To ensure safety when operating household electrical appliances, it is necessary to correctly calculate the cross-section of the power cable and wiring. Since an incorrectly selected cable cross-section can lead to a fire in the wiring due to a short circuit. This threatens to cause a fire in the building. This also applies to the choice of cable for connecting electric motors.
Current calculation
The current value is calculated by power and is necessary at the design (planning) stage of a dwelling - apartment, house.
- The value of this quantity depends on selection of power cable (wire), through which power consumption devices can be connected to the network.
- Knowing the voltage of the electrical network and the full load of electrical appliances, using the formula calculate the current that will need to be passed through the conductor(wire, cable). The cross-sectional area of the cores is selected based on its size.
If the electrical consumers in the apartment or house are known, it is necessary to perform simple calculations in order to correctly install the power supply circuit.
Similar calculations are performed for production purposes: determining the required cross-sectional area of the cable cores when connecting industrial equipment (various industrial electric motors and mechanisms).
Single-phase network voltage 220 V
Current strength I (in amperes, A) is calculated using the formula:
I=P/U,
where P is the electrical full load (must be indicated in the technical data sheet of the device), W (watt);
U – voltage of the electrical network, V (volts).
The table below shows load values of typical household electrical appliances and their current consumption (for voltage 220 V).
| electrical appliance | Power consumption, W | Current strength, A |
| Washing machine | 2000 – 2500 | 9,0 – 11,4 |
| Jacuzzi | 2000 – 2500 | 9,0 – 11,4 |
| Electric floor heating | 800 – 1400 | 3,6 – 6,4 |
| Stationary electric stove | 4500 – 8500 | 20,5 – 38,6 |
| microwave | 900 – 1300 | 4,1 – 5,9 |
| Dishwasher | 2000 - 2500 | 9,0 – 11,4 |
| Freezers, refrigerators | 140 - 300 | 0,6 – 1,4 |
| Electric meat grinder | 1100 - 1200 | 5,0 - 5,5 |
| Electric kettle | 1850 – 2000 | 8,4 – 9,0 |
| Electric coffee maker | 6z0 - 1200 | 3,0 – 5,5 |
| Juicer | 240 - 360 | 1,1 – 1,6 |
| Toaster | 640 - 1100 | 2,9 - 5,0 |
| Mixer | 250 - 400 | 1,1 – 1,8 |
| Hairdryer | 400 - 1600 | 1,8 – 7,3 |
| Iron | 900 - 1700 | 4,1 – 7,7 |
| Vacuum cleaner | 680 - 1400 | 3,1 – 6,4 |
| Fan | 250 - 400 | 1,0 – 1,8 |
| TV | 125 - 180 | 0,6 – 0,8 |
| Radio equipment | 70 - 100 | 0,3 – 0,5 |
| Lighting devices | 20 - 100 | 0,1 – 0,4 |
The figure shows diagram of the power supply device for an apartment with a single-phase connection to a 220 V network.
As can be seen from the figure, various electricity consumers are connected through corresponding machines to an electric meter and then to a general machine, which must be designed for the load of the devices with which the apartment will be equipped. The wire that supplies power must also satisfy the load of energy consumers.
Below is table for hidden wiring for a single-phase apartment connection diagram for selecting wires at a voltage of 220 V
| Wire core cross-section, mm 2 | Conductor core diameter, mm | Copper conductors | Aluminum conductors | ||
| Current, A | Power, W | Current, A | power, kWt | ||
| 0,50 | 0,80 | 6 | 1300 | ||
| 0,75 | 0,98 | 10 | 2200 | ||
| 1,00 | 1,13 | 14 | 3100 | ||
| 1,50 | 1,38 | 15 | 3300 | 10 | 2200 |
| 2,00 | 1,60 | 19 | 4200 | 14 | 3100 |
| 2,50 | 1,78 | 21 | 4600 | 16 | 3500 |
| 4,00 | 2,26 | 27 | 5900 | 21 | 4600 |
| 6,00 | 2,76 | 34 | 7500 | 26 | 5700 |
| 10,00 | 3,57 | 50 | 11000 | 38 | 8400 |
| 16,00 | 4,51 | 80 | 17600 | 55 | 12100 |
| 25,00 | 5,64 | 100 | 22000 | 65 | 14300 |
As can be seen from the table, the cross-section of the cores depends, in addition to the load, on the material from which the wire is made.
Three-phase network voltage 380 V
With a three-phase power supply, the current strength I (in amperes, A) is calculated by the formula:
I = P /1.73 U,
where P is power consumption, W;
U - network voltage, V,
since the voltage in a three-phase power supply circuit is 380 V, the formula will take the form:
I = P /657.4.
If a three-phase power supply with a voltage of 380 V is supplied to the house, the connection diagram will look as follows.
The cross-section of the cores in the power cable at various loads with a three-phase circuit with a voltage of 380 V for hidden wiring is presented in the table.
| Wire core cross-section, mm 2 | Conductor core diameter, mm | Copper conductors | Aluminum conductors | ||
| Current, A | Power, W | Current, A | power, kWt | ||
| 0,50 | 0,80 | 6 | 2250 | ||
| 0,75 | 0,98 | 10 | 3800 | ||
| 1,00 | 1,13 | 14 | 5300 | ||
| 1,50 | 1,38 | 15 | 5700 | 10 | 3800 |
| 2,00 | 1,60 | 19 | 7200 | 14 | 5300 |
| 2,50 | 1,78 | 21 | 7900 | 16 | 6000 |
| 4,00 | 2,26 | 27 | 10000 | 21 | 7900 |
| 6,00 | 2,76 | 34 | 12000 | 26 | 9800 |
| 10,00 | 3,57 | 50 | 19000 | 38 | 14000 |
| 16,00 | 4,51 | 80 | 30000 | 55 | 20000 |
| 25,00 | 5,64 | 100 | 38000 | 65 | 24000 |
To calculate the current in the power supply circuits of a load characterized by high reactive apparent power, which is typical for the use of power supply in industry:
- electric motors;
- chokes for lighting devices;
- welding transformers;
- induction furnaces.
This phenomenon must be taken into account when making calculations. In powerful devices and equipment, the share of reactive load is higher and therefore for such devices in calculations the power factor is taken equal to 0.8.
Content:
The movement of charged particles in a conductor in electrical engineering is called electric current. Electric current is not characterized only by the amount of electrical energy passing through the conductor, since in 60 minutes electricity equal to 1 Coulomb can pass through it, but the same amount of electricity can be passed through the conductor in one second.
What is current strength
When the amount of electricity flowing through a conductor over different time intervals is considered, it is clear that over a shorter period of time the current flows more intensely, so another definition is introduced into the characteristics of electric current - this is the current strength, which is characterized by the current flowing in the conductor per second of time. The unit of measurement for the magnitude of passing current in electrical engineering is the ampere.
In other words, the strength of the electric current in a conductor is the amount of electricity that has passed through its cross-section in a second of time, marked with the letter I. The strength of the current is measured in amperes - this is a unit of measurement that is equal to the strength of a constant current passing through endless parallel wires with the smallest circular sections separated by 100 cm and located in a vacuum, which causes an interaction on a meter of length of the conductor with a force = 2 * 10 minus 7 degrees of Newton for every 100 cm of length.
Experts often determine the magnitude of the passing current; in Ukraine (strum power) it is equal to 1 ampere, when 1 coulomb of electricity passes through the cross-section of the conductor every second.
In electrical engineering, you can see the frequent use of other quantities in determining the value of the passing current: 1 milliampere, which is equal to one / Ampere, 10 to the minus third power of Ampere, one microampere is ten to the minus sixth power of Ampere.
Knowing the amount of electricity passing through a conductor over a certain period of time, you can calculate the current strength (as they say in Ukraine - strumu force) using the formula:
When an electrical circuit is closed and has no branches, then the same amount of electricity flows per second at each place in its cross section. Theoretically, this is explained by the impossibility of accumulating electrical charges in any place in the circuit; for this reason, the current strength is the same everywhere.
This rule is also true in complex circuits when there are branches, but it applies to some sections of a complex circuit that can be considered as a simple electrical circuit.
How is current measured?
The magnitude of the current is measured with a device called an ammeter, and also for small values - a milliammeter and a microammeter, which can be seen in the photo below:
There is an opinion among people that when the current strength in a conductor is measured before the load (consumer), the value will be higher than after it. This is an erroneous opinion, based on the fact that supposedly some amount of force will be expended to bring the consumer into action. Electric current in a conductor is an electromagnetic process in which charged electrons participate; they move in a direction, but it is not the electrons that transmit energy, but the electromagnetic field that surrounds the conductor.
The number of electrons leaving the beginning of the chain will be equal to the number of electrons after the consumer at the end of the chain, they cannot be used up.
What types of conductors are there? Experts define the concept of “conductor” as a material in which particles with a charge can move freely. Almost all metals, acids and saline solutions have such properties in practice. A material or substance in which the movement of charged particles is difficult or even impossible is called insulators (dielectrics). Common dielectric materials are quartz or ebonite, an artificial insulator.
Conclusion
In practice, modern equipment works with large current values, up to hundreds or even thousands of amperes, as well as with small values. An example in everyday life of the current value in different devices can be an electric stove, where it reaches a value of 5 A, and a simple incandescent lamp can have a value of 0.4 A; in a photocell, the value of the passing current is measured in microamps. In urban public transport lines (trolleybus, tram), the value of the passing current reaches 1000 A.
Once upon a time, a lady, not very knowledgeable in electrical engineering, was told by an installer the reason for the loss of light in her apartment. It turned out to be a short circuit, and the woman demanded that it be extended immediately. You can laugh at this story, but it’s better to consider this trouble in more detail. Electrical specialists, even without this article, know what this phenomenon is, what it threatens and how to calculate the short circuit current. The information presented below is addressed to people who do not have a technical education, but, like everyone else, are not immune from troubles associated with the operation of equipment, machines, production equipment and the most common household appliances. It is important for every person to know what a short circuit is, what are its causes, possible consequences and methods of preventing it. This description cannot be completed without familiarization with the basics of electrical engineering science. A reader who does not know them may get bored and not read the article to the end.
Popular presentation of Ohm's law
No matter what the nature of the current in an electrical circuit, it occurs only if there is a potential difference (or voltage, it's the same thing). The nature of this phenomenon can be explained using the example of a waterfall: if there is a difference in levels, the water flows in some direction, and when not, it stands still. Even schoolchildren know Ohm's law, according to which the higher the voltage, the higher the current, and the lower, the higher the resistance included in the load:
I is the magnitude of the current, which is sometimes called “current strength,” although this is not a completely correct translation from German. Measured in Amperes (A).
In fact, the current itself does not have any force (that is, the cause of acceleration), which is precisely what manifests itself during a short circuit. This term has already become familiar and is used often, although teachers of some universities, having heard the words “current strength” from a student’s mouth, immediately give it a “failure”. “What about the fire and smoke coming from the wiring during a short circuit? - the persistent opponent will ask, “Isn’t this strength?” There is an answer to this remark. The fact is that ideal conductors do not exist, and their heating is due precisely to this fact. If we assume that R=0, then no heat would be released, as is clear from the Joule-Lenz law given below.
U is the same potential difference, also called voltage. It is measured in Volts (in our country V, abroad V). It is also called electromotive force (EMF).
R is electrical resistance, that is, the ability of a material to prevent the passage of current. For dielectrics (insulators) it is large, although not infinite, for conductors it is small. Measured in Ohms, but evaluated as a specific value. It goes without saying that the thicker the wire, the better it conducts current, and the longer it is, the worse. Therefore, resistivity is measured in Ohms multiplied by a square millimeter and divided by a meter. In addition, its value is affected by temperature; the higher it is, the greater the resistance. For example, a gold conductor 1 meter long and 1 square meter in cross section. mm at 20 degrees Celsius has a total resistance of 0.024 Ohm.
There is also a formula for Ohm's law for a complete circuit; the internal (own) resistance of the voltage source (EMF) is introduced into it.
Two simple but important formulas
It is impossible to understand the reason why short-circuit current occurs without mastering another simple formula. The power consumed by the load is equal (without taking into account reactive components, but more on them later) the product of current and voltage.
P - power, Watt or Volt-Amp;
U - voltage, Volt;
I - current, Ampere.
Power is never infinite, it is always limited by something, therefore, with its fixed value, as the current increases, the voltage decreases. The dependence of these two parameters of the operating circuit, expressed graphically, is called the current-voltage characteristic.
And one more formula necessary to calculate short-circuit currents is the Joule-Lenz law. It gives an idea of how much heat is generated when resisting a load, and is very simple. The conductor will heat up with an intensity proportional to the voltage and square of the current. And, of course, the formula is not complete without time; the longer the resistance heats up, the more heat it will release.
What happens in a circuit during a short circuit
So, the reader can consider that he has mastered all the main physical laws in order to understand what the magnitude (okay, let there be strength) of the short circuit current can be. But first you need to decide on the question of what, exactly, it is. KZ (short circuit) is a situation in which the load resistance is close to zero. Let's look at the formula of Ohm's law. If we consider its version for a section of the circuit, it is easy to understand that the current will tend to infinity. In the full version, it will be limited by the resistance of the EMF source. In any case, the short circuit current is very large, and according to the Joule-Lenz law, the greater it is, the more the conductor along which it runs heats up. Moreover, the dependence is not direct, but quadratic, that is, if I increases a hundredfold, then ten thousand times more heat will be released. This is the danger of the phenomenon, which sometimes leads to fires.
The wires become red-hot (or white-hot), and they transfer this energy to walls, ceilings, and other objects they touch, setting them on fire. If a phase in some device touches the neutral conductor, a short-circuit current occurs from the source, closed to itself. The combustible base of electrical wiring is a nightmare for fire inspectors and the reason for many fines imposed on irresponsible owners of buildings and premises. And the fault, of course, is not the Joule-Lenz and Ohm laws, but insulation that has dried out from old age, careless or illiterate installation, mechanical damage or overloading of the wiring.
However, the short circuit current, no matter how large it may be, is also not infinite. The amount of trouble it can cause is influenced by the duration of heating and the parameters of the power supply circuit.
AC circuits
The situations discussed above were of a general nature or concerned DC circuits. In most cases, power supply to both residential and industrial facilities is carried out from an alternating voltage network of 220 or 380 Volts. Troubles with DC wiring most often occur in cars.
There is a difference between these two main types of power supply, and a significant one. The fact is that the passage of alternating current is prevented by additional resistance components, called reactive and caused by the wave nature of the phenomena arising in them. Inductances and capacitances react to alternating current. The short circuit current of the transformer is limited not only by the active (or ohmic, that is, one that can be measured with a pocket tester) resistance, but also by its inductive component. The second type of load is capacitive. Relative to the active current vector, the vectors of the reactive components are deviated. The inductive current lags behind, and the capacitive current leads it by 90 degrees.
An example of the difference in behavior of a load with a reactive component is a conventional speaker. Some fans of loud music overload it until the diffuser knocks the magnetic field forward. The coil flies off the core and immediately burns out because the inductive component of its voltage decreases.
Types of short circuit
Short circuit current can occur in different circuits connected to different DC or AC sources. The simplest situation is with the usual plus, which suddenly connected with the minus, bypassing the payload.
But with alternating current there are more options. Single-phase short circuit current occurs when a phase is connected to the neutral or grounded. In a three-phase network, unwanted contact between two phases may occur. A voltage of 380 or more (when transmitting energy over long distances along power lines) volts can also cause unpleasant consequences, including an arc flash at the time of switching. All three (or four, together with the neutral) wires can be short-circuited at the same time, and the three-phase short circuit current will flow through them until the automatic protective equipment is triggered.
But that's not all. In the rotors and stators of electrical machines (motors and generators) and transformers, such an unpleasant phenomenon as an interturn short circuit sometimes occurs, in which adjacent wire loops form a kind of ring. This closed loop has extremely low AC resistance. The strength of the short circuit current in the turns increases, this causes heating of the entire machine. Actually, if such a disaster occurs, you should not wait until all the insulation melts and the electric motor begins to smoke. The windings of the machine need to be rewound; this requires special equipment. The same applies to those cases when, due to the “interturn” short circuit current of the transformer, a short circuit current has arisen. The less the insulation burns, the easier and cheaper it will be to rewind.
Calculation of the current value during a short circuit
No matter how catastrophic this or that phenomenon may be, its quantitative assessment is important for engineering and applied science. The short circuit current formula is very similar to Ohm's law, it just requires some explanation. So:
I short circuit = Uph / (Zn + Zt),
I short circuit - short circuit current value, A;
Uph - phase voltage, V;
Zn is the total (including reactive component) resistance of the short-circuited loop;
Zt is the total (including reactive component) resistance of the supply (power) transformer, Ohm.
Impedances are defined as the hypotenuse of a right triangle, the legs of which represent the values of active and reactive (inductive) resistance. It's very simple, you just need to use the Pythagorean theorem.
Somewhat more often than the short-circuit current formula, experimentally derived curves are used in practice. They represent the dependences of the magnitude of I short circuit. on the length of the conductor, the cross-section of the wire and the power of the power transformer. The graphs are a collection of exponentially descending lines, from which all that remains is to choose the appropriate one. The method provides approximate results, but its accuracy is well suited to the practical needs of power engineers.
How does the process work?
Everything seems to happen instantly. Something hummed, the light dimmed and then went out. In fact, like any physical phenomenon, the process can be mentally stretched, slowed down, analyzed and divided into phases. Before the onset of an emergency, the circuit is characterized by a steady current value that is within the rated mode. Suddenly the total resistance drops sharply to a value close to zero. Inductive components (electric motors, chokes and transformers) of the load seem to slow down the process of current growth. Thus, in the first microseconds (up to 0.01 sec), the short circuit current of the voltage source remains practically unchanged and even decreases slightly due to the onset of the transient process. At the same time, its EMF gradually reaches zero value, then passes through it and is established at some stabilized value, ensuring the occurrence of a large I short circuit. The current itself at the moment of the transient process is the sum of periodic and aperiodic components. The shape of the process graph is analyzed, as a result of which it is possible to determine a constant value of time, depending on the angle of inclination of the tangent to the acceleration curve at the point of its inflection (the first derivative) and the delay time, determined by the value of the reactive (inductive) component of the total resistance.
Short circuit shock current
The term “short circuit shock current” is often used in technical literature. You should not be afraid of this concept; it is not at all that scary and has no direct relation to electric shock. This concept means the maximum value of I short circuit. in an alternating current circuit, usually reaching its value half a cycle after an emergency situation occurs. At a frequency of 50 Hz, the period is 0.2 seconds, and its half is, respectively, 0.1 seconds. At this moment, the interaction of conductors located close to each other reaches its greatest intensity. The short-circuit shock current is determined by a formula that makes no sense to present in this article, which is not intended for specialists or even students. It is available in specialized literature and textbooks. In itself, this mathematical expression is not particularly difficult, but it requires rather voluminous comments that deepen the reader into the theory of electrical circuits.
Useful short notice
It would seem that the obvious fact is that a short circuit is an extremely bad, unpleasant and undesirable phenomenon. It can lead, at best, to a blackout of the facility, shutdown of emergency protective equipment, and at worst, to burnout of wiring and even a fire. Therefore, all efforts must be concentrated on avoiding this misfortune. However, calculating short-circuit currents has a very real and practical meaning. A lot of technical means have been invented that operate in high current modes. An example is a conventional welding machine, especially an arc welding machine, which during operation practically short-circuits the electrode to grounding. Another issue is that these modes are short-term in nature, and the power of the transformer allows them to withstand these overloads. When welding, huge currents pass at the point of contact of the end of the electrode (they are measured in tens of amperes), as a result of which enough heat is released to locally melt the metal and create a strong seam.
Protection methods
In the very first years of the rapid development of electrical engineering, when humanity was still bravely experimenting, introducing galvanic devices, inventing various types of generators, motors and lighting, the problem arose of protecting these devices from overloads and short-circuit currents. The simplest solution was to install fusible elements in series with the load, which were destroyed under the influence of resistive heat if the current exceeded the set value. Such fuses still serve people today; their main advantages are simplicity, reliability and low cost. But they also have disadvantages. The very simplicity of the “plug” (as the holders of fusible rates called it for their specific shape) provokes users after it burns out not to philosophize, but to replace the failed elements with the first wires, paper clips, or even nails that come to hand. Is it worth mentioning that such protection against short-circuit currents does not fulfill its noble function?
In industrial enterprises, automatic switches began to be used to de-energize overloaded circuits earlier than in residential switchboards, but in recent decades, “traffic jams” have been largely replaced by them. “Automatic machines” are much more convenient; you don’t have to change them, but turn them on after eliminating the cause of the short circuit and waiting for the thermal elements to cool down. Their contacts sometimes burn out, in which case it is better to replace them and not try to clean or repair them. More complex differential circuit breakers, at a high cost, do not last longer than conventional ones, but their functional load is wider; they turn off the voltage in the event of minimal current leakage “to the side”, for example, when a person is electrocuted.
In everyday life, experimenting with short circuits is not recommended.
Electrical energy carries a fairly high danger, from which neither workers at individual substations nor household appliances are protected. Short circuit current is one of the most dangerous types of electricity, but there are methods on how to control, calculate and measure it.
What it is
Short circuit current (SCC) is a sharply increasing shock electrical impulse. Its main danger is that, according to the Joule-Lenz law, such energy has a very high rate of heat release. As a result of a short circuit, wires may melt or certain electrical appliances may burn out.
Photo - timing diagramIt consists of two main components - the aperiodic current component and the forced periodic component.
Formula – periodic
Formula – aperiodic According to the principle, the most difficult thing to measure is the energy of aperiodic occurrence, which is capacitive, pre-emergency. After all, it is at the moment of the accident that the difference between the phases has the greatest amplitude. Also, its peculiarity is the non-typical occurrence of this current in networks. The diagram of its formation will help show the principle of operation of this flow.
The resistance of the sources due to the high voltage during a short circuit is short-circuited over a short distance or “short circuit” - that’s why this phenomenon got its name. There is a short circuit current of three-phase, two-phase and single-phase - here the classification occurs according to the number of closed phases. In some cases, the short circuit may be shorted between phases and to ground. Then, to determine it, you will need to separately take into account grounding.
Photo – result of short circuit You can also distribute short circuits according to the type of electrical equipment connection:
- With grounding;
- Without him.
To fully explain this phenomenon, we suggest considering an example. Let's say there is a specific current consumer that is connected to a local power line using a tap. With the correct circuit, the total voltage in the network is equal to the difference in EMF at the power source and the voltage reduction in local electrical networks. Based on this, Ohm's formula can be used to determine the short-circuit current:
R = 0; Ikz = Ɛ/r
Here r is the short-circuit resistance.
If you substitute certain values, you can determine the fault current at any point along the entire power line. There is no need to check the short circuit multiplicity here.
Calculation methods
Let's assume that a short circuit has already occurred in a three-phase network, for example, at a substation or on the windings of a transformer, how then the short circuit currents are calculated:
Formula - three-phase fault current Here U20 is the voltage of the transformer windings, and Z T is the resistance of a certain phase (which was damaged in the short circuit). If the voltage in the networks is a known parameter, resistance must be calculated.
Each electrical source, be it a transformer, a battery terminal, or electrical wires, has its own nominal resistance level. In other words, everyone has their own Z. But they are characterized by a combination of active resistances and inductive ones. There are also capacitive ones, but they are not important when calculating high currents. Therefore, many electricians use a simplified method for calculating this data: an arithmetic calculation of the direct current resistance in series-connected sections. When these characteristics are known, it will not be difficult to calculate the impedance for a section or an entire network using the formula below:
Full grounding formula Where ε is the emf, and r is the resistance value.
Considering that during overloads the resistance is zero, the solution takes the following form:
I = ε/r = 12 / 10 -2
Based on this, the short circuit strength of this battery is 1200 Amperes.
In this way, it is also possible to calculate the short-circuit current for a motor, generator and other installations. But in production it is not always possible to calculate acceptable parameters for each individual electrical device. In addition, it should be taken into account that in case of asymmetrical short circuits, the loads have a different sequence, which requires knowing cos φ and resistance to take into account. For the calculation, a special table GOST 27514-87 is used, where these parameters are indicated:
There is also the concept of a one-second short circuit, here the formula for the current strength during a short circuit is determined using a special coefficient:
Formula – short circuit coefficient It is believed that, depending on the cross-section of the cable, a short circuit can pass unnoticed by the wiring. The optimal short circuit duration is up to 5 seconds. Taken from Nebrat’s book “Calculation of short circuits in networks”:
| Section, mm 2 | Short circuit duration permissible for a specific type of wire | |||
| PVC insulation | Polyethylene | |||
| Copper veins | Aluminum | Copper | Aluminum | |
| 1,5 | 0,17 | No | 0,21 | No |
| 2,5 | 0,3 | 0,18 | 0,34 | 0,2 |
| 4 | 0,4 | 0,3 | 0,54 | 0,36 |
| 6 | 0,7 | 0,4 | 0,8 | 0,5 |
| 10 | 1,1 | 0,7 | 1,37 | 0,9 |
| 16 | 1,8 | 1,1 | 2,16 | 1,4 |
| 25 | 2,8 | 1,8 | 3,46 | 2,2 |
| 35 | 3,9 | 2,5 | 4,8 | 3,09 |
| 50 | 5,2 | 3 | 6,5 | 4,18 |
| 70 | 7,5 | 5 | 9,4 | 6,12 |
| 95 | 10,5 | 6,9 | 13,03 | 8,48 |
| 120 | 13,2 | 8,7 | 16,4 | 10,7 |
| 150 | 16,3 | 10,6 | 20,3 | 13,2 |
| 185 | 20,4 | 13,4 | 25,4 | 16,5 |
| 240 | 26,8 | 17,5 | 33,3 | 21,7 |
This table will help you find out the expected conditional duration of a short circuit in normal operation, the amperage on the busbars and various types of wires.
If there is no time to calculate data using formulas, then special equipment is used. For example, the Shch41160 indicator is very popular among professional electricians - this is a 380/220V phase-zero short-circuit current meter. The digital device allows you to determine and calculate the short-circuit strength in household and industrial networks. Such a meter can be purchased at special electrical stores. This technique is good if you need to quickly and accurately determine the current level of a loop or section of circuit.
The “Emergency Emergency” program is also used, which can quickly determine the thermal effect of a short circuit, loss rate and current strength. The check is carried out automatically, known parameters are entered and it calculates all the data itself. This is a paid project, the license costs about a thousand rubles.
Video: protecting the electrical network from short circuits
Protection and equipment selection guidelines
Despite the danger of this phenomenon, there is still a way to limit or minimize the likelihood of emergency situations. It is very convenient to use an electrical apparatus to limit short circuits; this can be a current-limiting reactor, which significantly reduces the thermal effect of high electrical impulses. But this option is not suitable for domestic use.
Photo - diagram of the short-circuit protection unit At home, you can often find the use of automatic circuit breakers and relay protection. These releases have certain restrictions (maximum and minimum network current), if exceeded, the power is turned off. The machine allows you to determine the permissible ampere level, which helps increase safety. The choice is made among equipment with a higher protection class than necessary. For example, in a 21-amp network, it is recommended to use a 25-amp circuit breaker.
Short circuit current
Figure 1 shows a diagram of connecting an electric incandescent lamp to an electrical network. If the resistance of this lamp r l = 240 Ohm, and the mains voltage U= 120 V, then according to Ohm’s law the current in the lamp circuit will be:
Figure 1. Short circuit diagram at the switch terminals
Let's look at a case where the wires going to an incandescent lamp are short-circuited through a very small resistance, for example, a thick metal rod with resistance r= 0.01 Ohm, accidentally falling on two wires. In this case, the network current passing to the point A, will branch along two paths: one large part of it will go along the metal rod - a path with low resistance, and the other, a small part of the current, will pass along a path with high resistance - an incandescent lamp.
An emergency mode of operation of a network, when, due to a decrease in its resistance, the current in it sharply increases compared to normal, is called short circuit.
Let us determine the strength of the short circuit current flowing through the metal rod:
In fact, in the event of a short circuit, the network voltage will be less than 120 V, since a large current will create a large current in the network and therefore the current flowing through the metal rod will be less than 12,000 A. But still this current will be many times higher than the current consumed previously an incandescent lamp.
Short circuit power at current I short circuit = 12,000 A will be:
P kz = U × I short circuit = 120 × 12,000 = 1,440,000 W = 1,440 kW.
Current passing through a conductor generates heat, and the conductor heats up. In our example, the cross-section of the wires of the electrical circuit was designed for a small current - 0.5 A. When the wires are closed, a very large current will flow through the circuit - 12,000 A. Such a current will cause the release of an enormous amount of heat, which will certainly lead to charring and burning of the wire insulation , melting of wire material, damage to electrical measuring instruments, melting of switch contacts, knife switches, and so on. The source of electrical energy powering such a circuit may also be damaged. Overheating of the wires may cause a fire.
Each electrical network is designed for its own normal current.
Due to the dangerous, destructive and sometimes irreparable consequences of a short circuit, it is necessary to observe certain conditions when installing and operating electrical installations in order to eliminate the causes of a short circuit. The main ones are the following:
1) the insulation of the wires must correspond to its purpose (network voltage and operating conditions);
2) the cross-section of the wires must be such that their heating under existing operating conditions does not reach a dangerous value;
3) laid wires must be reliably protected from mechanical damage;
4) connections and branches must be as reliably insulated as the wires themselves;
5) crossing wires must be done so that the wires do not touch each other;
6) wires must be laid through walls, ceilings and floors so that they are protected from dampness, mechanical and chemical damage and are well insulated.
Short circuit protection
To avoid a sudden, dangerous increase in current in an electrical circuit during a short circuit, the circuit is protected with fusible or automatic switches.
Fuses are a low-fusible wire connected in series to the network. When the current increases above a certain value, the fuse wire heats up and melts, as a result of which the electrical circuit automatically breaks and the current in it stops.
A circuit breaker is a more complex and expensive protection device than a fuse. However, unlike a fuse, it is designed for repeated operations to protect circuits during emergency operating conditions. Structurally, the circuit breaker is made in a dielectric housing with a tripping mechanism built inside. The release mechanism has fixed and moving contacts. The moving contact is spring-loaded; the spring provides force for quick release of the contacts. The release mechanism is activated by one of two releases: thermal or magnetic.
The thermal release is a bimetallic plate heated by flowing current. When a current flows above the permissible value, the bimetallic plate bends and activates the tripping mechanism. The response time depends on the current (time-current characteristic) and can vary from seconds to an hour. Unlike a fuse, a circuit breaker is ready for next use once the plate has cooled.
An electromagnetic release is an instantaneous release, which is a coil made of a conductor, the movable core of which can also actuate the release mechanism. The current passing through the switch flows through the solenoid winding and causes the core to retract when the specified current threshold is exceeded. An instantaneous release, unlike a thermal release, operates very quickly (fractions of a second), but at a much higher current: 2 ÷ 14 times the rated current.
Video 1. Short circuit
