When designing the electrical network of a new house, 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 the right choice circuit breaker according to the power consumption.
Briefly the principle of operation and purpose of circuit breakers
Automatic switch when short circuit Triggers almost instantly thanks to an electromagnetic release. At a certain excess of the rated current value, the heating bimetallic plate will turn off the voltage after some time, which can be found out from the current characteristic time graph.
This safety device protects the wiring from short circuits and overcurrents exceeding the calculated value for a given cross-section of the wire, which can heat the conductors to the melting point and ignite the insulation. To prevent this from happening, you need not only to choose the right protective switch that matches the power of the connected devices, but also to check whether the existing network can withstand such loads.
Appearance of a three-pole circuit breaker Wires must match the load
It often happens that in an old house a new electric meter, automatic machines, and RCDs are installed, but the wiring remains old. A lot of household appliances are bought, the power is summed up and an automatic machine is selected for it, which regularly holds the load of all switched on electrical appliances.
Everything seems to be correct, but suddenly the wire insulation begins to emit a characteristic odor and smoke, a flame appears, and the protection does not work. This can happen if the wiring parameters are not designed for such current.
Let's say the cross-section of the old cable core is 1.5mm², with a maximum permissible current limit of 19A. We assume that several electrical appliances were connected to it at the same time, making up a total load of 5 kW, which in current equivalent is approximately 22.7 A; it corresponds to a 25 A circuit breaker.
The wire will heat up, but this machine will remain on all the time until the insulation melts, which will lead to a short circuit, and the fire can already flare up in full swing.
Protect the weakest link in the electrical wiring
Therefore, before choosing a machine according to the load being protected, you need to make sure that the wiring will withstand this load.
According to PUE 3.1.4, the machine must protect the weakest section from overloads electrical circuit, or selected with a rated current corresponding to the currents of the electrical installations being connected, 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 choice of circuit breaker comes down to determining the appropriate 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 three-phase network, when choosing a machine, read the article about
Selection of circuit breakers for electrical installations(electric motors, transformers) with reactive load, as a rule, is not produced by power. The rating and type of current characteristics of the circuit breaker are selected according to the operating and starting current specified in the passport of this device.
Modern power supply to private houses and apartments is not recommended without circuit breakers. They provide security and guarantee long term wiring services. We will talk about choosing a circuit breaker in this article.
The main purpose of a circuit breaker is to protect the wiring from overheating and the insulation from melting. And it does this by turning off the power supply at those moments when the conductor heats up to critical temperatures due to the connection of an excessively high power load. The second task of the package operator is to disconnect the line during short-circuit currents. The goal is the same - to protect the wiring from destruction.
Turning off the power in a timely manner in the event of problems is very important, as it prevents damage to the wiring and fire. Therefore, choosing a circuit breaker is a responsible task. You need to choose according to the rules, and not according to the principle “so that it turns off less often.” This method may cause a fire. In general, the selection of a circuit breaker is carried out according to three parameters:
- denomination;
- breaking capacity (cut-off current);
- type of electromagnetic splitter (time-current characteristic).
Each parameter is important and is selected depending on the load connected to a particular line, the location of the electrical wiring relative to the distribution substations.
Types of circuit breakers
Circuit breakers are produced for single-phase and three-phase circuits. For a single-phase network there are two types of packets - single-pole and double-pole. Only the phase wire is connected to single-pole ones and, when triggered, only the phase is disconnected. It is recommended to install such machines in houses and apartments in rooms with normal operating conditions. Usually they are installed on lighting lines, socket groups, which are located in living rooms, corridors, and kitchens.
Circuit breakers - single-pole, double-pole and three-pole
Both phase and neutral wires are connected to two-pole circuit breakers. He breaks both chains. The degree of protection here is much higher since the shutdown is complete and not partial. Such a machine will ensure safety even if during an accident voltage reaches the neutral conductor. It is recommended to install two-pole circuit breakers on dedicated lines to which powerful household appliances are connected. They are also installed in rooms with difficult operating conditions. These include a bathroom, swimming pool, sauna.
For three-phase networks, three-pole and two-pole circuit breakers are used. All three phases are turned on to three-pole ones. Accordingly, they all turn off at the same time. Such packets are placed at the entrance to a house or apartment, as well as on the lines to which three-phase consumers are connected - a hob, an oven and other similar equipment. For the same consumers, four-pole circuit breakers can be installed. They will also disconnect the neutral wire.
On other power lines, on which one of the phases is used, two-pole packets are installed. Simultaneous disconnection of phase and zero is more preferable. And only on the lighting line can single-terminal circuits be installed.
Selecting a load current circuit breaker
When planning electrical wiring, the main task is to choose the correct circuit breaker rating. When current passes through a conductor, it begins to heat up. The more current passes through a conductor of the same cross-section, the more heat is generated. The circuit breaker's job is to cut off the power before the current draw becomes higher than is acceptable. Therefore, the rating of the circuit breaker must be less than the permissible wiring current.
The ratings of circuit breakers are standardized: 6 A, 10 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A and 63 A. In practice, six and ten amp options are almost never used anywhere - equipment in our homes is becoming more and more and small cross-section lines cannot cope with the load.
Choice of denomination
The circuit breaker is not selected based on the load, the power of the connected devices, or the current. These parameters are taken into account when choosing the conductor cross-section. And the choice of circuit breaker is made depending on the cross-section of the conductors. There is a special table that indicates the permissible load currents and the recommended rating of the circuit breaker. Using the table is simple: find the desired cross-section, in this line look for the rating of the circuit breaker. All.
| Cable cross-section | Recommended circuit breaker rating | Maximum operating current of the machine | Permissible continuous load current | Maximum load power | Scope of application |
|---|---|---|---|---|---|
| 1.5 mm2 | 10 A | 16 A | 19 A | 4.1 kW | Lighting and alarm |
| 2.5 mm2 | 16 A | 25 A | 27 A | 5.9 kW | Sockets, electric heated floor |
| 4 mm2 | 25 A | 32 A | 38 A | 8.3 kW | Water heaters, air conditioners, washing machines and dishwashers |
| 6 mm2 | 32 A | 50 A | 46 A | 10.1 kW | Electric stoves, ovens |
| 10 mm2 | 50 A | 63 A | 70 A | 15.4 kW | Entrances to a house, apartment |
How it all works
Looking at the table, the question arises: why is the rating of the machine so much less than the maximum permissible current load. The answer lies in the mechanics of the circuit breaker. It turns off only when the current in the circuit is 13% higher than the trip current.
For example, a 10 A machine will work when the current in the circuit is 16 A + 13% (2.08 A) = 18.08 A. That is, there is a small gap left to the permissible load. This gap is required to ensure the integrity of the insulation.
A modern power supply system for a house or apartment cannot do without automatic switches.
What will happen if you install a 16 A circuit breaker on a wire with a cross-section of 1.5 mm2? After all, its rating is lower than the permissible load current? Let's count. The current at which the packetizer will operate is 25 A + 3.25 A (13%) = 28.25 A. It is higher than the permissible long-term load current. Yes, it will rarely turn off, but after a while the insulation will melt and the wiring will have to be changed. Therefore, it is better to select a circuit breaker according to this table, and not according to the long-term permissible current.
Selection by load
If the power supply line is laid with a power reserve, and the load on it is far from the maximum, you can install a machine with a lower rating. In this case, it will protect not so much the line from overheating, but the equipment from short-circuit currents.
Choosing a circuit breaker based on load power is the wrong idea
The choice of the circuit breaker rating in this case can also be made using the same table. Just take the load power as a starting point. But let's repeat it again. This is the case if the line parameters can withstand a much greater load than exists.
Type of electromagnetic splitter (switch-off curve)
The next parameter by which the circuit breaker is selected is the type of electromagnetic splitter. It is responsible for the delay that occurs when triggered. It is necessary to avoid false shutdowns during the start of motors of various equipment.
When you turn on the motor of your refrigerator, dishwasher, or washing machine, the current in the circuit increases briefly. This phenomenon is called inrush currents, and they can exceed operating consumption by 10-12 times, but do not last long. Such a short-term increase does not cause harm. So, the electromagnetic splitter must have a delay that allows you to ignore these inrush currents. This characteristic is displayed in Latin letters B, C, D. This letter is placed before the rating of the circuit breaker (photo). Selecting a circuit breaker based on this criterion is not difficult. You just need to know the nature of the planned load:
Actually, choosing a circuit breaker in this case is simple. On the lighting line it is enough to install category B machines; on the rest you can install C.
Selecting the degree of protection against short-circuit currents (cut-off current)
The second function of the protective circuit breaker is to turn off the power when excess currents appear that occur during a short circuit (short circuit). Circuit breakers are designed for different values of these currents, and the characteristic that displays it is the breaking capacity or cut-off current. It shows at what short-circuit current the machine will still remain in working condition. The fact is that the burster does not fire instantly, because there is a delay in response to ignore starting overloads. During this delay, the contacts may melt and the device will become inoperable. So, the cut-off current or breaking capacity shows how much current the contacts can withstand without compromising performance.
In the household electrical network, circuit breakers are used with three degrees of protection against short-circuit currents: 4500 A, 6000 A, 10000 A. On the device body, these numbers are placed in a frame just below the rating of the machine. In terms of price, the difference is quite noticeable, but it is justified - more “resistant” bags use refractory materials, and they are much more expensive.
How to choose a circuit breaker in this case? The choice depends on the location of the network relative to the substation. If a house or apartment is located nearby, the short-circuit currents can be very large, therefore the breaking capacity should be at least 10,000 A. If the household is located in a rural area, the networks there are old and/or the supply occurs via an overhead network, an automatic circuit breaker with a breaking capacity of 4,500 A is sufficient In all other cases they set it to 6000 A.
Housing protection degree
The degree of protection of the case is in the characteristics. It is denoted by the Latin letters IP and two numbers. The first number shows how protected the device is from dust and foreign objects. Lowest protection (absent) - 0, the most high level— 6 (complete protection against long-term exposure). The second number indicates protection from moisture. Without protection - 0, maybe in the water for some time - 8. The decoding of the numbers is given in the table.
If electrical panel installed in an apartment, in a dry room, IP20 protection level is sufficient. On landings, a higher degree of protection is desirable. At least IP32. If the machine is installed outdoors, you should set it to at least IP55.
Expensive or cheap?
There are two price categories for circuit breakers in stores and markets. One part is produced by well-known brands and has a very respectable price tag. These are Schneider Electric, ABB, LeGrand and others. These brands have been on the market for a long time, have European roots and an established reputation. The quality of their products is always high, so those who do not like to take risks and can afford to spend a lot of money on assembling an electrical panel prefer to purchase products from these manufacturers.
Next to them there are usually the same machines, but they cost 2-5 times less. These are IEK (IEK), EKF (EKF), TDM (TDM), DEKRAFT (Derkaft), etc. These are Chinese machines, but produced in factories. Some brands (the same Dekraft) have European roots (in this case Germany), but production facilities are in China. These brands are also considered good and show stable results. So for those who are trying not to spend extra money, this is good option. Affordable and good in quality.
What you should not do is buy products from unknown manufacturers. Even if their price is very attractive and the seller praises them very much.
Available upon purchase famous brands pitfalls: there are too many fakes. Moreover, they are sold at almost the same price as the original and it is very difficult to distinguish them by external signs. The only thing you can focus on is less weight. In fakes less metal, some elements may be missing. Due to this, the weight is less. There may also be errors in the application of inscriptions; sometimes paints of other shades are used. To notice all this, you must first thoroughly study all the nuances of the originals on the official websites, or even better, hold them in your hands.
The calculation of circuit breakers is made based on the planned load in the electrical network or group circuit of the apartment. Also, the calculation of automatic machines can be done by section electric cable already installed and functioning in the apartment.
I would like to propose the calculation of circuit breakers in an apartment in two versions. Each option is used for different wiring conditions, but both options are subject to the rules, including those specified in the PUE.
Options for calculating circuit breakers
1.Option. Are you planning new wiring. In this case, the calculation of circuit breakers is carried out based on the planned power consumption of the apartment, the entire electrical network of the apartment as a whole, together with an analysis of the cross-section of the conductor cable cores.
2.Option. You already have functioning electrical wiring and you need, for example, to replace outdated machines with new ones.
Let's consider both of these options.
Calculation of circuit breakers for new electrical wiring
Before calculating, let's remember a little about what we need. First of all, to protect against short circuits and circuit overload. What does the circuit breaker protect? Protects electrical wiring and connection devices (sockets and switches) from overheating and fire.
Depending on the purpose of the circuit and its short circuit protection, we... Here we do without calculations. But now we’ll look at the calculation of exceeding the permissible load.
On the one hand, the circuit breaker must have a rated current or the current of the circuit breaker is greater than or equal to current at maximum load in the chain.
For example, your electrical circuit consists of 9 outlets with a planned maximum load of 3150 W. When I talk about maximum load, I mean that the planned appliances will be plugged into all sockets.
The current in the circuit will be equal to 14.3 Amperes. Calculation formula from school:
This means that the rated current of the circuit breaker can no longer be less than this current in the circuit. If it is less, then the machine will constantly knock out, and we do not need this.
Let's move on. On the other hand, the rated current of the circuit breaker cannot be unlimitedly large. We remember that the circuit breaker protects the cable from overheating. Therefore, the upper permissible value of the rated current of the circuit breaker must be such that the wires do not heat up, and this value is called permissible cable current, or rather, the permissible current of the conductors.
We find that the rated current of the circuit breaker must be less than or equal to the permissible current for the conductor.
As a result, we get a simple condition:
Where can I get the permissible TPG current?
The easiest and most reasonable way is to take the permissible conductor current (TCC) from Table 1.3.4. in PUE ed. 7.
Table: Maximum permissible current for apartment electrics for wires with PVC (polyvinyl chloride) and rubber insulation with copper conductors.
This table is not complete, but sufficient for residential wiring. Let me remind you that in apartment electrics, you cannot use wires with conductors thinner than 1.5 mm 2 and you cannot use cables with aluminum TPG thinner than 16 mm 2. (PUE, table 7.1.1)
Now the calculation of the circuit breaker for the new electrical wiring
Of course, the above formula does not provide an accurate calculation of the circuit breaker rating. It only shows its boundaries. We will carry out the calculation itself as follows (in quotes I will carry out the calculation for a conditional example of an electrical circuit of 9 outlets of 450 W each):
- We count the current in the circuit at maximum load ( 9×400W=3600W. 3600÷220=16.36 Ampere);
- According to the PUE table 1.3.4 (see above), we focus on the cross-section of the cable cores and select for wiring the cross-section of the cable cores one step larger, but not less than 1.5 mm 2. (according to the table, 1.5 mm is suitable, select 2.5 mm, since 2.0 is not on sale);
- Again, using the table, we look at the permissible current for the selected cable (25A);
- We find that according to the condition, namely (I network ≤I machine ≤I permissible cable current), 16.36 Amperes ≤I machine ≤25 Ampere).
- On sale for DIN rail, there are machines with a rating of 20 Amperes. We install it.
Another example of calculating a circuit breaker:
A circuit breaker must be installed at the input. According to the condition, the calculated network current is 27.5 Amperes. The input cable is copper, brand VVGng, cross-section 3×10.
1. Using the PUE table, we look at the permissible cable current. It is equal to 50 Amperes.
2. This means the rating of the circuit breaker should be:
27.5 A≤I of the machine≤50 Ampere.
There are 50 Ampere circuit breakers on sale. First select the machine: VA47-29 D50 2p 4.5 kA. By the way, how is their name structure deciphered?
Calculation of circuit breakers for functioning electrical wiring
Suppose you already have working electrical wiring and you need to install or replace circuit breakers. In this case, we calculate the machines based on the cross-section of the cables (or wires) of the circuit.
There are also two options here.
Option 1. The cross-section of all cables (wires) in the circuit is the same.
Note: the cable cross-section refers to the cross-section of the cable cores themselves. To calculate it, measure the diameter of the core and use a mathematical formula to calculate the cross-sectional area of the core.
In this case, the calculation of the machine repeats the calculation indicated above, only without calculating the maximum load.
Option 2. Electrical circuits use wires (cables) of different sections.
In this version, the calculation is also not complicated. The circuit breaker is selected according to the smallest cable cross-section according to table PUE 1.3.4. given above and the calculation algorithm given above.
Instructions
A correctly selected circuit breaker should trip in the event of a short circuit in the wiring or when a load significantly exceeds the design load. At the same time, it should not turn off if, for example, you turned on both washing machine. That is why select a circuit breaker specifically for your wiring and load.
Buy circuit breakers only in specialized stores, this will allow you to buy a high-quality product with a high degree of probability. Remember that the rated current of the circuit breaker should not be higher than the maximum current load allowed for your network. In other words, the machine should work, and the wiring should not burn out.
The parameters of the circuit breaker are calculated as follows. Let's assume you have enabled electric kettle power 2 kW. Divide the power, in watts, by 220, we get a current of 9.1 A. This figure can be rounded to 10 so that there is some margin in the calculations. This means that the circuit breaker must withstand a current of 10 amperes. Now calculate the total power of all electrical appliances you turn on at the same time and calculate, using the diagram above, the current strength. Let’s say you get 30 amperes, which means the circuit breaker must be designed for this current.
Having carried out the previous calculations, you have calculated what current the circuit breaker should be designed for. But you also need to know how much current your network can handle, and that depends on the wires you use. For example, if the wiring is made with 2.5 mm aluminum wire (the most common option), it can withstand a current of 24 amperes and a load of 5.2 kW. This means that in your case the circuit breaker should be designed for a current of slightly less than 24 amperes. If you use copper wire of this section, it will withstand a current of 30 amperes and a load of 6.6 kW. If you are using a wire of a different cross-section, find current and load data for it on the Internet.
When installing wiring in your house, ensure in advance that it is divided into several isolated circuits. For example, make separate lighting circuits and wiring to outlets. This will allow you to install a separate circuit breaker on each circuit, which will significantly improve overall safety. Electrical appliances such as electric water heaters, additionally turn on via RCD device protective shutdown, capable of protecting a person from injury electric shock. The RCD reacts to leakage, so if the phase wire is touched, it will immediately turn off the current.
When supplying electricity to an apartment, the following input switching devices can be installed on the floor electrical panel:
- fuses;
- batch switch;
- switch
An input circuit breaker (VA) is an automatic switch for supplying electricity from the supply network to the facility if an overload occurs in the circuit or a short circuit (short circuit) occurs. It differs from the listed devices in its larger rated current. The photo shows a shield with an introductory machine located on top of it.
Switchboard with circuit breaker
It is more correct to call the device an input circuit breaker. Since it is closer than other devices to the overhead line, the device must have an increased switching resistance (SSR), which characterizes the normal operation of the device when a short circuit occurs (the maximum current at which the circuit breaker is able to open the electrical circuit at least once). The indicator is indicated on the device labeling.
Types of input machines
The supply of electricity to an object depends on its needs and the electrical network diagram. In this case, the appropriate types of machines are selected.
Single pole
An input switch with one pole is used in a single-phase electrical network. The device is connected to power through terminal (1) at the top, and the bottom terminal (2) is connected to the outgoing wire (Fig. below).
Scheme of a single-pole circuit breaker
A machine with one pole is installed in the phase wire gap and disconnects it from the load when emergency situation(picture below). According to the principle of operation, it is no different from machines installed on outlet lines, but its current rating is higher (40 A).
Scheme of an introductory single-pole circuit breaker
The red supply phase is connected to it, and then to the meter, after which it is distributed to group machines. Neutral wire blue goes directly to the meter, and from it to the N bus, then connects to each line.
The input machine installed in front of the meter must be sealed.
The input circuit breaker protects the input cable from overheating. If a short circuit occurs on one of the branch lines from it, its circuit breaker will operate, and the other line will remain operational. This connection diagram allows you to quickly find and fix a fault in the internal network.
Bipolar
A two-terminal network is a block with two poles. They are equipped with an integrated lever and have a common lock between the shutdown mechanisms. This design feature is important, since the PUE prohibits breaking the neutral wire.
It is not allowed to install two single-terminal circuits instead of one double-terminal circuit.
An input machine with two poles is used for single-phase input due to the peculiarities of connection diagrams in old houses. A branch is made into the apartment from the riser of the interfloor electrical panel with a single-phase two-wire line. A housing electrician may accidentally swap the wires leading into the apartment. In this case, the neutral will be on the input single-phase circuit breaker, and the phase will be on the zero busbars.
To ensure a complete shutdown guarantee, it is necessary to de-energize the apartment panel using a two-terminal network. In addition, it is often necessary to change the package switch in the floor panel. Here it is more convenient to immediately install a two-pole input circuit breaker instead.
The apartment of the new house is supplied with a network with phase, neutral and grounding with a standard color coded. Here, too, the possibility of the wires being mixed up due to the low qualifications of the electrician or simply a mistake cannot be ruled out.
Another reason for installing a two-terminal network is to replace plugs. There are still plugs on the old apartment panels that are installed at phase and zero. The connection diagram remains the same.
PUEs prohibit the installation of fuses in neutral working wires.
In this situation, it is more convenient to install a two-terminal network, since there is no need to redo the circuit.
When connecting electricity to a private house using a TT scheme, a two-terminal network is necessary, since in such a system a potential difference may appear between the neutral and ground wire.
In Fig. Below is a diagram of connecting electricity to an apartment with a single-phase input through a two-pole circuit breaker.
Input circuit with two-pole circuit breaker
The supply phase is supplied to it, and then to the meter and to the fire protection grounding device of the RCD, after which it is distributed to group circuit breakers. The neutral wire goes directly to the meter, from it to the RCD, bus N, and then connects to the RCD of each line. The green neutral grounding conductor is connected directly to the PE bus, and from it it approaches the grounding contacts of sockets No. 1 and No. 2.
The input circuit breaker protects the input cable from overheating and short circuit. It can also work during a short circuit on a separate line if another machine there is faulty. The ratings of the meter and fire protection RCD are selected higher (50 A). In this case, the devices will also be protected from overloads by the input circuit breaker.
Three-pole
The device is used for a three-phase network to ensure simultaneous shutdown of all phases in case of overload or short circuit of the internal network.
Each terminal of the three-terminal network is connected in phase. In Fig. Below are pictures of him appearance and a circuit where for each circuit there are separate thermal and electromagnetic releases, as well as an arc suppressor.
Three-pole circuit breaker in a cabinet and its diagram
When connecting to a private house, the input circuit breaker is installed in front of the electric meter with 63 A protection (Fig. below). After the meter, an RCD is installed for a leakage current of 300 mA. This is due to the large length of the house's electrical wiring, where there is a high background leakage.
After the RCD, lines are separated from distribution buses (2) and (4) to sockets, lighting, as well as separate groups (6) for supplying voltage to extensions, three-phase loads and other powerful consumers.
Three-phase network of a private house
Automatic input calculation
Regardless of whether the machine is input or not, it is calculated by summing the currents of the lines going to the loads. To do this, the power of all connected consumers is determined. The rating is determined for the simultaneous inclusion of all electricity consumers. Based on this maximum current, the closest rating of the machine from the standard range is selected downward.
The power of the input circuit breaker depends on the rated current. With three-phase power, the power is determined by how the loads are connected.
It is also necessary to determine the number of switching devices. Only one switch is required per input, and then one for each line.
For powerful appliances such as electric boilers, water heaters, oven it is necessary to install separate machines. The panel must provide space for installing additional circuit breakers.
VA selection
The choice of device is made according to several parameters:
- Rated current. Exceeding it will cause the machine to trip due to overload. The rated current is selected based on the cross-section of the connected wiring. The permissible maximum current is determined for it, and then the nominal current for the machine is selected, having previously reduced it by 10-15%, leading to a standard series in the direction of decrease.
- Maximum short circuit current. The machine is selected according to the PKS, which must be equal to or exceed it. If the maximum short-circuit current is 4500 A, a 4.5 kA circuit breaker is selected. The switching class is selected for lighting - B (I start >I rated by 3-5 times), for powerful loads such as a heating boiler - C (I start >I rated by 5-10 times), for three phase motor large machine or welding machine – D (I start >I nom 10-12 times). Then the protection will be reliable, without false positives.
- Installed power.
- Neutral mode is a type of grounding. In most cases it is a TN system with different options(TN-C, TN-C-S, TN-S),
- The magnitude of the line voltage.
- Current frequency.
- Selectivity. The ratings of the machines are selected according to the distribution of loads in the lines, for example, an input machine - 40 A, an electric stove - 32 A, other powerful loads - 25 A, lighting - 10 A, sockets - 10 A.
- Power supply diagram. The machine is selected according to the number of phases: one- or two-pole for a single-phase network, three- or four-pole for a three-phase network.
- Manufacturer. In order to increase the degree of security, the machine is selected from well-known manufacturers and specialized stores.
The number of poles for a three-phase network is four. If available only three-phase loads with a delta connection diagram, you can use a three-pole circuit breaker.
The switch at the input must disconnect the phases and the working zero, since in the event of a leak on one of the phases to zero, there is a possibility of electric shock.
A three-pole machine can be used for a single-phase network: phase and zero are connected to two terminals, and the third will remain free.
Selecting an input circuit breaker depending on the type of grounding:
- TN-S system: supply neutral protective and working wires are separated from the substation to the consumer (Fig. a below). To simultaneously disconnect phases and zero, two-pole or four-pole input circuit breakers are used (depending on the number of phases at the input). If they have one or three poles, the neutral is carried out separately from the machines.
- TN-C system: the supply neutral protective and working wires are combined and pass to the consumer through a common conductor (Fig. b). The machine is installed single-pole or three-pole on the phase conductors, and zero is entered through the counter to the N bus.
As practice shows, connecting an input machine is not a difficult job. It is important to correctly calculate its power, think through the connection diagram and install it taking into account the features given in the article.