Short circuit. Causes and how to avoid it

One of the main causes of fire is a short circuit. This phrase is constantly heard, but what does it mean?

This is a connection of a ground wire or a neutral wire with a phase wire or two phase wires. This results in the interaction of two conductors with different potentials. It is called a short contact because it occurred without an electrical appliance.

When such wires are connected, a small explosion occurs. This is explained by a sharp jump in current strength, reaching an unacceptable value. Such a rapid increase in current leads to overheating of the wires and the formation of an electric arc between them, the temperature of which reaches 5000 degrees C.

Particularly spectacular is the short circuit of phase wires in a three-phase electrical network. If a person shorts the phases with a screwdriver, he may be thrown several meters away and may receive serious injuries and burns. The screwdriver will simply evaporate. In everyday conditions, there may not be a big explosion, but melting of the wire and insulation is guaranteed, and this is a direct path to the ignition of objects that are nearby.

It is important to remember that when a power line (power line) breaks due to a short circuit, a real explosion with an electromagnetic shock can occur. Therefore, under no circumstances should you approach the place where the line breaks.

The causes of a short circuit are known: old or damaged electrical wiring, incorrect installation (this is typical for amateurs who have little knowledge of electricity), defective insulation, electrical appliances that do not meet electrical safety conditions (again, old or damaged), loosening of wire connections, random line breaks.

All of the above reasons can be successfully combated if you follow some rules:

1. Do not use old wires with inadequate insulation.

2. Be careful when carrying out electrical installation work. Do not drill, tap, or cut walls in places where the power cable is laid.

3. Remove insulation during installation very carefully, do not cut the wire with a knife along the cores.

4. Make sure that the network is turned off when working with it. You need to post a sign on the sign saying “work in progress, do not turn on the electricity” or leave a person on duty.

5. Install protective devices shutdown - automatic ical switches, residual current devices, automatic devices.

6. Regularly monitor the condition of electrical points - sockets and switches. If necessary, replace them immediately.

7. Do not operate damaged electrical appliances from which sparks fly, with the exception of some tools, for example, which have carbon brushes - they spark a little during operation (this happens in a drill, jigsaw and other tools).

8. When installing wiring, do not run the wires in one large bundle; it is better to run them parallel next to each other or use special boxes.

Following these simple rules will significantly reduce the risk of short circuits and fires. And it is important to remember that it is better to entrust work with electricity to a professional electrician. Then life will be calmer and safer!

Hi all. I am very glad that you visited my site. And today, we will talk about what a short circuit is and what kind of short circuits there are.

A short circuit is a connection (contact) of two or more points (conductors) electrical circuit with different potential values.

Different potentials are when phase and zero are in the network AC, or plus and minus in a DC network.

Now let's look at what types of short circuit there are.

IN single-phase network There can only be two types of short circuit:

1. phase and zero - this type of short circuit very often occurs in simple everyday conditions. For example, with the onset of winter it becomes cold, and many people try to warm up with the help of electric heaters.

But few people pay attention to the sockets into which these same heaters are plugged in. It often happens that the sockets are not designed for the currents that the heaters consume, or often the sockets may have poor contact.

Because of this, sockets and plugs begin to heat up. As a result of prolonged heating, the insulation of the wires is destroyed. And at one fine moment two, already exposed, conductors may touch, and a short circuit will result.

2. phase and grounding - this is when the phase wire somehow begins to contact the grounded body of any electrical equipment. Be it an electric water heater, a lamp, a machine tool, and so on.

It also happens that the housing may be zeroed, then such a short circuit can be attributed to the first case.

But in situations in which a short circuit occurs, it can be much more:

1. single-phase circuit - phase and zero. I have already described this type above, so let’s move on to the next one.

2. two-phase - this is when two phases are connected to each other. Often happens on overhead power lines. This phenomenon has probably been seen by every person in his life. When there is a strong wind outside and begins to loosen the wires, and does not receive much fireworks. In industrial enterprises, such a short circuit often occurs in power circuits.

3. two-phase and ground - this, of course, happens less often, but it still happens. An example when two phases can connect to each other, and at the same time also contact the ground.

4. three-phase - this is when all three phases are somehow closed together. Such a short circuit will occur when some conductive object falls or touches all three phases at the same time.

What could be the consequences of short circuit currents?

During a short circuit, the current instantly increases, which leads to strong heating and melting of metals. Splashes of this metal scatter in all directions, and all this is accompanied by a bright flash and fire. Which can easily lead to a fire and very serious consequences.

In ordinary home conditions, if you do not choose the right short circuit protection, you can really lose a lot. Starting from your home and furniture, and ending with your own and the lives of the people living with you under the same roof.

In enterprises, short circuit currents can lead to emergency situations, damage to equipment, and people can also suffer from this. But enterprises usually use several protections at once, which practically eliminates the occurrence of short circuits.

That's all I wanted to say. If you have any questions, ask them in the comments. If the article was useful to you, then share it with your friends on social networks and subscribe for updates. Until next time.

Best regards, Alexander!

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 most simple circuit. 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 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 – internal resistance 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:

Human Defeat 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. This circuit breakers, designed for a certain 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.

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 for its prevention. 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-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 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 had general character or touched DC circuits. In most cases, power supply to both residential and industrial facilities is supplied from the network AC voltage 220 or 380 Volt. 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, the 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 vector active current 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 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 three-phase network may arise unwanted contact between two phases. Voltage of 380 or more (when transmitting energy to long distances along power lines) volts can also cause unpleasant consequences, including an arc flash at the moment 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 resistance in AC mains. 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, wire cross-section and power 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 impedance sharply decreases 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. The EMF gradually reaches it zero value, then passes through it and is established at some stabilized value, ensuring the flow 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. The concept means maximum value I short circuit in an alternating current circuit, usually reaching its value half a cycle after it occurs emergency. 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. Quite a lot has been invented technical means, operating in high mode current values. An example would be the usual welding machine, especially an arc one, which at the time of operation practically short-circuits the electrode with 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 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 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 automata with high cost do not last longer than usual, but their functional load is wider; they turn off the voltage in the event of minimal current leakage “to the side,” for example, if a person is electrocuted.

In everyday life, experimenting with short circuits is not recommended.

Short circuit is one of the dangers of the technosphere

Even a person far from electricity, at least once in his life, has encountered a phenomenon called a “short circuit.” In order to protect yourself, your loved ones, as well as your home and electrical appliances from this process, you should carefully understand its nature, causes and varieties.

Concept and characteristics of short circuit

From the point of view of electrical engineering, a short circuit is a phenomenon in which the resistance of an electrical circuit consisting of several wires is extremely insignificant, and it can be compared with the resistance of the wires themselves. In this case, according to Ohm’s law, the current strength will exceed its nominal value several times at once, and this will happen almost in an instant. This, in turn, will lead to the electrical circuit breaking much earlier than a critical increase in the temperature of the wires occurs.

Main causes of short circuit

As practice shows, a short circuit most often occurs due to the fact that for some reason the external insulation of wires or electrical equipment is damaged. This, in turn, may be associated with the gradual aging of the main elements of the electrical circuit, and with its mechanical damage, and even with a lightning strike. In addition, in recent years At enterprises, cases have become more frequent when a short circuit was the result of unscrupulous maintenance of electrical equipment by the relevant services.

Artificial closure

However, in the work of factories and plants there may come a time when there will be a need to cause this phenomenon artificially. In particular, an intentional short circuit is often used in the circuit of transformer substations that operate by reducing the current. For this purpose, special equipment is used - short circuiters, which act as a kind of controllers. If any damage occurs on the line or in the transformer itself, this device will artificially cause a short circuit, the circuit will be broken and there will be no serious consequences (for example, a fire).

Consequences of a short circuit

This phenomenon leads to very serious consequences. Firstly, quite often it is accompanied by the failure of electrical installations and the occurrence of fires in them. Secondly, due to a sharp increase in current in the circuit, individual parts of the cable may be subjected to mechanical impact, resulting in mechanical and thermal damage. Thirdly, quite often a short circuit is accompanied by a significant voltage drop in the circuit or in its individual sections. This, in turn, leads to deterioration in the performance of electrical equipment. Finally, fourthly, this phenomenon has an extremely negative impact on nearby appliances, wires and other electrical equipment.

Short circuit protection methods

Short circuit protection includes the whole complex measures, the starting point of which is the prevention of damage to power lines and equipment. In addition, to prevent a fire, special devices are used - fusible switches, which, when shorted, burn out and open the electrical circuit.

Compliance with safety regulations as the main way to prevent short circuits

The power of a short circuit depends on many factors, the main one of which is the current strength in the circuit. At the same time, it should be remembered that any similar phenomenon represents a potential danger to humans, therefore, when working with electricity, you should strictly adhere to safety regulations.