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 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). It is also called electromotive force (EMF).
R - electrical resistance, that is, the ability of the 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 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 sq. mm at 20 degrees Celsius has a total resistance of 0.024 Ohm.
There is also the formula of Ohm's law for a complete circuit, 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, inaccurate 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 an alternating voltage network of 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 fact is that the passage of alternating current is prevented by 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. A single-phase short-circuit current 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 over long distances through power lines) volts 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 the rotors and stators of electrical machines (motors and generators) and transformers, sometimes such an unpleasant phenomenon as an interturn 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, V;
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 of the 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 process. 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 a constant value of 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 a conventional welding machine, especially an arc welding machine, which almost short-circuits the electrode with grounding at the time of operation. 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 mankind was still boldly 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 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?
At industrial enterprises, circuit breakers began to be used to de-energize overloaded circuits 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.
Consider a special case of parallel connection of conductors - the so-called short circuit. It is called the parallel inclusion in the circuit of a conductor with very little resistance. Consider 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, and the closed switch in the right diagram is a conductor with very little resistance. Therefore, according to the definition, on the right diagram there is a lamp short circuit.
Let, for example, the voltage of the current source be selected so that when the switch is open, both lamps do not glow very brightly - at full dimmer (therefore, in the first diagram they are half painted over). If the switch is closed, then 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 can lead to overheating of the wires and a fire.
Let's explain why the left lamp lights up brighter. Recall that when the conductors are connected in parallel, their total resistance becomes less than the smaller of them, that is, even less than the resistance of the switch (which already has almost zero). According to Ohm's law, a decrease in resistance leads to an increase in current. And the increase in current, according to the Joule-Lenz law, leads to a stronger incandescence 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 the lamp is zero (more precisely, very small). This voltage is clearly not enough to keep the lamp glowing, so it goes out.
|
Used to protect electrical appliances from short circuits. circuit breakers. Their purpose is to turn off the power if the current increases more than the permissible value. In the picture on the right you see auto a fuse with a screw base like a lamp. Such fuses (colloquially "plugs") are screwed into special cartridges that are mounted on the wall.
There are also fuses. In them, the main part is a thin (about 0.1 mm in diameter) wire made of tin or lead (see the figure below). In the case of a strong increase in current, it melts almost instantly, and the circuit opens, interrupting the current. Unlike "reusable" automatic fuses, fuses are disposable electrical appliances.
|
If we assume that the wires that supply 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 can see that the resistivity of lead is about 10 times greater than that of aluminum. Therefore, the resistance of the wire is about 1000 times 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 strength in them is the same. Since, according to the Joule-Lenz law, Q \u003d 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, and the electrical wiring remains intact. Currently, fuses are practically not used in technology, giving way to automatic ones.
A short circuit is formed due to the short circuit of two wires of the circuit, which are connected to different contacts (this is a plus and a minus). In this case, this happens through a small resistance, which can be compared with the resistance of the wire itself. In this case, the current may exceed the nominal value several times. To prevent fire, the electrical circuit must be broken before the wires reach a critical temperature.
What is a short circuit?
Every day, wherever we are, we close the electrical circuit. In this case, nothing dangerous happens, since when you connect the plug of electrical equipment to a socket, electrical energy is converted into:
- mechanical energy;
- thermal power.
These types of closure can be conditionally called "long". A short circuit is, in simple terms, a kind of energy that is expressed in the form of a spark, pop or fire. This is a state where the resistance of the load itself becomes less than the resistance of the power source. In the event of a short circuit, the current increases instantly, which leads to a strong release of heat. This - in turn - can lead to the melting of the wiring and its subsequent ignition. Such a short circuit can not only disrupt the performance of an electrical circuit element, but also lead to a decrease in the input voltage of other consumers.
In normal operating mode, current flows between the phase and neutral wires only when a load is connected, which limits it to a safe level for electrical wiring. How does a short circuit happen? In cases where there is a violation of the insulating coating, leading to a short circuit of plus and minus, the current bypasses the load and flows between these wires. This type of contact is called "short", due to the fact that it bypasses electrical appliances.
A metallic short circuit is a short circuit in which contact resistance is not taken into account. It is possible only if it is specially prepared by means of a bolted connection of current-carrying parts.
Short-circuit current is such a current that appears due to damage to the insulation of current-carrying parts with different electrical potentials. It can also arise simply by accidentally connecting conductive parts with the same potentials.
The shock short-circuit current is the maximum current that occurs during a three-phase short circuit.
The short circuit mode is such a state of a two-terminal network when its outputs are connected to each other using a conductor with zero resistance. In this mode, the secondary winding is short-circuited. When conducting such an experiment, it is possible to determine the amount of losses in the windings of the transformer itself.
It is also worth knowing that the short circuit voltage of a transformer is the voltage that must be applied to the winding when the second one is closed. And then the rated current will begin to flow in the last winding.
How to detect and prevent it?
You can recall the well-known Ohm's law, which says: "The current in the circuit is directly proportional to the voltage and inversely proportional to the resistance." It is on the latter that it is worth paying close attention in this case. Due to the fact that the wiring resistance is very small, it is considered to be equal to "0". In the case of a short circuit, its value - on the contrary - is very large, since current begins to flow in a closed circuit.
In order to prevent a short circuit, it is necessary to periodically measure the resistance of the wiring. If you can’t do it yourself, then you should seek help from specialists. They will professionally carry out all measurements related to wiring, as well as help to test instrument current transformers, which will also protect your equipment and increase fire safety.
Any person whose work is related to the maintenance of electrical engineering is very well aware of the troubles that a short circuit (short circuit) is fraught with. Sometimes it is considered to be damage. This is not true. A short circuit is a process, or, if you like, an emergency operation of a section of an electrical installation. But the consequences of it really lead to damage. The generally accepted definition reads: “A short circuit is a direct connection of two or more points in an electrical circuit with different potentials. It 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 recall the principles of the functioning of the circuit elements. Imagine the simplest 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 the 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, since 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 of time is limited by the resistance of the lamp, conductors, EMF source. In other words, the current does not grow indefinitely, but corresponds to a steady state.
But for some reason, the insulation in the circuit section is damaged. For example, the 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 water "isthmus" on the lamp. Usually this amount is so insignificant that it is not taken into account in the calculations (with the exception of specialized calculations).
The result is an almost infinite increase in current, determined by the classical Ohm's law. In this case, short circuit power is often mentioned. It is determined by the limiting value of the electric current that the power source is capable of delivering before failure. By the way, that is why it is forbidden to wire (short) the opposite contacts of the batteries.
Although in the example we are considering removing the resistance of the lamp from the circuit due to the ingress of water on it, there are many reasons for a short circuit. For example, if we talk about the same scheme, then k.z. It 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, into 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 summarized: short circuits can be with or without earth. It does not affect ongoing processes.
What kind of damage was discussed at the beginning of the article? As you know, the higher the value of the current flowing through the sections of the circuit, the greater their heating. With sufficient source power at short circuit. some sections of the circuit simply burn out, turning into copper dust (for copper elements).
Short circuit protection is quite simple and effective. Messages about damage due to a short circuit arise, first of all, due to incorrectly selected parameters of protection devices, incorrect selectivity. If we are talking about a 220 V household circuit, then they use it with an excessive increase in current, the electromagnetic release inside breaks the circuit.
Topic: what is a short circuit in an electrical circuit, what are the consequences of a short circuit.
Many have heard about an electrical short circuit, but not everyone knows the essence of this phenomenon. Let's deal with this. So, if you delve into the phrase “short circuit” itself, then you can understand that some kind of process is taking place in which something closes along a short, namely the shortest path for the flow of electric current (electric charges in a conductor). Simply put, there is a path through which electricity flows, its current of charges. These are various electrical circuits, conductors of electricity. The longer this path, the more obstacles the charges need to overcome, the greater the electrical resistance of this path. And from Ohm's law, it is known that the greater the resistance of the circuit, the less current will be in it (at a certain voltage value). Therefore, on the shortest path, there will be the maximum possible current, and this path will be short if the ends of the power source itself are shorted.
In general, we have, for example, an ordinary car battery (in a charged state). If you connect a light bulb designed for battery voltage (12 volts) to it, then as a result of the passage of a current of a certain value through this lamp, we will receive light and heat radiation. The lamp has a certain electrical resistance, which limits the current flowing through this circuit. To intentionally short circuit, we just need to take a piece of wire and connect it to the ends of the battery leads (parallel to the lamp). This wire has very little resistance compared to a lamp. Therefore, there is no special restriction that would prevent the movement of charged particles. And as soon as we close such a circuit, we get our short circuit. A large current will immediately flow through the wire, which can simply heat and melt this piece of wire.
As a result of such a short circuit, the conductor (its insulation) will ignite, up to a fire, if this conductor, by its ignition, transfers fire to flammable things that are nearby. In addition, such a sharp, spasmodic current flow can be harmful to the battery itself. It also starts to heat up at this time. And as you know, batteries really do not like excessive heat. At a minimum, their service life is significantly reduced after this, and as a maximum, they fail and even catch fire and explode. If such a short circuit occurs, for example, with a lithium battery in a phone (which has no electronic protection inside), strong heating occurs for several seconds, followed by a flame and an explosion.
There are some batteries that are originally designed to deliver high currents (traction batteries), but even with them a complete short circuit can lead to big trouble. Well, what happens to the voltage during a short circuit? From school physics it should be known that the greater the current, the greater the voltage drop in this section of the circuit. Therefore, when no load is connected to the power supply, the maximum voltage value can be seen on it (this is the EMF of the power supply, its electromotive force). As soon as we load this power supply, a certain voltage drop immediately appears. And the greater the load, the greater the voltage drop. Since, in the event of a short circuit, the circuit resistance is practically zero, and the current strength will be the maximum possible, the voltage drop across the power source will also be maximum (near zero).
This we considered the option of a complete short circuit, which occurs directly at the terminals of the power source. Yes, here's what else to add about it. In the case of a battery, there will be a large current load on the internal parts and chemicals of the battery itself (electrolyte, plates, leads). In the event of a short circuit on such power sources as electric generators, the current load falls on the windings of these generators, which leads to its excessive heating and damage (well, those circuits that operate in the generator after this winding). A short circuit at the terminals of various power supplies leads to overheating and failure of the electrical circuits of the current sources themselves and the secondary winding of the transformer.
A short circuit can occur in the electrical wiring circuit itself, the circuit. In this case, the consequences are also extremely negative. But at the same time, the current strength will already be, as a rule, slightly less than in the case of a short circuit at the output of the power source. For example, there is a sound amplifier circuit. Suddenly, due to poor insulation of the speakers themselves, a short circuit occurs at the sound output of this amplifier. As a result, the output transistors, the microcircuit in the last stages of sound amplification, will most likely burn out. The power supply itself in this case may not even be affected, since an excessive current load may not reach it. I think you got the point of the short circuit.
P.S. In any case, the phenomenon of an electrical short circuit leads to disastrous consequences. Normal fuses, circuit breakers, protection circuits, etc. are usually used to protect against this. Their task is to quickly break the electrical circuit with a sharp increase in current strength. That is, an ordinary fuse is, as it were, the weakest link in all electrical circuits. As soon as the current strength has increased sharply, the fusible link simply melts and breaks the circuit. This in most cases results in other circuits in the circuit remaining undamaged.
