Short circuit. Physics. Short circuit in the house

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.

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-Ampere;

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 has occurred. 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 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.

We often hear “There is a short circuit”, “There is a short in the circuit”. It is immediately clear that something unplanned and bad happened. But why is the circuit short and not long? Let's put an end to the uncertainty and figure out what exactly happens when there is a short circuit in the electrical circuit.

What is a short circuit (SC)

An electric stingray swims in the ocean and is not happy short circuit, completely dispensing with knowledge of Ohm's law. For us, to understand the nature and causes of a short circuit, this law is simply necessary. So, if you haven’t already, let’s read about Ohm’s law, current, voltage, resistance and other wonderful physical concepts.

Now that you know all this, you can give the definition of a short circuit from physics and electrical engineering:

Short circuit- this is a connection of two points of an electrical circuit with different potentials, which is not provided for by the normal operating mode of the circuit and leads to a critical increase in current strength at the junction.

A short circuit leads to the formation of destructive currents that exceed permissible values, failure of devices and damage to wiring. Why is this happening? Let us examine in detail what happens in the circuit during a short circuit.

Let's take the simplest chain. It contains a current source, resistance and wires. Moreover, the resistance of the wires can be neglected. Such a diagram is quite sufficient to understand the essence of the short circuit.

In a closed circuit, Ohm's law applies: current is directly proportional to voltage and inversely proportional to resistance. In other words, the lower the resistance, the greater the current .

More precisely, for our circuit Ohm’s law will be written in the following form:

Here r is the internal resistance of the current source, and the Greek letter epsilon denotes the emf of the source.

What is meant by short circuit current? If resistance R in our circuit will not be, or it will be very small, then the current strength will increase, and a short circuit current will flow in the circuit:

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Types of short circuits and their causes

In everyday life, short circuits occur:

single-phase– when the phase wire is shorted to zero. Such short circuits happen most often;
two-phase– when one phase closes to another;
three-phase– when three phases are closed at once. This is the most problematic type of short circuit.

For example, on Sunday morning, your neighbor behind the wall connects phase and neutral in the socket by plugging in a hammer drill. This means that the circuit is closed and the current flows through the load, that is, through the device plugged into the outlet.

If a neighbor connects the phase and neutral wires in the socket without connecting the load, then a short circuit will occur in the circuit, but you will be able to sleep longer.

For those who do not know, for a better understanding it will be useful to read what phase and zero are in electricity.

A short circuit is called a short circuit, since the current in such a circuit closure seems to follow a short path, bypassing the load. A controlled or long circuit is the usual, familiar to everyone, plugging in devices into a socket.

Short circuit protection

First, about what consequences a short circuit can cause:

Damage to a person by electric shock and generated heat.
Fire.
Failure of devices.
Power outage and lack of internet at home. As a result, there is a forced need to read books and dine by candlelight.

As you can see, a short circuit is an enemy and a pest that needs to be fought. What are the methods of short circuit protection?

Almost all of them are based on quickly opening the circuit when a fault is detected. This can be done using various short circuit protection devices.

Almost all modern electrical appliances have fuses. High current simply melts the fuse and the circuit breaks.

The apartments use short circuit protection circuit breakers. These are circuit breakers designed for a specific operating current. When the current increases, the machine is triggered, breaking the circuit.

To protect industrial electric motors from short circuits, special relays are used.

Now you can easily define a short circuit, at the same time you know about Ohm’s law, as well as phase and zero in electricity. We wish everyone not to cause short circuits! And if you are stuck in your head and have absolutely no energy for any work, our student service will always help you cope with it.

And finally, a video about how NOT to handle electric current.

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 voltage drop 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 , previously consumed by 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 by fuses or circuit breakers.

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 release 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 solenoid (a coil made of a copper 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

Let's consider a special case of parallel connection of conductors - the so-called short circuit. It is called the parallel connection of a conductor with very low resistance in a circuit. Let's look at an example.
Let the lamps and the switch be connected as shown in the diagrams. Note that the switch and the second lamp are connected in parallel, in addition, the closed switch in the right diagram is a conductor with very little resistance. Therefore, according to the definition, In the right diagram there is a short circuit in the lamp.

Suppose, for example, that the voltage of the current source is selected so that when the switch is open, both lamps do not glow very brightly - at half intensity (that is why in the first diagram they are half shaded). If the switch is closed, the left lamp will burn brightly, and the right lamp will go out altogether. Thus, increasing the brightness of the left lamp indicates to us that When there is a short circuit in the circuit, the current increases sharply. According to the Joule-Lenz law, an increase in current strength can lead to overheating of the wires and a fire.
Let us explain why the left lamp lights up brighter. Let us remember that when conductors are connected in parallel, their total resistance becomes less than the smaller one, that is, even less than the resistance of the switch (for which it is already almost zero). According to Ohm's law, a decrease in resistance leads to an increase in current. And an increase in current, according to the Joule-Lenz law, leads to a stronger heating of the spiral of the left lamp.
Let us now explain why the right lamp goes out. Since when the conductors are connected in parallel, the voltage on each of them is the same, the voltages on the right lamp and on the switch are the same. According to Ohm's law, U=I·R. As we found out in the previous paragraph, the resistance of this connection is almost zero, that is, R»0. Substituting zero into the formula, we get: U=I·0=0. That is, the voltage on the switch and lamp is zero (more precisely, very small). This voltage is clearly not enough to keep the lamp glowing, so it goes out.

To protect electrical appliances from short circuits they are used circuit breakers. Their purpose is to turn off the electricity if the current increases above the permissible value. In the picture on the right you see auto fuse with a screw base like a lamp. Such fuses (colloquially “plugs”) are screwed into special sockets that are mounted on the wall.
There are also fuses. The main part in them is a thin (about 0.1 mm in diameter) wire made of tin or lead (see figure below). In the event of a strong increase in current, it melts almost instantly and the circuit opens, interrupting the current. Unlike "reusable" fuses, fuses are disposable electrical devices.

If we assume that the wires supplying current to the apartment wiring are made of aluminum and have a diameter of 1 mm, then the cross-sectional area of the lead wire will be 100 times smaller. In addition, looking at the table, we see that the resistivity of lead is about 10 times greater than that of aluminum. Therefore, the resistance of the wire is approximately 1000 times greater than the resistance of an aluminum wire of the same length.
Since the wire and the fuse (that is, the wire inside it) are connected in series, the current in them is the same. Since, according to the Joule-Lenz law, Q = I2Rt, therefore, the amount of heat released in the wire at each moment of time is 1000 times greater than in the wire. That is why the wire melts, but the electrical wiring remains intact. Currently, fuses are practically not used in technology, giving way to automatic ones.

Any person whose work involves servicing electrical equipment knows very well about the troubles that a short circuit (short circuit) poses. It is sometimes thought to represent damage. This is wrong. A short circuit is a process, or, if you like, an emergency mode of operation of any part of an electrical installation. But its consequences really lead to damage. The generally accepted definition is: “A short circuit is a direct connection of two or more points in an electrical circuit that have different potentials. Is an abnormal (unintended) mode of operation.”

To understand what exactly happens in the circuit at the moment when a short circuit occurs there, it is necessary to remember the principles of operation of the circuit elements. Let's imagine a simple circuit consisting of two conductors and a load (for example, a light bulb). Under normal conditions, there is a directed movement of charged elementary particles in a conductor, due to the constant influence of the source. They move from one pole of the source to the other through two sections of wire and a lamp. Accordingly, the lamp emits light because the particles do a certain amount of work in it.

When the direction of movement is constantly changing, but in this case it is not important. The number of electrons passing through a certain section of the circuit per unit time is limited by the resistance of the lamp, conductors, and EMF source. In other words, the current does not grow indefinitely, but corresponds to a steady state.

But for some reason the insulation on a section of the circuit is damaged. For example, a lamp was flooded with water. In this case, it decreases. As a result, the current flowing through the circuit is limited by the total resistance of the power source, wires and the water “isthmus” on the lamp. Usually this amount is so insignificant that it is not taken into account in calculations (with the exception of specialized calculations).

The result is an almost infinite increase in current, determined by the classical Ohm's law. Short circuit power is often mentioned in this case. It is determined by the limiting value of the electric current that the power source is capable of delivering before failure. By the way, this is why it is forbidden to connect (short-circuit) the opposite contacts of the batteries.

Although in the example we are considering the removal of lamp resistance from the circuit due to water getting into it, there are many reasons for a short circuit. For example, if we talk about the same circuit, then short circuit. can also occur if the insulation of at least one wire is broken and it comes into contact with the ground. In this case, the current from the power source will follow the path of least resistance, that is, to the ground, which has a huge capacity. Damage to the insulation of two wires at once and their contact will lead to the same result.

The above can be generalized: short circuits can be with or without ground. This does not affect the ongoing processes.

What kind of damage were discussed at the beginning of the article? As is known, the higher the current flowing through sections of the circuit, the greater their heating. With sufficient source power during short circuit. some sections of the chain simply burn out, turning into copper dust (for copper elements).

Short circuit protection is quite simple and effective. Reports of damage due to short circuits arise primarily due to incorrectly selected parameters of protection devices and incorrect selectivity. If we are talking about a 220 V household circuit, then when the current increases excessively, an electromagnetic release located inside breaks the circuit.