What is rated current in electrical engineering. How to calculate the rated current of a circuit breaker? Ratings of circuit breakers

To control electrical wiring in an apartment or house, special protective devices are used that turn off the electricity when the network is overloaded. Characteristics such as load current and network voltage determine the ratings of circuit breakers.

Types of devices

There are several types of devices that can monitor wiring and, if necessary, cut off electrical power. They are:

Miniature (mini-models);
Air (open design);
Enclosed molded case switches;
RCD (Residual Current Devices);
Automatic switches additionally equipped with RCD (differential).

Miniature devices are designed to work in networks with light loads; as a rule, they do not have additional adjustment functions. This model range is represented by circuit breakers with a breaking capacity designed for a misfire current from 4.5 to 15 kA. Therefore, they are used most often in household wiring, since higher current strength is required for production capacities.

Photo - model with a nominal value of 32 A

Models produced by Schneider Electric are very popular. There are machines on sale with ratings from 2 to 125 A, which allows you to select a separate device even for a small group of devices, for example, for connecting lighting or other electrical equipment (sconce, electric kettle, etc.).

If devices with a higher rating are required, say, to control the operation of electrical networks to which powerful consumers are connected, air-type circuit breakers are selected. Their cutoff current rating is an order of magnitude higher than that of miniature models. As a rule, they are produced in a three-pole design, but now many companies, including IEK, produce four-pole models.

Installation of automatic switches is carried out in a special cabinet where DIN rails are installed for their fastening. Distribution cabinets with the appropriate protection class (at least IP55) can be placed in open space (poles, street switchboards, etc.). The waterproof housing, made of refractory materials, ensures the proper level of safety.

The model line of these circuit breakers allows a slight deviation (up to 10%) from the specified characteristics. The biggest advantage of these machines over miniature ones is the ability to customize the operating parameters of the device.

Photo - option for low-voltage networks

For this purpose, special inserts are used, with which you can control the current strength at the contacts. In other words, when installing a calibrated insert on the active contact, it becomes possible to change the parameters of the switch, which in some conditions makes it possible to expand the nominal characteristics. Regardless of the range of action and ratings, circuit breakers have the same size across the entire model range, the only dimension that changes is the width (modularity). It depends on the number of poles (there can be 2 or more).

Automatic switches are mounted in a vertical position, with the exception of devices designed over 5000 A and 6300 A. They can be used for installation in open areas or in special switchboards. The advantage of such devices is the presence of additional contacts and connections, which significantly expands the scope of use and installation possibilities.

Enclosed circuit breakers are manufactured in a cast housing made of refractory material. This makes them completely sealed and suitable for use in extreme conditions. On average, the range of such machines is used with a current of up to 200 Amperes and a voltage of up to 750 Volts. Based on their operating principle, they are divided into the following types:

Adjustable;
Thermal;
Electromagnetic.

Depending on the needs, you need to choose the optimal operating principle of the devices. Electromagnetic type devices are considered the most accurate, since they determine the rms value of active currents and are triggered in the event of a short circuit. This allows you to prevent all negative consequences in advance.

Photo – solid cast IEC

Any of the listed types of devices can be manufactured in one of four standard sizes, with a cut-off current in the range from 25 to 150 A. The design can be two, three and four poles, which allows them to be used when connecting to the power supply network of both residential and production premises.

Electromagnetic machines have proven themselves to be excellent devices that can control the operation of the motors of machine tools or other equipment. A distinctive feature is the ability to withstand current impulses of up to 70,000 Amperes. The rated operating current is indicated on the device body.

Photo – AE series machine gun

RCDs cannot be considered independent devices for protecting networks from overvoltage. It is recommended to use them either in tandem with automatic machines, or immediately buy a switch equipped with an additional protection device (differential automatic devices). At the same time, during installation of wiring, the RCD is installed in front of the machines, and not vice versa. Otherwise, the device may simply burn out due to high short-circuit current pulses.

Video: load switches

Machine denominations (calculation according to the table)

To select the correct ratings for home and industrial circuit breakers, a special table is used:

Current (A)	Network power with 1 phase (kW)	Power of 3-phase network (kW)	Permissible wire cross-sections (mm 2)
-	-	-	copper	aluminum
1	0,2	0,5	1	2,5
2	0,4	1,1	1	2,5
3	0,7	1,6	1	2,5
4	0,9	2,1	1	2,5
5	1,1	2,6	1	2,5
6	1,3	3,2	1	2,5
8	1,7	5,1	1,5	2,5
10	2,2	5,3	1,5	2,5
16	3,5	8,4	1,5	2,5
20	4,4	10,5	2,5	4
25	5,5	13,2	4	6
32	7	16,8	6	10
40	8,8	21,1	10	16
50	11	26,3	10	16
63	13,9	33,2	16	25
80	17,6	52,5	25	35
100	22	65,7	35	50

Calculating the ratings of circuit breakers is also very simple. You need to select a group of devices, for example, it will be a kettle, a lamp, a refrigerator, after which you need to find out their power to determine the rated current. Let's use Ohm's law: I=P/U, Where:

I – current consumed by the equipment (A);
P – equipment power (W);
U – mains voltage (V).

For example, we have a kettle with a power of 1.5 kW (1500 W), a lamp - 100 W, a refrigerator - 300 W; in total, the total value will be 1.9 kW (1900 W), we calculate the rated current: I = 1900/220 = 8.6. The closest automatic device in terms of operating current is 10 A. Naturally, in practice this figure will be higher; modern wiring must be designed for a load current of at least 16 A.

A slight overestimation of the parameters will not cause harm, but an underestimation may result in a short circuit and fire. Experts recommend that when there are a large number of amperes, use not one powerful machine, but several with an average rating - this ensures greater operational reliability.

Devices for shutting off electricity during overloads and short circuits are installed at the entrance to any home network. It is necessary to correctly calculate the current ratings of circuit breakers, otherwise their operation will be ineffective. Do you agree?

We will tell you how to calculate the parameters of the machine, according to which this protective device is selected. From our article you will learn how to choose the device required to protect the electrical network. Taking into account our advice, you will purchase an option that will clearly operate at a dangerous moment for wiring.

To ensure the correct selection of trip device ratings, an understanding of their operating principles, conditions and response times is necessary.

The operating parameters of circuit breakers are standardized by Russian and international regulatory documents.

Basic elements and markings

The design of the switch includes two elements that react when the current exceeds the established range of values:

The bimetallic plate, under the influence of the passing current, heats up and, bending, presses on the pusher, which disconnects the contacts. This is “thermal protection” against overload.
The solenoid, under the influence of a strong current in the winding, generates a magnetic field that presses on the core, which then acts on the pusher. This is a “current protection” against short circuit, which reacts to such an event much faster than the plate.

Types of electrical protection devices have markings that can be used to determine their main parameters.

Each circuit breaker is marked with its main characteristics. This allows you to avoid confusing devices when they are installed in the panel

The type of time-current characteristic depends on the setting range (the magnitude of the current at which operation occurs) of the solenoid. To protect wiring and devices in apartments, houses and offices, switches of type “C” or, much less common, “B” are used. There is no particular difference between them for everyday use.

Type “D” is used in utility rooms or carpentry in the presence of equipment with electric motors that have high starting power.

There are two standards for disconnect devices: residential (EN 60898-1 or GOST R 50345) and a more stringent industrial (EN 60947-2 or GOST R 50030.2). They differ slightly and machines of both standards can be used for residential premises.

In terms of rated current, the standard range of automatic machines for domestic use contains devices with the following values: 6, 8, 10, 13 (rare), 16, 20, 25, 32, 40, 50 and 63 A.

Time-current response characteristics

In order to determine the speed of operation of the machine during an overload, there are special tables depending on the shutdown time on the coefficient of excess of the nominal value, which is equal to the ratio of the existing current strength to the rated one:

K = I / I n.

A sharp drop down in the graph when the range coefficient value reaches from 5 to 10 units is due to the operation of the electromagnetic release. For type “B” switches this occurs at a value from 3 to 5 units, and for type “D” – from 10 to 20.

The graph shows the dependence of the response time range of type “C” circuit breakers on the ratio of the current strength to the value that is set for this switch

With K = 1.13, the machine is guaranteed not to disconnect the line within 1 hour, and with K = 1.45, it is guaranteed to disconnect within the same time. These values are approved in clause 8.6.2. GOST R 50345-2010.

To understand how long it takes for the protection to operate, for example, at K = 2, you need to draw a vertical line from this value. As a result, we get that, according to the above graph, the shutdown will occur in the range from 12 to 100 seconds.

Such a large spread of time is due to the fact that heating of the plate depends not only on the power of the current passing through it, but also on the parameters of the external environment. The higher the temperature, the faster the machine operates.

Rules for choosing denomination

The geometry of intra-apartment and house electrical networks is individual, so there are no standard solutions for installing switches of a certain rating. The general rules for calculating the permissible parameters of machines are quite complex and depend on many factors. It is necessary to take them all into account, otherwise an emergency situation may be created.

The principle of indoor wiring

Internal electrical networks have a branched structure in the form of a “tree” - a graph without cycles. Compliance with this design principle is called, according to which all types of electrical circuits are equipped with protective devices.

This improves the stability of the system in the event of an emergency and simplifies the work to eliminate it. It is also much easier to distribute the load, connect energy-intensive devices and change the wiring configuration.

At the base of the graph there is an input machine, and immediately after the branching, group switches are placed for each individual electrical circuit. This is a standard scheme proven over the years

The functions of the input circuit breaker include monitoring the general overload - preventing the current from exceeding the permitted value for the object. If this happens, there is a risk of damage to the external wiring. In addition, it is likely that protective devices outside the apartment, which are already part of the common property or belong to the local power grid, will be triggered.

The functions of group machines include current control on individual lines. They protect the cable in a dedicated area and the group of electricity consumers connected to it from overload. If such a device does not operate during a short circuit, then it is insured by an input circuit breaker.

Even for apartments with a small number of electrical consumers, it is advisable to install a separate line for lighting. When you turn off the circuit breaker of another circuit, the light will not go out, which will allow you to eliminate the problem in more comfortable conditions. In almost every panel, the nominal value of the input machine is less than the amount on the group ones.

Total power of electrical appliances

The maximum load on the circuit occurs when all electrical appliances are turned on at the same time. Therefore, usually, the total power is calculated by simple addition. However, in some cases this figure will be less.

For some lines, the simultaneous operation of all electrical appliances connected to it is unlikely, and sometimes impossible. Homes sometimes specifically place restrictions on the operation of powerful devices. To do this, you need to remember not to turn them on at the same time or use a limited number of sockets.

The probability of simultaneous operation of all office equipment, lighting and auxiliary equipment (kettles, refrigerators, fans, heaters, etc.) is very low, therefore, when calculating the maximum power, a correction factor is used

When electrifying office buildings, the empirical simultaneity coefficient is often used for calculations, the value of which is taken in the range from 0.6 to 0.8. The maximum load is calculated by multiplying the sum of the powers of all electrical appliances by a factor.

There is one subtlety in the calculations - it is necessary to take into account the difference between the rated (total) power and consumed (active), which are related by the coefficient (cos ( f)).

This means that for the device to operate, a power current equal to the consumed divided by this coefficient is required:

Ip = I/cos(f)

I p – rated current strength, which is used in load calculations;
I is the current consumed by the device;
cos(f)<= 1.

Usually the rated current is indicated immediately or through the indication of the cos (f) value in the technical data sheet of the electrical device.

For example, the coefficient value for fluorescent light sources is 0.9; for LED lamps – about 0.6; for ordinary incandescent lamps - 1. If the documentation is lost, but the power consumption of household devices is known, then for guarantee take cos (f) = 0.75.

How to select a circuit breaker according to the load power is written in, the contents of which we advise you to familiarize yourself with.

Selection of core cross-section

Before laying a power cable from the distribution panel to a group of consumers, it is necessary to calculate the power of electrical appliances when they operate simultaneously. The cross-section of any branch is selected according to calculation tables depending on the type of wiring metal: copper or aluminum.

Wire manufacturers provide similar reference materials to their products. If they are missing, then they are guided by the data from the reference book “Rules for the Construction of Electrical Equipment” or produce.

However, consumers often play it safe and choose not the minimum acceptable cross-section, but one step larger. So, for example, when buying a copper cable for a 5 kW line, choose a core cross-section of 6 mm 2, when according to the table a value of 4 mm 2 is sufficient.

The reference table presented in the PUE allows you to select the required cross-section from the standard range for various operating conditions of the copper cable

This is justified for the following reasons:

Longer service life of a thick cable, which is rarely subjected to the maximum permissible load for its cross-section. Re-wiring is not an easy and expensive job, especially if the room has been renovated.
The bandwidth reserve allows you to seamlessly connect new electrical appliances to the network branch. So, you can add an additional freezer to the kitchen or move the washing machine there from the bathroom.
The start of operation of devices containing electric motors produces strong starting currents. In this case, a voltage drop is observed, which is expressed not only in the blinking of the lighting lamps, but can also lead to breakdown of the electronic part of the computer, air conditioner or washing machine. The thicker the cable, the smaller the voltage surge will be.

Unfortunately, there are many cables on the market that are not made according to GOST, but according to the requirements of various specifications.

Often the cross-section of their cores does not meet the requirements or they are made of conductive material with greater resistance than required. Therefore, the actual maximum power at which permissible heating of the cable occurs is less than in the standard tables.

This photo shows the differences between cables made according to GOST (left) and according to TU (right). There is an obvious difference in the cross-section of the conductors and the tightness of the insulating material.

Calculation of the circuit breaker rating for cable protection

The machine installed in the panel must ensure that the line is disconnected when the current power goes beyond the range permitted for the electric cable. Therefore, it is necessary to calculate the maximum permissible rating for the switch.

According to the PUE, the permissible long-term load of copper cables laid in boxes or in the air (for example, over a suspended ceiling) is taken from the table above. These values are intended for emergency situations when there is a power overload.

Some problems begin when relating the rated power of the switch to the long-term permissible current, if this is done in accordance with the current GOST R 50571.4.43-2012.

A fragment of clause 433.1 of GOST R 50571.4.43-2012 is given. There is an inaccuracy in formula “2”, and for a correct understanding of the definition of the variable In, you need to take into account Appendix “1”

Firstly, deciphering the variable In as rated power is misleading, if you do not pay attention to Appendix “1” to this paragraph of GOST. Secondly, there is a typo in formula “2”: the coefficient of 1.45 was added incorrectly, and this fact is stated by many experts.

According to clause 8.6.2.1. GOST R 50345-2010 for household switches rated up to 63 A, the conditional time is 1 hour. The set tripping current is equal to the nominal value multiplied by a factor of 1.45.

Thus, according to both the first and modified second formulas, the rated current of the circuit breaker must be calculated using the following formula:

I n<= I Z / 1,45

I n – rated current of the machine;
I Z – long-term permissible cable current.

Let's calculate the ratings of switches for standard cable sections for a single-phase connection with two copper conductors (220 V). To do this, we divide the long-term permissible current (when laying through the air) by a tripping coefficient of 1.45.

Let's choose a machine so that its face value is less than this value:

Section 1.5 mm 2: 19 / 1.45 = 13.1. Rating: 13 A;
Section 2.5 mm 2: 27 / 1.45 = 18.6. Rating: 16 A;
Section 4.0 mm 2: 38 / 1.45 = 26.2. Rating: 25 A;
Section 6.0 mm 2: 50 / 1.45 = 34.5. Rating: 32 A;
Section 10.0 mm 2: 70 / 1.45 = 48.3. Rating: 40 A;
Section 16.0 mm 2: 90 / 1.45 = 62.1. Rating: 50 A;
Section 25.0 mm 2: 115 / 1.45 = 79.3. Denomination: 63 A.

13A circuit breakers are rarely on sale, so devices with a rated power of 10A are often used instead.

Cables based on aluminum cores are now rarely used when installing internal wiring. There is also a table for them that allows you to select a section based on load

In a similar way, for aluminum cables we calculate the ratings of the machines:

Section 2.5 mm 2: 21 / 1.45 = 14.5. Rating: 10 or 13 A;
Section 4.0 mm 2: 29 / 1.45 = 20.0. Rating: 16 or 20 A;
Section 6.0 mm 2: 38 / 1.45 = 26.2. Rating: 25 A;
Section 10.0 mm 2: 55 / 1.45 = 37.9. Rating: 32 A;
Section 16.0 mm 2: 70 / 1.45 = 48.3. Rating: 40 A;
Section 25.0 mm 2: 90 / 1.45 = 62.1. Rating: 50 A.
Section 35.0 mm 2: 105 / 1.45 = 72.4. Denomination: 63 A.

If the power cable manufacturer declares a different dependence of the permissible power on the cross-sectional area, then it is necessary to recalculate the value for the switches.

The formulas for the dependence of current on power for single-phase and three-phase networks are different. Many people who have devices designed for 380 Volts make a mistake at this stage

How to determine the technical parameters of a circuit breaker by marking, in detail. We recommend that you read the educational material.

Prevention of overload from consumer work

Sometimes a machine is installed on the line with a rated power significantly lower than what is necessary to ensure that the electrical cable remains operational.

It is advisable to reduce the rating of the switch if the total power of all devices in the circuit is significantly less than the cable can withstand. This happens if, for safety reasons, when some of the devices were removed from the line after installation of the wiring.

Then reducing the rated power of the machine is justified from the position of its faster response to emerging overloads.

For example, when an electric motor bearing jams, the current in the winding increases sharply, but not to short circuit values. If the machine reacts quickly, the winding will not have time to melt, which will save the engine from an expensive rewinding procedure.

They also use a value less than the calculated value due to strict restrictions on each circuit. For example, for a single-phase network, a 32 A switch is installed at the entrance to an apartment with an electric stove, which gives 32 * 1.13 * 220 = 8.0 kW of permissible power. Suppose that when wiring the apartment, 3 lines were organized with the installation of group circuit breakers with a nominal value of 25 A.

If the number of group circuit breakers installed in the distribution board is large, then they must be signed and numbered. Otherwise you might get confused

Let's assume that there is a slow increase in load on one of the lines. When the power consumption reaches a value equal to the guaranteed tripping of the group switch, only (32 – 25) * 1.45 * 220 = 2.2 kW will remain for the remaining two sections.

This is very little relative to total consumption. With such a distribution panel design, the input circuit breaker will turn off more often than devices on the lines.

Therefore, in order to maintain the principle of selectivity, it is necessary to install switches with a rating of 20 or 16 amperes in the areas. Then, with the same imbalance in power consumption, the other two links will account for a total of 3.8 or 5.1 kW, which is acceptable.

Let's consider the possibility with a rating of 20A using the example of a separate line dedicated to the kitchen.

The following electrical appliances are connected to it and can be turned on simultaneously:

Refrigerator with a rated power of 400 W and a starting current of 1.2 kW;
Two freezers, power 200 W;
Oven, power 3.5 kW;
When operating an electric oven, it is allowed to additionally turn on only one additional device, the most powerful of which is an electric kettle, consuming 2.0 kW.

A twenty-amp machine allows you to pass current for more than an hour with a power of 20 * 220 * 1.13 = 5.0 kW. A guaranteed shutdown in less than one hour will occur with a current flow of 20 * 220 * 1.45 = 6.4 kW.

In the kitchen, refrigeration equipment and the stove must have a permanent connection to electricity. If there is a risk of excess current, then the simultaneous operation of other devices can be eliminated by allocating only two sockets for them

When the oven and electric kettle are turned on simultaneously, the total power will be 5.5 kW or 1.25 parts of the machine’s nominal value. Since the kettle does not work for long, it will not turn off. If at this moment the refrigerator and both freezers turn on, the power will be 6.3 kW or 1.43 parts of the nominal value.

This value is already close to the guaranteed tripping parameter. However, the likelihood of such a situation occurring is extremely low and the duration of the period will be insignificant, since the operating time of the motors and the kettle is short.

The starting current that occurs when starting the refrigerator, even in the sum of all operating devices, will not be enough to trigger the electromagnetic release. Thus, under the given conditions, a 20 A circuit breaker can be used.

The only caveat is the possibility of increasing the voltage to 230 V, which is permitted by regulatory documents. In particular, GOST 29322-2014 (IEC 60038:2009) defines the standard voltage as 230 V with the possibility of using 220 V.

Now most networks supply electricity with a voltage of 220 V. If the current parameter is adjusted to the international standard of 230 V, then the ratings can be recalculated in accordance with this value.

Conclusions and useful video on the topic

Switch device. Selecting an input machine depending on the connected power. Power distribution rules:

Selecting a switch based on cable capacity:

Calculating the rated current of a circuit breaker is a complex task, for which many conditions must be taken into account. The ease of maintenance and safety of the local electrical network depends on the installed machine.

If you have doubts about the ability to make the right choice, you should contact experienced electricians.

Please write comments in the block below. Tell us about your own experience in selecting circuit breakers. Share useful information and photos on the topic of the article, ask questions.

Types of devices

There are several types of devices that can monitor wiring and, if necessary, cut off electrical power. They are:

Miniature (mini-models);
Air (open design);
Enclosed molded case switches;
RCD (Residual Current Devices);
Automatic switches additionally equipped with RCD (differential).

Photo - model with a nominal value of 32 A

Photo - option for low-voltage networks

Adjustable;
Thermal;
Electromagnetic.

Photo – solid cast IEC

Photo – AE series machine gun

Video: load switches

Machine denominations (calculation according to the table)

To select the correct ratings for home and industrial circuit breakers, a special table is used:

Current (A)	Network power with 1 phase (kW)	Power of 3-phase network (kW)	Permissible wire cross-sections (mm 2)
-	-	-	copper	aluminum
1	0,2	0,5	1	2,5
2	0,4	1,1	1	2,5
3	0,7	1,6	1	2,5
4	0,9	2,1	1	2,5
5	1,1	2,6	1	2,5
6	1,3	3,2	1	2,5
8	1,7	5,1	1,5	2,5
10	2,2	5,3	1,5	2,5
16	3,5	8,4	1,5	2,5
20	4,4	10,5	2,5	4
25	5,5	13,2	4	6
32	7	16,8	6	10
40	8,8	21,1	10	16
50	11	26,3	10	16
63	13,9	33,2	16	25
80	17,6	52,5	25	35
100	22	65,7	35	50

I – current consumed by the equipment (A);
P – equipment power (W);
U – mains voltage (V).

When designing the electrical network of a new home, to connect new powerful devices, in the process of modernizing the electrical panel, it is necessary to select a circuit breaker for reliable electrical safety.

Some users are careless about this task, and can without hesitation connect any available machine, as long as it works, or when choosing, they are guided by the following criteria: cheaper, so that it won’t cost too much, or more powerful, so that it won’t break the bank again.

Very often, such negligence and ignorance of the basic rules for choosing the rating of a safety device leads to fatal consequences. This article will introduce the basic criteria for protecting electrical wiring from overload and short circuit, in order to be able to correctly select a circuit breaker according to the power consumption of electricity.

Briefly the principle of operation and purpose of circuit breakers

In the event of a short circuit, the circuit breaker operates almost instantly thanks to the electromagnetic release. At a certain excess of the rated current value, the heating bimetallic plate will turn off the voltage after some time, which can be found out from the current characteristic time graph.

This safety device protects the wiring from short circuits and overcurrents exceeding the calculated value for a given wire cross-section, which can heat the conductors to the melting point and cause the insulation to ignite. To prevent this from happening, you need not only to choose the right protective switch that matches the power of the connected devices, but also to check whether the existing network can withstand such loads.

Appearance of a three-pole circuit breaker

Wires must match the load

It often happens that in an old house a new electric meter, automatic machines, and RCDs are installed, but the wiring remains old. A lot of household appliances are bought, the power is summed up and an automatic machine is selected for it, which regularly holds the load of all switched on electrical appliances.

Everything seems to be correct, but suddenly the wire insulation begins to emit a characteristic odor and smoke, a flame appears, and the protection does not work. This can happen if the wiring parameters are not designed for such current.

Let's say the cross-section of the old cable core is 1.5mm², with a maximum permissible current limit of 19A. We assume that several electrical appliances were connected to it at the same time, making up a total load of 5 kW, which in current equivalent is approximately 22.7 A; it corresponds to a 25 A circuit breaker.

The wire will heat up, but this machine will remain on all the time until the insulation melts, which will lead to a short circuit, and the fire can already flare up in full swing.

Protect the weakest link in the electrical wiring

Therefore, before choosing a machine according to the load being protected, you need to make sure that the wiring will withstand this load.

According to PUE 3.1.4, the machine must protect the weakest section of the electrical circuit from overloads, or be selected with a rated current corresponding to the currents of the connected electrical installations, which again implies their connection with conductors with the required cross-section.

If you ignore this rule, you should not blame an incorrectly designed machine and curse its manufacturer if a weak link in the electrical wiring causes a fire.

Melted wire insulation

Calculation of the machine's nominal value

We assume that the wiring is new, reliable, correctly calculated, and meets all requirements. In this case, the selection of a circuit breaker comes down to determining a suitable rating from a typical range of values, based on the calculated load current, which is calculated by the formula:

where P is the total power of electrical appliances.

This means active load (lighting, electric heating elements, household appliances). This calculation is completely suitable for a home electrical network in an apartment.

Let's say the power calculation is made: P = 7.2 kW. I=P/U=7200/220=32.72 A. Select a suitable 32A machine from a range of values: 1, 2, 3, 6, 10, 16, 20, 25, 32, 40, 63, 80, 100.

This rating is slightly less than the calculated value, but it is practically impossible for all electrical appliances in the apartment to be turned on at the same time. It is also worth considering that in practice, the operation of the machine begins with a value 1.13 times greater than the nominal value, due to its time-current characteristics, that is, 32 * 1.13 = 36.16 A.

To simplify the selection of a circuit breaker, there is a table where the ratings of the circuit breakers correspond to the power of single-phase and three-phase loads:

Current circuit breaker selection table

The denomination found using the formula in the above example is closest in terms of power value, which is indicated in the red highlighted cell. Also, if you want to calculate the current for a three-phase network when choosing a machine, read the article about

The selection of circuit breakers for electrical installations (electric motors, transformers) with reactive loads, as a rule, is not made based on power. The rating and type of current characteristics of the circuit breaker are selected according to the operating and starting current specified in the passport of this device.

The current passing through the circuit breaker is determined by the well-known Ohm's law by the magnitude of the applied voltage divided by the resistance of the connected circuit. This theoretical position of electrical engineering is the basis for the operation of any machine.

In practice, the network voltage, for example, 220 volts, is maintained by automatic devices of the energy supply organization within the limits specified by state standards, and changes slightly within this range. If it goes beyond the GOST limits, it is considered a malfunction or an accident.

The circuit breaker cuts into the phase wire of the power supply to lamps, sockets and other consumers. When an electric razor and then a washing vacuum cleaner are powered from the outlet, in both cases current flows through the machine in a closed circuit between phase and zero.

But, in the first case it will be relatively small, and in the second - significant: these devices differ in resistance. They create different loads. Its value is constantly monitored by the machine’s protection, turning it off in case of deviations from the norm.

How does current flow through a circuit breaker?

Structurally, the machine is created in such a way that the current acts on sequentially located elements. These include:

wire connection terminals with clamping screws;

power contacts with moving and stationary parts;

bimetallic thermal release plate;

short circuit current cut-off electromagnet;

connecting conductors.

The current path through the circuit breaker is shown in the picture with red arrows.

The power movable contacts are pressed against the stationary ones, creating a continuous electrical circuit only after the operator turns the control lever manually. A mandatory condition for inclusion is the absence of emergency situations in the switched circuit. If they appear, the automatic shutdown protections immediately begin to work. There is no other way to turn on the machine.

But you can break these contacts, de-energizing the supply of phase potential to consumers, in two ways:

manually by returning the control lever to its original position;

automatically from protection activation.

How the structural elements of a circuit breaker are created and work

Power contacts

They, like the entire design of the circuit breaker, are designed to transmit strictly limited power. It cannot be exceeded, because otherwise the machine will fail and burn out.

The technical characteristic that limits the maximum power passing through the power contacts is an indicator called “Limiting breaking capacity”. It is designated by the index “Icu”.

The value of the maximum breaking capacity of a circuit breaker is set during its design from a standard range of currents, usually measured in kiloamperes. For example, Icu can be 4 or 6 or even 100 or more kA.

This value is indicated directly on the front side of the machine body, as well as other characteristics of the current value settings.

So, an electric current from zero to 4000 amperes can safely pass through the power contacts of the machine shown in the picture. The AV itself will normally withstand it and will turn it off if an emergency occurs within the connected electrical wiring with consumers.

For this purpose, a distinction has been made between currents flowing through power contacts into:

1. nominal and working;

2. emergency, including overload and short circuits.

What is the rated current of the circuit breaker

Any machine is created to operate under certain technical conditions. It must reliably ensure the passage of the operating load current flowing both through the electrical wiring and through the connected consumers.

When choosing a machine for a household network, users often take into account the conductive properties of the wiring or only the power of electrical appliances, making a mistake: it is necessary to comprehensively analyze both of these issues. Because a switch is an automatic device that is already configured to operate when certain current values are reached.

When these conditions have not yet occurred, and the operating current through the machine is less. than the lower shutdown limit, the power contacts are reliably closed. The upper limit of this operating range is usually called the rated current, denoted In.

The number “16” shown in the picture means that currents passing through the power contacts, up to and including 16 amperes, will be reliably transmitted by the circuit breaker to connected consumers through electrical wires.

How protections work

All currents greater than the rated value lead to protection activation. They are called operation currents and are designated Iav.

For automatic shutdown, two types of devices are mounted inside the machine body, operating according to different shutdown principles:

1. heating and bending the bimetal to disengage the mechanical latch;

2. knocking out the latch with a mechanical blow from the electromagnet core.

Thermal release

It works by bending a bimetallic composite plate when heated by a current passing through it, and is cooled by dissipating heat to the environment.

The thermal energy generated by the electric current through the passing bimetal is applied to this release. Its value, as we know from the Joule-Lenz law, depends on:

1. electrical resistance of the circuit;

2. the strength of the flowing current;

3. and the time of its exposure.

Of these three parameters, electrical resistance practically does not change in a steady process. It is taken into account only in theoretical calculations. When the load is switched, the current changes sharply. Therefore, two other parameters are more important:

1. magnitude of electric current;

2. the time it occurs.

To take into account its value with proper configuration and operation of the circuit breaker, special devices are used - resistance meters of this loop.

Their measurement makes it possible to take into account the correction introduced by the additional resistance of the wires, and therefore accurately take into account the currents passing in emergency mode through the power contacts and protection of the circuit breaker.

How a circuit breaker is tested for currents passing through it

After manufacturing in production until installation in an electrical circuit, products from any manufacturer can be transported over long distances or stored in warehouses for a long time. During this time, a decrease in its quality is possible due to violation of technical characteristics.

Therefore, when installed in a circuit, before putting it into operation, circuit breakers must be tested for serviceability, which is usually called loading.

To do this, a special circuit for loading the machine is assembled in the electrical laboratory or one of the many designs of stationary or portable stands is used.

The circuit breaker is tested according to the rated current indicated on the housing. It must withstand its value for a long time.

Then the machine is subjected to overloads and short-circuit currents, which it must withstand during operation. At the same time, the following are clearly measured and recorded:

1. currents of thermal release and current cutoff protection;

2. the time the machine is turned off from the moment of simulating an emergency situation.

Some designs of machines allow you to adjust the output parameters during loading. For example, certain types of thermal releases have a screw fastening, which allows you to adjust the response setting of the bimetallic plate within certain limits.

All measured characteristics are recorded with high accuracy by measuring instruments and entered into the inspection report and compared with GOST requirements. After their analysis, a certificate is issued with a conclusion on suitability.

Loading the machine under load allows you to identify defects and prevent possible fires and electrical injuries.

Thus, the currents passing through circuit breakers are taken into account during design, production, testing and operation. For this purpose, terms have been introduced that take into account the requirements of GOST:

rated current;

overload;

short circuit current;

protection operation current;

fault shutdown time.