Rated current of machines. Existing current ratings of circuit breakers

The main characteristic of automatic protective equipment is the rated currents of the circuit breakers. This parameter is displayed on the device body along with the manufacturer's trademark and serial number. This value represents the maximum. The time for this current to flow through the circuit breaker can be infinitely long, without disconnecting the protected circuit. If the nominal value is exceeded, the machine is triggered and the protected circuit is opened.

Machine parameters

The main current ratings are standardized and represent the following range of values: 6, 10, 16, 20, 25, 32, 40, 50, 63, 80 and 100 amperes. These values ​​correspond exactly to their value when the temperature surrounding the machines does not exceed 30 degrees. If the temperature increases, the corresponding rated current will decrease.

This can happen when several devices are installed in an electrical panel at a very close distance from each other. Mutual heating occurs, as a result of which the value of the switched electric current decreases. In order to take this error into account, special correction factors can be used.

When several consumers are connected to the network at once, inrush currents may arise for a short time, many times exceeding the rated current of the circuit breaker. To avoid spontaneous shutdowns at such moments, it is practiced to use automatic machines with different time-current characteristics.

Time-current characteristic

When considering the rated currents of automatic machines, we must not forget about the time-current characteristic of their operation. Here the time of switching off the circuit and the strength of the current flowing through it depend. The actual flowing electric current is correlated with the nominal current and shows how many times it exceeds it. That is, with the same current, shutdown can occur in different ways.

There are several types of circuit breakers, depending on the time-current characteristics. Each type shows how many times it exceeds the current value. The scope of application of this or that type of circuit breaker also depends on this.

In some device models there is a C marking, meaning the ability to release as quickly as possible. This characteristic of the machines is a graph where the rated current is indicated horizontally, and the operating time of the device is indicated vertically.

Selecting a circuit breaker

To protect electrical circuits, various safety devices are used - RCDs, AVs, differential circuit breakers - which are largely similar in their purpose and operating principle. Some are a “set” consisting of several devices placed in one housing.

But they are all united by a common feature - protecting lines and connected equipment from current “surprises”, more precisely, exceeding the calculated value of a given parameter for a specific “thread” or the entire circuit. In this article we will look at the existing current ratings of circuit breakers.

People who do not understand the true purpose of the AV and the meaning of current protection are often guided by the principle “more is better” (in this case, the setting value is implied). This approach can lead to the fact that, figuratively speaking, everything around will melt and ignite, but it will not work. That is why for each circuit, AB is selected individually, after determining its parameters.

Calculation of the rated current value

What is taken into account:

• Voltage (V). In everyday life it is mainly 220/50.
• Total line load (W). It is determined by adding the powers of all connected devices (devices).

AB denominations

All current values ​​are in A.

1. Non-tunable AB. These are the simplest machines, installed mainly on individual threads. Their denominations are set by the manufacturer - 6, 10 and 16.
2. Adjustable AV. With rare exceptions, you can change the current in them by selecting the required value. AB denominations are 20, 25, 32, 40, 50, 63, 80 and 100.

When choosing a circuit breaker, you should read its data sheet. It indicates, in addition to the main characteristics, a correction factor. It is important if the device may overheat. For example, installation in rooms with high temperatures, dense layout of the power panel, and so on.

If you need to choose a circuit breaker for your electrical panel, you must first study its characteristics. It is important to know what the load will be on him. The most basic characteristic that one focuses on when purchasing this device is the rated current. It is indicated on the body of the circuit breaker in large numbers and letters.

The principle of determining the nominal indicator

The explanatory dictionary of the Russian language by Academician Ozhegov explains the meaning of the word “nominal”, as designated, called, but not fulfilling its duties, purpose, that is, fictitious.

This definition quite accurately explains the electrical terms of rated voltage, current and power. They seem to be there, assigned and defined, but in fact they only serve as guidelines for electricians. The actual numerical expressions of these parameters in reality differ from the assigned values.

For example, we are all very familiar with a single-phase alternating network with a voltage of 220 volts, which is considered nominal. In fact, its value according to GOST can only reach the upper limit of 252 volts. This is how the state standard works.

The same picture can be seen with the rated current.

The basis for choosing its value is the maximum possible thermal heating of electrical conductors (including their insulation), which must operate reliably under load for an indefinitely long time. At rated current, a thermal balance is maintained between:

• heating of conductors from the temperature effect of electric charges, described by the Joule-Lenz law;
• cooling due to the removal of part of the heat to the environment.

In this case, heat Q1 should not affect the mechanical and strength characteristics of the metal, and heat Q2 should not affect the change in the chemical and dielectric properties of the insulation layer.

Even if the rated current value is slightly exceeded, after a certain period of time it will be necessary to remove the voltage from the electrical equipment to cool the metal of the current conductor and insulation. Otherwise, their electrical properties will be disrupted and breakdown of the dielectric layer or metal deformation will occur.

Any electrical equipment (including current sources, its consumers, connecting wires and systems, protective devices) is calculated, designed and manufactured to operate at a certain rated current. Its value is indicated not only in the technical factory documentation, but also on the housing or nameplates of the electrical equipment.

The above photograph clearly shows the rated current values ​​of 2.5 and 10 amperes, which are made by stamping during the manufacture of an electrical plug. In order to standardize equipment, GΟST 6827−76 introduced a variety of rated current values ​​at which almost all electrical installations must operate.

Selection of protective device

Since the rated current determines the possibility of long-term operation of electrical equipment without any damage, all current protective devices are configured to operate when it is exceeded.

In practice, situations quite often occur when an overload occurs in the power supply circuit for a short period for various reasons. In this case, the temperature of the metal conductor and the insulation layer do not have time to reach the limit when a violation of their electrical properties occurs.

For these reasons, the overload zone is allocated to a separate area, which is limited not only by its size, but also by its duration. When the critical temperature values ​​of the insulation layer and conductor metal are reached, the voltage from the electrical installation must be removed to cool it. These functions are performed by the so-called overload protection operating on a thermal principle:

• circuit breakers;
• thermal releases.

They perceive the thermal load and are configured to turn it off with a certain time delay. The setting of the protections that perform “instantaneous” load cutoff lies slightly above the overload current. The term "instantaneous" actually defines action in the shortest possible period of time. For today's fastest current protections, cutoff is performed in just under 0.02 seconds.

The operating current in normal power mode is most often less than the nominal value.

In the example given, the case for alternating current circuits is analyzed. In constant voltage circuits there is no fundamental difference in the relationships between the operating, rated current and the choice of settings for the operation of protections.

The main characteristic of a circuit breaker is the rated current. They are the most widely used in the protection of industrial devices and household electrical networks. Such switches combine in their design:

• thermal releases operating with a time delay;
• current cut-off, which very quickly turns off the emergency mode.

At the same time, automatic switches are manufactured for rated current and voltage. Based on their size, protective devices are selected for operation in the specific conditions of a certain circuit.

For this purpose, the standards define 4 types of time-current characteristics for different designs of machines. They are designated by the Latin letters A, B, C, D and are designed to guarantee the shutdown of accidents with a rated current ratio of 1.3 to 14.

Based on the time-current characteristic, taking into account the ambient temperature, the circuit breaker is selected for a specific type of load, For example:

• semiconductor devices;
• lighting systems;
• circuits with mixed loads and moderate starting currents;
• chains with high overload capacity.

The time-current characteristic can consist of three zones of action, as shown in the picture, or two (without the middle one).

The rated current value can be found on the machine body. The picture shows a switch marked with 100 amperes.

This means that it will work (turn off) not from the rated current (100 A), but from its excess. Let’s say that if the cutoff of the machine is set to a multiple of 3.5, then a current of 100x3.5 = 350 amperes or more will be stopped by it without a time delay.

When the thermal release is set to a multiple of 1.25, then when the value reaches 100x1.25 = 125 amperes, the shutdown will occur after some time, for example, one hour. In this case, the circuit will work with overload during this period.

It should be taken into account that other factors also influence the shutdown time of the machine. factors associated with maintaining temperature protection:

• environmental conditions;
• degree of filling of the distribution panel with equipment;
• possibility of heating or cooling from external sources.

To determine the basic electrical parameters of protections and wires, the load applied to them must be taken into account. To do this, it is calculated based on the rated power of the devices connected to the operation, taking into account their occupancy factor.

For example, a dishwasher, a multicooker, an electric oven and a microwave oven are connected to the socket group located in the kitchen, which consume a total power of 5660 watts in normal mode (taking into account the frequency of switching on).

The rated voltage of the household network is 220 volts. The load current that will pass through the wires and protective devices should be determined by dividing the power by the voltage. I=5660/220=25.7 A.

Next, you need to look at the table of a number of rated currents for electrical equipment. There is no automatic switch for this current. But manufacturers produce 25 amp circuit breakers. Its value most closely matches our objectives. Therefore, it should be taken as the basis of a protective device for electrical wiring of consumers of the socket group.

After this, you need to decide on the wire material and cross-section. Take copper as a basis, since aluminum wiring, even for domestic purposes, is no longer popular due to its performance characteristics.

Electricians' reference books provide tables for selecting wires from different materials according to current load. Take this case taking into account the fact that the wiring is carried out with a separate cable with polyethylene insulation, hidden in the wall groove. Temperature limits should be taken as corresponding to room conditions.

The table will provide information that the minimum permissible cross-section of a standard copper wire for our case is 4 mm square. You can’t take less, but it’s better to increase it.

Sometimes the task arises of selecting the protection rating for already working wiring. In this case, it is quite justified to determine the load current of the consumer network with an electrical measuring instrument and compare it with that calculated by the above theoretical method.

In this way, the term “rated current” helps electricians navigate the technical characteristics of electrical equipment.

Automatic switch IEK. Thermal current – ​​32 A

A circuit breaker has several other names among people - circuit breaker, plug, bag, or simply circuit breaker.

What we are talking about is in the picture on the left. This is the most budget model.

This article will discuss the technical characteristics of circuit breakers, what they are, and how to choose them in various cases.

We can absolutely say that anyone who carefully reads this article can be considered an expert on circuit breakers. At least to a first approximation, sufficient for practical work and understanding of processes.

I have already written several articles on this topic on the blog, and I will post links along the way.

Circuit breaker functions

From the name it is clear that this switch, which turns off automatically. That is, myself, in certain cases. From the second name - circuit breaker - it is intuitively clear that this is some kind of automatic device that protects something.

Now more details. The circuit breaker trips and turns off in two cases - in case of overload by current, and in the case short circuit (short circuit).

Overcurrent occurs due to faulty consumers, or when there are too many consumers. Short circuit is a mode when all the power of the electrical circuit is spent on heating the wires, while the current in this circuit is the maximum possible. More details will follow.

In addition to protection (automatic shutdown), machines can be used to manually turn off the load. That is, like a switch or a regular “advanced” switch with additional options.

Another important function (this goes without saying) is the connection terminals. Sometimes, even if the protection function is not particularly needed (and it never hurts), the terminals of the circuit breaker can be very useful. For example, as shown in the article.

Number of poles

Depending on the number of poles, the machines are:

1. Single pole(1p, 1p). This is the most common type. It stands in a circuit and protects one wire, one phase. This is shown at the beginning of the article.
2. Bipolar(2p, 2p). In this case, these are two single-pole circuit breakers, with a combined switch (handle). As soon as the current through one of the machines exceeds the permissible value, both will turn off. These are mainly used to completely disconnect a single-phase load when both the zero and the phase break. It is the two-pole circuit breakers that are used at the entrance to our apartments.
3. Three-pole(3p, 3p). Used to break and protect three-phase circuits. Just as in the case of two-pole ones, these are actually three single-pole circuit breakers, with a common on/off handle.
4. Four-pole(4p, 4p). They are rare, they are installed mainly at the input of three-phase switchgears (switchgears) to break not only the phases (L1, L2, L3), but also the working zero (N). Attention! Under no circumstances should the protective grounding (PE) wire be broken!

Circuit breaker current

Automatic currents come from the following series:

0,5, 1, 1,6, 2, 3,15, 4, 5, 6 , 8, 10 , 13, 16 , 20, 25 , 32 , 40 , 50, 63.

The denominations most often used in everyday life are highlighted in bold. There are other denominations, but we won’t talk about them now.

This current for the circuit breaker is rated. If it is exceeded, the switch will turn off. True, not immediately, as stated below:

Time-current characteristics

Obviously, the machine does not always turn off instantly, and sometimes it needs to “think and make a decision”, or give the load a chance to return to normal.

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The time-current characteristic shows after what time and at what current the machine will turn off. These characteristics are also called tripping curves or current-time characteristics. Which is more precise, since it depends on the current after what time the machine turns off.

Tripping curves or current-time characteristics

Let me explain these graphs. As I said above, the circuit breaker has two types of protection - thermal (against overcurrent) and electromagnetic (against short circuit). In the graph, the operation of thermal protection is a section that smoothly descends. Electromagnetic – the curve abruptly breaks down.

The thermal one works slowly (for example, if the current is twice the nominal value, the machine will go out in about a minute), and the electromagnetic one works instantly. For graph IN this instant “starts” when the current exceeds the nominal value by 3-5 times, for the category WITH– 6-10 times, for D(not shown because it is not used in everyday life) - 10-20 times.

How it works - you can imagine what will happen if the current exceeds the nominal value by 5 times, and the protection is with the “C” characteristic, as in all houses. The machine will only go off after 1.5-9 seconds, depending on your luck. In 9 seconds the insulation will melt and the wiring will need to be changed. In this case, therefore, short circuit is better than overload.

Selecting a circuit breaker. Basic Rule

It is necessary to select a circuit breaker based on the cross-sectional area of ​​the wire that this circuit breaker protects (which is connected after this circuit breaker). And the cross-section of the wire is based on the maximum current (power) of the load.

The algorithm for selecting a circuit breaker is as follows:

1. We determine the power and current of the line consumers that will be fed through the machine. The current is calculated using the formula I=P/220, where 220 is the rated voltage, I is the current in amperes, P is the power in watts. For example, for a 2.2 kW heater the current will be 10 A.
2. Select the wire according to the table. A cable with a conductor cross section of 1.5 mm² is suitable for our heater. In the worst conditions in a single-phase network, it holds a current of up to 19A.
3. We choose a machine so that it is guaranteed to protect our wire from overload. For our case - 13A. If you install a machine with such a rated thermal current, then at a current of 19A (one and a half times higher), the machine will work in about 5-10 minutes, judging by the time-current characteristics.

Is it a lot or a little? Considering that the cable also has thermal inertia and cannot instantly melt, this is normal. But considering that the load cannot just increase its current by one and a half times, and in these minutes a fire can occur - this is a lot.

Therefore, for a current of 10 A, it is better to use a wire with a cross-section of 2.5 mm² (the current with an open installation is 27 A), and a 13 A machine (if it is exceeded by 2 times, it will work in about a minute). This is for those who want to play it safe.

The main rule will be this:

The wire current must be greater than the current of the machine, and the current of the machine must be greater than the load current

Iload< Iавт < Iпров

This refers to maximum currents.

And if there is such a possibility, the rating of the machine should be shifted towards the load current. For example, the maximum load current is 8 Amperes, the maximum wire current is 27A (2.5mm2). The machine should be chosen not for 13 or 16, but for 10 Amperes.

Here is the machine selection table:

Table for selecting a circuit breaker based on cable cross-section

The choice of circuit breaker clearly depends on the cable cross-section. If the current of the machine is selected more than necessary, then the cable may overheat due to the flow of high current. If the machine is selected correctly, then if the current exceeds it will turn off and the cable will not be damaged.

Pay attention to the cable routing methods (installation type). Depending on where the cable is laid, the current of the selected circuit breaker may differ by 2 times!

According to the table, we have the initial cable cross-section, and select a circuit breaker for it. For us, as electricians, the first three columns of the table are most important.

Now - how to choose a circuit breaker if the power of the devices is known?

Table for selecting a circuit breaker based on load power

Table of consumption and current of the circuit breaker according to the power of devices

It can be seen that the manufacturer recommends different time-current characteristics for different electrical appliances. Where the load is purely active (different types of heaters), the characteristics of the machine “B” are recommended. Where there are electric motors - “C”. Well, where powerful engines with difficult starting are used - “D”.

Table of dependence of the current of the circuit breaker (fuse) on the cross-section

And here is how the Germans treat the circuit breaker current depending on the cross-sectional area of ​​the wire:

As you can see, the Germans are playing it safe and are providing for a larger reserve compared to us.

Although, perhaps this is because the table was taken from instructions from “strategic” industrial equipment.

How does a circuit breaker work?

A bonus is the device of the protective circuit breaker, several photos of the circuit breaker, which is given at the beginning of the article.

Circuit breaker device. As you can see, the device is not simple. Upper (fixed) contact – right

Circuit breaker. One second before the trash can)

As always, I will be glad to have questions and comments on the article in the comments!

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.