Choosing a circuit breaker: types and characteristics of automatic machines

Vasily Borutsky
Checked by a specialist: Vasily Borutsky
Author: Julia Polyakova
Last update: November 2019

Surely many of us wondered why the circuit breakers so quickly replaced outdated fuses from the electrical circuits? The activity of their implementation is justified by a number of very convincing arguments, among which the opportunity to buy this type of protection, which ideally matches the time-current data of specific types of electrical equipment.

Doubt what kind of machine you need and don’t know how to choose it right? We will help you find the right solution - the article discusses the classification of these devices. As well as important characteristics that you should pay close attention to when choosing a circuit breaker.

To make it easier for you to deal with machines, the article material is supplemented by visual photos and useful video recommendations from experts.

Classification of circuit breakers

The machine almost instantly disconnects the line entrusted to it, which eliminates damage to the wiring and network-powered equipment. After a shutdown has been completed, the branch can be immediately restarted without having to replace the safety device.

Usually circuit breakers they are selected according to four key parameters - rated breaking capacity, number of poles, time-current characteristic, rated operating current.

Rated breaking capacity

This characteristic indicates the permissible short circuit current (short circuit) at which the circuit breaker will trip and, by opening the circuit, will disconnect the wiring and devices connected to it.

According to this parameter, three types of machines are divided - 4.5 kA, 6 kA, 10 kA.

  1. 4.5 kA automatic machines (4500 A) usually used to prevent damage to power lines of private residential facilities. The resistance of the wiring from the substation to the fault location is approximately 0.05 Ohms, which gives a current limit of about 500 A.
  2. 6 kA (6000 A) devices they are used to protect against short-circuiting of the residential sector, public places where the line resistance can reach 0.04 Ohm, which increases the likelihood of getting a short circuit to 5.5 kA.
  3. 10 kA circuit breakers (10,000 A) used to protect electrical installations for industrial use. A current of up to 10,000 A can occur in a short circuit located close to the substation.

Before choosing the optimal circuit breaker modification, it is important to understand whether short-circuit currents exceeding 4.5 kA or 6 kA are possible?

Circuit breaker marking
The rated breaking capacity is indicated in the documentation for the switch and on the case in the form of a code - 4500A, 600A, 10000A or 4.5kA, 6kA, 10kA. On the front of the device there is information about the manufacturer, model, rated voltage, consisting of time-current characteristic data, operating current

Turning off the machine occurs when the set values ​​are shorted. Most often for domestic use, circuit breakers of 6000 A modification are used.

The 4500 A models are practically not used to protect modern electric networks, and in some countries they are forbidden to operate.

If you are interested in how to correctly transfer Amperes to Watts, we recommend that you familiarize yourself with the material set forth in the next article.

How to correctly choose a circuit breaker
When registering by a KZ automatic device, the electromagnetic coil disconnects (situation A). When the rated currents are exceeded, a bimetallic plate opens the network (situation B)

The operation of the circuit breaker is to protect the wiring (and not the equipment and users) from the short circuit and from the melting of the insulation when currents pass above the nominal values.

By the number of poles

This characteristic indicates the maximum possible number of wires that can be connected to the AV to protect the network.

Their shutdown occurs when an emergency occurs (during exceeding the permissible current indicators or exceeding the level of the time-current curve).

This characteristic indicates the maximum possible number of wires that can be connected to the AV to protect the network. Their shutdown occurs when an emergency occurs (during exceeding the permissible current indicators or exceeding the level of the time-current curve).

Single pole circuit breakers

A single-pole type switch is the simplest modification of the machine. It is designed to protect individual circuits, as well as single-phase, two-phase, three-phase wiring. It is possible to connect 2 wires to the circuit breaker design - a power wire and an outgoing one.

The functions of this class of device include only the protection of the wire from fire. The neutral wiring itself is placed on the zero bus, thereby bypassing the machine, and the ground wire is connected separately to the ground bus.

Single pole switch
A single-pole AV connection is made with a single-wire cable, but sometimes two-wire cables are used. They connect power to the top of the machine, and the protected line to the bottom, which simplifies installation. Installation takes place on an 18 mm din rail

A single-pole circuit breaker does not perform the function of an input, since when it is forced to turn off, the phase line breaks and the neutral is connected to a voltage source, which does not give a 100% guarantee of protection.

Bipolar Circuit Breakers

When it is necessary to completely disconnect the wiring network from voltage, a two-pole automatic device is used.

It is used as an introduction, when during a short circuit or a network failure, all electrical wiring is de-energized at the same time. This allows you to carry out timely repair work, chain modernization is absolutely safe.

Bipolar machines are used in cases where a separate switch is needed for a single-phase electrical appliance, for example, a water heater, boiler, machine.

Bipolar machine
The connection of a two-pole circuit breaker takes into account the electrical protection circuit using a 1- or 2-wire cable (the number of cores depends on the circuitry). Mounting on a 36 mm DIN rail

The machine is connected to the protected device using 4 wires, two of which are power wires (one of them is directly connected to the network, and the second supplies power with a jumper) and two are outgoing wires that require protection, and they can be 1-, 2- 3 wire.

Three Pole Circuit Breakers

To protect a three-phase 3- or 4-wire network, three-pole machines are used. They are suitable for connection by the type of a star (the middle wire is left unprotected, and the phase wires are connected to the poles) or a triangle (with a missing central wire).

In the event of an accident on one of the lines, the other two are disconnected on their own.

Three-pole circuit breaker
Connection of a three-pole AB is made by 1-, 2-, 3-wire wires. Installation requires a 54 mm wide DIN rail

A three-pole switch serves as an input and common for all types of three-phase loads. Often, modifications are used in industry to provide electric motors with current.

Up to 6 wires are connected to the model, 3 of them are represented by phase wires of a three-phase power supply network. The remaining 3 are protected. They represent three single-phase or one three-phase wiring.

Four Pole Circuit Breakers

To protect a three-, four-phase mains, for example, a powerful motor connected according to the principle of “stars with a zero point removed”, a four-pole circuit breaker is used. It is used as an input switch to a three-phase four-wire network.

How to choose a four-pole circuit breaker
A four-pole switch is connected with a 1-, 2-, 3-, 4-wire cable, the circuit depends on the type of connection, install the housing on a 73 mm wide din rail

It is possible to connect eight wires to the machine case, of which three are phase wires of the power supply network (+ from one zero) and four are represented by outgoing wires (3 phase + 1 neutral).

Single-phase consumers are powered by a voltage of 220 V, which can be obtained by taking one of the phases and the neutral conductor (neutral) of the electrical network. That is, in this case, in addition to the three phases of the electric network, there is another conductor - zero, therefore, for the protection and switching of such an electric network, four-pole circuit breakers are installed that break all four conductors.

By time-current characteristic

AB can have the same rate rated load power, but the characteristics of electricity consumption by devices may be different.

Power consumption can flow unevenly, depending on the type and load, as well as when you turn on, turn off or constant operation of a device.

Fluctuations in power consumption can be quite significant, and the range of their changes is wide. This leads to the shutdown of the machine due to exceeding the rated current, which is considered a false disconnection of the network.

In order to exclude the possibility of inappropriate fuse tripping during non-emergency standard changes (increase in current strength, power change), machines with certain time-current characteristics (VTX) are used.

This allows the operation of circuit breakers with the same current parameters with arbitrary permissible loads without false trips.

The BTX show how long the circuit breaker will trip and what indicators of the ratio of the current strength and the direct current of the machine will be.

Features of machines with characteristic B

The machine with the specified characteristic turns off in 5-20 seconds. The current indicator is 3-5 rated currents of the machine. These modifications are used to protect the circuits supplying household standard appliances.

Most often, the model is used to protect the wiring of apartments, private houses.

Feature C - Operation Principles

The machine with the nomenclature designation C is switched off in 1-10 seconds at 5-10 rated currents.

Circuit breakers of this group are used in all areas - in everyday life, construction, industry, but they are most in demand in the field of electrical protection of apartments, houses, residential premises.

Operation of circuit breakers with characteristic D

D-class machines are used in industry and are represented by three-pole and four-pole modifications. They are used to protect powerful electric motors and various 3-phase devices.

The response time AB is 1-10 seconds at a current multiple of 10-14, which allows it to be effectively used to protect various wiring.

Graph of time-current characteristics
The bottom of the graph shows the multiplicity of the nominal current values, along the vertical line - the shutdown time. For characteristic B, the trip occurs when 3-5 times the current rises above the rated current, for C - 5-10 times, for D - 10-14 times

Powerful industrial engines work exclusively with AB with characteristic D.

You may also be interested in readingmarking circuit breakers in our other article.

Rated operating current

In total there are 12 modifications of machines, differing in rated operating current - 1A, 2A, 3A, 6A, 10A, 16A, 20A, 25A, 32A, 40A. The parameter is responsible for the speed of operation of the machine when the current rises above the nominal value.

Machine selection by power
The table illustrates the maximum power of each modification of the machine, based on the connection diagram and the network voltage. The maximum return of the circuit breaker occurs when the load is connected according to the triangle circuit

The choice of the switch according to the specified characteristic is made taking into account the power of the wiring, the permissible current that the wiring can withstand in normal mode. If the current value is unknown, it is determined using formulas using data on the cross section of the wire, its material and method of laying.

Automatic machines 1A, 2A, 3A are used to protect circuits with low currents. They are suitable for providing electricity to a small number of devices, for example, lamps or chandeliers, low-power refrigerators and other devices whose total power does not exceed the capabilities of the machine.

Switch 3A is effectively used in industry if it is tri-phase connected in the form of a triangle.

Switches 6A, 10A, 16A can be used to provide electricity to individual circuits, small rooms or apartments.

These models are used in industry, with their help they supply electric motors, solenoids, heaters, welding machines connected by a separate line.

Three-, four-pole circuit breakers 16A are used as input devices for a three-phase power supply circuit. In production, devices with a D-curve are preferred.

Automatic machines 20A, 25A, 32A are used to protect the wiring of modern apartments, they are able to provide washing machines, heaters, electric dryers and other equipment with high power with electricity. Model 25A is used as an introductory machine.

Switches 40A, 50A, 63A belong to the class of devices with high power.They are used to provide electricity to large-capacity power equipment in the home, industry, and civil engineering.

Selection and calculation of circuit breakers

Knowing the characteristics of AB, you can determine which machine is suitable for a particular purpose. But before choosing the optimal model, it is necessary to make some calculations with which you can accurately determine the parameters of the desired device.

Step # 1 - determining the power of the machine

When choosing a machine, it is important to consider the total power of the connected devices.

For example, you need an automatic machine to connect kitchen appliances to the power supply. Let's say a coffee maker (1000 W), a refrigerator (500 W), an oven (2000 W), a microwave oven (2000 W), an electric kettle (1000 W) will be connected to the outlet. The total power will be equal to 1000 + 500 + 2000 + 2000 + 1000 = 6500 (W) or 6.5 kV.

Instrument Power Table
The table shows the rated power of some household appliances necessary for their operation. According to the regulatory data, the cross section of the power wire is selected for their power supply and an automatic machine for protecting the wiring

If you look at the table of automatic devices according to the connection power, take into account that the standard wiring voltage under domestic conditions is 220 V, then a single-pole or two-pole 32A circuit breaker with a total power of 7 kW is suitable for operation.

It should be noted that a large power consumption may be required, since during operation it may be necessary to connect other electrical appliances that were not initially taken into account. In order to foresee this situation, in the calculations of total consumption, a raising factor is used.

Suppose, by adding additional electrical equipment, an increase in power of 1.5 kW was required. Then you need to take the coefficient 1.5 and multiply it by the calculated design power.

In calculations it is sometimes advisable to use a reduction factor. It is used when the simultaneous use of several devices is impossible.

Suppose the total wiring power for the kitchen was 3.1 kW. Then the reduction factor is 1, since the minimum number of devices connected at the same time is taken into account.

If one of the devices cannot be connected with the others, then the reduction factor is taken less than unity.

Step # 2 - calculation of the rated power of the machine

The rated power is the power at which the wiring is not disconnected.

It is calculated by the formula:

M = N * CT * cos (φ),


  • M - power (watts);
  • N - mains voltage (Volt);
  • ST - amperage that can pass through the machine (Ampere);
  • cos (φ) - the cosine of the angle, taking the value of the angle of shift between phases and voltage.

The cosine value is usually equal to 1, since there is practically no shift between the phases of the current and voltage.

From the formula we express CT:

CT = M / N,

We have already determined the power, and the mains voltage is usually 220 volts.

If the total power is 3.1 kW, then:

CT = 3100/220 = 14.

The resulting current will be 14 A.

For the calculation at a three-phase load, the same formula is used, but angular shifts that can reach large values ​​are taken into account. Usually they are indicated on the connected equipment.

Step # 3 - calculation of rated current

The rated current can be calculated according to the documentation for the wiring, but if it is not there, then it is determined based on the characteristics of the conductor.

For calculations, the following data is required:

In domestic conditions, wiring is usually located in the wall.

Wire cross section measurement with a micrometer
To calculate the cross-sectional area, you need a micrometer or vernier caliper. Only conductive conductors should be measured, not wire and insulation

Having made the necessary measurements, we calculate the cross-sectional area:

S = 0.785 * D * D,


  • D Is the diameter of the conductor (mm);
  • S - conductor cross-sectional area (mm2).

Next, use the table below.

Conductor power and current table
Having determined what material the conductor cores were made of, and having calculated the cross-sectional area, it is possible to determine the current and power indicators that the electrical wiring can withstand. Data shown for wiring hidden in the wall.

Based on the data obtained, we select the operating current of the machine, as well as its nominal value. It should be equal to or less than the operating current. In some cases, it is allowed to use machines with a rating that exceeds the current wiring current.

Step # 4 - determining the current-time characteristic

To correctly determine the VTX, it is necessary to take into account the starting currents of the connected loads.

The necessary data can be found using the table below.

Table of time-current characteristics of devices
The table shows some types of electrical devices, as well as the frequency of inrush current and pulse duration in seconds

According to the table, you can determine the current strength (in Amperes) when the device is turned on, as well as the period through which the limiting current will occur again.

For example, if we take an electric meat grinder, the power of which is 1.5 kW, calculate the working current for it from the tables (this will be 6.81 A) and, given the multiplicity of the starting current (up to 7 times), we get the current value 6.81 * 7 = 48 (A).

A current of this force flows with a frequency of 1-3 seconds. Given the VTK schedules for class B, you can see that when overloaded, the circuit breaker will trip in the first seconds after starting the meat grinder.

Obviously, the multiplicity of this device corresponds to class C, therefore, an automatic machine with characteristic C must be used to ensure the operation of an electric meat grinder.

For domestic needs, switches that meet the characteristics of B and C are usually used. In industry, equipment with large multiple currents (motors, power supplies, etc.) generates current up to 10 times, so it is advisable to use D-modifications of the device.

However, the power of such devices, as well as the duration of the starting current, should be taken into account.

Autonomous automated switches differ from ordinary ones in that they are installed in separate switchboards.

The functions of the device include protecting the circuit from unexpected voltage surges, power outages on an entire or a specific part of the network.

Conclusions and useful video on the topic

The choice of AB according to the current characteristic and an example of calculating the current are considered in the following video:

The calculation of the rated current AB is demonstrated in the following video:

Automatic machines are mounted at the entrance of a house or apartment. They are located in durable plastic boxes. The presence of AB in the home electrical circuit is a guarantee of security. Devices allow timely disconnecting the power line if the network parameters exceed a predetermined threshold.

Given the basic characteristics of circuit breakers, as well as making correct calculations, you can make the right choice of this device and its installation.

If you have knowledge or experience in performing electrical work, please share it with our readers. Leave your comments about the choice of the circuit breaker and the nuances of its installation in the comments below.

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Visitors Comments
  1. Vadim lipinsky

    The question arose - can a single-pole machine be used as an input? I heard that this option is not entirely applicable, because it can not provide reliable protection due to the fact that during the shutdown only the phase line breaks, and the “neutral” still remains energized. But I have never met categorical opposing opinions. Thank!

    • Sirozh

      But can’t you read?

    • Expert
      Vasily Borutsky

      Good afternoon, Vadim.

      To ensure safe replacement of the meter, PUEs require breaking all the wires that are suitable for the meter (a screenshot of the item is attached). There are also typical schemes of input panels with counters - in order not to describe, I attached a screenshot. By the way, on circuits without a counter, they cost one introductory circuit breaker.

      Regarding the neutral, which remains energized - among the inhabitants there is an opinion that the potential of the "neutral wire" is "zero". In fact, the voltage of this conductor can reach tens of volts during phase distortions (there were cases when the voltage "zero" reached 90 V). When a broken phase wire falls to zero, the phase potential is taken to "zero" (until the protection works).

      Attached photos:
  2. Alexander

    A good post - in the most detailed way everything is written about the choice of machines. He bookmarked. But this is in theory. In practice, I choose an automatic machine depending on the required power (I am approximately calculating the current strength - that's all). I use automatic machines, as a rule, from the IEK company or (shown in the photo in the post) ABB - the best in my opinion. And another note: it is necessary to observe selectivity - each circuit breaker below the circuit should be lower than the previous one in terms of current strength - otherwise it will not work. And this is very important for safety.

    • Expert
      Vasily Borutsky

      Good afternoon, Alexander.

      Theory and practice "merge" in the development of power supply schemes for workshops, enterprises - apartments, cottages are clicked by the designers, like seeds. It is impossible to choose an economically viable option of providing electricity to several hundred machines connected by technological chains, the range of parts, and the production program, following your advice.

      Regarding selectivity, the issue is also quite complicated. For example, the workshop is provided with electricity via 10 cable lines of 0.4 kV. And what kind of automatic machines, one asks, will you put on TP-10 / 0.4 kV, realizing your theory of selectivity?

      To understand the complexity of the task, I attached a screenshot of several points of the PUE devoted to selectivity. There are others.

      Attached photos:
  3. Artyom

    If we talk about choosing the rating of the machine according to the total load of consumers, it must be indicated that the cable for the received total load must be appropriate. The rating of the machine is selected solely depending on the cable section, because the machine is no longer needed for anything other than to protect the cable itself from overheating. So, if the cable to the kitchen sockets goes 3x2.5, at least you plug in all the appliances in the house, and the machine should be no more than 16A. Otherwise, overheating of the cable, melting of the insulation, fire.

    • Expert
      Vasily Borutsky

      Good afternoon, Artem! The principles of choosing a circuit breaker you set out professionally, but the 16-ampere rating of the machine is not tied to the material of the cores. If they are aluminum, then everything is fine. True, the laying conditions affect. If the conductors are copper, then a permissible current of 21 amperes appears when laying in one pipe - I chose the column of the table that you used. A screenshot of the corresponding lines of the PUE tables - attached.

      Attached photos:
  4. Uncle Vasya

    I do not agree with the last comment. Firstly, the maximum current causing destruction of the wire varies greatly.For copper wire made in accordance with GOST, it can be 30 amperes. Secondly, the cost of connected equipment can many times exceed the cost of a piece of wire. And the task is not to protect copper or aluminum eyeliner, but to protect devices whose failure can cause catastrophic consequences.

    • Expert
      Vasily Borutsky

      Good afternoon, Uncle Vasya 🙂 Artyom set out the principles of choosing a vending machine correctly - the network branch connected to the vending machine is protected (screenshot of the PUE item - attached). Concerning the accounting of material of veins - you are right. Protecting expensive equipment is another story. Here you must consider the presence of built-in protections.

      Attached photos:
  5. Good afternoon, I correctly understand that according to your table, an automatic machine with a rating of 25A is suitable for a VVG 3x2.5 cable, and a rating of 16A for 3x1.5. I ask in connection with the fact that no comic debates surf the Internet in connection with this issue ... I tried to figure out the PUE, but it did not work out too well.

    Many advise you to install a 16A machine on a 3x2.5 cable, and increase the number of groups, thereby increasing the cost of wiring, what is the best argument in this case, which you can refer to to confirm your table.

    Do I understand correctly that the rating of the machine must be higher than the permissible continuous current in table 1.3.4. and if so, why? Thank you in advance for your response.

  6. “A single-pole circuit breaker does not perform the function of an input, since when it is forced to disconnect, the phase line breaks and the neutral is connected to a voltage source, which does not give a 100% guarantee of protection." “The author, where does the neutral source come from?”