Standard range of circuit breakers. How currents in circuit breakers are taken into account. How to calculate the required current rating of circuit breakers
The main purpose of circuit breakers used in household electrical networks is to timely disconnect consumers in the event of a short circuit or exceeding the rated current value as a result of an overload.
In order to be able to perform both of these functions, anyone must be equipped with two types of releases, one of which (electromagnetic) reacts to a sharp increase in current, and the other (thermal) opens the circuit in the event of an unacceptable increase in the temperature of the conductors.
Failure of the protection to perform its functions can lead to excessive heating of network elements, their destruction or fire. Therefore, during installation or repair, it is extremely important to correctly select and install its protection devices.
It is quite natural that the rated current and voltage, as well as the shutdown time, are different for different circuit breakers.
In order to understand the criteria and rules that usually guide the installation of these devices, you should take a closer look at their technical characteristics.
Most of these characteristics can be determined simply by looking at the corresponding markings on the circuit breaker housing.
Rated current
This characteristic shows the maximum permissible current that can flow through this device for a long time without triggering the thermal release.
To calculate this value for a separate single-phase line, you need to divide the total power of electrical appliances connected to it by the network voltage. For example, with a consumer power of 3 kW, the rated current will be equal to:
Rated voltage
Everything is very simple here; for a single-phase network you should choose machines with a rating of 230 V, and for a three-phase network - 380 V.
Type of time-current characteristic
This characteristic shows the dependence of the speed of disconnection of the release contacts on the amount of current flowing through them. The fact is that some devices used in household electrical networks have quite large inrush currents, much higher than the rated currents.
In household electrical networks, circuit breakers are used that have three types of time-current characteristics:
- B. Designed for power supply and lighting lines.
- C. The most common type of switches, used to power consumers that are more powerful than in the previous case (including motors with low starting currents). Such a machine can be used as an incoming disconnecting device in the power supply system of an apartment or private house.
- Circuit breakers with this type of characteristic are used mainly to protect electric motors.
As can be seen from the figure, even if the current in the circuit exceeds its nominal value ten times, the machine with characteristic D will turn off the circuit with a delay of 1 to 2 s.
Rated breaking capacity
This characteristic shows what maximum current value can be turned off by this machine.
Number of poles
In single-phase networks, switches with one or two poles are used. To install a three-phase motor in the power circuit, three-pole devices are used, and to protect consumers of a four-wire three-phase network (with a dedicated zero), four-pole circuit breakers are used.
If all of the above values are determined, then choosing a circuit breaker for installation in an apartment is not particularly difficult. However, there are a few more important points that you should pay attention to when installing a protection system consisting of several switches.
- To ensure maximum reliability, it is better to use machines from one manufacturer.
- Current ratings of switches must be selected in such a way as to ensure selectivity of the automatic protection system. That is, in the event of an emergency, it should turn off only the part of the network where such a situation occurred. To ensure this parameter, it is necessary to select devices with a large rated current as general machines.
- To avoid purchasing a low-quality device or one that does not meet the stated characteristics, it is better not to buy it from unverified suppliers. For this purpose, there are specialized stores or, ideally, certified dealers from reliable manufacturers.
The current passing through the circuit breaker is determined by the well-known Ohm's law by the magnitude of the applied voltage divided by the resistance of the connected circuit. This theoretical position of electrical engineering is the basis for the operation of any machine.
In practice, the network voltage, for example, 220 volts, is maintained by automatic devices of the energy supply organization within the limits specified by state standards, and changes slightly within this range. If it goes beyond the GOST limits, it is considered a malfunction or an accident.
The circuit breaker cuts into the phase wire of the power supply to lamps, sockets and other consumers. When an electric razor and then a washing vacuum cleaner are powered from the outlet, in both cases current flows through the machine in a closed circuit between phase and zero.
But, in the first case it will be relatively small, and in the second - significant: these devices differ in resistance. They create different loads. Its value is constantly monitored by the machine’s protection, turning it off in case of deviations from the norm.
How does current flow through a circuit breaker?
Structurally, the machine is created in such a way that the current acts on sequentially located elements. These include:
wire connection terminals with clamping screws;
power contacts with moving and stationary parts;
bimetallic thermal release plate;
short circuit current cut-off electromagnet;
connecting conductors.
The current path through the circuit breaker is shown in the picture with red arrows.
The power movable contacts are pressed against the stationary ones, creating a continuous electrical circuit only after the operator turns the control lever manually. A mandatory condition for inclusion is the absence of emergency situations in the switched circuit. If they appear, the automatic shutdown protections immediately begin to work. There is no other way to turn on the machine.
But you can break these contacts, de-energizing the supply of phase potential to consumers, in two ways:
manually by returning the control lever to its original position;
automatically from protection activation.
How the structural elements of a circuit breaker are created and work
Power contacts
They, like the entire design of the circuit breaker, are designed to transmit strictly limited power. It cannot be exceeded, because otherwise the machine will fail and burn out.
The technical characteristic that limits the maximum power passing through the power contacts is an indicator called “Limiting breaking capacity”. It is designated by the index “Icu”.
The value of the maximum breaking capacity of a circuit breaker is set during its design from a standard range of currents, usually measured in kiloamperes. For example, Icu can be 4 or 6 or even 100 or more kA.
This value is indicated directly on the front side of the machine body, as well as other characteristics of the current value settings.
So, an electric current from zero to 4000 amperes can safely pass through the power contacts of the machine shown in the picture. The AV itself will normally withstand it and will turn it off if an emergency occurs within the connected electrical wiring with consumers.
For this purpose, a distinction has been made between currents flowing through power contacts into:
1. nominal and working;
2. emergency, including overload and short circuits.
What is the rated current of the circuit breaker
Any machine is created to operate under certain technical conditions. It must reliably ensure the passage of the operating load current flowing both through the electrical wiring and through the connected consumers.
When choosing a machine for a household network, users often take into account the conductive properties of the wiring or only the power of electrical appliances, making a mistake: it is necessary to comprehensively analyze both of these issues. Because a switch is an automatic device that is already configured to operate when certain current values are reached.
When these conditions have not yet occurred, and the operating current through the machine is less. than the lower shutdown limit, the power contacts are reliably closed. The upper limit of this operating range is usually called the rated current, denoted In.
The number “16” shown in the picture means that currents passing through the power contacts, up to and including 16 amperes, will be reliably transmitted by the circuit breaker to connected consumers through electrical wires.
How protections work
All currents greater than the rated value lead to protection activation. They are called operation currents and are designated Iav.
For automatic shutdown, two types of devices are mounted inside the machine body, operating according to different shutdown principles:
1. heating and bending the bimetal to disengage the mechanical latch;
2. knocking out the latch with a mechanical blow from the electromagnet core.
Thermal release
It works by bending a bimetallic composite plate when heated by a current passing through it, and is cooled by dissipating heat to the environment.
The thermal energy generated by the electric current through the passing bimetal is applied to this release. Its value, as we know from the Joule-Lenz law, depends on:
1. electrical resistance of the circuit;
2. the strength of the flowing current;
3. and the time of its exposure.
Of these three parameters, electrical resistance practically does not change in a steady process. It is taken into account only in theoretical calculations. When the load is switched, the current changes sharply. Therefore, two other parameters are more important:
1. magnitude of electric current;
2. the time it occurs.
To take into account its value with proper configuration and operation of the circuit breaker, special devices are used - resistance meters of this loop.
Their measurement makes it possible to take into account the correction introduced by the additional resistance of the wires, and therefore accurately take into account the currents passing in emergency mode through the power contacts and protection of the circuit breaker.
How a circuit breaker is tested for currents passing through it
After manufacturing in production until installation in an electrical circuit, products from any manufacturer can be transported over long distances or stored in warehouses for a long time. During this time, a decrease in its quality is possible due to violation of technical characteristics.
Therefore, when installed in a circuit, before putting it into operation, circuit breakers must be tested for serviceability, which is usually called loading.
To do this, a special circuit for loading the machine is assembled in the electrical laboratory or one of the many designs of stationary or portable stands is used.
The circuit breaker is tested according to the rated current indicated on the housing. It must withstand its value for a long time.
Then the machine is subjected to overloads and short-circuit currents, which it must withstand during operation. At the same time, the following are clearly measured and recorded:
1. currents of thermal release and current cutoff protection;
2. the time the machine is turned off from the moment of simulating an emergency situation.
Some designs of machines allow you to adjust the output parameters during loading. For example, certain types of thermal releases have a screw fastening, which allows you to adjust the response setting of the bimetallic plate within certain limits.
All measured characteristics are recorded with high accuracy by measuring instruments and entered into the inspection report and compared with GOST requirements. After their analysis, a certificate is issued with a conclusion on suitability.
Loading the machine under load allows you to identify defects and prevent possible fires and electrical injuries.
Thus, the currents passing through circuit breakers are taken into account during design, production, testing and operation. For this purpose, terms have been introduced that take into account the requirements of GOST:
rated current;
overload;
short circuit current;
protection operation current;
fault shutdown time.
In this article we will look at the main characteristics of circuit breakers that you need to know in order to correctly navigate when choosing them - these are rated current and time current characteristics of circuit breakers.
Let me remind you that this publication is part of a series of articles and videos devoted to electrical protection devices from the course
The main characteristics of the circuit breaker are indicated on its body, where the manufacturer's trademark or brand and catalog or serial number are also applied.
The most important characteristic of a circuit breaker is rated current. This is the maximum current (in Amperes) that can flow through the circuit breaker indefinitely without disconnecting the protected circuit. When the flowing current exceeds this value, the machine is triggered and opens the protected circuit.
A number of rated current values of circuit breakers are standardized and are:
6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100A.
The rated current of the machine is indicated on its body in amperes and corresponds to the ambient temperature of +30˚С. As the temperature increases, the rated current decreases.
When some consumers, for example, refrigerators, vacuum cleaners, compressors, etc., are connected to the electrical network, inrush currents briefly arise in the circuit, which can be several times higher than the rated current of the machine. For a cable, such short-term surges of current are not dangerous.
Therefore, so that the machine does not turn off every time with a small short-term increase in the current in the circuit, machines with different types of time-current characteristics are used.
Thus, the following main characteristic:
time-current characteristic of circuit breaker operation- this is the dependence of the shutdown time of the protected circuit on the strength of the current flowing through it. The current is indicated as a ratio to the rated current I/Inom, i.e. how many times the current flowing through the circuit breaker exceeds the rated current for a given circuit breaker.
The importance of this characteristic lies in the fact that machines with the same will turn off differently (depending on the type of time-current characteristic). This makes it possible to reduce the number of false alarms by using circuit breakers with different current characteristics for different types of load,
Let's consider the types of time-current characteristics:
— Type A(2-3 rated current values) are used to protect circuits with long electrical wiring and to protect semiconductor devices.
— Type B(3-5 rated current values) are used to protect circuits with a low inrush current multiplicity with a predominantly active load (incandescent lamps, heaters, furnaces, general-purpose lighting networks). Indicated for use in apartments and residential buildings, where the loads are mainly active.
— Type C(5-10 rated current values) are used to protect circuits of installations with moderate inrush currents - air conditioners, refrigerators, home and office socket groups, gas-discharge lamps with increased inrush current.
— Type D(10-20 rated current values) are used to protect circuits supplying electrical installations with high starting currents (compressors, lifting mechanisms, pumps, machine tools). They are installed mainly in industrial premises.
— Type K(8-12 rated current values) are used to protect circuits with inductive loads.
— Type Z(2.5-3.5 rated current values) are used to protect circuits with electronic devices sensitive to overcurrents.
In everyday life they are usually used with the characteristics B,C and very rarely D. The type of characteristic is indicated on the body of the machine with a Latin letter in front of the rated current value.
The marking “C16” on the circuit breaker will indicate that it has an instantaneous trip type C (that is, it trips at a current value of 5 to 10 values of the rated current) and a rated current of 16 A.
The time-current characteristic of a circuit breaker is usually given in the form of a graph. The horizontal axis indicates the multiple of the rated current value, and the vertical axis indicates the operation time of the machine.
The wide range of values on the graph is due to the spread of parameters of circuit breakers, which depend on temperature - both external and internal, since the circuit breaker is heated by the electric current passing through it, especially in emergency modes - by overload current or short circuit current (SC).
The graph shows that with a value of I/In≤1, the circuit breaker shutdown time tends to infinity. In other words, as long as the current flowing through the circuit breaker is less than or equal to the rated current, the circuit breaker will not trip (trip).
The graph also shows that the greater the I/In value (i.e., the more current flowing through the circuit breaker exceeds the rated current), the faster the circuit breaker will turn off.
When a current flows through the circuit breaker, the value of which is equal to the lower limit of the operating range of the electromagnetic release (3In for “B”, 5In for “C” and 10In for “D”), it must turn off in a time of more than 0.1 s.
When a current flows equal to the upper limit of the operating range of the electromagnetic release (5In for “B”, 10In for “C” and 20In for “D”), the circuit breaker will turn off in less than 0.1s. If the main circuit current is within the instantaneous trip current range, the circuit breaker trips with either little or no time delay (less than 0.1 s).
Circuit breakers
Definition: (This definition of a circuit breaker is in GOST R 50030.2.)
Automatic switch (abbreviated as switch or allegorically automatic) - mechanical switching device , allowing you to turn on, pass and turn off electric current under normal conditions; turn on and pass electrical energy for a specified period of time and disconnect a circuit under certain abnormal circuit conditions.
Purpose:
(functionality (application) of a circuit breaker)
- Protection function.
The circuit breaker serves to protect electrical networks and various energy consumers from short circuit currents (SC) and overload, as well as from unacceptable voltage drops.
To protect against short circuits, an electromagnetic release can be used (in switches from 630A and above, semiconductor or electronic releases).
For protection in the overload zone, a thermal release is used (for circuit breakers from 630A or more, semiconductor or electronic releases are used for this purpose).
To protect against unacceptable voltage drops, an undervoltage release or a zero-voltage release (optional) is activated.
2. Control function.
Automatic switches allow manual and automatic infrequent operational switching on/off of the circuit.
For manual switching, the manufacturer sets the number of operating cycles for a certain time. Often, the higher the amperage of the circuit breaker, the fewer switching operations are allowed.
To remotely control the network, the switch must be equipped with an independent release (an additional custom option) or an electromagnetic drive.
3. Role of circuit breaker.
Reusable object protection. The fuse provides only one-time protection (afterwards you need to change the melted insert or replace the fuse itself). Although the switch implies control of the circuit, a switch is usually installed for these purposes. The contact group of the switch tolerates a limited number of switchings, and the cost of the switch is much lower.
Regulatory documents (GOST) according to which low-voltage circuit breakers are produced
We list the standards that regulate the production and testing of low-voltage circuit breakers:
GOST R 50345-99 - standard for household circuit breakers (original translation of the international standard IEC 60898).
GOST R 50030.2-99 - standard for industrial circuit breakers (original Russian translation of IEC 60947.2).
GOST 9098-78 is a regulatory document for low-voltage air circuit breakers (the current Union standard).
IEC (English abbreviation IEK) - International Electrotechnical Commission.
Classification of circuit breakers:
Circuit breakers can be classified according to the following characteristics (not a complete list of parameters is given, but a partial one):
1. By application category: A and B.
A - non-selective circuit breakers (operation at short-circuit currents occurs without a time delay);
B - selective circuit breakers (in short-circuit conditions a short-term specified time delay is provided).
Selectivity:
Overcurrent selectivity is when, when connecting two circuit breakers in series, designed to protect against short circuits, the load-side circuit breaker disconnects the contacts without tripping the second circuit breaker.
The meaning of selectivity:
Switches with rated currents from 1000A are selective; they are installed in front of an industrial complex, they protect further branching circuits and energy consumers. Let's assume that a short circuit occurs in one of the branches of the circuit; when a switch of 1000 A or higher is automatically activated, the entire object will be completely cut off. To prevent this, this machine is given selectivity, that is, a certain period of time is set after which (just in case) it will work. And during this time, a circuit breaker with a lower amperage is triggered, which will turn off the specific branch with the resulting short circuit. In this case, the industrial facility operates without one branch, the selectivity switch does not operate.
2. By type of current: direct current, alternating current; for alternating and direct currents.
An example of an AC circuit breaker: AE 2056 circuit breaker.
An example of a circuit breaker for alternating and direct currents: circuit breaker BA 04 36.
Range (line) of rated currents for low-voltage circuit breakers (numbers - switch amperage):
1.6A; 2.5A; 4A; 6.3A; 10A; 16A; 25A; 31.5A; 40A; 50A; 63A
80A; 100A; 125A; 160A; 200A; 250A; 320A; 400A; 500A; 630A; 800A
1000A; 1600A; 2000A; 2500A; 4000A; 5000A; 6300A
Switches up to 63A can be installed in apartment panels and floor panels.
The following chain of amperages (80 - 800 amperes) is typical for switches used in industry in input distribution devices.
Next come the amperages of circuit breakers (over 1000A), which are installed in front of large industrial facilities; they often have selectivity (input circuit breakers).
3. According to the medium in which the shutdown occurs: air, vacuum, gas.
All circuit breakers of the BA series are air operated
4. By the number of poles: single-pole, two-pole, three-pole and four-pole.
5. According to the presence of current limitation: current-limiting and non-current-limiting.
Current-limiting circuit breakers (also non-selective) are:
— fast-acting (response time does not exceed 0.005 s);
— normal (switch-off time in the range from 0.02 to 0.1 s).
Non-current-limiting circuit breakers (also selective) allow you to adjust the time before the contacts disengage (no more than 1 second).
6. By types of releases: with maximum current release (MCR), with independent release (NR), with minimum voltage release (MRN) and with zero voltage release (NVR).
7. By type of drive: with a manual drive, with a motor (electromagnetic) drive, with a spring drive.
8. By installation method: stationary, retractable, plug-in.
9. According to the degree of protection from water (moisture, suspended water dust) and solid objects (tools, fingers, probes, nails, etc.) provided by the shell (circuit breaker housing) according to GOST 14254-96.
Design (principle of operation) of a circuit breaker
The circuit breaker is assembled from several components: the machine body, switching device, control mechanism, arc chutes, overcurrent releases and additional assembly units (shunt release, auxiliary contacts, undervoltage release, zero voltage release).
The body of the machine is made of dielectric material and guarantees a specified degree of protection from atmospheric influences and contact with live or mechanical parts of solid foreign bodies.
The switching device consists of moving and fixed contacts that engage (switch operation) and disengage (automatic or manual shutdown). A circuit breaker pole is made up of a pair of contacts, the number of poles can vary from one to four, and an arc chute is mounted in each pole.
Nowadays, contacts at the adhesion point are often made of silver-based cermets. The use of silver is due to its high electrical conductivity and lack of oxidation under normal conditions.
The control mechanism is an independent manual drive, which guarantees instant closing and opening of the main contacts. The control element is a handle or button.
The arc chute must ensure arc extinguishing under various network modes.
There are two types of arc extinguishing devices used in circuit breakers: semi-closed and open.
In the semi-closed version, the machine is closed with a casing in which slots are made to allow hot gases to escape. The emission zone of ionized gases reaches a length of only a few centimeters from the outlet openings. This solution is used for low-voltage equipment that is mounted with other devices, in distribution boards, and at manual circuit breakers.
At currents of 100 kA and above, open-type chambers with a large emission zone are used.
In circuit breakers, a deionic arc-extinguishing grid consisting of metal plates is widely used. In AC circuits with voltages up to 690 V, such devices are capable of extinguishing an arc with a current of up to 50 kA. In DC circuits with voltages up to 440 V, arc chutes made of steel plates successfully extinguish arcs with currents up to 55 kA. Extinguishing the arc occurs quite calmly with minimal release of heated ionized gases.
Overcurrent releases (MRT). A circuit breaker often uses a combined release - electromagnetic (instantaneous) and thermal releases.
The principle of operation of an electromagnetic release is that a load current is supplied to a coil with a winding made of copper wire. During normal operation, the current does not cause the core to move; but when short-circuit currents are high, the core is pulled or pushed out of the coil and acts on the tripping mechanism.
The thermal release is a bimetallic plate, which is made of two pressed metals with different linear expansions. When current is passed through the plate, it heats up and bends. When an overload occurs (currents exceeding the nominal value by 1.1 times or higher), the plate heats up sufficiently and acts on the release mechanism. The heating process can last from several minutes to an hour - the time after which the contacts open.
Manufacturers (factories) of circuit breakers
A few examples:
- "Kursk Electrical Equipment Plant" TM "KEAZ" (Kursk, Russia);
- company "IEK" (InterEnergoKomplekt in the past);
- electrical engineering company "EKF electrotechnica";
- Ulyanovsk plant of low-voltage equipment "Kontactor",
- "Divnogorsk plant of low-voltage circuit breakers" (TM "DZNVA"),
- international corporation "General Electric" (General Electric, a division of GE Consumer & Industrial Power Protection).
To select a circuit breaker based on load power, it is necessary to calculate the load current and select the rating of the circuit breaker greater than or equal to the obtained value. The current value expressed in amperes in a single-phase 220 V network usually exceeds the load power value expressed in kilowatts by 5 times, i.e. if the power of the electrical receiver (washing machine, light bulb, refrigerator) is 1.2 kW, then the current that will flow in the wire or cable is 6.0 A (1.2 kW * 5 = 6.0 A). Calculated at 380 V, in three-phase networks, everything is similar, only the current value exceeds the load power by 2 times.
Power factor
This is a dimensionless physical quantity that characterizes the consumer of alternating electric current from the point of view of the presence of a reactive component in the load. Power factor shows how much the alternating current flowing through a load is out of phase relative to the voltage applied to it.
Numerically, the power factor is equal to cosine of this phase shift or cos φ
We take cosine phi from Table 6.12 of the regulatory document SP 31-110-2003 “Design and installation of electrical installations of residential and public buildings”
Table 1. Cos φ value depending on the type of power receiver
Let's take our power receiver with a power of 1.2 kW. as a household single-phase refrigerator at 220V, we take cos φ from the table as 0.75 as a motor from 1 to 4 kW.
Let's calculate the current I=1200 W / 220V * 0.75 = 4.09 A.
Now the most correct way to determine the current of an electrical receiver— take the current value from the nameplate, passport or operating instructions. There are rating plates on almost all electrical appliances.
EKF circuit breakers
The total current in the line (for example, a socket network) is determined by summing the current of all electrical receivers. Based on the calculated current, we select the closest rating of the automatic circuit breaker, upwards. In our example, for a current of 4.09A it will be a 6A machine.
It is very important to note that choosing a circuit breaker based only on the load power is a gross violation of fire safety requirements and can lead to fire of the cable or wire insulation and, as a result, a fire. When choosing, it is also necessary to take into account the cross-section of the wire or cable.
Based on the load power, it is more correct to choose the cross-section of the conductor. The selection requirements are set out in the main regulatory document for electricians called PUE (Electrical Installation Rules), or more precisely in Chapter 1.3. In our case, for a home electrical network, it is enough to calculate the load current as indicated above, and in the table below select the cross-section of the conductor, provided that the resulting value is lower than the long-term permissible current corresponding to its cross-section.
Selecting a machine according to cable cross-section
Let us consider the problem of choosing circuit breakers for home electrical wiring in more detail, taking into account fire safety requirements. The necessary requirements are set out in Chapter 3.1 “Protection of electrical networks up to 1 kV,” since the network voltage in private houses, apartments, and cottages is 220 or 380V.
Calculation of the cross-section of cable and wire cores Voltage 220V.
– single-phase network is used mainly for sockets and lighting.
380V. - These are mainly distribution networks - power lines running along the streets, from which houses are connected by branches.
According to the requirements of the above chapter, internal networks of residential and public buildings must be protected from short-circuit currents and overloads. To fulfill these requirements, protection devices called circuit breakers were invented.
Automatic circuit breaker
This is a mechanical switching device capable of turning on, carrying currents in the normal state of the circuit, and also turning on, conducting for a specified time and automatically turning off currents in the specified abnormal state of the circuit, such as short circuit and overload currents.
Short circuit (SC)
an electrical connection of two points of an electrical circuit with different potential values, not provided for by the design of the device and disrupting its normal operation. A short circuit can occur as a result of a violation of the insulation of current-carrying elements or mechanical contact of non-insulated elements. Also, a short circuit is a condition when the load resistance is less than the internal resistance of the power source.
Overload current
– exceeding the rated value of the continuous permissible current and causing overheating of the conductor. Protection against short-circuit currents and overheating is necessary for fire safety, to prevent the fire of wires and cables, and as a result of a fire in the house.
Continuous permissible current of cable or wire
- the amount of current that constantly flows through the conductor and does not cause excessive heating.
The value of continuous permissible current for conductors of different sections and materials is presented below. The table is a combined and simplified version applicable for household power supply networks, tables No. 1.3.6 and 1.3.7 PUE.
Selecting a circuit breaker based on short-circuit current
The selection of a circuit breaker for protection against short circuit (short circuit) is carried out based on the calculated value of the short circuit current at the end of the line. The calculation is relatively complicated, the value depends on the power of the transformer substation, the cross-section of the conductor and the length of the conductor, etc.
From the experience of carrying out calculations and designing electrical networks, the most influential parameter is the length of the line, in our case the length of the cable from the panel to the outlet or chandelier.
Because in apartments and private houses this length is minimal, then such calculations are usually neglected and circuit breakers with characteristic “C” are selected; you can, of course, use “B”, but only for lighting inside an apartment or house, because such low-power lamps do not cause a high inrush current, and already in the network for kitchen appliances with electric motors, the use of machines with characteristic B is not recommended, because the machine may be triggered when the refrigerator or blender is turned on due to a jump in the starting current.
Selecting a machine based on the long-term permissible current (LTC) of the conductor
The selection of a circuit breaker for protection against overload or overheating of a conductor is carried out based on the DDT value for the protected section of the wire or cable. The rating of the machine must be less than or equal to the DDT value of the conductor indicated in the table above. This ensures automatic shutdown of the machine when the DDT in the network is exceeded, i.e. part of the wiring from the machine to the last electrical receiver is protected from overheating, and as a result, from fire.
Example of circuit breaker selection
We have a group from the panel to which we plan to connect a dishwasher -1.6 kW, a coffee maker - 0.6 kW and an electric kettle - 2.0 kW.
We count the total load and calculate the current.
Load = 0.6+1.6+2.0=4.2 kW. Current = 4.2*5=21A.
We look at the table above; all conductor cross-sections except 1.5 mm2 for copper and 1.5 and 2.5 for aluminum are suitable for the current we have calculated.
We choose a copper cable with cores with a cross section of 2.5 mm2, because... It makes no sense to buy a cable with a larger cross-section for copper, and aluminum conductors are not recommended for use, and may already be prohibited.
We look at the scale of denominations of the machines produced - 0.5; 1.6; 2.5; 1; 2; 3; 4; 5; 6; 8; 10; 13; 16; 20; 25; 32; 40; 50; 63.
The circuit breaker for our network is suitable for 25A, since it is not suitable for 16A because the calculated current (21A) exceeds the rating of the 16A circuit breaker, which will cause it to trip when all three power receivers are turned on at once. A 32A machine is not suitable because it exceeds the DDT of the 25A cable we selected, which can cause overheating of the conductor and, as a result, a fire.
Summary table for selecting a circuit breaker for a single-phase 220 V network.
| Rated current of the circuit breaker, A. | Power, kWt. | Current, 1 phase, 220V. | Cross-section of cable cores, mm2. |
| 16 | 0-2,8 | 0-15,0 | 1,5 |
| 25 | 2,9-4,5 | 15,5-24,1 | 2,5 |
| 32 | 4,6-5,8 | 24,6-31,0 | 4 |
| 40 | 5,9-7,3 | 31,6-39,0 | 6 |
| 50 | 7,4-9,1 | 39,6-48,7 | 10 |
| 63 | 9,2-11,4 | 49,2-61,0 | 16 |
| 80 | 11,5-14,6 | 61,5-78,1 | 25 |
| 100 | 14,7-18,0 | 78,6-96,3 | 35 |
| 125 | 18,1-22,5 | 96,8-120,3 | 50 |
| 160 | 22,6-28,5 | 120,9-152,4 | 70 |
| 200 | 28,6-35,1 | 152,9-187,7 | 95 |
| 250 | 36,1-45,1 | 193,0-241,2 | 120 |
| 315 | 46,1-55,1 | 246,5-294,7 | 185 |
Summary table for selecting a circuit breaker for a three-phase 380 V network.
| Rated current automatic switch, A. | Power, kWt. | Current, 1 phase 220V. | Core cross-section cable, mm2. |
| 16 | 0-7,9 | 0-15 | 1,5 |
| 25 | 8,3-12,7 | 15,8-24,1 | 2,5 |
| 32 | 13,1-16,3 | 24,9-31,0 | 4 |
| 40 | 16,7-20,3 | 31,8-38,6 | 6 |
| 50 | 20,7-25,5 | 39,4-48,5 | 10 |
| 63 | 25,9-32,3 | 49,2-61,4 | 16 |
| 80 | 32,7-40,3 | 62,2-76,6 | 25 |
| 100 | 40,7-50,3 | 77,4-95,6 | 35 |
| 125 | 50,7-64,7 | 96,4-123,0 | 50 |
| 160 | 65,1-81,1 | 123,8-124,2 | 70 |
| 200 | 81,5-102,7 | 155,0-195,3 | 95 |
| 250 | 103,1-127,9 | 196,0-243,2 | 120 |
| 315 | 128,3-163,1 | 244,0-310,1 | 185 |
| 400 | 163,5-207,1 | 310,9-393,8 | 2x95* |
| 500 | 207,5-259,1 | 394,5-492,7 | 2x120* |
| 630 | 260,1-327,1 | 494,6-622,0 | 2x185* |
| 800 | 328,1-416,1 | 623,9-791,2 | 3x150* |
* - double cable, two cables connected in parallel, for example 2 cables VVGng 5x120
Results
When choosing a machine, it is necessary to take into account not only the load power, but also the cross-section and material of the conductor.
For networks with small protected areas from short-circuit currents, circuit breakers with characteristic “C” can be used
The rating of the machine must be less than or equal to the long-term permissible current of the conductor.
