The selection and installation of circuit breakers is one of the most important stages in the formation of wiring in both residential and non-residential premises. It is this device that is able to protect the electrical network from overload and short circuit, stopping the supply of current during overheating or exceeding its limit values.
How does a circuit breaker work
The principle of operation of any circuit breaker is based on two types of protection. This is thermal protection and electromagnetic protection. In modern AB, both types of protection are combined, and such devices are indicated by a special term - circuit breakers with a combined release.
Thermal protection
The thermal protection of the circuit breaker is triggered in situations when operating electrical appliances are connected to the network, the total power of which exceeds the maximum allowable for this network (or its segment). As a rule, this happens when serious consumers are turned on at once, such as an electric kettle, heater, washing machine, welding machine, etc. on wiring, not designed for such loads. Due to the huge number of electrons, the thermal energy that arises inside the conductor (in this case, wires) simply does not have time to dissipate, so the temperature of the conductor gradually increases. The plate installed in the circuit breaker also heats up, and at a certain moment under the influence of high temperature it begins to deform, causing the release to trip and thus de-energize the network.
Attempts to return the toggle switch of the circuit breaker to its working position in most cases are unsuccessful: until the temperature of the wire and the plate drops to normal values, the power supply cannot be restored.
Electromagnetic protection
In the event of a short circuit, when the current strength increases at lightning speed and causes a temperature jump that can melt the wiring and cause a fire, the thermal protection simply does not have time to work, so the electromagnetic protection circuit breaks into action instantly. A fast magnetic flux inside a special solenoid pushes the core, which causes the circuit to trip. Inevitably, the high-temperature electric arc arising in this case is extinguished in a special chamber consisting of many independent plates, therefore, the breaker case does not melt.
It will be possible to supply power to the network only after the electrical device that has caused a short circuit is found and turned off. It is enough to consistently disconnect from the network each of the devices that worked at the time of the circuit.
Selection of circuit breaker by power
In order for the circuit breaker to fully fulfill its purpose in case of problems, it is necessary to be very careful about the choice of such a device. On the shelves of electric stores you can find several categories of circuit breakers at once, and on each of them you can see completely different current ratings. To determine which of the devices is suitable for a particular wiring, you can use Ohm’s law, well known from the school curriculum, one of the wordings of which says: "The current strength in a circuit section is directly proportional to the voltage and inversely proportional to the electrical resistance of this circuit section."
This is expressed by the equally well-known formula I = P / U, which is quite acceptable for calculations in household energy.
I in this case is the current in amperes, the values of which are indicated on the circuit breakers: 10A, 25A or 40A.
P is the power. Each value must be calculated based on the number of electrical appliances used in a particular wiring section.
U is the voltage in the network, represented by a fixed number of 220 volts.
Example power calculation AB
An example is the selection of a circuit breaker in terms of power for a large kitchen. As a place in which many energy-intensive consumers are used:
- The kitchen includes a fridge, microwave, electric kettle, washing machine and small TV. First you need to find out the total power of electrical appliances (this information is available in the operating instructions or duplicated on the nameplates or labels on the devices themselves). The most common indicators are approximately the following: a refrigerator - 200W, a microwave - 900W, an electric kettle - 1800W, an electric oven - 2400W, a washing machine - 2000W, a split system - 900W, a TV - 50W. The total power of all devices is 8250W.
- The voltage in the network is known - it is 220V.
- 8250W, that is, P, must be divided into 220V, that is, U.
- The result is 37.5A - it is precisely this current that the machine will have to pass through itself. The closest to the required indicators from commercially available devices is a 40A circuit breaker.
Not everyone has the opportunity for one reason or another to make such calculations. In such cases, you can use the table to select a circuit breaker by power . Where to find her? The table of ratings of current machines looks like this:
If there is no desire to spend time searching for the power values of the equipment, a table of another type will come in handy:
Selection of circuit breaker for cut-off current
In addition to the rating, each of the circuit breakers is marked with a letter denoting the so-called instantaneous trip current. Among machines used in everyday life, there are machines with the following designations:
- In - supersensitive devices designed for use by low-current consumers. This means that such an automaton can work not only with a short circuit, but also when starting a conventional air conditioner, considering its starting current to exceed the nominal value. That is why such devices are not used on conventional lines.
- C is the most common group of circuit breakers, the rated cutoff current of which allows not to turn off the network when using many modern household appliances, including such powerful consumers as heaters, air conditioners or washing machines. The choice of a circuit breaker in terms of power is mainly carried out precisely among the devices of this group.
- D - machines are suitable for those who use appliances with high starting currents (for example, electric motors or welding machines). With the sensible organization of the home electrical network, they are responsible strictly for a certain line unconnected with ordinary household loads.
Selection of circuit breaker by number of poles
The installation of AB, opening several poles at once, is most often used at industrial facilities or in common house electrical panels. In domestic conditions, single-pole circuit breakers are mainly used.
Single pole circuit breakers
When installing a household electrical panel, to which a single-phase line is connected, its contents are made up of single-pole circuit breakers that are connected to phase disruption and do not affect the zero wire, mounted on a special bus. Provide protection for lines of sockets and lighting from short circuits and overheating.
Bipolar Circuit Breakers
In household energy, they are used as input devices that can open two wires at once - both phase and zero. When choosing a circuit breaker in terms of power, it should be borne in mind that the rating of such a device must correspond to the total load created by all consumers in the house or apartment - both electrical appliances and all lighting lines.
In the photo - bipolar circuit breaker 40A.
Three Pole Circuit Breakers
Such devices belong to the class of semi-industrial and are extremely rare in domestic conditions. The main scope of their use is three-phase networks. There are also devices on four poles, but they are used less and less in everyday life.
Wire cross section
With proper wiring, you should not be limited to just choosing a circuit breaker in terms of power. The cross section of the wire being laid is also important. Since an incorrect choice of wire thickness, even with the correct choice of a circuit breaker, can lead to very unpleasant situations in which constant overheating will cause the machine to operate continuously.
Accordingly, it is necessary to choose the wire cross section for power, observing certain requirements. The most important thing to consider in any situation is the rule of permissible heating.
Permissible Heating Rule
Unshakable physical quantities help to follow it, namely resistance.
"Resistance is a physical quantity characterizing the property of a conductor to impede the passage of electric current and is equal to the ratio of the voltage at the ends of the conductor to the strength of the current flowing through it."
In the field of electrical appliances, sockets and switches familiar to a simple inhabitant, this is explained by the fact that a certain part of the electricity transmitted through the wires is spent on heating these very wires, which is caused by their resistance. And an increase in current strength will inevitably cause an increase in the resistance of the wiring, and this, in turn, will lead to voltage drops. Therefore, the cross-section of the wires must correspond to the permissible losses and heating. Of course, you can build house wiring from wires of large cross-section (for example, 4 or 6 mm 2 ) and not think about the problem of overheating at all, however, given the considerable cost of cables with copper cores, this option is far from affordable for everyone.
The easiest way to select the wire cross section for power is to use the table:
Most often, copper cables are used in wiring, aluminum is mainly used for lead-in lines. This is connected with a whole list of the advantages of copper over aluminum, including: service life, conductivity, strength, ease of installation, etc. Of course, copper wires are more expensive than aluminum, but overpayment, especially with proper selection, will not be so noticeable.
During installation, you should also take into account the peculiarities of the wiring arrangement - external or internal. These nuances are regulated by another table.
Using this data, as well as a table of ratings of current-controlled machines, it will be much easier to determine the required power of AB. The approximate power of each of the electrical appliances is also easy to find. For example, a 10A circuit breaker is selected for a wire cross section of 0.75 mm2, which corresponds to a load of 1.3 kW.
It is also important to determine the maximum permissible cable length and the loads corresponding to this length, but this is not very applicable to home conditions, where the distances are not so great.