Surely many of us have wondered why circuit breakers so quickly replaced outdated fuses from electrical circuits? The activity of their implementation is justified by a number of very convincing arguments, including the opportunity to buy this type of protection, which ideally matches the time-current data of specific types of electrical equipment.
Do you doubt which machine you need and don’t know how to choose it correctly? We will help you find the right solution - the article discusses the classification of these devices. As well as important characteristics that you should pay close attention to when choosing a circuit breaker.
To make it easier for you to understand the machines, the article’s material is supplemented with visual photos and useful video recommendations from experts.
The machine almost instantly disconnects the line entrusted to it, which eliminates damage to the wiring and equipment powered from the network. After the shutdown has been completed, the branch can be restarted immediately without replacing the safety device.
If you have knowledge or experience performing electrical work, please share it with our readers. Leave your comments about choosing a circuit breaker and the nuances of installing it in the comments below.
The table shows that at currents up to 1.13*Iн the machine will not work. If an overload of the circuit occurs 13% more than the rated current (1.13 * In), the circuit breaker will turn off no earlier than in an hour, and if there is an overload of up to 45% (1.45 In), the thermal release of the machine must operate within one hour ( i.e. it can work in an hour). Thus, in the current range of 1.13-1.45 from the rated current In, the thermal release of the machine will operate in a time from several minutes to several hours. From all this it follows that when choosing a circuit breaker, it is worth considering not only its rated current, but also the value of the thermal release setting, which should not exceed the long-term permissible current for the protected line.
What happens if you don’t take into account the thermal release setting when choosing a machine? For convenience, let's look at an example:
Let's take the most common rating of the machine - 16 A, the overload current at which the machine will operate within an hour will be equal to 16 * 1.45 = 23.2 A (a table was presented above, from which it can be seen that the value of the thermal release setting is 1.45 rated current). Accordingly, it is for this current that it is worth selecting the cable cross-section. From table 1.3.4. we select a suitable cross-section: for hidden electrical wiring made of copper - this is at least 2.5 mm 2 (maximum overload current 27 A).
In a similar way, you can carry out calculations for a 10 A machine. The current at which the machine will turn off within an hour will be equal to 10·1.45 = 14.5A. According to the table, this current corresponds to a cable with a cross section of 1.5 mm 2.
Very often, installers neglect this rule and, to protect a line with a cross-section of 2.5 mm2, install a circuit breaker with a rating of 25 A (after all, the line can withstand a current of 25 A for a long time). But they forget that the unswitched current of such a machine is 25 * 1.13 = 28.25 A, and this is already more than the long-term permissible overload current. The current at which the machine will turn off within an hour will be 25*1.45=36.25 A!!! With such a current and for such a time, the cable will overheat and burn out.
Also, do not forget that the majority of the cable market consists of cables manufactured not according to GOST, but according to specifications. It follows from this that their actual cross-section will be underestimated. By purchasing a cable manufactured according to specifications, instead of a cable with a core cross-section of 2.5 mm 2, you can get a cable with an actual core cross-section of less than 2.0 mm 2!
Here is an example of what can happen if the rules for choosing the cross-section of the cable and machine are neglected:
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Table for selecting machines by power
An extended table for selecting circuit breakers by power, including three-phase star and delta connections, allows you to select a circuit breaker that matches the power consumption. To work with the table, that is, to select a machine corresponding to the power, it is enough to know this power, select a value in the table greater than or equal to this power value. In the leftmost column you will see the rated current of the machine corresponding to the selected power.
Next, above the selected power, you will see the type of connection of the machine, the number of poles and the voltage used. If the selected power corresponds to several power values in the table for example, a power of 6.5 kW can be obtained by connecting a single-phase 32A machine, connecting a three-pole 6A machine with a three-phase trigon and connecting a four-pole 10A machine with a three-phase star, you should select the connection method available to you. That is, when choosing a machine for a power of 6.5 kW in the absence of a three-phase power supply, you need to choose only from a single-phase connection, where a single-pole and two-pole 32A machine will be available. Following the link in the table for a specific power corresponding to the connection capabilities is carried out to a circuit breaker corresponding in rated current and number of poles with time current characteristic C. In the event that a different cut-off characteristic is needed, you can select a machine with a different characteristic, links to which are located on the page of each machine.
Selection of machines by power and connection
| Connection type => | Single phase introductory |
Three-phase triangle |
Three-phase star |
||
| Machine polarity => | Single pole machine |
Bipolar machine |
Three-pole machine |
Four-pole machine |
|
| Supply voltage => | 220 volt | 220 volt | 380 Volt | 220 volt | |
| V | V | V | V | ||
| Automatic 1A > | 0.2 kW | 0.2 kW | 1.1 kW | 0.7 kW | |
| 0.4 kW | 0.4 kW | 2.3 kW | 1.3 kW | ||
| Automatic 3A > | 0.7 kW | 0.7 kW | 3.4 kW | 2.0 kW | |
| Automatic 6A > | 1.3 kW | 1.3 kW | 6.8 kW | 4.0 kW | |
| Automatic 10A > | 2.2 kW | 2.2 kW | 11.4 kW | 6.6 kW | |
| Automatic 16A > | 3.5 kW | 3.5 kW | 18.2 kW | 10.6 kW | |
| Automatic 20A > | 4.4 kW | 4.4 kW | 22.8 kW | 13.2 kW | |
| Automatic 25A > | 5.5 kW | 5.5 kW | 28.5 kW | 16.5 kW | |
| Automatic 32A > | 7.0 kW | 7.0 kW | 36.5 kW | 21.1 kW | |
| Automatic 40A > | 8.8 kW | 8.8 kW | 45.6 kW | 26.4 kW | |
| Automatic 50A > | 11 kW | 11 kW | 57 kW | 33 kW | |
| Automatic 63A > | 13.9 kW | 13.9 kW | 71.8 kW | 41.6 kW | |
An example of selecting a machine based on power
One of the ways to select a circuit breaker is to select the circuit breaker based on load power. The first step, when choosing a machine based on power, the total power of loads connected on a permanent basis to the automatically protected wiring/network is determined. The resulting total power is increased by the consumption coefficient, which determines the possible temporary excess of power consumption due to the connection of other, initially unaccounted for electrical appliances.
As an example, we can cite kitchen electrical wiring designed to connect an electric kettle (1.5 kW), microwave (1 kW), refrigerator (500 Watt) and extractor hood (100 Watt). The total power consumption will be 3.1 kW. To protect such a circuit, you can use a 16A circuit breaker with a rated power of 3.5 kW. Now imagine that a coffee machine (1.5 kW) was installed in the kitchen and connected to the same electrical wiring.
The total power removed from the wiring when connecting all the specified electrical appliances in this case will be 4.6 kW, which is more than the power of a 16 Amp circuit breaker, which, when all devices are turned on, will simply turn off due to excess power and leave all devices without power, including the refrigerator. To reduce the likelihood of such situations occurring, an increasing consumption factor is used. In our case, when connecting a coffee machine, the power increased by 1.5 kW, and the consumption coefficient became 1.48 (rounded to 1.5). That is, to be able to connect an additional device with a power of 1.5 kW, the calculated power of the network must be multiplied by a factor of 1.5, resulting in 4.65 kW of power that can be obtained from the wiring.
At choosing a machine based on power It is also possible to use a reducing consumption factor. This coefficient determines the difference in power consumption, in the direction of reduction, from the total calculated due to the non-use of all electrical appliances included in the calculation at the same time. In the previously discussed example of kitchen wiring with a power of 3.1 kW, the reduction factor will be equal to 1, since the kettle, microwave, refrigerator and hood can be turned on simultaneously, and in the case of considering wiring with a power of 4.6 kW (including a coffee machine), the reduction factor can be equal to 0.67 if it is impossible to turn on the electric kettle and coffee machine at the same time (for example, there is only one socket for both devices and there are no tees in the house)
Thus, in the first step, the calculated power of the protected wiring is determined, and the increasing (increase in power when connecting new electrical appliances) and decreasing (impossibility of simultaneously connecting some electrical appliances) coefficients are determined.
When choosing a machine, it is preferable to use the power obtained by multiplying the increasing factor by the calculated power, while naturally taking into account the capabilities of the electrical wiring (the cross-section of the wire must be sufficient to transmit such power).
Rated power of the machine
The rated power of the machine, that is, the power whose consumption in the wiring protected by the circuit breaker will not lead to the machine being turned off, is calculated in the general case using the formula, which can be described by the phrase => “Power = Voltage times Current times cosine Phi”, where voltage is alternating voltage of the electrical network in Volts, current strength is the current flowing through the machine in Amperes and cosine phi is the value of the trigonometric function Cosine for the angle phi (angle phi is the shift angle between the phases of voltage and current). Since in most cases the choice of a machine based on power is made for domestic use, where there is practically no shift between the phases of current and voltage caused by reactive loads such as electric motors, the cosine is close to 1 and the power can be approximately calculated as voltage multiplied by current.
Since the power has already been determined, from the formula we obtain the current, namely the current that corresponds to the calculated power by dividing the power in Watts by the network voltage, that is, by 220 Volts.
In our example with a power of 3.1 kW (3100 Watt), the current obtained is 14 Amperes (3100 Watt/220 Volt = 14.09 Ampere). This means that when connecting all of the specified devices with a total power of 3.1 kW, a current approximately equal to 14 Amperes will flow through the circuit breaker.
After determining the current strength by power consumption, the next step in choosing a circuit breaker is selecting a circuit breaker by current
To select a machine based on the power of a three-phase load, the same formula is used, taking into account the fact that the shift between the phases of voltage and current in a three-phase load can reach large values and, accordingly, it is necessary to take into account the cosine value. In a large number of cases, a three-phase load is marked indicating the value of the cosine of the phase shift, for example, on the marking plate of an electric motor you can see which is the one involved in the calculation of the cosine of the phase shift angle. Accordingly, when calculating a three-phase load, the power indicated on the nameplate of the connected three-phase, 380 Volt, electric motor is 7 kW, the current is calculated as 7000/380/0.6 = 30.07
The resulting current is the sum of the currents in all three phases, that is, one phase (per pole of the machine) accounts for 30.07/3~10 Amperes, which corresponds to the choice of a three-pole machine D10 3P. Characteristic D in this example was chosen due to the fact that when starting the electric motor, while the motor rotor is spinning, the currents significantly exceed the rated values, which can lead to the switching off of the circuit breaker with characteristic B and characteristic C.
Maximum circuit breaker power
The maximum power of the circuit breaker, that is, the power and, accordingly, the current that the circuit breaker can pass through itself and not turn off, depends on the ratio of the current flowing through the circuit breaker and the rated current of the circuit breaker, specified in the technical data of the circuit breaker. This ratio can be called reduced current, which is a dimensionless coefficient that is no longer related to the rated current of the machine. The maximum power of the circuit breaker depends on the time-current characteristics, the reduced current and the duration of the reduced current flowing through the circuit breaker, which is described in the section Time-current characteristics of circuit breakers.
Maximum short-term power of the machine
The maximum short-term power of the machine can be several times higher than the rated power, but only for a short time. The magnitude of the excess and the time that the circuit breaker will not turn off the load in case of such an excess are described by characteristics (operation curves) designated by a Latin letter, or indicated in the marking of the circuit breaker by a number indicating the rated current of the circuit breaker.
Not a single electrical device, not a single electrical appliance, should be used without automatic protective equipment. An automatic circuit breaker (AB) is installed for a specific device, or for a group of consumers connected to the same line. In order to correctly answer the question of what power corresponds, for example, to a machine with a rating of 25A, you should first become familiar with the design of the circuit breaker and the types of protective devices.
Structurally, AB combines mechanical, thermal and electromagnetic releases that operate independently of each other.
Mechanical release
Designed to turn the machine on/off manually. Allows you to use it as a switching device. Used during repair work to de-energize the network.
Thermal release (TR)
This part of the circuit breaker protects the circuit from overload. Current passes through the bimetallic strip, heating it. Thermal protection is inertial, and can briefly pass currents exceeding the operation threshold (In). If the current exceeds the rated current for a long time, the plate heats up so much that it deforms and turns off the AV. After the bimetallic plate has cooled (and the cause of the overload has been eliminated), the machine is turned on manually. In a 25A machine, the number 25 indicates the TP response threshold.
Electromagnetic release (ER)
Breaks the electrical circuit during a short circuit. The overcurrents generated during a short circuit require an instant response from the protective device, therefore, unlike a thermal release, an electromagnetic release is triggered instantly, in a fraction of a second. Switching off occurs due to the passage of current through the winding of a solenoid with a movable steel core. The solenoid, when activated, overcomes the resistance of the spring and turns off the moving contact of the circuit breaker. To disconnect due to a short circuit, currents exceeding In from three to fifty times are required, depending on the type of circuit breaker.
Types of AV according to current-time characteristics
Let's ignore industrial electronics and motor protection devices with built-in thermal relays, and consider the most common types of circuit breakers:
- Characteristic B - when In is three times higher, the TR is triggered in 4-5 s. ER triggers when In is exceeded three to five times. They are used in lighting networks or when connecting a large number of low-power consumers.
- Characteristic C is the most common type of AB. TR is triggered in 1.5 s when In is five times exceeded, ER is triggered when In is 5-10 times exceeded. They are used for mixed networks that include devices of various types, including those with low inrush currents. The main type of circuit breakers for residential and administrative buildings.
- Characteristic D - machines with the highest overload capacity. Used to protect electric motors and energy consumers with high starting currents.
The ratio of AV ratings and consumer power
To determine how many kilowatts can be connected through a circuit breaker of a certain power, use the table:
| automatic 220v, A | power, kWt | |
|---|---|---|
| single-phase | three-phase | |
| 2 | 0,4 | 1,3 |
| 6 | 1,3 | 3,9 |
| 10 | 2,2 | 6,6 |
| 16 | 3,5 | 10,5 |
| 20 | 4,4 | 13,2 |
| 25 | 5,5 | 16,4 |
| 32 | 7,0 | 21,1 |
| 40 | 8,8 | 26,3 |
| 50 | 11,0 | 32,9 |
| 63 | 13,9 | 41,4 |
To calculate the power of the introductory machine at home, use a coefficient of 0.7 of the total power of consumers.
When determining the load capacity of a circuit breaker, it is important to take into account not only its rating, but also the overload characteristic. This will help avoid false alarms when starting up powerful electrical appliances.
When designing the electrical network of a new home, to connect new powerful devices, in the process of modernizing the electrical panel, it is necessary to select a circuit breaker for reliable electrical safety.
Some users are careless about this task, and can without hesitation connect any available machine, as long as it works, or when choosing, they are guided by the following criteria: cheaper, so that it won’t cost too much, or more powerful, so that it won’t break the bank again.
Very often, such negligence and ignorance of the basic rules for choosing the rating of a safety device leads to fatal consequences. This article will introduce the basic criteria for protecting electrical wiring from overload and short circuit, in order to be able to correctly select a circuit breaker according to the power consumption of electricity.
Briefly the principle of operation and purpose of circuit breakers
In the event of a short circuit, the circuit breaker is triggered almost instantly thanks to the electromagnetic splitter. At a certain excess of the rated current value, the heating bimetallic plate will turn off the voltage after some time, which can be found out from the current characteristic time graph.
This safety device protects the wiring from short circuits and overcurrents exceeding the calculated value for a given wire cross-section, which can heat the conductors to the melting point and cause the insulation to ignite. To prevent this from happening, you need not only to choose the right protective switch that matches the power of the connected devices, but also to check whether the existing network can withstand such loads.
Appearance of a three-pole circuit breaker
Wires must match the load
It often happens that in an old house a new electric meter, automatic machines, and RCDs are installed, but the wiring remains old. A lot of household appliances are bought, the power is summed up and an automatic machine is selected for it, which regularly holds the load of all switched on electrical appliances.
Everything seems to be correct, but suddenly the wire insulation begins to emit a characteristic odor and smoke, a flame appears, and the protection does not work. This can happen if the wiring parameters are not designed for.
Let's say the cross-section of the old cable core is 1.5mm², with a maximum permissible current limit of 19A. We assume that several electrical appliances were connected to it at the same time, making up a total load of 5 kW, which in current equivalent is approximately 22.7 A; it corresponds to a 25 A circuit breaker.
The wire will heat up, but this machine will remain on all the time until the insulation melts, which will lead to a short circuit, and the fire can already flare up in full swing.
Protect the weakest link in the electrical wiring
Therefore, before choosing a machine according to the load being protected, you need to make sure that the wiring will withstand this load.
According to PUE 3.1.4, the machine must protect the weakest section of the electrical circuit from overloads, or be selected with a rated current corresponding to the currents of the connected electrical installations, which again implies their connection with conductors with the required cross-section.
If you ignore this rule, you should not blame an incorrectly designed machine and curse its manufacturer if a weak link in the electrical wiring causes a fire.
Melted wire insulation
Calculation of the machine's nominal value
We assume that the wiring is new, reliable, correctly calculated, and meets all requirements. In this case, the selection of a circuit breaker comes down to determining a suitable rating from a typical range of values, based on the calculated load current, which is calculated by the formula:
where P is the total power of electrical appliances.
This means active load (lighting, electric heating elements, household appliances). This calculation is completely suitable for a home electrical network in an apartment.
Let's say the power calculation is made: P = 7.2 kW. I=P/U=7200/220=32.72 A. Select a suitable 32A machine from a range of values: 1, 2, 3, 6, 10, 16, 20, 25, 32, 40, 63, 80, 100.
This rating is slightly less than the calculated value, but it is practically impossible for all electrical appliances in the apartment to be turned on at the same time. It is also worth considering that in practice, the operation of the machine begins with a value 1.13 times greater than the nominal value, due to its time-current characteristics, that is, 32 * 1.13 = 36.16 A.
To simplify the selection of a circuit breaker, there is a table where the ratings of the circuit breakers correspond to the power of single-phase and three-phase loads:
Current circuit breaker selection table
The denomination found using the formula in the above example is closest in terms of power value, which is indicated in the red highlighted cell. Also, if you want to calculate the current for a three-phase network when choosing a machine, read the article about
The selection of circuit breakers for electrical installations (electric motors, transformers) with reactive loads, as a rule, is not made based on power. The rating and type are selected according to the operating and starting current specified in the passport of this device.
Long gone are the days of ceramic plugs that were screwed into home electrical panels. Currently, various types of circuit breakers that perform protective functions are widely used. These devices are very effective against short circuits and overloads. Many consumers have not yet fully mastered these devices, so the question often arises which machine should be installed at 15 kW. The reliable and durable operation of electrical networks, appliances and equipment in a house or apartment completely depends on the choice of machine.
Basic functions of machines
Before choosing an automatic protective device, you need to understand the principles of its operation and capabilities. Many consider the main function of the machine to be the protection of household appliances. However, this judgment is absolutely wrong. The machine does not react in any way to devices connected to the network; it is triggered only during short circuits or overloads. These critical conditions lead to a sharp increase in current strength, causing overheating and even fire of cables.
A special increase in current strength is observed during a short circuit. At this moment, its value increases to several thousand and the cables are simply not able to withstand such a load, especially if its cross-section is 2.5 mm2. With such a cross-section, an instant fire occurs in the wire.
Therefore, a lot depends on the correct choice of machine. Accurate calculations, including calculations, make it possible to reliably protect the electrical network.
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Types of slot machines
Classification of circuit breakers occurs according to the following parameters:
- number of poles;
- rated and limit currents;
- the type of electromagnetic release used;
- maximum power switching capacity.
Let's look at it in order.
Number of poles
The number of poles is the number of phases that the machine is capable of protecting. Depending on the number of poles, machines can be:
Rated and limiting currents
Everything is simple here - such a current strength at which the machine will open the circuit. At the rated current and even a little more than stated, work will be carried out, but only when the limit current is exceeded by 10–15% will a shutdown occur. This is due to the fact that quite often the starting currents exceed the maximum possible currents for a short period of time, so the machine has a certain reserve of time, after which the circuit will open.
Type of electromagnetic release
This is a part of the machine that allows you to open the circuit in the event of a short circuit, as well as in the event of an increase in current (overload) by a certain number of times. Releases are divided into several categories, let's look at the most popular ones:
- B - opening when the rated current is exceeded by 3–5 times;
- C - when exceeded by 5–10 times;
- D - when exceeded by 10–20 times.
Maximum power switching capacity. This is the value of the short circuit current (determined in thousands of amperes) at which the machine will remain operational after the circuit opens due to a short circuit.
Selection of the optimal cable cross-section
Each cable, like a machine, has a certain permitted load current. Depending on the cross-section and material of the cable, the load current also varies. To select a machine according to cable cross-section, use the table.
It should be noted that it is permissible to choose a cable with a small margin, but not a packet switch! The machine must match the planned load! In accordance with the rules for electrical installations 3.1.4, the setting currents of the circuit breakers should be selected those that will be less than the calculated currents of the selected zones.
Let's look at an example: in a certain area, the electrical wiring is laid with a cable with a cross-section of 2.5 mm square, and the load is 12 kW, in this case, when installing a machine (at a minimum current) of 50 A, the wiring will ignite, since a wire with this cross-section is designed for an allowed current of 27 A, and much more passes through it. In this case, the circuit does not break, since the machine is adapted to these currents, but the wire is not; the automation will turn off the machine only in the event of a short circuit.
Neglecting this rule can lead to serious consequences!
Important! First, you should calculate the power of the consumers, and then select a conductor of the appropriate cross-section, and only after that select an automatic machine (packet). The rated current of the packet must be less than the maximum current allowed for the wire of this cross-section.
It is thanks to this principle that the wiring will never overheat and, therefore, no fire will occur.
Calculation of consumer power
Each electrical network in an apartment or house can be divided into sections (rooms). Depending on what devices are planned to be used in a particular area, electrical wiring calculations are made. Typically, the electrical wiring zones for each machine are divided among themselves into each room of the apartment or house. One section of wiring for one room, the second for another, and the third for the kitchen and bathroom. In this situation, such powerful consumers as electric stoves, ovens, water heaters, and heating boilers stand apart. This technique requires a dedicated power line, so in modern homes designed for use with electric stoves, a separate circuit breaker is installed to provide power to the device.
Calculating the required current for a particular section of wiring is quite simple. To do this, use the formula I=P/U, according to which I is the current strength, P is the power (in watts) of all operating electrical appliances on this line, U is the network voltage (standard - 220 volts). To calculate, you need to add up the power of those electrical appliances that you plan to use on the line, and then divide the resulting sum by 220. From here we get the current strength, according to which you will need to select a cable of a certain cross-section.
As an example, let’s take an area (room) and calculate for it a machine and a cable of the required cross-section. The following will work simultaneously in the room:
- vacuum cleaner (1300 W);
- electric iron (1000 W);
- air conditioning (1300 W);
- computer (300 W).
Let's add these indicators (1300+1000+1300+300 = 3900 W) and divide them by 220 (3900/220 = 17.72). It turns out that the current strength is 17.72, we select the optimal cable cross-section for this based on the table, take a copper cable with a cross-section of 2.5 mm or 4 mm square (be sure to take it with a reserve) and a circuit breaker with a rated protection current of 20 amperes.
It is worth mentioning that you should not choose a circuit breaker with an overestimated rated current, since if the electrical network is overloaded (exceeding the continuous-permissible current for a particular wire), the wiring will start to catch fire. The rating of the machine must correspond to the value of the continuous-permissible current of the conductor or be less.
Experienced electricians repeatedly say that you should not install cables with a small cross-section because they are cheap; you should choose a cable with a reserve to avoid overloading the electrical section and causing a fire in the wiring. But choosing a powerful machine gun is contraindicated!
The wiring is installed once, it is difficult to replace it, but replacing the switch in the event of a significantly increased load is much easier.
At the moment, more and more powerful electrical appliances are appearing, so it’s worth taking care in advance in case you decide to use a more powerful vacuum cleaner or add some additional device to the room.
Nuances
In general, readers should not have any questions regarding the selection of packages according to the cable cross-section, but there are some subtleties that we did not mention above.
- A machine with which type of electromagnetic release to choose
In everyday life, machines of categories “B” and “C” are most often used.
This is due to the fastest possible operation of package switches when the rated current is exceeded. This is extremely important when using appliances such as electric kettles, toasters and irons. Depending on the type of equipment used, you should choose a specific category; it is advisable to give preference to category “B” switches. - A machine with what maximum switching power should you choose?
It depends on the location of the electricity input from the substation to the apartment; if in close proximity, then you should choose one with a switching capacity of 10,000 amperes, otherwise for city apartments there are enough devices for 5,000–6,000 amperes. You can play it safe and choose the option of 10,000 amperes; ultimately, this indicator only affects whether the machine will be operational after a short circuit. - Which type of wire to choose: aluminum or copper
We strongly do not recommend purchasing aluminum conductors. Copper wiring is more durable and can handle higher currents.
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What are circuit breakers for and how do they work?
Modern AVs have two degrees of protection: thermal and electromagnetic. This allows you to protect the line from damage as a result of prolonged excess of the flowing current of the rated value, as well as a short circuit.
The main element of the thermal release is a plate made of two metals, which is called bimetallic. If it is exposed to a current of increased power for a sufficiently long time, it becomes flexible and, acting on the disconnecting element, causes the circuit breaker to operate.
The presence of an electromagnetic release determines the breaking capacity of the circuit breaker when the circuit is exposed to short-circuit overcurrents, which it cannot withstand.
An electromagnetic type release is a solenoid with a core, which, when a high power current passes through it, instantly moves towards the disconnecting element, turning off the protective device and de-energizing the network.
This makes it possible to protect the wire and devices from an electron flow, the value of which is much higher than that calculated for a cable of a particular cross-section.
What is the danger of a cable mismatch with the network load?
Selecting the correct power circuit breaker is a very important task. An incorrectly selected device will not protect the line from a sudden increase in current.
But it is equally important to choose the correct cross-section of the electrical cable. Otherwise, if the total power exceeds the rated value that the conductor can withstand, this will lead to a significant increase in the temperature of the latter. As a result, the insulating layer will begin to melt, which can lead to a fire.
To more clearly imagine the consequences of a mismatch between the wiring cross-section and the total power of the devices connected to the network, let’s consider this example.
New owners, having bought an apartment in an old house, install several modern household appliances in it, giving a total load on the circuit equal to 5 kW. The current equivalent in this case will be about 23 A. In accordance with this, a 25 A circuit breaker is included in the circuit. It would seem that the choice of the circuit breaker in terms of power was made correctly, and the network is ready for operation. But some time after turning on the appliances, smoke appears in the house with a characteristic smell of burnt insulation, and after a while a flame appears. The circuit breaker will not disconnect the network from the power supply - after all, the current rating does not exceed the permissible one.
If the owner is not nearby at this moment, the melted insulation will cause a short circuit after some time, which will finally trigger the machine, but the flames from the wiring may already spread throughout the house.
The reason is that although the power calculation of the machine was done correctly, the wiring cable with a cross-section of 1.5 mm² was designed for 19 A and could not withstand the existing load.
So that you do not have to take out a calculator and independently calculate the cross-section of electrical wiring using formulas, we present a standard table in which it is easy to find the desired value.
Weak link protection
So, we are convinced that the calculation of the circuit breaker should be made based not only on the total power of the devices included in the circuit (regardless of their number), but also on the cross-section of the wires. If this indicator is not the same along the electrical line, then we select the section with the smallest cross-section and calculate the machine based on this value.
The PUE requirements state that the selected circuit breaker must provide protection for the weakest section of the electrical circuit, or have a current rating that will correspond to a similar parameter for the installations connected to the network. This also means that the connection must be made using wires with a cross-section that can withstand the total power of the connected devices.
How to select the wire cross-section and rating of the circuit breaker - in the following video:
If a careless owner ignores this rule, then in the event of an emergency that arises due to insufficient protection of the weakest section of the wiring, he should not blame the selected device and scold the manufacturer - only he himself will be to blame for the current situation.
How to calculate the rating of a circuit breaker?
Let's assume that we took into account all of the above and selected a new cable that meets modern requirements and has the required cross-section. Now the electrical wiring is guaranteed to withstand the load from switched on household appliances, even if there are quite a lot of them. Now we proceed directly to the selection of a circuit breaker based on current rating. Let's remember the school physics course and determine the calculated load current by substituting the corresponding values into the formula: I=P/U.
Here I is the value of the rated current, P is the total power of the installations included in the circuit (taking into account all consumers of electricity, including light bulbs), and U is the network voltage.
To simplify the selection of a circuit breaker and save you from the need to use a calculator, we present a table that shows the ratings of the circuit breakers that are included in single-phase and three-phase networks and the corresponding total load power.
This table will make it easy to determine how many kilowatts of load correspond to which rated current of the protective device. As we can see, a 25 Ampere circuit breaker in a network with a single-phase connection and a voltage of 220 V corresponds to a power of 5.5 kW, for a 32 Ampere circuit breaker in a similar network - 7.0 kW (this value is highlighted in red in the table). At the same time, for an electrical network with a three-phase delta connection and a rated voltage of 380 V, a 10 Amp circuit breaker corresponds to a total load power of 11.4 kW.
Visually about the selection of circuit breakers in the video:
Conclusion
In the presented material, we talked about why electrical circuit protection devices are needed and how they work. In addition, taking into account the information presented and the tabular data provided, you will not have any difficulty with the question of how to choose a circuit breaker.
When assembling an electrical panel or connecting new large household appliances, the home master will definitely encounter such a problem as the need to select circuit breakers. They provide electrical and fire safety, so choosing the right machine is the key to the safety of you, your family and your property.
What is the machine used for?
A machine is installed in the power supply circuit to prevent overheating of the wiring. Any wiring is designed to carry a certain current. If the current passed exceeds this value, the conductor begins to heat up too much. If this situation persists for a sufficient period of time, the wiring begins to melt, resulting in a short circuit. A circuit breaker is installed to prevent this situation.
The second task of the circuit breaker is to turn off the power when a short circuit current (SC) occurs. When a short circuit occurs, the currents in the circuit increase many times over and can reach thousands of amperes. To prevent them from destroying the wiring and damaging the equipment included in the line, the circuit breaker must turn off the power as quickly as possible - as soon as the current exceeds a certain limit.
In order for the protective circuit breaker to properly perform its functions, it is necessary to correctly select the machine according to all parameters. There are not many of them - only three, but you need to deal with each one.
What types of circuit breakers are there?
To protect conductors of a single-phase 220 V network, there are single-pole and double-pole disconnecting devices. To single-pole wires, only one conductor is connected - phase, to double-pole wires, both phase and neutral. Single-pole circuit breakers are installed on 220 V indoor lighting circuits, on socket groups in rooms with normal operating conditions. They are also installed on some types of load in three-phase networks, connecting one of the phases.
For three-phase networks (380 V) there are three and four poles. These circuit breakers (the correct name is a circuit breaker) are installed on a three-phase load (ovens, hobs and other equipment that operate on a 380 V network).
In rooms with high humidity (bathroom, bathhouse, swimming pool, etc.) two-pole circuit breakers are installed. They are also recommended for installation on powerful appliances - washing machines, dishwashers, boilers, ovens, etc.
It’s just that in emergency situations - in the event of a short circuit or insulation breakdown - phase voltage can reach the neutral wire. If a single-pole device is installed on the power line, it will disconnect the phase wire, and the zero with dangerous voltage will remain connected. This means there is still a possibility of electric shock when touched. That is, the choice of machine is simple - single-pole switches are installed on some lines, and double-pole switches on others. The specific amount depends on the network condition.
For a three-phase network, there are three-pole circuit breakers. Such a machine is installed at the entrance and at consumers, to which all three phases are supplied - an electric stove, a three-phase hob, an oven, etc. The remaining consumers are equipped with two-pole circuit breakers. They must disconnect both phase and neutral.
Example of three-phase network wiring - types of circuit breakers
The choice of circuit breaker rating does not depend on the number of wires connected to it.
Deciding on the denomination
Actually, from the functions of the circuit breaker, the rule for determining the rating of the circuit breaker follows: it must operate until the current exceeds the capabilities of the wiring. This means that the current rating of the machine must be less than the maximum current that the wiring can withstand.
Based on this, the algorithm for selecting a circuit breaker is simple:
- for a specific area.
- See what maximum current this cable can withstand (see the table).
- Next, from all the ratings of the circuit breakers, we select the nearest smaller one. The ratings of the machines are tied to the permissible long-term load currents for a particular cable - they have a slightly lower rating (see the table). The list of denominations looks like this: 16 A, 25 A, 32 A, 40 A, 63 A. From this list you choose the appropriate one. There are smaller values, but they are practically not used anymore - we have too many electrical appliances and they have considerable power.
Example
The algorithm is very simple, but it works flawlessly. To make it clearer, let's look at an example. Below is a table that shows the maximum permissible current for conductors that are used at. Recommendations regarding the use of machines are also given there. They are given in the column “Nominal current of the circuit breaker”. This is where we look for the ratings - it is slightly less than the maximum permissible for the wiring to work normally.
| Cross section of copper wires | Permissible continuous load current | Maximum load power for single-phase network 220 V | Rated current of circuit breaker | Circuit breaker current limit | |
|---|---|---|---|---|---|
| 1.5 sq. mm | 19 A | 4.1 kW | 10 A | 16 A | lighting and alarm |
| 2.5 sq. mm | 27 A | 5.9 kW | 16 A | 25 A | socket groups and electric heated floor |
| 4 sq.mm | 38 A | 8.3 kW | 25 A | 32 A | air conditioners and water heaters |
| 6 sq.mm | 46 A | 10.1 kW | 32 A | 40 A | electric stoves and ovens |
| 10 sq. mm | 70 A | 15.4 kW | 50 A | 63 A | opening lines |
In the table we find the selected wire cross-section for this line. Suppose we need to lay a cable with a cross-section of 2.5 mm 2 (the most common when laying to medium-power devices). A conductor with this cross-section can withstand a current of 27 A, and the recommended rating of the machine is 16 A.
How will the circuit work then? As long as the current does not exceed 25 A, the machine does not turn off, everything works as normal - the conductor heats up, but not to critical values. When the load current begins to increase and exceeds 25 A, the machine does not turn off for some time - perhaps these are starting currents and they are short-lived. It turns off if the current exceeds 25 A by 13% for a sufficiently long time. In this case, if it reaches 28.25 A. Then the power supply will work and de-energize the branch, since this current already poses a threat to the conductor and its insulation.
Power calculation
Is it possible to choose a machine based on load power? If only one device is connected to the power line (usually large household appliances with high power consumption), then it is permissible to make a calculation based on the power of this equipment. You can also choose an introductory machine based on power, which is installed at the entrance to a house or apartment.
If we are looking for the rating of the input circuit breaker, we need to add up the power of all devices that will be connected to the home network. Then the found total power is substituted into the formula, and the operating current for this load is found.
After we have found the current, select the nominal value. It may be either slightly more or slightly less than the found value. The main thing is that its shutdown current does not exceed the maximum permissible current for this wiring.
When can you use this method? If the wiring is laid with a large margin (this is not bad, by the way). Then, in order to save money, you can automatically install switches that correspond to the load, and not the cross-section of the conductors. But once again we draw your attention to the fact that the long-term permissible current for the load must be greater than the maximum current of the circuit breaker. Only then will the choice of circuit breaker be correct.
Selecting the breaking capacity
The selection of a packager based on the maximum permissible load current is described above. But the network circuit breaker must also turn off when a short circuit (short circuit) occurs in the network. This characteristic is called breaking capacity. It is displayed in thousands of amperes - this is the order currents can reach during a short circuit. Selecting a machine based on its breaking capacity is not very difficult.
This characteristic shows at what maximum value of the short-circuit current the circuit breaker remains operational, that is, it will not only be able to turn off, but will also work after being turned on again. This characteristic depends on many factors and for accurate selection it is necessary to determine the short-circuit currents. But for wiring in a house or apartment, such calculations are done very rarely, and are based on the distance from the transformer substation.
If the substation is located close to the entrance to your house/apartment, take a circuit breaker with a breaking capacity of 10,000 A; for all other city apartments, 6,000 A is enough. If the house is located in a rural area or you are choosing a circuit breaker for a summer residence, it may well be enough and a breaking capacity of 4,500 A. The networks here are usually old and short-circuit currents are not large. And since the price increases significantly with increasing breaking capacity, the principle of reasonable savings can be applied.
Is it possible to install bags with a lower breaking capacity in city apartments? In principle, it is possible, but no one guarantees that after the first short circuit you will not have to change it. He may have time to turn off the network, but will be inoperative. In the worst case scenario, the contacts will melt and the machine will not have time to turn off. Then the wiring will melt and a fire may occur.
Type of electromagnetic release
The machine must operate when the current rises above a certain level. But short-term overloads periodically occur in the network. They are usually associated with inrush currents. For example, such overloads can be observed when turning on the refrigerator compressor, washing machine motor, etc. The circuit breaker should not turn off during such temporary and short-term overloads, because they have a certain delay for operation.
But if the current has increased not because of an overload but because of a short circuit, then during the time that the circuit breaker “wait”, its contacts will melt. This is what an electromagnetic automatic release is for. It operates at a certain current value, which can no longer be an overload. This indicator is also called cut-off current, since in this case the circuit breaker cuts off the line from the power supply. The magnitude of the operating current can be different and is displayed in letters that appear in front of the numbers indicating the rating of the machine.
There are three most popular types:
What characteristics should you choose? In this case, the choice of a circuit breaker is also based on the distance of your household from the substation and the state of the electrical networks; the choice of a circuit breaker is carried out using simple rules:
- With the letter “B” on the body they are suitable for dachas, houses in villages and towns that receive power supply through air ducts. They can also be installed in apartments of old houses in which the internal electrical network has not been reconstructed. These circuit breakers are not always on sale; they cost a little more than category C, but can be delivered to order.
- Bags with a “C” on the body are the most widely used option. They are installed in networks with normal condition, suitable for apartments in new buildings or after major renovations, in private houses near the substation.
- Class D is installed in enterprises and workshops with equipment with high starting currents.
That is, in essence, the choice of a circuit breaker in this case is simple - type C is suitable for most cases. It is available in stores in a large assortment.
Which manufacturers should you trust?
And finally, let’s pay attention to the manufacturers. The choice of a circuit breaker cannot be considered complete if you have not thought about which brand of circuit breakers you will buy. You definitely shouldn’t take on unknown companies - electrical engineering is not a field where you can conduct experiments. Learn more about choosing a manufacturer in the video.
Any electrical circuit in an apartment or house must be protected by a circuit breaker against overloads and short circuit overcurrents. This simple truth can be clearly demonstrated in any electrical panel of an apartment, floor panel, input distribution panel of a house, etc. electrical cabinets and boxes.
The question is not whether to install a circuit breaker or not, the question is how to calculate the circuit breaker so that it correctly performs its tasks, works when needed and does not interfere with the stable operation of electrical appliances.
Examples of circuit breaker calculations
You can read the theory of circuit breaker calculations in the article:. Here are some practical examples of calculating circuit breakers in the electrical circuit of a house or apartment.
Example 1. Calculation of the introductory machine at home
Let's start with examples of calculating circuit breakers from a private house, namely, we will calculate the input circuit breaker. Initial data:
- Network voltage Un = 0.4 kV;
- Estimated power Рр = 80 kW;
- Power factor COSφ = 0.84;
1st calculation:
To select the rating of the circuit breaker, we consider the load current rating of a given electrical network:
Iр = Рр / (√3 × Un × COSφ) Iр = 80 / (√3 × 0.4 × 0.84) = 137 A
2nd calculation
To avoid false tripping of the circuit breaker, the rated current of the circuit breaker (thermal release current) should be selected 10% more than the planned load current:
- I current of the release = Iр × 1.1
- It.r = 137 × 1.1 = 150 A
Calculation result: Based on the calculations made, we select a circuit breaker (according to PUE-85 clause 3.1.10) with the release current closest to the calculated value:
- I rated = 150 Ampere (150 A).
This choice of circuit breaker will allow the electrical circuit of the house to operate stably in operating mode and only operate in emergency situations.
Example 2. Calculation of a kitchen group circuit breaker
In the second example, we will calculate which circuit breaker should be chosen for the kitchen electrical wiring, which is correctly called the kitchen electrical wiring socket. It could be the kitchen of an apartment or a house, it makes no difference.
Similar to the first example, the calculation consists of two calculations: calculation of the load current of the kitchen electrical circuit and calculation of the thermal release current.
Load current calculation
Initial data:
- Mains voltage Un = 220 V;
- Estimated power Рр = 6 kW;
- Power factor COSφ = 1;
That is, if the kitchen has 10 sockets for 10 household appliances (stationary and portable), you need to take into account that all 10 appliances will not work at the same time.
Usage rate
- Write down the planned household appliances on a piece of paper.
- Next to the device, place its power according to its passport.
- Sum up all the power of the devices according to the passport. This Calculation.
- Think about what appliances can work simultaneously: kettle + toaster, microwave + blender, kettle + microwave + toaster, etc.
- Calculate the total powers of these groups. Calculate the average total power of groups of simultaneously switched on devices. It will be Pnominal(rated power).
- Divide Calculation on Pnominal, get the kitchen utilization rate.
In fact, in the theory of calculations, the utilization factor inside the house (without utility networks) and apartment is assumed to be equal to one if the number of sockets is no more than 10. This is true, but in practice, it is the utilization factor that allows modern kitchen household appliances to operate on old electrical wiring.
Note:
In the theory of calculations, 1 household outlet is planned for 6 sq. meters of apartment (house). Wherein:
- utilization factor = 0.7 – for sockets from 50 pcs.;
- utilization factor = 0.8 – sockets 20-49 pcs.;
- utilization factor = 0.9 – sockets from 9 to 19 pcs.;
- utilization factor=1.0 – sockets ≤10pcs.
Let's go back to the kitchen circuit breaker. We calculate the kitchen load current rating:
- Iр = Рр / 220V;
- Iр = 6000 / 220 = 27.3 A.
Release current:
- Icalc.= Iр×1.1=27.3×1.1=30A
Based on the calculations made, we choose 32 Amperes for the kitchen.
Conclusion
The given example of calculating a kitchen turned out to be somewhat overestimated; usually 16 amperes are enough if you consider that the stove, washing machine, and dishwasher are put into separate groups.
These examples of calculating circuit breakers for group circuits only show the general principle of calculations, and do not include the calculation of engineering circuits including the operation of pumps, machines and other motors of a private house.
Photo gallery of circuit breakers
The purpose of a circuit breaker (hereinafter referred to as AB) is to protect electrical wiring and electrical equipment from short circuits (hereinafter referred to as short circuit) and overload. If you do not use such switches in the network, then over time an accident may occur, that is, a short circuit in electrical wiring, electrical appliances or power tools. If not a short circuit, then an overload in the operation of electrical equipment.
In the first and second cases, the wire or cable will heat up, which means the insulation will melt. The wires will short out, a short circuit will occur, which means fire, sparks and ultimately a fire.
To prevent this from happening, AVs are used as protection against possible unpleasant consequences.
How does AB protect electrical wiring and electrical appliances and tools? If, simply put, there is a special device inside this switch that provides instantaneous shutdown of the voltage supply if there is a short circuit or overload problem.
AB are:
- single-pole, only one phase is connected to it, used where the electricity consumer is 220 V;
- bipolar, two opposite phases or phase and zero are connected to it. As soon as a problem occurs in one of the phases (exceeding the current value), two circuit breakers are switched off at once. Such machines are not used in everyday life;
- three-pole, used where there is a three-phase power transmission system. For example, when entering a cottage, apartment buildings;
- four-pole, used in switchgears (RU), for breaking 3 phases and zero, not used in everyday life.
Selecting a circuit breaker by current
According to the rated current AB
The industry produces a wide variety of circuit breakers with rated current: 0.5A; 1A; 1.6A; 2A; 3.15A; 4A; 5A; 6A; 10A; 16A; 20A; 25A; 32A; 40A; 50A; 63A. In everyday life, it is mainly used from 6A to 40A.
When purchasing an AV, you need to choose a rating so that it operates until the moment when the current does not exceed the capabilities of the wiring.
Therefore, you need to know what cross-section you need to lay the wire (cable) to the consumer or group of consumers and their power. The nominal value of AB will depend on this.
Table 1.
Selecting AV based on short circuit current
You can purchase AVs with short circuit ratings: 3,000, 4,500, 6,000, 10,000 Amperes. The choice of AV with the required rating depends on the length of the cable or overhead line from the transformer substation (TS) to your house, apartment or cottage.
If the transformer substation is located nearby, then the short-circuit currents are very high, so you need to purchase a circuit breaker with a cutoff of 10,000 A. In the private sector of households, there is a large length of overhead power lines, so you need to use a circuit breaker with a short-circuit current of 4,500 A. In other cases, the average value is - 6,000 A.
Electromagnetic release
An electromagnetic release is a part inside the AB that, in the event of a short circuit (SC), opens the electrical circuit. Releases are divided into categories. We will look at the categories that are used most often:
B – the circuit opens when the rated current is exceeded by 3 to 5 times;
C – exceeded by 5–10 times;
D – exceeded by 10–20 times.
Selection of circuit breaker by power: table
To select a circuit breaker based on power (P), you need to calculate the load current using the formula, then, using the data obtained, select a circuit breaker of a larger value.
Example of selecting an automatic switch
First you need to calculate the sum of all powers for which you need to select an AB. A wire is connected to the automatic switch in the apartment panel, which goes to the kitchen, where a 2.2 kW kettle, a 700 W microwave oven, and a 720 W bread maker are connected through sockets. The total power of electricity consumers is 3,620 W = 3.62 kW. We will calculate the current using the formula:
I – current consumption;
P – total power of consumers;
U – network voltage.
I = 3,620/220 = 16.4A
As you can see, the consumed load current is 16.4 A. And based on this, you can select AB. You can take a 16 A automatic machine, but it will work at the very limit. Any machine is designed in such a way that the specified rated current is increased by 13% and if overloaded it will work for some time. Why take an AB that will work to the limit. You need to take it with a reserve. The next AB rating is 20 A.
To determine a more accurate load, you need to look at the passport or take data from the nameplate, which is on all electrical appliances.
Look at the power table to select an AB by rating.
Table 2.
| Connection type | Single phase 220 V, | Three-phase (delta), 380 V | Three-phase (star), 220 V |
| Machine rating, A | |||
| 1 | 200 W | 1 100 W | 700 W |
| 2 | 400 W | 2,300 W | 1,300 W |
| 3 | 700 W | 3,400 W | 2,000 W |
| 6 | 1,300 W | 6,800 W | 4,000W |
| 10 | 2,200 W | 11,400 W | 6,600 W |
| 16 | 3,500 W | 18,200 W | 10,600 W |
| 20 | 4,400 W | 22,800 W | 13,200 W |
| 25 | 5,500 W | 28,500 W | 16,500 W |
| 32 | 7,000 W | 36,500 W | 21 100 W |
| 40 | 8,800 W | 45,600 W | 26,400 W |
| 50 | 11,000 W | 57,000 W | 33,000 W |
| 63 | 13,900 W | 71,800 W | 41,600 W |
Selection of machine according to cable cross-section - table
Industry produces specific sections of wire or cable. Each conductor section has a certain current load. Using a certain cross-section, you can also select a circuit breaker (AB) according to its rating. If you are not sure about the cross-section of a particular wire or cable, then this matter can be calculated with.
The easiest way is to use a table where you can immediately determine which AB you need. The table contains data without taking into account the length of the wire (cable).
Table 3.
| Circuit breaker current, A | Wire cross-section, mm² | power, kWt | ||
| Copper | Aluminum | 220 V | 380 V (cos φ = 0.8) | |
| 5 | 1 | 2,5 | 1,1 | 2,6 |
| 6 | 1 | 2,5 | 1,3 | 3,2 |
| 10 | 1,5 | 2,5 | 2,2 | 5,3 |
| 16 | 1,5 | 2,5 | 3,5 | 8,4 |
| 20 | 2,5 | 4 | 4,4 | 10,5 |
| 25 | 4 | 6 | 5,5 | 13,2 |
| 32 | 6 | 10 | 7 | 16,8 |
| 40 | 10 | 16 | 8,8 | 21,1 |
| 50 | 10 | 16 | 11 | 26,3 |
| 63 | 16 | 25 | 13,9 | 33,2 |
The main thing in selecting the AV and the cross-section of the wire (cable) is that the current of the automatic switch is less than the permissible current of the conductor.
Do not forget that before choosing a wire (cable), you need to know the total power of the electricity consumer and only lastly AB.
Conclusion
You learned how to choose the right AB from this article. Before purchasing automatic switches, you should already know which manufacturers produce quality products. Choose only trusted companies.