What is the rated operating current of a circuit breaker

Operating principle of the circuit breaker

The main function of circuit breakers is to protect the insulation of wires and power cables from destruction under the influence of short circuit currents. These devices are not able to protect people from electric shock; they only protect the network and equipment. The operation of automatic switches ensures normal operation of the wiring, completely eliminating the threat of fire.

When choosing a machine, you must take into account that the overestimated characteristics of the device will facilitate the passage of currents that are critical for wiring. In this case, the protected area will not be disconnected, which will lead to melting or fire of the insulation. If the characteristics of the machine are underestimated, the line will constantly break when starting powerful equipment. Automatic machines fail very quickly due to contacts sticking under the influence of too high currents.

The main operating elements of the machines are those that directly break the circuit in critical situations. They are divided into the following types:

• Electromagnetic releases. They react almost instantly to short circuit currents and cut off the desired area within 0.01 or 001 seconds. The design includes a coil with a spring and a core that retracts under the influence of high currents. During retraction, the core activates a spring associated with a release device.
• Thermal bimetallic releases. Provide network overload protection. They ensure that the circuit breaks when a current passes that does not correspond to the maximum operating parameters of the cable. Under the influence of high current, the bimetallic strip bends and causes the release to trip.

Most machines used in everyday life use an electromagnetic and thermal release. A well-coordinated combination of these two elements ensures reliable operation of the protective equipment.

Rated current in electrical engineering

Search for an article using the following words:

Rated current is the maximum current that is allowed, subject to the heating conditions of conductive parts and insulation, upon which the equipment will be able to operate for an unlimited period. Rated current is one of the most important parameters of any electrical equipment, be it sockets, transformers or power lines. At rated current, a constant balance of heat exchange is maintained between the heating of conductors when exposed to electric charges and their cooling due to partial temperature removal to the external environment. In order to correctly select the necessary associated equipment, it is important to be able to correctly determine the rated current.

The principle of determining the rated current If you need to find the value of the rated current for a conductor, you can use a specialized table. It indicates the current values ​​that can destroy the conductor. If you need to find the value of the rated current for electric motors included in the structure of any structures, then it is best to use the formulas. If you need to determine the rated current value for a fuse, you need to know the power for which it is designed. To carry out calculations and measurements you will need: a caliper, a voltmeter , a device data sheet and a table of the dependence of the rated current on the cross-section of the conductors.

In order to standardize equipment, GOST 6827-76 introduced a number of rated current values ​​at which almost all electrical installations must operate.

How to determine the rated current by cross-section First, you need to determine the material from which the conductor (wire) is made. The most in demand are aluminum and copper wires with a round cross-section. Measure its diameter using a caliper and find the cross-sectional area. To do this, multiply 3.14 by the square of the diameter and divide by 4. The formula is as follows: S=3.14•D²/4. You can figure out the type of wire you are dealing with. It can be single-core, two-core or three-core. Then refer to the table and find out the rated current for this wire. It is important to remember that exceeding the specified values ​​will cause the wire to burn out.

How to determine the rated current of a fuse The fuse device always indicates its power with a deviation of approximately 20%. Knowing the voltage in the network into which it must be inserted (can be measured with a voltmeter), you need to divide the calculated power of the device in watts by the mains voltage. The fuse is used to protect the conductor from destruction if the rated current values ​​are exceeded. How to determine the rated current of an electric motor To determine the rated current of a DC motor, you need to know its rated power, the voltage of the source to which it is connected, and its efficiency. All values ​​can be found in the technical documents. The mains source voltage is measured with a voltmeter. Next, you need to alternately divide the power into voltage and efficiency in fractions. The formula looks like this: I=P/(U•η). You will find the current value in amperes. It is also interesting to know that the maximum value of the rated current can be the short circuit current.

How to choose the right protective device based on rated current If the current value in the circuit is below the rated value, then it will be impossible to achieve the maximum power of the device. If the current strength, on the contrary, turns out to be greater than the rated one, then the circuit will be broken. The rated current must pass through the contacts of the circuit without consequences - in the longest possible time period. All current protective devices must be configured to operate when it is exceeded. Overload protective devices can operate on a thermal principle. These are fuses and thermal releases. They react to the thermal load and, after withstanding a certain time, turn it off. It is also possible to install protective devices that perform “instantaneous” load cutoff. Its shutdown time is 0.02 seconds. The choice of protective device is critical for AC systems. Settings of the circuit breaker according to the rated current To protect household electrical networks and various industrial devices, switches that operate on the principle of current cut-off and thermal releases are quite common. Any circuit breaker is made for rated current and voltage values. It is based on their values ​​that protective devices are selected. There are 4 types of time-current characteristics for various machines. Their designations are A, B, C, D. They are designed to disconnect during accidents at a current ratio of 1.3 to 14. Such switches are selected for a specific type of load: • lighting systems; • semiconductors; • schemes with mixed loads; • chains that can withstand heavy overloads. Factors influencing the speed of shutdown of the machine: the environment, the degree of filling of the panel and the likelihood of heating or cooling with the participation of extraneous sources.

Current ratings of automatic machines table

The need to select circuit breakers arises during the design of electrical networks in new homes, as well as when connecting devices and equipment with higher power. Thus, during further operation, reliable electrical safety of objects is ensured.

Negligence in choosing a device with the necessary parameters leads to serious negative consequences. Therefore, before choosing an automatic protective device, you must make sure that the installed wiring can withstand the planned load. In accordance with the PUE, the circuit breaker must provide overload protection for the weakest section of the circuit. Its rated current must match the current of the connected device. Accordingly, conductors are selected with the required .

To calculate the current power of the machine, you need to use the formula: I=P/U, where P is the total power of all electrical appliances in the apartment. Having calculated the required current, you can choose the most suitable machine. The table greatly simplifies the calculations, with the help of which you can select a circuit breaker depending on specific operating conditions. The calculation of the machine based on current power is carried out mainly for electrical installations - electric motors, transformers and other devices with a reactive load.

Machine denominations (calculation according to the table)

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

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, our kettle has 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 10A. Naturally, in practice this figure will be higher; modern wiring must be designed for a load current of at least 16A.

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.

Table of dependence of machine power on wire cross-section

Each electrical wiring is divided into certain groups. Accordingly, each group uses an electrical wire or cable with a certain cross-section, and protection is provided by a circuit breaker with the most suitable rating.

The table will help you select a circuit breaker and cable cross-section depending on the expected load of the electrical network, calculated in advance. The table helps you make the right choice of machine based on load power. When calculating current loads, it should be remembered that load calculations for one consumer and a group of household appliances differ from each other. When making calculations, it is necessary to take into account the difference between single-phase and three-phase power.

The circuit breaker is designed to protect the electrical network to which consumers are connected. In this case, the total power of consumers should not exceed the power of the machine itself. Therefore, it is necessary to correctly select the machine according to the load power. How can this be done, is there one way to choose or are there several?

Selecting a machine based on load power (current)

Although the main purpose of the machine is to protect electrical wiring, under certain conditions it is advisable to calculate the machine based on the load current. This is possible in cases where the line extending from the machine is intended to power one specific electrical appliance. In household networks, this could be an electric stove or air conditioner, some kind of machine, electric boiler, etc. As a rule, we know the rated current of an electrical appliance, or we can calculate it by knowing the load power. Since the wiring is selected with a certain margin, in this case the rating of the machine is usually less than what we would get by calculating the permissible current of the wire. Therefore, in case of any short circuits inside the electrical device or its overload, our protection will work, protecting it from further destruction.

Selection methods

Let’s say right away that there are several ways. But no matter which one you choose, first of all you need to determine the total load on the network. How to calculate this indicator? To do this, you will have to deal with all the household appliances that are installed on the power supply section. In order not to be unfounded, we will give an example of a network into which a large number of household appliances are usually connected. It's a kitchen.

So, in the kitchen there is usually:

• Refrigerator with power consumption of 500 W.
• Microwave oven – 1 kW.
• Electric kettle – 1.5 kW.
• Hood – 100 W.

This is almost a standard set, which can be a little larger or a little smaller. Adding up all these indicators, we get the total power of the site, which is equal to 3.1 kW. And now here are the methods for determining the load and the choice of machine itself.

Tabular method

This is the easiest option to choose the right circuit breaker. To do this, you will need a table in which you can select a machine (single- or three-phase) based on the total indicator. Here's the selection table below:

Everything is quite simple here. Most importantly, you need to understand that the calculated total power may not be the same as in the table. Therefore, the calculated indicator will have to be increased to the tabular one. From our example it can be seen that the power consumption of the site is 3.1 kW. There is no such indicator in the table, so we take the nearest larger one. And this is 3.5 kW, which corresponds to a 16-amp machine.

Graphic method

This is practically the same as the tabular one. Only instead of a table, a graph is used here. They are also freely available on the Internet. As an example, we give one of these.

On the graph, circuit breakers with current load indicator are located horizontally, and the power consumption of the network section is located vertically. To determine the power of the circuit breaker, you must first find the calculated power consumption on the vertical axis, and then draw a horizontal line from it to the green column that determines the rated current of the machine. You can do this yourself with our example, which shows that our calculation and selection was done correctly. That is, this power corresponds to a machine with a load of 16A.

Circuit breakers. Selection, calculation of circuit breaker.

The control handle is used as a switching position indicator. The unit for adjusting the non-operation current of the thermal release is a thermoelement with temperature compensation thermobimetal and an adjustment device.

The adjusting device consists of a system of levers and an adjusting screw.

Terms and Definitions

Electromagnetic circuit breaker release

designed to protect circuits from short circuit current, it is an electromagnet that, at a certain current, instantly attracts an armature, resulting in the circuit breaker tripping. Many modern switches have a semiconductor trip unit, which acts as an electromagnetic release.

Thermal release of circuit breaker

- a thermal relay that responds to the amount of heat generated in its heating element and protects the circuit from overload.

Combination release

— the release, which provides protection against overload and short circuits, is a combination of two releases: thermal and electromagnetic.

Undervoltage release

- an electromagnet that is triggered when the voltage disappears or when it decreases to the release set point.

Shunt release

- an electromagnet that triggers and turns off the circuit breaker when an impulse is supplied from a key or control button.

Non-adjustable circuit breaker

- an automatic circuit breaker that does not have the ability to regulate the release setting during operation. The circuit breaker release is adjusted by the manufacturer to a certain rated current.

Adjustable circuit breaker

- a device that has the ability, by acting on a mechanical system or a special device, to adjust the response time of the release.

Selective circuit breaker

- a device that operates with a time delay and allows selective protection of networks by installing circuit breakers with different time delays: the smallest at the consumer and increasing stepwise towards the power source.

Circuit breaker parameters

Rated current

- current, the passage of which is permissible for an unlimited time.

Rated voltage

- voltage at which a switch of this type can be used.

Maximum switchable current

- a current that can be turned off by a circuit breaker without causing any damage to it.

Rated current of release

- current, the passage of which for an unlimited time does not cause the release to operate.

Release setting current

- the smallest current, upon the passage of which the release is triggered.

Current setting

— setting the circuit breaker for a given operating current.

Current cut-off

— current setting of the electromagnetic release for instantaneous operation.

Operating principle

The operation of turning on and off the switches is carried out by moving the handle to the “1” and “0” positions, respectively. When overload or short circuit currents exceed the operating current setting, the contact system is automatically switched off.

The switches are turned off under the action of the releases regardless of whether the handle is held manually in the on position or not. The free release mechanism provides instantaneous closing and opening of the contact system under automatic and manual control.

The switch is turned on after automatic operation by moving the handle to position “0”, which is done by charging, and then by turning it to position “1” - turning on.

Protective characteristics of circuit breakers

According to GOST R 50345-99, circuit breakers are divided into the following types based on instantaneous tripping current:

B

: from 3·
In
to 5·
In
(where
In
is the rated current);

C

: 5·
In
to 10·
In
;

D

: 10·
In
to 20·
In
.

Rice.
26. Trip diagrams of circuit breakers of different types (the area of ​​instantaneous tripping currents is shaded)
For European manufacturers, the classification may be slightly different. In particular, there is an additional type A

(2·
In
to 3·
In
). Some manufacturers have additional shutdown curves. For example, ABB has circuit breakers with K and Z curves.

Automatic switch AB25

Single-pole installation circuit breakers of the AB25 brand are designed for automatic shutdown of electrical circuits or individual receivers in the event of overloads and short circuits.

Device

.
In the plastic case (Fig. 27)
a metal bracket with a contact and a screw clamp for connecting the wire is fixedly fixed. The moving contact is mounted on a brass lever, which is pressed in the center by a spring, and the end rests against a bimetallic plate. This plate is welded to the terminal with a screw clamp attached to it for connecting the second wire. To create reliable contact, the bimetallic plate and the lever are connected by a flexible copper conductor.

Rice.
27. Design of the AB-25 circuit breaker
When the machine is turned on, the handle is set to the upper position and its protrusion releases the lever, which rotates under the action of a spring and closes the contacts.

Shutdown

. When a short circuit or overload occurs, the machine is switched off as follows. The current heats the bimetallic plate and, bending downwards, it releases the end of the lever, which rotates under the action of a spring and opens the contacts.

The spark that occurs between the contacts is extinguished in the arc chamber.

Reactivation

. When the AB25 automatically turns off, the handle remains in the “on” position, so to turn the machine on again, you must first lower it to the “off” position and then move it to the upper position again.

Due to this design of the drive, the machine is equipped with an operation indicator (plastic rod with a spring). When the machine is turned on, as well as when turned off manually, the pointer is recessed into the housing. When automatically switched off, the end of the lever pushes it out of the socket and becomes clearly visible.

Selection of circuit breakers

The selection of circuit breakers is made based on rated voltage and current, subject to the following conditions:

Unom.a.

≥
Unom.s.
;
Inom.
a ≥
Idolt
;

where Unom.a.

— rated voltage of the circuit breaker;
Unom.s.
— nominal network voltage;
Inom.a.
— rated current of the circuit breaker;
Idlit
- long-term design current of the circuit.

In addition, the following must be correctly selected: rated current of the releases Inom.rast.

;
current setting of the electromagnetic release element of the combined release Iset.el.magn.
;
rated current setting of a thermal release or thermal element of a combined release - Inom.set.therm.
.

The rated currents of the electromagnetic, thermal or combined release must be no less than the rated current of the motor:

Inom.rast.

≥
Inom.
dv. The setting current of the electromagnetic release (cut-off) or the electromagnetic element of the combined release, taking into account the inaccuracy of the release and deviations of the actual starting current from the catalog data, is selected from the condition:

Iset el.magn.

≥ 1,25
Istart.
;

where Istart.

— motor starting current.

For a group of engines:

, where is the sum of the rated currents of simultaneously operating motors and other elements creating current in the circuit, protected by a switch, until the moment of starting the motor (group of motors), which gives the greatest increase in the starting current; Istart

— starting current of the motor (or a group of motors started simultaneously), which gives the greatest increase in starting current.

Rated current setting of the thermal release or thermal element of the combined release:

Inom.set heat.

≥
Inom. dv
.

The settings of the circuit breaker releases are also selected to protect the circuits of other electrical receivers of the power supply system, for example, circuits of control and measuring instruments, etc. Of course, if this becomes necessary, since in most cases, to protect devices and other similar low-power electrical receivers for reasons sensitivity it turns out that it is necessary to use fuses.

It must be taken into account that if a circuit breaker with an electromagnetic release is installed in the circuits of electrical receivers, when turned on, inrush current surges do not occur, then there is no need to tune out these surges. In this case, the setting current of the electromagnetic release should be selected as low as possible.

Operation of protective devices

Circuit breakers are inspected at least once a year or every 2000 starts, as well as after each automatic shutdown. Nagar

and
soot
from the inside of the switch is removed with a cloth moistened with acetone.

Upon examination

check the tightness of the screws, the integrity of the springs, the condition of the contacts, and lubricate the hinges. They also pay attention to the serviceability of the protective casings in which the launchers are located. If the seal is broken, dust and dirt can enter the device, which increase the resistance of the contact surfaces and cause their heating and corrosion, and these worsen the condition of the insulation, which leads to its aging, breakdown, and, consequently, to an accident.

Periodically check the correct operation of the relay and disconnection of the circuit breakers from the action of thermal or electromagnetic releases. Fuses require constant monitoring, replacement of blown fuse links and timely repairs. Reliable and safe operation of electrical installations depends on their serviceability and correct selection of inserts. Only calibrated fuse links should be used. Using random insertion wires may cause accidents and fires. To speed up the selection and replacement of a blown insert, each fuse must have a clear number on the rated current. When maintaining electrical equipment, minor repairs are often carried out.

Nuances of choice

Today it is necessary to take into account the fact that the number of convenient household appliances is limited, and every person tries to acquire new devices, thereby making their life easier. This means that by increasing the number of equipment, we increase the load on the network. Therefore, experts recommend using a multiplying factor when calculating the power of the machine.

Let's return to our example. Imagine that the owner of the apartment purchased a 1.5 kW coffee machine. Accordingly, the total power indicator will be equal to 4.6 kW. Of course, this is more power than the circuit breaker we selected (16A). And if all the devices are turned on at the same time (plus the coffee machine), the machine will immediately reset and disconnect the circuit.

You can recalculate all the indicators, buy a new machine and reinstall it. In principle, this is all easy. But it will be optimal if you foresee this situation in advance, especially since it is standard these days. It is difficult to predict exactly what additional household appliances can be installed. Therefore, the simplest option is to increase the total calculated indicator by 50%. That is, use a multiplying factor of 1.5. Let's go back to our example again, where the end result will be like this:

3.1x1.5=4.65 kW. Let's return to one of the methods for determining the current load, in which it will be shown that for such an indicator you will need a 25 ampere machine.

For some cases, a reduction factor can be used. For example, there is not enough sockets for all devices to work simultaneously. This could be one socket for an electric kettle and a coffee machine. That is, it is not possible to turn on these two devices at the same time.

Attention! When it comes to increasing the current load on a network section, it is necessary to change not only the machine, but also check whether the electrical wiring can withstand the load, for which the cross-section of the laid wires is considered. If the cross-section does not meet the standards, then it is better to change the wiring.

Selecting a three-phase machine

Three-phase circuit breakers designed for a 380-volt network cannot be ignored in this article. Moreover, they are indicated in the tables. Here is a slightly different approach to selection, which is based on a preliminary calculation of the current load. Here is a simplified version of it.

• First, the total power of all devices and lighting sources that are connected to the machine is determined.
• The result obtained is multiplied by a factor of 1.52. This is the load current.
• Next, select the circuit breaker according to the table.

But keep in mind that the rated current must be at least 15% greater than the calculated current. This is the first one. Secondly, this calculation can only be used if the three phases of the consumption network have the same load or close to the same indicator. If on one of the phases the load is greater than on the other two, then the machine is selected precisely according to this high load. But keep in mind that to calculate the load in this case, a coefficient of 4.55 is used, since one phase is taken into account.

Related posts:

Modern power supply to private houses and apartments is not recommended without circuit breakers. They provide safety and guarantee long service life of the wiring. We will talk about choosing a circuit breaker in this article.

The main purpose of a circuit breaker is to protect the wiring from overheating and the insulation from melting. And it does this by turning off the power supply at those moments when the conductor heats up to critical temperatures due to connecting an excessively high power load. The second task of the package operator is to disconnect the line during short-circuit currents. The goal is the same - to protect the wiring from destruction.

Turning off the power in a timely manner in the event of problems is very important, as it prevents damage to the wiring and fire. Therefore, choosing a circuit breaker is a responsible task. You need to choose according to the rules, and not according to the principle “so that it turns off less often.” This method may cause a fire. In general, the selection of a circuit breaker is carried out according to three parameters:

• denomination;
• breaking capacity (cut-off current);
• type of electromagnetic splitter (time-current characteristic).

Each parameter is important and is selected depending on the load connected to a particular line, the location of the electrical wiring relative to the distribution substations.

Selection of circuit breakers

Since an increase in current strength above the rated value (overload) entails disruptions in the operation of devices, in this case it is necessary to provide for de-energizing the circuit.

The task is performed by the following protection devices:

• fuses: contain a fusible insert - when overheated, it melts and the circuit opens;
• automatic switches (VA).

VA consists of two parts:

1. thermal release . Bimetallic plate that opens contacts when heated. The response time can be tens of minutes;
2. electromagnetic release (coil with solenoid). Triggers almost instantly (0.02 s) when the current reaches a certain value.

The operating threshold of the electromagnetic release for different consumers also requires an individual one. Some fail even with the slightest overload, others can withstand 14 times the excess of In. Therefore, they produce 4 classes of VA, differing in the setting of the electromagnetic device for breaking the circuit (cut-off current setting): A, B, C and D.

The class is selected according to the type of consumers:

1. semiconductor elements. Class A, the most sensitive: cut-off current - 2 times higher than the rated current;
2. sockets, lighting circuits and others where inrush currents are absent or small. Class B: cut-off current - 3 times higher than the rated current;
3. input devices for household electrical networks. Class C: cut-off current - 5 times higher than the rated current. Such VAs are not used alone: ​​they ensure the security of the network as a whole, while each group (sockets, lighting) is additionally protected by a class B VA. That is, a class C VA insures class B machines, but in the event of an overload in one of the groups, the entire the network is not de-energized (selectivity);
4. input devices for networks of buildings and structures, circuits with high starting currents (electric motors act as consumers). Class D: cut-off current - 10 times higher than the rated current. At the entrance to the building, such a VA also plays the role of a selective one - it insures circuit breakers on floors and in individual rooms.

When an overload is less than the cut-off current setting, a current in excess of the rated current flows through the circuit for some time (before the thermal release is activated).

This is taken into account, for example, when choosing an RCD, officially called a “residual current switch”. This is another protective device that de-energizes the circuit when a current leak is detected, thereby preventing electrical injury to the user.

Types of circuit breakers

Circuit breakers are produced for single-phase and three-phase circuits. For a single-phase network there are two types of packets - single-pole and double-pole. Only the phase wire is connected to single-pole ones and, when triggered, only the phase is disconnected. It is recommended to install such machines in houses and apartments in rooms with normal operating conditions. Usually they are installed on lighting lines, socket groups, which are located in living rooms, corridors, and kitchens.

Circuit breakers - single-pole, double-pole and three-pole

Both phase and neutral wires are connected to two-pole circuit breakers. He breaks both chains. The degree of protection here is much higher since the shutdown is complete and not partial. Such a machine will ensure safety even if during an accident voltage reaches the neutral conductor. It is recommended to install two-pole circuit breakers on dedicated lines to which powerful household appliances are connected. They are also installed in rooms with difficult operating conditions. These include a bathroom, swimming pool, sauna.

For three-phase networks, three-pole and two-pole circuit breakers are used. All three phases are turned on to three-pole ones. Accordingly, they all turn off at the same time. Such packets are placed at the entrance to a house or apartment, as well as on the lines to which three-phase consumers are connected - a hob, an oven and other similar equipment. For the same consumers, four-pole circuit breakers can be installed. They will also disconnect the neutral wire.

On other power lines, on which one of the phases is used, two-pole packets are installed. Simultaneous disconnection of phase and zero is more preferable. And only on the lighting line can single-terminal circuits be installed.

How does a circuit breaker work?

The main task of a circuit breaker (circuit breaker) is to capture excessive currents in the electrical network and instantly de-energize it

No matter what category the circuit breaker belongs to, it must be able to quickly de-energize the electrical network and thereby prevent damage to cables

Therefore, the main function of the circuit breaker is:

• Triggered in case of power overload. Everything here is quite simple, and if there is an excessively large load on the network, for example, due to a large number of connected electrical appliances in the house, the circuit breaker should trip and de-energize the home electrical network. If this does not happen, and the machine does not cope with its task, then the electrical wiring in the house may catch fire;
• React to overcurrent caused by a short circuit in the electrical wiring. Everything here is also clear. In the event of a short circuit, the electrical wiring is subjected to strong heating, and where it is thin, as is known, it breaks, therefore, if the machine does not work, the electrical wiring may be damaged and caught fire.

You should know that each circuit breaker is rated for a different amperage. The response time of the machine depends on the magnitude of the power supply overload. If this is a short circuit, then the circuit breaker will operate instantly, literally in a matter of seconds. If the overload is not too large, then the machine and electrical wiring can heat up for hours.

As for the design of the circuit breaker and its operating principle, it is based on a bimetallic plate through which electric current passes. If it is too large, for which the machine is not designed, then the plate begins to heat up, which ultimately leads to the operation of the circuit breaker.

Circuit breakers “B” and “C” - what is the difference, categories of circuit breakers

Those people who are upgrading their home electrical network are often interested in the question of how exactly circuit breakers of categories “B” and “C” differ, because they are most often installed in household networks. The main difference between the machines “B” and “C” is the sensitivity of the electromagnetic release.

The letters A, B, C, D and K, Z indicate the characteristics of the release installed in the circuit breaker:

A - circuit breakers of this category have the highest sensitivity. If the rated current on the line where the category “A” circuit breaker will be installed exceeds 30%, the circuit breaker will turn off.

B - machines of this category are triggered when the rated current load is 3-5 times higher. Category “B” circuit breakers are designed for installation in electrical networks with no or minimal starting current (electric motors, etc.). In simple words, category “B” machines are more sensitive to passing current, and can work when starting powerful electric motors.

C - standard type automatic circuit breakers with an even greater overload capacity than class “B” circuit breakers. They are switched off if the rated current passing through the machine becomes 5-10 times higher. The response time of a category “C” machine is about 1.5 seconds. Such machines are designed to provide protection for general-purpose electrical networks.

Category D machines are rarely used in everyday life. Most often, these circuit breakers are used in electrical networks with large starting loads. Well, the last categories of machines are “K” and “Z”, they are used for special purposes, for example, to protect lines to which electronic devices are connected.

Selecting a load current circuit breaker

When planning electrical wiring, the main task is to choose the correct circuit breaker rating. When current passes through a conductor, it begins to heat up. The more current passes through a conductor of the same cross-section, the more heat is generated. The circuit breaker's job is to cut off the power before the current draw becomes higher than is acceptable. Therefore, the rating of the circuit breaker must be less than the permissible wiring current.

The ratings of circuit breakers are standardized: 6 A, 10 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A and 63 A. In practice, six and ten amp options are almost never used anywhere - equipment in our homes is becoming more and more and small cross-section lines cannot cope with the load.

Choice of denomination

The circuit breaker is not selected based on the load, the power of the connected devices, or the current. These parameters are taken into account when choosing the conductor cross-section. And the choice of circuit breaker is made depending on the cross-section of the conductors. There is a special table that indicates the permissible load currents and the recommended rating of the circuit breaker. Using the table is simple: find the desired cross-section, in this line look for the rating of the circuit breaker. All.

How it all works

Looking at the table, the question arises: why is the rating of the machine so much less than the maximum permissible current load. The answer lies in the mechanics of the circuit breaker. It turns off only when the current in the circuit is 13% higher than the trip current.

For example, a 10 A machine will work when the current in the circuit is 16 A + 13% (2.08 A) = 18.08 A. That is, there is a small gap left to the permissible load. This gap is required to ensure the integrity of the insulation.

A modern power supply system for a house or apartment cannot do without automatic switches.

What will happen if you install a 16 A circuit breaker on a wire with a cross-section of 1.5 mm2? After all, its rating is lower than the permissible load current? Let's count. The current at which the packetizer will operate is 25 A + 3.25 A (13%) = 28.25 A. It is higher than the permissible long-term load current. Yes, it will rarely turn off, but after a while the insulation will melt and the wiring will have to be changed. Therefore, it is better to select a circuit breaker according to this table, and not according to the long-term permissible current.

Selection by load

If the power supply line is laid with a power reserve, and the load on it is far from the maximum, you can install a machine with a lower rating. In this case, it will protect not so much the line from overheating, but the equipment from short-circuit currents.

Choosing a circuit breaker based on load power is the wrong idea

The choice of the circuit breaker rating in this case can also be made using the same table. Just take the load power as a starting point. But let's repeat it again. This is the case if the line parameters can withstand a much greater load than exists.

What it is?

In (according to the PUE - permissible continuous current) is the maximum current strength that allows arbitrary operation of an electrical device, not limited in time, that is, not leading to overheating of its current-carrying parts.

When In occurs, two conditions are met:

1. the heat release in the conductors and its removal into the surrounding space are balanced;
2. the generated heat does not cause damage to the mechanical and chemical properties of the materials necessary for the operation of the device.

When the nominal value is exceeded, an imbalance is observed in favor of heat release: the temperature of the conductive parts increases, followed by melting of the insulation.

This poses a risk of fire and short circuit. Metal elements lose strength and become deformed. All components of the power supply system, from the generator or current source to the consumer, are designed for a certain In during design. This applies not only to devices, but also to wires, connecting elements, etc.

The value of In is indicated in the equipment passport. Also, this parameter, along with other most important ones, is often marked on the body or nameplate of the device. Most preferred: 1; 1.6; 2.5; 4; 6.3 A and multiples thereof.

In addition to the values ​​​​given in the regulatory document, it is allowed to use:

• for transformers: 15, 30, 60, 75, 120 A and multiples thereof;
• for existing devices (by agreement between the customer and the manufacturer): 1400, 2240 A;
• for converters and transformers for them (also by agreement between the manufacturer and the customer): 37.5, 75 and 150 kA.

Type of electromagnetic splitter (switch-off curve)

The next parameter by which the circuit breaker is selected is the type of electromagnetic splitter. It is responsible for the delay that occurs when triggered. It is necessary to avoid false shutdowns during the start of motors of various equipment.

When you turn on the motor of a refrigerator, dishwasher or washing machine, the current in the circuit increases briefly. This phenomenon is called inrush currents, and they can exceed operating consumption by 10-12 times, but they do not last long. Such a short-term increase does not cause harm. So, the electromagnetic splitter must have a delay that allows you to ignore these inrush currents. This characteristic is displayed in Latin letters B, C, D. This letter is placed before the rating of the circuit breaker (photo). Selecting a circuit breaker based on this criterion is not difficult. You just need to know the nature of the planned load:

Actually, choosing a circuit breaker in this case is simple. On the lighting line it is enough to install category B machines; on the rest you can install C.

Video on the topic

About calculating the rated current of circuit breakers in the video:

Without understanding the value of the rated current, it is impossible not only to correctly design, but also to operate the electrical network.

It is important to remember that exceeding the current strength in the wires above the rated value in the absence or incorrect selection of circuit breakers can lead to a fire or equipment failure. An example is imported boilers and geysers, in which circuit boards burn out during power surges.

Source



Selecting the degree of protection against short-circuit currents (cut-off current)

The second function of the protective circuit breaker is to turn off the power when excess currents appear that occur during a short circuit (short circuit). Circuit breakers are designed for different values ​​of these currents, and the characteristic that displays it is the breaking capacity or cut-off current. It shows at what short-circuit current the machine will still remain in working condition. The fact is that the burster does not fire instantly, because there is a delay in response to ignore starting overloads. During this delay, the contacts may melt and the device will become inoperable. So, the cut-off current or breaking capacity shows how much current the contacts can withstand without compromising performance.

In the household electrical network, circuit breakers are used with three degrees of protection against short-circuit currents: 4500 A, 6000 A, 10000 A. On the device body, these numbers are placed in a frame just below the rating of the machine. In terms of price, the difference is quite noticeable, but it is justified - more “resistant” bags use refractory materials, and they are much more expensive.

How to choose a circuit breaker in this case? The choice depends on the location of the network relative to the substation. If a house or apartment is located nearby, the short-circuit currents can be very large, therefore the breaking capacity should be at least 10,000 A. If the household is located in a rural area, the networks there are old and/or the supply occurs via an overhead network, an automatic circuit breaker with a breaking capacity of 4,500 A is sufficient In all other cases they set it to 6000 A.

Housing protection degree

The degree of protection of the case is in the characteristics. It is denoted by the Latin letters IP and two numbers. The first number shows how protected the device is from dust and foreign objects. The lowest level of protection (absent) is 0, the highest level is 6 (complete protection from long-term exposure). The second number indicates protection from moisture. Without protection - 0, maybe in the water for some time - 8. The decoding of the numbers is given in the table.

If the electrical panel is installed in an apartment, in a dry room, the degree of protection IP20 is sufficient. On landings, a higher degree of protection is desirable. At least IP32. If the machine is installed outdoors, you should set it to at least IP55.

Expensive or cheap?

There are two price categories for circuit breakers in stores and markets. One part is produced by well-known brands and has a very respectable price tag. These are Schneider Electric, ABB, LeGrand and others. These brands have been on the market for a long time, have European roots and an established reputation. The quality of their products is always high, so those who do not like to take risks and can afford to spend a lot of money on assembling an electrical panel prefer to purchase products from these manufacturers.

Next to them there are usually the same machines, but they cost 2-5 times less. These are IEK (IEK), EKF (EKF), TDM (TDM), DEKRAFT (Derkaft), etc. These are Chinese machines, but produced in factories. Some brands (the same Dekraft) have European roots (in this case Germany), but production facilities are in China. These brands are also considered quite good and show stable results. So for those who try not to spend extra money, this is a good option. Affordable and good quality.

What you should not do is buy products from unknown manufacturers. Even if their price is very attractive and the seller praises them very much.

There are also pitfalls when buying well-known brands: there are too many fakes. Moreover, they are sold at almost the same price as the original and it is very difficult to distinguish them by external signs. The only thing you can focus on is less weight. Counterfeits contain less metal and may be missing some elements. Due to this, the weight is less. There may also be errors in the application of inscriptions; sometimes paints of other shades are used. To notice all this, you must first thoroughly study all the nuances of the originals on the official websites, or even better, hold them in your hands.

Automatic switch IEK. Thermal current - 32 A

The circuit breaker has several other names among the 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 you can see that this is a switch

which turns off
automatically
.
That is, himself
, 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 case of
short circuit (SC)
.

Overcurrent occurs due to faulty consumers, or when there are too many consumers. A short circuit is a mode when all the power of an 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.

DETERMINATION OF DESIGN CURRENTS OF OPERATING MODE

2015-05-26 14601
Rated current I

rated - the highest current (rms value) that a device or conductor is capable of carrying for a long time at a given voltage, rated frequency and rated air temperature, while the temperature of the parts of the device should not exceed the permissible limit established for long-term operation.

The operating mode of devices and conductors according to their load is divided into normal and heavy.

Under normal mode

electrical installations understand an operating mode in which the values ​​of its parameters do not exceed the limits acceptable under given operating conditions. In normal mode, all elements of this electrical installation function without forced shutdowns and without overloads.

Heavy mode

is called a mode when a part of the connections is forced to be disconnected due to their damage or in connection with preventive repairs, when the operating currents of other connections can noticeably increase. In this case, the most severe mode is taken as the design one, when the greatest current flows in the electrical installation.

When choosing the cross-section of conductors based on the economic current density, they proceed from the normal operating mode without taking into account short-term overloads, and according to the heating condition, from the conditions of the heavy duty mode.

Thus, to select devices and conductors in normal modes, you need to know the values ​​of the operating currents of connections of normal I

working norms and heavy
I
working modes.

In general, the current strength can be determined by the formula:

where S

load
–
total load power, in kV. A, (the value can be determined using the ordered chart method or the demand coefficient);

U

nom – rated voltage, in kV.

The formulas below for calculating currents in individual elements of the power system are mainly used for approximate calculations when their load is unknown. If the load on the elements differs from the rated one, then to calculate operating and emergency currents it is necessary to take into account the actual load in operating and emergency modes.

Let's consider some specific cases of determining the calculated operating currents.

For connecting generators and synchronous compensators

,
the calculated normal operating current is taken equal to the corresponding rated current
where P

nom
–
rated power of the generator, in kW;

cos φ

nom – rated power factor of the generator.

For synchronous motors at rated field current and asynchronous motors

where P

nom
–
rated engine power, in kW;

U

nom – rated voltage of the supply network, in kV

cos φ

nom – rated power factor of the engine;

η

nom – rated engine efficiency.

There is practically no heavy duty duty for generators, synchronous compensators and motors, since the permissible continuous overcurrent does not exceed 5% (with a voltage drop of 5%), while the heavy duty current

For connections of power transformers, the calculated operating current of the normal mode must be equal to the rated current of the transformer, less or more than it, depending on the purpose and method of redundancy of the transformer.

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:

How to calculate the rated current of a circuit breaker?

Greetings, dear readers of the site.
In the previous series of articles, we studied in detail the purpose, design and principle of operation of a circuit breaker, analyzed its main characteristics and connection diagrams, now, using this knowledge, we will come to the issue of choosing circuit breakers. In this post we will look at how to calculate the current rating of a circuit breaker.

This article continues the series of publications RCD circuit breakers - a detailed guide . In the following publications I plan to analyze in detail how to choose a cable cross-section, consider the calculation of the electrical wiring of an apartment using a specific example with the calculation of the cable cross-section, the choice of ratings and types of machines, and the breakdown of wiring into groups. At the end of the series of articles on circuit breakers there will be a detailed step-by-step comprehensive algorithm for their selection.

Do you want to not miss the release of these materials? Then subscribe to the site news, the subscription form is on the right and at the end of this article.

So let's get started.

Electrical wiring in an apartment or house is usually divided into several groups.

The group line feeds several consumers of the same type and has a common protection device. In other words, these are several consumers that are connected in parallel to one power cable from the electrical panel and a common circuit breaker is installed for these consumers.

The wiring of each group is carried out with an electric cable of a certain cross-section and is protected by a separate circuit breaker.

To calculate the rated current of the machine, it is necessary to know the maximum operating current of the line, which is allowed for its normal and safe operation.

The maximum current that a cable can withstand without overheating depends on the cross-sectional area and material of the cable conductor (copper or aluminum), as well as on the method of wiring (open or hidden).

It is also necessary to remember that the circuit breaker serves to protect electrical wiring, not electrical appliances, from overcurrents. That is, the machine protects the cable that is laid in the wall from the machine in the electrical panel to the outlet, and not the TV, electric stove, iron or washing machine that are connected to this outlet.

Therefore, the rated current of the circuit breaker is selected, first of all, based on the cross-section of the cable used, and then the connected electrical load is taken into account. The rated current of the machine must be less than the maximum permissible current for a cable of a given cross-section and material.

The calculation for a group of consumers differs from the calculation of a single consumer network.

Let's start with the calculation for a single consumer.

1.A. Calculation of current load for a single consumer

In the passport for the device (or on the plate on the case) we look at its power consumption and determine the calculated current:

There are two different types of resistance in an AC circuit - active and reactive. Therefore, load power is characterized by two parameters: active power and reactive power.

The power factor cos φ characterizes the amount of reactive energy consumed by the device. Most household and office equipment have an active load (they have no or little reactance), for which cos φ = 1.

Refrigerators, air conditioners, electric motors (for example, a submersible pump), fluorescent lamps, etc., along with the active component, also have a reactive component, so cos φ must be taken into account for them.

1.B. Calculation of current load for a group of consumers

The total load power of a group line is determined as the sum of the powers of all consumers in a given group.

That is, to calculate the power of a group line, you need to add up the powers of all devices in this group (all devices that you plan to turn on in this group).

We take a sheet of paper and write down all the devices that we plan to connect to this group (i.e. to this wire): iron, hair dryer, TV, DVD player, table lamp, etc.):

When calculating a group of consumers, the so-called demand coefficient Kc , which determines the probability of simultaneous inclusion of all consumers in the group over a long period of time. If all electrical appliances in a group operate simultaneously, then Kc = 1.

In practice, usually all devices do not turn on at the same time. In general calculations for residential premises, the demand coefficient is taken depending on the number of consumers from the table shown in the figure.

The power of consumers is indicated on the plates of electrical appliances, in their passports; in the absence of data, you can take it according to the table (RM-2696-01, Appendix 7.2), or look at similar consumers on the Internet:

Based on the design power, we determine the total design power:

We determine the calculated load current for a group of consumers:

The current calculated using the above formulas is obtained in amperes.

2. Select the rating of the circuit breaker.

For internal power supply of residential apartments and houses, modular circuit breakers are mainly used.

We select the rated current of the machine equal to the design current or the nearest larger one from the standard range:

6, 10, 16, 20, 25, 32, 40, 50, 63 A.

If you choose a machine with a lower rating , then the circuit breaker may trip at full load in the line.

If the selected rated current of the machine is greater than the maximum possible current of the machine for a given cable cross-section, then it is necessary to select a cable of a larger cross-section, which is not always possible, or such a line must be divided into two (if necessary, more) parts, and conduct the entire above calculation first.

It must be remembered that for the lighting circuit of home wiring, cables of 3 × 1.5 mm2 are used, and for the socket circuit - with a cross section of 3 × 2.5 mm2. This automatically means limiting the power consumption for the load supplied through such cables.

It also follows from this that circuit breakers with a rated current of more than 10A cannot be used for lighting lines, and for socket lines - more than 16A. Lighting switches are produced for a maximum current of 10A, and sockets for a maximum current of 16A.

Watch the detailed video How to calculate the rated current of a circuit breaker
I recommend

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.

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 off in about a minute), and the electromagnetic one works instantly. For schedule B

this moment “begins” when the current exceeds the nominal value by 3-5 times, for category
C
- 6-10 times, for
D
(not shown, since 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.

Rated current

Rated current

- the maximum permissible current under heating conditions of conductive parts and insulation, at which the equipment can operate indefinitely.

The rated current is one of the main parameters of almost any electrical equipment (switches, transformers, power lines, buses, etc.) and is indicated in its passport.

The PUE uses the term permissible continuous current

[1] for selecting heating conductor cross-sections.

A number of rated currents of electrical equipment, A (according to GOST 6827-76).

1. This series applies to electrical equipment and electrical energy receivers, for which the main parameter is the rated current.

2. By agreement between the consumer and the manufacturer, it is allowed to use currents of 37,500, 75,000 and 150,000 A for converter units and transformers intended for them.

3. The current values ​​indicated in brackets are not used in new developments.

Selecting a circuit breaker

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 by 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.

Tired of it? Maybe this will be interesting:

Continuation of the article:
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 an automatic 13 A (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

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:

What is rated current in electrical engineering

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, purposes, 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.

Principle of determining 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 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, GOST 6827-76 introduced a number of rated current values ​​at which almost all electrical installations must operate.

How to select a protective device based on rated current

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 the metal conductor are reached, the voltage from the electrical installation must be removed to cool it.

These functions are performed by overload protection operating on a thermal principle:

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 completed 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.

How the circuit breaker is configured to operate at rated current

In the protection of industrial devices and household electrical networks, the most widely used circuit breakers are those that combine in their design:

thermal releases operating with a time delay;

current cut-off, which very quickly turns off the emergency mode.

In this case, circuit breakers are manufactured for rated voltage and current. 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 certain type of load, for example:

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 designation can be found on the machine body. The picture shows a switch on which the value of 100 amperes is indicated.

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.