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 when it overheats or exceeds its established limit values.
How a circuit breaker works
The principle of operation of any circuit breaker is based on two types of protection. These are thermal protection and electromagnetic protection. In modern ABs, both types of protection are combined, and such devices are designated by a special term - circuit breakers with a combined release.
Heat protection
The thermal protection of the circuit breaker is triggered in situations where working 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 such as an electric kettle, heater, washing machine, welding machine, etc. are turned on at the same time. on wiring, not designed for such loads. The thermal energy that arises inside the conductor (in this case, the wire) simply does not have time to dissipate due to the huge number of electrons, 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 operate and thereby de-energizing 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 plate drops to normal values, the power supply cannot be restored.
Electromagnetic protection
In case of a short circuit, when the current grows 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 comes into action, opening the circuit instantly. The fast magnetic flux inside the special solenoid pushes out the core, which causes the circuit to turn off. The high-temperature electric arc that inevitably occurs in this case is extinguished in a special chamber consisting of many independent plates, so the circuit breaker body does not melt.
It will be possible to supply power to the network only afterthe electrical appliance that caused the short circuit was found and disconnected. It is enough just to sequentially disconnect from the network each of the devices that were working at the time of the closure.
Power circuit breaker selection
In order for the circuit breaker to fully fulfill its purpose in the event of problems, it is necessary to carefully consider the choice of such a device. On the shelves of electrical 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, known from the school curriculum, one of the formulations 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 section of the circuit."
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 strength in amperes, the values of which are indicated on the cases of circuit breakers: 10A, 25A or 40A.
P - power. Everyone must calculate this value based on the number of electrical appliances used in a certain section of the wiring.
U - mains voltage, represented by a fixed number of 220 volts.
Example of AB power calculation
An example is the selection of a power circuit breaker for a large kitchen. Like a place wheremany energy-intensive consumers are used:
- The kitchen is equipped with a fridge, microwave, electric kettle, washing machine and a small TV. First you need to find out the total power of electrical appliances (this information is available in the instruction manual or duplicated on nameplates or stickers on the devices themselves). Most often, the indicators are approximately the following: refrigerator - 200W, microwave - 900W, electric kettle - 1800W, electric oven - 2400W, washing machine - 2000W, split system - 900W, TV - 50W. The total power of all devices is 8250W.
- The mains voltage is known - it is 220V.
- 8250W, i.e. P, must be divided by 220V, i.e. U.
- The result is 37.5A - it is this current that the machine will have to pass through itself. The closest commercially available device to the required performance 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 the circuit breaker by power. Where to find it? The table of ratings of automatic machines for current looks like this:
If you don't want to spend time looking for vehicle power values, another type of table will come in handy:
Selection of circuit breaker by cutoff current
In addition to the face value, each of the circuit breakers is marked with a letter,denoting the so-called instantaneous trip current. Among those used in everyday life there are machines with the following designations:
- B - ultra-sensitive devices designed for the use of low-current consumers. This means that such an automatic machine can work not only in case of a short circuit, but also when starting a conventional air conditioner, considering its starting current as exceeding the nominal value. That is why such devices are not used on ordinary lines.
- С - the most common group of circuit breakers, the rated cut-off current of which allows you to keep the network connected when using many modern household appliances, including such powerful consumers as heaters, air conditioners or washing machines. The choice of a circuit breaker by power is mainly carried out among the devices of this group.
- D - machines are suitable for those who use electrical appliances with high starting currents (for example, electric motors or welding machines). With a sensible organization of the home electrical network, they are strictly responsible for a certain line, unrelated to ordinary household loads.
Selection of circuit breaker by number of poles
Installation of AB, opening several poles at once, is most often used at industrial facilities or in general 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 main contentsbecome single-pole circuit breakers, connected to a phase break and not affecting the zero wire, mounted on a special bus. Provide protection for outlet lines and lighting from short circuits and overheating.
Two-pole 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.
The photo shows a 40A bipolar circuit breaker.
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 with four poles, but they are used less and less in everyday life.
Wire section
With proper wiring, you should not be limited to just choosing a circuit breaker for power. The cross section of the wire being laid is also important. Since the incorrect choice of wire thickness, even with the correct choice of the circuit breaker, can lead to very unpleasant situations in which constant overheating will cause the machine to constantly trip.
Accordingly, and choose the wire cross-section according to powershould be subject to certain requirements. The most important thing to consider in all situations is the rule of allowable heat.
Tolerable heat rule
Unshakable physical quantities help to follow it, namely resistance.
"Resistance is a physical quantity that characterizes the property of a conductor to prevent 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 layman, this is due to the fact that a certain part of the electricity transmitted through wires is spent on heating these same wires, which is caused by their resistance. And an increase in current 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 allowable losses and heating. Of course, you can build wiring for your home from large wires (for example, 4 or 6 mm2) and not think about the problem of overheating at all, however, given the considerable cost of cables with copper conductors, this option not everyone can afford it.
The easiest way to select the wire cross-section by power is to use the table:
Most often, copper cables are used in the formation of wiring, aluminum is mainly used for introductory lines. This is due to a whole list of advantages of copper over aluminum,among which: service life, conductivity, strength, ease of installation, etc. Of course, copper wires are more expensive than aluminum ones, but the overpayment, especially with proper selection, will not be so noticeable.
When installing, you should also take into account the features of the location of the wiring - external or internal. These nuances are regulated by another table.
Using this data, as well as a table of current ratings for automatic machines, it will be much easier to determine the required AB power. It is also easy to find out the approximate power of each of the electrical appliances. For example, a 10A circuit breaker is selected for a wire section of 0.75 mm2, which corresponds to a load of 1.3 kW.
It is also important to determine the maximum allowable cable length and the loads corresponding to this length, but this is not very applicable to home conditions where distances are not so great.
