If you need to choose a circuit breaker for the electrical panel, you must first study its characteristics. It is important to know what the load will be on it. The most basic characteristic that they are guided by when buying this device is the rated current. It is she who is indicated on the body of the circuit breaker in large numbers and letters.
Content
- The principle of determining the nominal indicator
- Selection of a protective device
- Circuit breakers
- Features of electrical wiring
The principle of determining the nominal indicator
The explanatory dictionary of the Russian language of academician Yzhegov explains the meaning of the word "nominal", as designated, named, but not fulfilling its duties, appointment, that is, fictitious.
This definition quite accurately explains the electrical terms of rated voltage, current and power. They seem to be there, assigned and defined, but in fact serve only as guidelines for electricians. Actual numerical expressions of these parameters in reality differ from the assigned values.
For example, we are all familiar with a variable single-phase network with a voltage of 220 volts, which is considered nominal. In fact, its value according to GΟST can only reach the upper limit of 252 volts. This is how the state standard works.
The same picture can be seen with the rated current.
The basis for the choice of its value was taken the maximum possible thermal heating of electrical conductors (including their insulation), which must operate reliably under load for an unlimited time. At rated current, a thermal balance is maintained between:
- heating of conductors from the temperature effect of electric charges, described by the action of the Joule-Lenz law;
- cooling due to the removal of part of the heat into the environment.
In this case, the heat Q1 should not affect the mechanical and strength characteristics of the metal, and Q2 - on the change in the chemical and dielectric properties of the insulation layer.
Even with a slight excess of the nominal current value, after a certain period of time, it will be necessary to remove the voltage from the electrical equipment to cool the metal of the current lead and insulation. Otherwise, their electrical properties will be violated and a breakdown of the dielectric layer or deformation of the metal will occur.
Any electrical equipment (including power sources, their consumers, connecting wires and systems, protective devices) is calculated, designed and manufactured for operation at a certain nominal current. Its value is indicated not only in the technical factory documentation, but also on the body or nameplates of electrical equipment.
The above photo clearly shows the nominal current values of 2.5 and 10 amperes, which are made by stamping in the manufacture of an electrical plug. In order to standardize the equipment, GOST 6827-76 has put in place many values of rated currents at which almost all electrical installations should operate.
Selection of a protective device
Since the rated current determines the possibility of long-term operation of electrical equipment without any damage, then all protective devices for current are configured to operate when it is exceeded.
In practice, situations are quite common when an overload occurs for a short period in the power supply circuit for various reasons. In this case, the temperature of the metal of the conductor and the insulation layer does not have time to reach the limit when their electrical properties are violated.
For these reasons, the overload zone is allocated into a separate area, which is limited not only by the size, but also by the duration of the action. When the critical temperature values of the insulation layer and the metal of the conductor are reached, the voltage from the electrical installation must be removed to cool it. These functions are performed by the so-called thermal overload protection:
- circuit breakers;
- thermal releases.
They perceive the heat load and are set to shutdown with a certain time delay. The setting of the protections that perform "instantaneous" cut-off of the load is slightly higher than the overload current. The term "instantaneous" actually defines the action in the shortest possible period of time. For today's fastest overcurrent protections, cutoff is performed in a time slightly less than 0.02 seconds.
The operating current in the normal mode of supply is most often less than the nominal value.
In the given example, the case is analyzed for AC circuits. In DC voltage circuits, there is no fundamental difference in the relationship between the operating, rated current and the choice of settings for the protection operation.
Circuit breakers
The main characteristic of the circuit breaker is the rated current. In the protection of industrial devices and household power grids, they are the most widespread. Such switches combine in their design:
- thermal time-delayed releases;
- current cutoff, very quickly disconnecting the emergency mode.
At the same time, automatic switches for rated current and voltage are manufactured. By their size, protective devices are chosen for operation in the specific conditions of a certain circuit.
For this, the standards define 4 types of time-current characteristics for different designs of machines. They are denoted by the Latin letters A, B, C, D and are created for guaranteed tripping of faults with a multiplicity of rated current from 1.3 to 14.
The circuit breaker, according to the time-current characteristic, taking into account the temperature of its environment, is selected for a certain type of load, for example:
- semiconductor devices;
- lighting systems;
- circuits with mixed loads and moderate inrush currents;
- chains with high overload capacity.
The time-current characteristic can consist of three zones of action, as shown in the picture, or two (without the middle).
The designation of the rated current can be found on the body of the machine. The picture shows a switch labeled with a value of 100 amperes.
This means that it will operate (disconnect) not from the rated current (100 A), but from its excess. Suppose if the cut-off of the machine is set to a multiplicity of 3.5, then a current of 100x3.5 = 350 amperes or more will be stopped by it without a time delay.
When the thermal release is set to a multiplicity of 1.25, then when a value of 100x1.25 = 125 amperes is reached, the shutdown will occur after some time, for example, one hour. In this case, the circuit will work with overload during this period.
It should be borne in mind that other factors associated with maintaining the temperature regime of protection:
- environmental conditions;
- the degree of filling the switchboard with equipment;
- the possibility of heating or cooling from extraneous sources.
Features of electrical wiring
To determine the main electrical parameters of protections and wires, the load applied to them must be taken into account. To do this, it is calculated according to the rated power of the devices connected to the operation, taking into account the coefficient of their employment.
For example, a dishwasher, multicooker, an electric oven and a microwave oven, which consume a total power in normal mode of 5660 watts (taking into account frequency of inclusions).
The rated voltage of the household network is 220 volts. It is necessary to determine the load current that will pass through the wires and protective devices by dividing the power by the voltage. I = 5660/220 = 25.7 A.
Next, you need to look at the table of a number of rated currents for electrical equipment. It does not have a circuit breaker for such a current. But manufacturers produce machines for 25 amperes. Its value is closest to our objectives. Therefore, it should be taken as the basis for a protective device for wiring consumers of the outlet group.
After that, you need to decide on the material of the wires and the cross section. Take copper as a basis, since aluminum wiring, even for domestic purposes, is no longer popular because of its performance.
Electricians' handbooks contain tables for the selection of wires from different materials for current load. Take this case, taking into account the fact that the wiring is carried out with a separate PE-insulated cable hidden in the wall groove. Take temperature limits corresponding to room conditions.
The table will provide information that the minimum allowable cross-section of a standard copper wire for our case is 4 mm square. You cannot take less, but it is better to increase it.
Sometimes the problem arises of selecting the protection rating for an already working wiring. In this case, it is quite justified to determine the load current of the consumers' network with an electrical measuring tool and compare it with that calculated by the above theoretical method.
In this way, the term "rated current" helps electricians navigate the technical characteristics of electrical equipment.
