Calculation of the machine by power: the purpose of the device, the principle of operation, the selection of the rating according to the table

When carrying out electrical work, safety should always be the main criterion. After all, a lot depends on this, up to the life and health of a person. And the reason for such an event does not matter at all. In any case, it is necessary to choose the right protective devices. It is in this regard that you will have to calculate the machine in terms of power, taking into account some important nuances.

Circuit breakers

Everyone who has come across electrical wiring has heard of circuit breakers or automatic machines. First of all, a competent electrician will always advise you to treat the choice of such an important part of the power grid with particular scrupulousness. Since later this very simple device can save you from many troubles.

It does not matter what kind of electrical work is carried out - whether new wiring is installed in a newly built house, the old one is replaced, the shield is modernized or a separate branch is being laid for too energy-intensive devices - in any case, special attention must be paid to the selection of the machine in terms of power and other parameters.

Purpose of the device

Any modern machine has two degrees of protection. It means that he can help in two, the most common situations.

1. The first, implies overheating of the wiring as a result of the passage of currents through it, more than the nominal. It is easy to guess what this can lead to: cable burnout, and as a result, a short circuit or fire in general.
2. The second situation that the circuit breaker is capable of preventing is a short circuit, due to which the power the current in the circuit can increase by huge values, and this is fraught, at best, with the failure of everything electrical equipment. In the worst case, the fire of electrical engineering, and from it the entire room. There is no need to talk about the integrity of the wiring.

Thus, the machine is able to protect not only personal property, but in some cases also life. Although for this it is necessary to carry out a competent calculation of the circuit breaker in terms of power and a number of other parameters. And also, you should not take an automatic machine "with a margin", since at critical values ​​of currents in the network, it may simply not work, which is tantamount to its absence.

As for the protection of a person from electric shock as a result of touching live parts, it is preferable to use an RCD.

Principle of operation

The main task of the protective switch is to cut off the supply of electric current from the supply cable to the consumer's network. This happens thanks to the releases located in the body of the machine. Moreover, there are two types of such parts:

1. Electromagnetic, representing a coil, a spring and a core, which, when the rated currents are exceeded, is drawn in and disconnects the contacts through the spring. This happens almost instantly - from 0.01 to 0.001 seconds, which can provide reliable protection.
2. Bimetallic thermal - they are triggered when currents exceed the limit values. In this case, the bimetallic plate, which is the basis of such a release, bends and the contacts break.

For more reliable disconnection, in most modern models of automatic machines, they try to use both types of releases.

Types of AB and their features

Given the variety of power grids and certain situations, machines can be of different types. The principle of their operation is not significantly different - all the same releases work, but depending on the situation and a number of other nuances, different variations of them are used.

So, for a standard single-phase network with a voltage of 220 volts, single-pole and two-pole AB are produced. The former are capable of breaking only one wire - a phase. The latter can work with both phase and zero. Of course, the second option is preferable. Especially when it comes to rooms with high humidity. Of course, a single-pole machine will quite cope with its task, but situations may arise when burned-out wires are shorted together. In this case, of course, the phase will be cut off, but the neutral wire will be energized, which can be extremely dangerous.

For three-phase networks with a voltage of 380 volts, three- or four-pole machines are used. They must be installed both at the entrance and directly in front of the consumer. As is clear, such machines cut off all three phases connected to them. In rare cases, it is possible to use one- or two-pole protective devices to cut off, respectively, one or two phases.

Choosing a protective device

Of course, any automaton will perfectly cope with the tasks assigned to it - this is beyond doubt if it is in good working order. But the fact is that it is necessary to select AB taking into account several parameters.

If the selected automaton is too “weak”, then constant false positives will occur. Conversely, a model that is too “strong” will be of rather dubious utility.

Load power

One of the possibilities to choose a protective device is to select an automatic machine according to the load power. To do this, you need to know the value of the load current. And already from these data, choose the appropriate denomination. The easiest way (and more precisely) do this using Ohm's law by the formula:

I = P / U,

where P is the power of the consumer (refrigerator, microwave oven, washing machine, etc.) ), and U is the mains voltage.

For example, the consumer will be taken 1.5 kW, and the mains voltage is usual 220 V. Having these data, substituting them into the formula, you get:

I = 1500/220 = 6.8 A.

In the case of a three-phase 380 volt network, the voltage will be 380 V.

Based on Ohm's law, you can easily calculate the power of the load, from which you can select the required rating of the machine. However, do not forget that when choosing AB in this way, it is necessary to add up the load of all consumers.

There is another formula for choosing a current circuit breaker, but it is a little more complicated, but the final result will be much more accurate. In practice, this is not important, but for informational purposes it is still worth citing it:

I = P / U * cos φ.

The values ​​of I, P, U will be the same as in Ohm's law, but cos φ is the power factor that takes into account the reactive component in the load. This value helps to determine table 6.12 of the normative document SP 31-110-2003 "Design and installation of electrical installations of residential and public buildings."

For example, the data will be used the same, i.e. consumer 1.5 kW, and the voltage is still 220 V. According to the table, cos φ will be equal to 0.65, as for computers. Hence:

I = 1500 W / 220 V * 0.65 = 4.43 A.

Cable section

Choosing an automatic machine based only on the power of the load will be an unforgivable mistake, which can be expensive. After all, if you do not take into account the cross-section of the cable, then all meaning in the selection of the machine is lost. However, the obtained load values ​​and AB rating can help in the selection of the required cable.

To do this, you do not need to make any calculations, since it is enough to use table No. 1.3.6 and 1.3.7 of the PUE, where the concept of long-term permissible current means a voltage passing through a conductor for a long time, which does not cause excessive voltage heating. Simply put, the calculated load power can be taken as this value. And get the required section of copper or aluminum wire.

Short-circuit current

Although it took some calculations to choose a circuit breaker in terms of power, they were extremely simple. This cannot be said at all about the calculations when choosing an automatic machine for short-circuit currents.

But when selecting the nominal AB for a house, cottage, apartment or office, such calculations will be superfluous, since the main indicator, which especially affects the data, is the length of the conductor. But in such situations, it is extremely small to significantly affect the result. Therefore, such calculations are carried out only when designing substations and other similar structures, where the cable length is significant.

Therefore, when choosing a circuit breaker, models with the designation "C" are usually purchased, where the values ​​of inrush currents are taken into account.

Selection of denomination

The selection of the circuit breaker rating must meet certain requirements. More specifically, the machine must operate before the currents can exceed the permissible wiring values. From this it follows that the rating of the machine should be slightly less than the current strength that the wiring can withstand.

Selecting the correct AB is quite simple. Moreover, there is a table of ratings of automatic machines for current, and this greatly simplifies the task.

Based on all this, you can compose an algorithm, according to which it is easiest to choose an automatic machine of the required denomination:

• For a single section, the cross-section and material of the wire are calculated.
• From the table, the value of the maximum current that the cable can withstand is taken.
• It remains with the help of the table only to choose an automatic machine with a value slightly less than the long-term permissible current.

The table contains five ratings AB 16 A, 25 A, 32 A, 40 A, 63 A, from which the protective device will be selected. Machines of lower values ​​are practically not used, since the loads of modern consumers simply will not allow this to be done. Thus, having the necessary values, it is very easy to choose an automaton corresponding to a specific case.