Once, a fitter told a lady who was not very knowledgeable in electrical engineering 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 lengthened 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 below is addressed to people who do not have a technical education, but, like everyone else, are not immune from the 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 to prevent it. You can not do in this description without getting acquainted with the basics of electrical science. A reader who does not know them may get bored and not finish reading the article to the end.

Popular exposition of Ohm's law

No matter what the nature of the current in an electrical circuit is, it only occurs if there is a potential difference (or voltage, it's the same thing). The nature of this phenomenon can be explained by the example of a waterfall: if there is a level difference, the water flows in some direction, and when not, it stands still. Even schoolchildren know Ohm's law, according to which the current is the greater, the higher the voltage, and the less, 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 competent translation from German. It is measured in Amperes (A).

In fact, the current itself does not have power (that is, the cause of acceleration), which is exactly what manifests itself during a short circuit. This term has already become familiar and is often used, although teachers of some universities, having heard the words “current strength” from the lips of a student, immediately put “bad”. “But what about the fire and smoke coming from the wiring during a short circuit? - the stubborn opponent will ask, - Is this not strength? There is an answer to this comment. 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 below.

U is the same potential difference, also called voltage. It is measured in Volts (we have V, abroad V). He is also called electromotive force(EMF).

R- 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. It is measured in ohms, but is estimated 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 it is. Therefore, resistivity is measured in ohms multiplied by square millimeter and divided by the metre. In addition, its value is affected by temperature, the higher it is, the more resistance. For example, a gold conductor 1 meter long and 1 sq. mm at 20 degrees Celsius has a total resistance of 0.024 Ohm.

There is also Ohm's law formula for complete chain, the internal (intrinsic) resistance of the voltage source (EMF) is introduced into it.

Two simple but important formulas

It is impossible to understand the reason why a short circuit current occurs without mastering another simple formula. The power consumed by the load is equal (excluding reactive components, but about them later) to 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 working circuit, expressed graphically, is called the current-voltage characteristic.

And one more formula necessary in order to calculate the 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 magnitude of the voltage and the square of the current. And, of course, the formula is not complete without time, the longer the resistance is heated, the more heat it will release.

What happens in the circuit during a short circuit

So, the reader can consider that he has mastered all the main physical laws in order to figure out what the magnitude (okay, let it be) of the short circuit current can be. But first you need to decide on the question of what, in fact, it is. A short circuit (short circuit) is a situation in which the load resistance is close to zero. We 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 larger it is, the more the conductor along which it goes is heated. 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 a phenomenon that sometimes leads to fires.

The wires glow red hot (or white hot), they transfer this energy to walls, ceilings and other objects that they touch and set them on fire. If the phase in some device touches the neutral conductor, a short-circuit current of the source occurs, closed to itself. The combustible base of electrical wiring is a nightmare for fire inspectors and the cause of many fines imposed on irresponsible owners of buildings and premises. And the fault, of course, is not the laws of Joule-Lenz and Ohm, but the insulation dried up from old age, carelessly or illiterate installation, mechanical damage or overloading of the wiring.

However, the short-circuit current, no matter how large, is also not infinite. The size of the troubles that he can do is affected by the duration of heating and the parameters of the power supply scheme.

AC circuits

The situations discussed above were of a general nature or concerned DC circuits. In most cases, both residential and industrial facilities are supplied with electricity from the network. AC voltage 220 or 380 volts. Trouble with DC wiring most often occurs in cars.

There is a difference between these two main types of power supply, and a significant one. The point is that the passage alternating current prevent additional components of resistance, called reactive and due to 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 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 rejected. The inductive current lags behind, and the capacitive current leads it by 90 degrees.

An example of the difference in the behavior of a load that has a reactive component is a conventional speaker. Some lovers of loud music overload it until the magnetic field knocks the diffuser 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 sources of direct or alternating current. The easiest way is with the usual plus, which suddenly connected with the minus, bypassing the payload.

But with alternating current, there are more options. Single phase current a short circuit occurs when a phase is connected to a neutral or grounded. In a three-phase network, unwanted contact between two phases may occur. A voltage of 380 or more (when transmitting power to long distances on power lines) volt can also cause unpleasant consequences, including an arc flash at the time of switching. It can close all three (or four, together with the neutral) wires at the same time, and the three-phase short circuit current will flow through them until the protective automatics work.

But that's not all. In rotors and stators electrical machines(motors and generators) and transformers, sometimes such an unpleasant phenomenon as an inter-turn short circuit 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 the entire machine to heat up. Actually, if such a disaster occurs, you should not wait until all the insulation melts and the electric motor smokes. The windings of the machine need to be rewound, this requires special equipment. The same applies to those cases when, due to the “interturn”, a short-circuit current of the transformer has arisen. The less the insulation burns, the easier and cheaper rewinding will be.

Short circuit current calculation

No matter how catastrophic this or that phenomenon, its quantitative assessment is important for engineering and applied science. The short circuit current formula is very similar to Ohm's law, it just needs some explanation. So:

I short circuit = Uph / (Zn + Zt),

I k.z. - value of short circuit current, A;

Uph- phase voltage, AT;

Zn is the total (including the reactive component) resistance of the short-circuited loop;

Zt is the total (including the reactive component) resistance of the power transformer (power), Ohm.

Impedances are defined as the hypotenuse of a right triangle, the legs of which are the values ​​of active and reactive (inductive) resistance. It's very simple, you need to use the Pythagorean theorem.

Somewhat more often than the short-circuit current formula, experimentally derived curves are used in practice. They represent dependences of the value of I k.z. on the length of the conductor, the cross section of the wire and the power power transformer. Charts are a collection of descending exponential lines, from which it remains only to choose the appropriate one. The method gives approximate results, but its accuracy is well suited to the practical needs of power supply engineers.

How is the process

Everything seems to happen instantly. Something hummed, the light dimmed and immediately 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 the emergency moment, the circuit is characterized by a steady current value that is within the nominal mode. Suddenly, the impedance drops sharply to a value close to zero. The inductive components (electric motors, chokes and transformers) of the load at the same time, as it were, 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 slightly decreases due to the onset of the transient. At the same time, its EMF gradually reaches zero, then passes through it and is set at some stabilized value, which ensures the flow of a large I short circuit. The current itself at the time of the transient process is the sum of the periodic and aperiodic components. The shape of the process graph is analyzed, as a result of which it is possible to determine constant value time, depending on the angle of inclination of the tangent to the acceleration curve at the point of its inflection (first derivative) and the delay time, determined by the value of the reactive (inductive) component of the total resistance.

Surge short-circuit current

In the technical literature, the term "short-circuit shock current" is often found. You should not be afraid of this concept, it is not at all so terrible and has nothing to do with electric shock. This concept means the maximum value of I k.z. in the alternating current circuit, which usually reaches its value in half a period after the emergency situation has arisen. At a frequency of 50 Hz, the period is 0.2 seconds, and its half is 0.1 seconds, respectively. At this moment, the interaction of conductors located close to each other reaches its maximum intensity. The shock short-circuit current is determined by the formula, which in this article, intended not for specialists and not even for students, does not make sense. 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 circuit

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 de-energization of the facility, shutdown of emergency protective equipment, and, at worst, to burnout of the wiring and even a fire. Therefore, all efforts must be focused on avoiding this scourge. However, the calculation of short-circuit currents has a very real and practical meaning. A lot of technical means have been invented that operate in the mode of high current values. An example is the usual welding machine, especially arc, which at the time of operation almost short-circuits the electrode with grounding. Another issue is that these regimes are of a short-term nature, and the power of the transformer makes it possible to withstand these overloads. When welding at the point of contact of the end of the electrode, huge currents pass (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 boldly experimenting, introducing galvanic devices, inventing different kinds generators, motors and lighting, the problem arose of protecting these devices from overloads and short circuit currents. Its simplest solution was to install fusible elements in series with the load, which were destroyed by resistive heat if the current exceeded the set value. Such fuses serve people today, their main advantages are simplicity, reliability and low cost. But they also have disadvantages. The very simplicity of the “cork” (as the holders of fusible rates were called for their specific shape) provokes users, after its burnout, not to philosophize slyly, but to replace the failed elements with the first wires, paper clips, and 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 plants for de-energizing overloaded circuits circuit breakers began to be used earlier than in apartment shields, but in recent decades, "plugs" have been largely replaced by them. "Automatic machines" are much more convenient, they can not be changed, but turned on by 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 automata at a high cost do not last longer than usual ones, but functionally their load is wider, they turn off the voltage in case of minimal current leakage “to the side”, for example, when a person is struck by an electric current.

In everyday life, experimenting with a short circuit is not recommended.

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The strength of the short circuit current at the motor terminals depends on the power of the supply transformer, the length and cross section of the connecting cable.

The strength of short-circuit currents is usually calculated using the no - resistance method. The essence of the method is to determine the resistance value of individual elements of the system, then the total (total) resistance of the entire network from the power supply source to the fault site, and by the magnitude of the total resistance - the short circuit current strength. In this case, the resistance of individual sections of the network with different voltages is referred to the voltage of the short circuit.

The strength of the short-circuit current that occurs in the switched section can be limited by introducing a resistance into this section, and the operating current of the rotor must pass through such a resistance only during switching. The latter is necessary in order not to have too large heat losses.

If the short circuit current exceeds 2000 A/V.

Measurement of short-circuit currents in galvanic cells with low internal resistance difficult because the connection measuring device(milliammeter or ammeter) increases the resistance of the circuit and distorts the true value of the current.

Determine the short-circuit current of the battery if, at a current of 2 A, a power of 24 W is released in the external circuit, and at a current of 5 A, a power of 30 W. Find maximum power, which can stand out in the chain.

To calculate the strength of the short circuit current in networks with voltages above 1000 V, it is necessary to know the resistances of generators, transformers, transmission lines and other elements.

To limit the strength of the short circuit current that occurs when an arc is formed between the electrodes in the electrostatic precipitator, and to extinguish the arc in the circuit primary winding high-voltage transformer connected in series buffer resistance (rheostat) BS.

FROM; / - short circuit current strength, A; R - conductor resistance, Ohm; t - short circuit time, s; cpr - heat capacity of the conductor material, J / / (kg - K); mr - mass of the conductor, kg.

The values ​​of the short circuit current calculated in this way make it possible to check the correspondence of the catalog values ​​of the thermal (thermal) and dynamic (shock) stability of the selected devices to the calculated values ​​of the short circuit currents.

What formula is used to determine the short circuit current.

Ijmn, close to the strength of the short circuit current, the generator does not produce more current with a further increase in the rotor speed.