this, the algorithm for selecting a circuit breaker is simple:

• for a specific area.
• See what maximum current this cable can withstand (see the table).
• Next, from all the ratings of the circuit breakers, we select the nearest smaller one. The ratings of the machines are tied to the permissible long-term load currents for a particular cable - they have a slightly lower rating (see the table). The list of denominations looks like this: 16 A, 25 A, 32 A, 40 A, 63 A. From this list you choose the appropriate one. There are smaller values, but they are practically not used anymore - we have too many electrical appliances and they have considerable power.

Example

The algorithm is very simple, but it works flawlessly. To make it clearer, let's look at an example. Below is a table that shows the maximum permissible current for conductors that are used at. Recommendations regarding the use of machines are also given there. They are given in the column “Nominal current of the circuit breaker”. This is where we look for the ratings - it is slightly less than the maximum permissible for the wiring to work normally.

In the table we find the selected wire cross-section for this line. Suppose we need to lay a cable with a cross-section of 2.5 mm 2 (the most common when laying to medium-power devices). A conductor with this cross-section can withstand a current of 27 A, and the recommended rating of the machine is 16 A.

How will the circuit work then? As long as the current does not exceed 25 A, the machine does not turn off, everything works as normal - the conductor heats up, but not to critical values. When the load current begins to increase and exceeds 25 A, the machine does not turn off for some time - perhaps these are starting currents and they are short-lived. It turns off if the current exceeds 25 A by 13% for a sufficiently long time. In this case, if it reaches 28.25 A. Then the power supply will work and de-energize the branch, since this current already poses a threat to the conductor and its insulation.

Power calculation

Is it possible to choose a machine based on load power? If only one device is connected to the power line (usually large household appliances with high power consumption), then it is permissible to make a calculation based on the power of this equipment. You can also choose an introductory machine based on power, which is installed at the entrance to a house or apartment.

If we are looking for the rating of the input circuit breaker, we need to add up the power of all devices that will be connected to the home network. Then the found total power is substituted into the formula, and the operating current for this load is found.

After we have found the current, select the nominal value. It may be either slightly more or slightly less than the found value. The main thing is that its shutdown current does not exceed the maximum permissible current for this wiring.

When can you use this method? If the wiring is laid with a large margin (this is not bad, by the way). Then, in order to save money, you can automatically install switches that correspond to the load, and not the cross-section of the conductors. But once again we draw attention to the fact that the long-term permissible current for the load must be greater than the maximum current of the circuit breaker. Only then will the choice of circuit breaker be correct.

Selecting the breaking capacity

The selection of a packager based on the maximum permissible load current is described above. But the network circuit breaker must also turn off when a short circuit (short circuit) occurs in the network. This characteristic is called breaking capacity. It is displayed in thousands of amperes - this is the order currents can reach during a short circuit. Selecting a machine based on its breaking capacity is not very difficult.

This characteristic shows at what maximum value of the short-circuit current the circuit breaker remains operational, that is, it will not only be able to turn off, but will also work after being turned on again. This characteristic depends on many factors and for accurate selection it is necessary to determine the short-circuit currents. But for wiring in a house or apartment, such calculations are done very rarely, and are based on the distance from the transformer substation.

If the substation is located close to the entrance to your house/apartment, take a circuit breaker with a breaking capacity of 10,000 A; for all other city apartments, 6,000 A is enough. If the house is located in a rural area or you are choosing a circuit breaker for a summer residence, it may well be enough and a breaking capacity of 4,500 A. The networks here are usually old and short-circuit currents are not large. And since the price increases significantly with increasing breaking capacity, the principle of reasonable savings can be applied.

Is it possible to install bags with a lower breaking capacity in city apartments? In principle, it is possible, but no one guarantees that after the first short circuit you will not have to change it. He may have time to turn off the network, but will be inoperative. In the worst case scenario, the contacts will melt and the machine will not have time to turn off. Then the wiring will melt and a fire may occur.

Type of electromagnetic release

The machine must operate when the current rises above a certain level. But short-term overloads periodically occur in the network. They are usually associated with inrush currents. For example, such overloads can be observed when turning on the refrigerator compressor, washing machine motor, etc. The circuit breaker should not turn off during such temporary and short-term overloads, because they have a certain delay for operation.

But if the current has increased not because of an overload but because of a short circuit, then during the time that the circuit breaker “wait”, its contacts will melt. This is what an electromagnetic automatic release is for. It operates at a certain current value, which can no longer be an overload. This indicator is also called cut-off current, since in this case the circuit breaker cuts off the line from the power supply. The magnitude of the operating current can be different and is displayed by letters that appear in front of the numbers indicating the rating of the machine.

There are three most popular types:

What characteristics should you choose? In this case, the choice of a circuit breaker is also based on the distance of your household from the substation and the state of the electrical networks; the choice of a circuit breaker is carried out using simple rules:

• With the letter “B” on the body they are suitable for dachas, houses in villages and towns that receive power supply through air ducts. They can also be installed in apartments of old houses in which the internal electrical network has not been reconstructed. These circuit breakers are not always on sale; they cost a little more than category C, but can be delivered to order.
• Bags with a “C” on the body are the most widely used option. They are installed in networks with normal condition, suitable for apartments in new buildings or after major renovations, in private houses near the substation.
• Class D is installed in enterprises and workshops with equipment with high starting currents.

That is, in essence, the choice of a circuit breaker in this case is simple - type C is suitable for most cases. It is available in stores in a large assortment.

Which manufacturers should you trust?

And finally, let’s pay attention to the manufacturers. The choice of a circuit breaker cannot be considered complete if you have not thought about which brand of circuit breakers you will buy. You definitely shouldn’t take on unknown companies - electrical engineering is not a field where you can conduct experiments. Learn more about choosing a manufacturer in the video.

Not a single electrical device, not a single electrical appliance, should be used without protective automatic equipment. An automatic circuit breaker (AB) is installed for a specific device, or for a group of consumers connected to the same line. In order to correctly answer the question of what power corresponds, for example, to a machine with a rating of 25A, you should first become familiar with the design of the circuit breaker and the types of protective devices.

Structurally, AB combines mechanical, thermal and electromagnetic releases that operate independently of each other.

Mechanical release

Designed to turn the machine on/off manually. Allows you to use it as a switching device. Used during repair work to de-energize the network.

Thermal release (TR)

This part of the circuit breaker protects the circuit from overload. Current passes through the bimetallic strip, heating it. Thermal protection is inertial, and can briefly pass currents exceeding the operation threshold (In). If the current exceeds the rated current for a long time, the plate heats up so much that it deforms and turns off the AV. After the bimetallic plate has cooled (and the cause of the overload has been eliminated), the machine is turned on manually. In a 25A machine, the number 25 indicates the TP response threshold.

Electromagnetic release (ER)

Breaks the electrical circuit during a short circuit. The overcurrents generated during a short circuit require an instant response from the protective device, therefore, unlike a thermal release, an electromagnetic release is triggered instantly, in a fraction of a second. Switching off occurs due to the passage of current through the winding of a solenoid with a movable steel core. The solenoid, when activated, overcomes the resistance of the spring and turns off the moving contact of the circuit breaker. To disconnect due to a short circuit, currents exceeding In from three to fifty times are required, depending on the type of circuit breaker.

Types of AV according to current-time characteristics

Let's ignore industrial electronics and motor protection devices with built-in thermal relays, and consider the most common types of circuit breakers:

• Characteristic B - when In is three times higher, the TR is triggered in 4-5 s. ER triggers when In is exceeded three to five times. They are used in lighting networks or when connecting a large number of low-power consumers.
• Characteristic C is the most common type of AB. TR is triggered in 1.5 s when In is five times exceeded, ER is triggered when In is 5-10 times exceeded. They are used for mixed networks that include devices of various types, including those with low inrush currents. The main type of circuit breakers for residential and administrative buildings.
• Characteristic D - machines with the highest overload capacity. Used to protect electric motors and energy consumers with high starting currents.

The ratio of AV ratings and consumer power

To determine how many kilowatts can be connected through a circuit breaker of a certain power, use the table:

To calculate the power of the introductory machine at home, use a coefficient of 0.7 of the total power of consumers.

When determining the load capacity of a circuit breaker, it is important to take into account not only its rating, but also the overload characteristic. This will help avoid false alarms when starting powerful electrical appliances.

When assembling a distribution panel for a three-phase network, 3-pole circuit breakers are used. If a network overload occurs or a short circuit occurs, such a machine will disconnect three phases at once.

How many poles are there?

Single-pole, two-pole, three-pole and four-pole circuit breakers

In the distribution panel of an apartment or house, single-pole circuit breakers are most often used. Their task is to disconnect the phase conductor, thereby interrupting the supply of electricity to the circuit. Differential circuit breakers and RCDs disconnect both the phase and the working zero at the same time, because their operation may be due to a violation of the integrity of the wiring. The input circuit breaker in such a panel should always be two-pole.

Three-phase current is used by enterprises to power powerful units that require a voltage of 380 volts. Sometimes a four-core cable (three phases and a working zero) is supplied to a residential building or office. Due to the fact that equipment designed for such voltage is not used in these rooms, the three phases are separated in the distribution panel and a voltage of 220 is obtained between each phase and the working zero.

For such panels, 3-pole and four-pole circuit breakers are used. They are triggered when the rated load on any of the three wires is exceeded and turn them off all at the same time, and in the case of a four-pole wire, the working zero is additionally turned off.

Why use two and four poles

The input circuit breaker must completely disconnect all phases and the working zero, because One of the wires of the input cable may leak to zero and if it is not disconnected using a single-pole or 3-pole circuit breaker, there is a possibility of electric shock.

Leakage at 3-pole circuit breaker

The figure shows that in this case the entire working zero in the network is energized. If you use an input circuit breaker that disconnects phase and zero, this can be avoided; therefore, the use of four-pole and two-pole circuit breakers for three-phase and single-phase electrical networks is safer.

Each 3-pole circuit breaker is three single-pole circuit breakers that operate simultaneously. One phase is connected to each terminal of a 3-pole circuit breaker.

Diagram of a 3-pole circuit breaker

As can be seen from the diagram, each circuit has separate electromagnetic and thermal releases, and the body of the 3-pole circuit breaker has separate arc extinguishers.

A 3-pole circuit breaker can also be used in a single-phase power supply. In this case, the phase and neutral wires are connected to two terminals of the switch, and the third terminal remains empty (signal).

Price

3-pole circuit breakers, depending on the manufacturer, also differ in price. In the table below you can compare the cost of such electrical installation products from the most popular brands in the Russian Federation: IEK, Legrand, Schnider Electric and ABB:

Cost table for 3-pole circuit breakers leading on the Russian market

Video about switch polarity and connection methods

The video will be useful for beginners who want to understand the differences and functionality of single-pole, double-pole, 3-pole and 4-pole circuit breakers. How to connect them correctly and in what cases one or another machine should be used.

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To select a circuit breaker based on load power, it is necessary to calculate the load current and select the rating of the circuit breaker greater than or equal to the obtained value. The current value expressed in amperes in a single-phase 220 V network usually exceeds the load power value expressed in kilowatts by 5 times, i.e. if the power of the electrical receiver (washing machine, light bulb, refrigerator) is 1.2 kW, then the current that will flow in the wire or cable is 6.0 A (1.2 kW * 5 = 6.0 A). Calculated at 380 V, in three-phase networks, everything is similar, only the current value exceeds the load power by 2 times.

Power factor

This is a dimensionless physical quantity that characterizes the consumer of alternating electric current from the point of view of the presence of a reactive component in the load. Power factor shows how much the alternating current flowing through a load is out of phase relative to the voltage applied to it.
Numerically, the power factor is equal to cosine of this phase shift or cos φ

We take cosine phi from Table 6.12 of the regulatory document SP 31-110-2003 “Design and installation of electrical installations of residential and public buildings”

Table 1. Cos φ value depending on the type of power receiver

Let's take our power receiver with a power of 1.2 kW. as a household single-phase refrigerator at 220V, we take cos φ from the table as 0.75 as a motor from 1 to 4 kW.
Let's calculate the current I=1200 W / 220V * 0.75 = 4.09 A.

Now the most correct way to determine the current of an electrical receiver— take the current value from the nameplate, passport or operating instructions. There are rating plates on almost all electrical appliances.

EKF circuit breakers

The total current in the line (for example, a socket network) is determined by summing the current of all electrical receivers. Based on the calculated current, we select the closest rating of the automatic circuit breaker, upwards. In our example, for a current of 4.09A it will be a 6A machine.

It is very important to note that choosing a circuit breaker based only on the load power is a gross violation of fire safety requirements and can lead to fire of the cable or wire insulation and, as a result, a fire. When choosing, it is also necessary to take into account the cross-section of the wire or cable.

Based on the load power, it is more correct to choose the cross-section of the conductor. The selection requirements are set out in the main regulatory document for electricians called PUE (Electrical Installation Rules), or more precisely in Chapter 1.3. In our case, for a home electrical network, it is enough to calculate the load current as indicated above, and in the table below select the cross-section of the conductor, provided that the resulting value is lower than the long-term permissible current corresponding to its cross-section.

Selecting a machine according to cable cross-section

Let us consider the problem of choosing circuit breakers for home electrical wiring in more detail, taking into account fire safety requirements. The necessary requirements are set out in Chapter 3.1 “Protection of electrical networks up to 1 kV,” since the network voltage in private houses, apartments, and cottages is 220 or 380V.

Voltage 220V.

– single-phase network is used mainly for sockets and lighting.
380V. - These are mainly distribution networks - power lines running along the streets, from which houses are connected by branches.

According to the requirements of the above chapter, internal networks of residential and public buildings must be protected from short-circuit currents and overloads. To fulfill these requirements, protection devices called circuit breakers were invented.

Automatic circuit breaker

This is a mechanical switching device capable of turning on, carrying currents in the normal state of the circuit, and also turning on, conducting for a specified time and automatically turning off currents in the specified abnormal state of the circuit, such as short circuit and overload currents.

Short circuit (SC)

an electrical connection of two points of an electrical circuit with different potential values, not provided for by the design of the device and disrupting its normal operation. A short circuit can occur as a result of a violation of the insulation of current-carrying elements or mechanical contact of non-insulated elements. Also, a short circuit is a condition when the load resistance is less than the internal resistance of the power source.

Overload current

– exceeding the rated value of the continuous permissible current and causing overheating of the conductor. Protection against short-circuit currents and overheating is necessary for fire safety, to prevent the fire of wires and cables, and as a result of a fire in the house.

Continuous permissible current of cable or wire

- the amount of current that constantly flows through the conductor and does not cause excessive heating.

The value of continuous permissible current for conductors of different sections and materials is presented below. The table is a combined and simplified version applicable for household power supply networks, tables No. 1.3.6 and 1.3.7 PUE.

Selecting a circuit breaker based on short-circuit current

The selection of a circuit breaker for protection against short circuit (short circuit) is carried out based on the calculated value of the short circuit current at the end of the line. The calculation is relatively complicated, the value depends on the power of the transformer substation, the cross-section of the conductor and the length of the conductor, etc.

From the experience of carrying out calculations and designing electrical networks, the most influential parameter is the length of the line, in our case the length of the cable from the panel to the outlet or chandelier.

Because in apartments and private houses this length is minimal, then such calculations are usually neglected and circuit breakers with characteristic “C” are selected; you can, of course, use “B”, but only for lighting inside an apartment or house, because such low-power lamps do not cause a high inrush current, and already in the network for kitchen appliances with electric motors, the use of machines with characteristic B is not recommended, because the machine may be triggered when the refrigerator or blender is turned on due to a jump in the starting current.

Selecting a machine based on the long-term permissible current (LTC) of the conductor

The selection of a circuit breaker for protection against overload or overheating of a conductor is carried out based on the DDT value for the protected section of the wire or cable. The rating of the machine must be less than or equal to the DDT value of the conductor indicated in the table above. This ensures automatic shutdown of the machine when the DDT in the network is exceeded, i.e. part of the wiring from the machine to the last electrical receiver is protected from overheating, and as a result, from fire.

Example of circuit breaker selection

We have a group from the panel to which we plan to connect a dishwasher -1.6 kW, a coffee maker - 0.6 kW and an electric kettle - 2.0 kW.

We count the total load and calculate the current.

Load = 0.6+1.6+2.0=4.2 kW. Current = 4.2*5=21A.

We look at the table above; all conductor cross-sections except 1.5 mm2 for copper and 1.5 and 2.5 for aluminum are suitable for the current we have calculated.

We choose a copper cable with cores with a cross section of 2.5 mm2, because... It makes no sense to buy a cable with a larger cross-section for copper, and aluminum conductors are not recommended for use, and may already be prohibited.

We look at the scale of denominations of the machines produced - 0.5; 1.6; 2.5; 1; 2; 3; 4; 5; 6; 8; 10; 13; 16; 20; 25; 32; 40; 50; 63.

The circuit breaker for our network is suitable for 25A, since it is not suitable for 16A because the calculated current (21A) exceeds the rating of the 16A circuit breaker, which will cause it to trip when all three power receivers are turned on at once. A 32A machine is not suitable because it exceeds the DDT of the 25A cable we selected, which can cause overheating of the conductor and, as a result, a fire.

Summary table for selecting a circuit breaker for a single-phase 220 V network.

Summary table for selecting a circuit breaker for a three-phase 380 V network.

* - double cable, two cables connected in parallel, for example 2 cables VVGng 5x120

Results

When choosing a machine, it is necessary to take into account not only the load power, but also the cross-section and material of the conductor.

For networks with small protected areas from short-circuit currents, circuit breakers with characteristic “C” can be used

The rating of the machine must be less than or equal to the long-term permissible current of the conductor.