The correct use of protective devices prevents electric shock, the emergence and development of emergency situations. The correct choice of the machine according to the load power is necessary to create new and modernize the operating power supply networks.
Why are circuit breakers needed and how do they work
Devices in this category are capable of detecting excessive consumption of electrical energy. This occurs when a short circuit occurs or when powerful or reactive loads are connected. In such situations, the circuit breaker cuts off the 220 (380) V power supply without user intervention.
To perform the marked functions in the standard design, two technologies are used. With a rapid increase in current above a level determined by calculation, the solenoid creates a magnetic field that moves the rod. Through a mechanical drive, this assembly opens the contact group. Node parameters are calculated taking into account starting loads in order to eliminate false alarms.
The second protection is organized using a well-known phenomenon - heating the conductor by passing current. The corresponding section of the chain is created from a bimetallic plate. With increasing temperature, it changes shape until the contact ruptures. Some models of machines have a special adjustment to adjust the sensitivity level.
What is the danger of cable mismatch network load
Possible problems are easier to understand with a specific example. Initial data:
- standard power supply with alternating voltage U = 220 V;
- the apartment has an old aluminum wiring (cross-section 2.5 mm square.);
- the amperage of the machine is 30 A;
- connect 6 convectors of 750 watts and one iron of 850 watts.
Payment:
- total power (R) of consumers - 5 350 W;
- the current (I) in the circuit is calculated by the formula I = P / U = 5 350/220 = 24.32 A).
The machine will not work in this situation (30> 24.32A). Such a current will greatly heat the aluminum wire and melt the insulation. A circuit destroyed by a short circuit will have to be restored, which is difficult when installing networks inside building structures. In a worse situation, a fire will destroy significant material assets.
According to the PUE standards for a conductor with the considered parameters, the load power when connected to a single-phase 220V network should not exceed 4.4 kW. The corresponding current limit is 20 A. “Twisting”, oxides, and other defects in the joints can contribute to the negative development of the situation.
Circuit Weak Link Protection
In addition to the cross-section, when choosing a suitable cable product, they pay attention to real operating conditions. Normalized values are given for heating to a temperature not exceeding + 60 ° C. When installing the line on a site near a country house, it is necessary to provide protection against humidity and other adverse external influences.
Carefully check all parts of the electrical network. The basic rule is reliable protection, taking into account the performance of the site with the worst parameters. It should be borne in mind that copper is designed for the same cross section for high loads compared to aluminum. The purity of the metal is of particular importance.As impurities increase, conductivity worsens, and losses for useless and dangerous heating increase.
Intra-apartment wiring device
In such real estate objects, the following standard solutions are used to create a high-quality power supply system:
- the introductory machine must be set to the counter;
- behind the control device, a common residual current device (RCD) is mounted;
- further equip automatic circuit breakers (AB) individual lines.
RCD prevents accidents that cause leakage currents. In some situations, it prevents electric shock. However, comprehensive protective measures are performed using circuit breakers. Be sure to use effective grounding.
As a rule, it is convenient to put several groups in the kitchen for even distribution of loads. It is especially carefully recommended to select the distribution of powerful consumers:
- hobs;
- ovens;
- heating boilers, boilers, flow heaters;
- electric convectors, heat guns;
- air conditioners.
The wiring diagram has a tree structure. From the center line of the “trunk” make the necessary branches of the “branch” for connecting sockets and switches.
How to calculate the circuit breaker rating
The main function of AB is to protect the wiring. For this reason, first ensure compliance with the circuit breaker rating in amperes, cross-section and material (copper, aluminum) of the cable cores.
Determining the total capacity of consumers
How much electricity is needed for the functioning of lamps and other products, noted in the accompanying documentation. Power is indicated on the case. These data can be obtained on the official website of the manufacturer. However, simply adding kilowatts is not enough.
Active and rated component
The simple calculation algorithm shown in the example describes the resistive load situation. It is this component (active power - P) that is indicated in the technical passport of the corresponding product. It is determined by the meter for regular payments of energy consumed.
However, when connecting a machine or other equipment with an electric drive, it will be necessary to take into account the inductive component. Similarly, if there is a capacitor in the circuit.
Formulas and explanations:
- P = S * cos ϕ;
- Q = S * sin ϕ;
- S = P / cos ϕ;
- ϕ is the angle between the vectors P and S (phase shift).
The reactive component (Q) denotes a cyclical exchange of energy between the power source and the load. The sum of the vectors P and Q will help determine the total apparent power (S).
Increased starting currents
The inclusion of a powerful pump (another reactive load) is accompanied by an inrush and a subsequent oscillatory process with a transition to normal operation. The pulse duration, as a rule, does not exceed 1.5-2 seconds. Such a duration is insufficient for heating the bimetallic plate. But this may be enough to move the stem of the solenoid.
The list shows typical levels of exceeding the nominal value that activate the shutdown of the electromagnetic coil. In parentheses are the time delays before breaking the circuit with a bimetallic plate (sec):
- A - 30% (20-30);
- B - 200% (4-5);
- C - 5 times (1.5);
- D - 10 times (0.4).
The corresponding modes are taken into account when creating relevant standards. To prevent an erroneous trip, you must choose the appropriate type of machine.
Demand factor
This correction factor (Ks) is used to account for loads in real operating conditions: Estimated = S * Ks. Its value (interval from 0 to 1) indicates the number of connected consumers. This method is convenient to use when creating office and production projects, which implies the use of the same type of equipment: machine tools, computers, etc.
For homework, it’s not difficult to draw the right conclusion based on an elementary check. It is difficult to imagine a situation when in one room at the same time they use air conditioning for cooling, and they heat the air with a fan heater.
Power consumption calculation
The above formulas with a phase shift are used to correct inductive and capacitive loads. Resistive count on passport data without recount. The value of cos ϕ is taken from the accompanying documentation.
The current strength can be calculated as follows:
- P / U - constant power supplies, resistive loads;
- P / (U * cos ϕ) = P / (220 * cos ϕ) - one phase, ~ 220V, consumer reactive characteristics;
- P / (U * √3 * cos ϕ) = P / (380 * 1.7321 * cos ϕ) - three-phase network ~ 380V, inductive (capacitive) parameters of the equipment.
You can measure real voltages using a multimeter. The methodology for performing work operations is given in the official manufacturer's instructions.
Core section selection
The necessary information on load capacities is contained in the official documentation of cable manufacturers. It is recommended to choose a larger section from the series to avoid overheating and damage during operation. According to the current rules, conductors with an area of 1.5 mm or more are suitable for residential premises.
Given the increased energy supply of modern real estate, the minimum network capacity will not be enough. Experts advise providing for the connection of additional equipment as part of a subsequent modernization.
Rating of current circuit breakers
The limit value of the rating is determined by the formula Inom ≤ Ipr / 1.45, where Ipr is the current permissible in a continuous mode for a particular wiring. If you plan to install the network, proceed as follows:
- specify the connection scheme of consumers;
- collect passport data of equipment, measure voltage;
- according to the presented scheme, they are calculated separately, the currents in the individual circuits are summed;
- for each group you need to choose a machine that will withstand the corresponding load;
- determine cable products with a suitable conductor cross section.
If the networks are installed in gates and covered with plaster, disassembly is too difficult. In this case, the selection of an automatic machine over the cable section is used. Begin by assessing the load capacities of existing lines. The result obtained is used to evaluate suitable models of protective devices. Next, distribute consumers into groups taking into account the total capacity (sharing).
Denomination Rules
For correct conclusions, it is necessary to take into account the features of the connected equipment. If, according to the calculation, the total current is 19 amperes, users prefer to buy the device at 25A. This solution suggests the possibility of applying additional loads without significant restrictions.
However, in some situations it is better to choose a 20A circuit breaker. This provides a relatively shorter time to turn off the power when the current rises (temperature rises) with a bimetallic disconnector. Such a precaution will help to maintain the integrity of the motor windings when the rotor is blocked by a jammed drive.
Different response times are useful to ensure the selective operation of protective equipment. Devices with less delay are installed on the lines. In an emergency, only the damaged part is disconnected from the electricity. The introductory machine does not have time to turn off. Power supply through other circuits is useful for maintaining lighting, alarm, and other engineering systems in a healthy state.
Selection of a machine for power
Separate work operations are simplified by using specialized calculators. Such programs provide free information and reference sites.But the selection of an automatic machine for power will have to be done on the basis of real equipment.
Typical Algorithm:
- specify the initial consumption data of individual devices;
- distributed into groups, specify the total values;
- The results obtained are used to select protective equipment.
This parameter indicates the preservation of functionality in case of multiple excess current loads (times):
- B (3-5);
- C (5-10);
- D (40-50).
Requirements for the models of machines are formed taking into account the nominal and class of switching capabilities. Protective equipment is chosen with a margin to ensure operability in the event of an emergency.
Tabular way
In the reference materials indicate how much load power can be applied if the machines are installed in different power networks. Example for a 2 A model (values in kW):
- 220 V, 1 (2) poles, single-phase connection - 0.4;
- 380 V, 3 poles., “Triangle” - 2,3;
- 380 V, 4 poles., "Star" - 1.3.
The result must be increased to the nearest value in the model range for reliability.
Graphical way
This technique applies similar principles. But the test parameters are presented in clear graphical form.
The nuances of choice
In any case, the choice of a circuit breaker for current (power) is done with a margin. Experts recommend using a multiplying factor of 1.4-1.6. At the same time check the wiring ability to withstand the maximum load.
Calculation of the machine for the cross-section of wiring
For a typical household network, data can be taken from the following table:
| Conductor cross section, mm sq. | Permissible load power, W | Rating of the switch, And | ||
| Copper | Aluminum | 220 A, 1 phase | 380 V, 3 phases | |
| 1,5 | 2,5 | 2 200 | 5 300 | 10 |
| 2,5 | 4 | 4 400 | 10 500 | 20 |
| 4 | 6 | 5 500 | 13 200 | 25 |
Choosing a cable cross-section machine helps protect wiring. This technique is recommended by experienced professionals. If the area is unknown, it is calculated according to a typical geometric formula taking into account the measured diameter (D) of the conductor: S = (π * D2) / 4 = 0.785 * D2.
Formula for calculating current and voltage power
For calculations by these parameters, the definitions of total (S), active (P) and reactive (Q) power are used. The following formulas are suitable for calculating single-phase 220 V networks:
- S = U * I;
- P = U * I * cos ϕ;
- Q = U * I * sin ϕ.
The source data for the calculation can be taken from the directories. Also apply the measurement results.
Active load
Incandescent lamps and heaters do not have reactive characteristics. Such loads do not displace the phases of currents and voltages. Power is consumed fully at twice the frequency.
Capacitive load
When a capacitor is connected to an alternating current network, energy is exchanged in both directions. This process is not accompanied by useful work.
Negative reactive load
The explanations presented deal with the ideal situation. However, in reality, each reactive element has a certain electrical resistance. Do not forget about the corresponding losses in the connecting wires and other components of the circuit.
With significant values of the capacitive (induction) component, the noted problems must be taken into account. In some schemes, in addition to increasing the load capacity of the machines, additional compensation components are used.
What currents do machines calculate
The power of the protective device is selected according to the wiring current (calculated or tabular value), taking into account the consumption of the connected load. The rating of the machine is chosen less in order to maintain the integrity of the power line during operation. On different sections of the network, conductors of the corresponding section are installed, guided by the principles of the tree structure.
A deliberate decrease in the circuit breaker rating is permissible when connecting loads with reduced power consumption. In this embodiment, the use of lines with a large current margin is implied. Such a solution better protects the connected equipment from damage.
