What is a short circuit: definition, explanation for dummies. What is a short circuit and overload currents in the electrical wiring of a house or apartment?

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.

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-Amp;

U - voltage, Volt;

I - current, Ampere.

Power is never infinite, it is always limited by something, therefore, with its fixed value, as the current increases, the voltage decreases. The dependence of these two parameters of the operating circuit, expressed graphically, is called the current-voltage characteristic.

And one more formula necessary to calculate short-circuit currents is the Joule-Lenz law. It gives an idea of how much heat is generated when resisting a load, and is very simple. The conductor will heat up with an intensity proportional to the voltage and square of the current. And, of course, the formula is not complete without time; the longer the resistance heats up, the more heat it will release.

What happens in a circuit during a short circuit

So, the reader can consider that he has mastered all the main physical laws in order to understand what the magnitude (okay, let there be strength) of the short circuit current can be. But first you need to decide on the question of what, exactly, it is. KZ (short circuit) is a situation in which the load resistance is close to zero. Let's look at the formula of Ohm's law. If we consider its version for a section of the circuit, it is easy to understand that the current will tend to infinity. In the full version, it will be limited by the resistance of the EMF source. In any case, the short circuit current is very large, and according to the Joule-Lenz law, the greater it is, the more the conductor along which it runs heats up. Moreover, the dependence is not direct, but quadratic, that is, if I increases a hundredfold, then ten thousand times more heat will be released. This is the danger of the phenomenon, which sometimes leads to fires.

The wires become red-hot (or white-hot), and they transfer this energy to the 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, carelessly 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 protection system 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 value of I short circuit. on the length of the conductor, the cross-section of the wire and the power of the power transformer. The graphs are a collection of exponentially descending lines, from which all that remains is to choose the appropriate one. The method provides approximate results, but its accuracy is well suited to the practical needs of power engineers.

How does the process work?

Everything seems to happen instantly. Something hummed, the light dimmed and then went out. In fact, like any physical phenomenon, the process can be mentally stretched, slowed down, analyzed and divided into phases. Before the onset of an emergency, the circuit is characterized by a steady current value that is within the rated mode. Suddenly the total resistance drops sharply to a value close to zero. Inductive components (electric motors, chokes and transformers) of the load seem to slow down the process of current growth. Thus, in the first microseconds (up to 0.01 sec), the short circuit current of the voltage source remains practically unchanged and even decreases slightly due to the onset of the transient process. At the same time, its EMF gradually reaches zero value, then passes through it and is established at some stabilized value, ensuring the occurrence of a large I short circuit. The current itself at the moment of the transient process is the sum of periodic and aperiodic components. The shape of the process graph is analyzed, as a result of which it is possible to determine a constant value of time, depending on the angle of inclination of the tangent to the acceleration curve at the point of its inflection (the first derivative) and the delay time, determined by the value of the reactive (inductive) component of the total resistance.

Short circuit shock current

The term “short circuit shock current” is often used in technical literature. You should not be afraid of this concept; it is not at all that scary and has no direct relation to electric shock. This concept means the maximum value of I short circuit. in an alternating current circuit, usually reaching its value half a cycle after an emergency situation occurs. At a frequency of 50 Hz, the period is 0.2 seconds, and its half is, respectively, 0.1 seconds. At this moment, the interaction of conductors located close to each other reaches its greatest intensity. The short-circuit shock current is determined by a formula that makes no sense to present in this article, which is not intended for specialists or even students. It is available in specialized literature and textbooks. In itself, this mathematical expression is not particularly difficult, but it requires rather voluminous comments that deepen the reader into the theory of electrical circuits.

Useful short notice

It would seem that the obvious fact is that a short circuit is an extremely bad, unpleasant and undesirable phenomenon. It can lead, at best, to a blackout of the facility, shutdown of emergency protective equipment, and at worst, to burnout of wiring and even a fire. Therefore, all efforts must be concentrated on avoiding this misfortune. However, calculating short-circuit currents has a very real and practical meaning. A lot of technical means have been invented that operate in high current modes. An example is a conventional welding machine, especially an arc welding machine, which during operation practically short-circuits the electrode to grounding. Another issue is that these modes are short-term in nature, and the power of the transformer allows them to withstand these overloads. When welding, huge currents pass at the point of contact of the end of the electrode (they are measured in tens of amperes), as a result of which enough heat is released to locally melt the metal and create a strong seam.

Protection methods

In the very first years of the rapid development of electrical engineering, when humanity was still bravely experimenting, introducing galvanic devices, inventing various types of generators, motors and lighting, the problem arose of protecting these devices from overloads and short-circuit currents. The simplest solution was to install fusible elements in series with the load, which were destroyed under the influence of resistive heat if the current exceeded the set value. Such fuses still serve people today; their main advantages are simplicity, reliability and low cost. But they also have disadvantages. The very simplicity of the “plug” (as the holders of fusible rates called it for their specific shape) provokes users after it burns out not to philosophize, but to replace the failed elements with the first wires, paper clips, or even nails that come to hand. Is it worth mentioning that such protection against short-circuit currents does not fulfill its noble function?

In industrial enterprises, automatic switches began to be used to de-energize overloaded circuits earlier than in residential switchboards, but in recent decades, “traffic jams” have been largely replaced by them. “Automatic machines” are much more convenient; you don’t have to change them, but turn them on after eliminating the cause of the short circuit and waiting for the thermal elements to cool down. Their contacts sometimes burn out, in which case it is better to replace them and not try to clean or repair them. More complex differential circuit breakers, at a high cost, do not last longer than conventional ones, but their functional load is wider; they turn off the voltage in the event of minimal current leakage “to the side”, for example, when a person is electrocuted.

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

A short circuit is formed due to the short circuit of two wires of the circuit, which are connected to different contacts (this is plus and 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 can exceed the rated 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 an electrical equipment plug to a socket, electrical energy is converted into:

mechanical energy;
thermal power.

These types of closures can be conventionally called “long”. A short circuit is, in simple terms, a type of energy that is expressed as a spark, bang or fire. This is a condition when the resistance of the load itself becomes less than the resistance of the power source. When a short circuit occurs, the current instantly increases, which leads to a strong release of heat. This, in turn, can lead to melting of the wiring and its subsequent fire. Such a short circuit can not only disrupt the functionality of an electrical circuit element, but also lead to a decrease in the input voltage for 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 occur? In cases where there is a violation of the insulating coating, leading to a short circuit between the 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 transient resistance is not taken into account. This is possible only if it is specially prepared using bolted connections of live parts.

Short circuit current is a current that appears due to damage to the insulation of live parts that have different electrical potentials. It can also arise simply from the accidental connection of conductive parts with the same potentials.

The short circuit shock current is the maximum current value that occurs during a three-phase short circuit.

Short circuit mode is 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 states: “The current in a circuit is directly proportional to the voltage and inversely proportional to the resistance.” It is precisely the latter that is worth paying close attention to in this case. Due to the fact that the wiring resistance is very small, it is usually considered equal to “0”. In the case of a short circuit, its value, on the contrary, is very large, since current begins to flow in the closed circuit.

In order to prevent short circuits, it is necessary to periodically measure the wiring resistance. If you cannot do this on your own, then you should seek help from specialists. They will carry out all measurements related to wiring at a professional level, and will also help test instrument current transformers, which will also protect your equipment and increase fire safety.

The normal steady-state operating mode of an electrical installation is considered to be one whose parameters are within normal limits. Short circuit current (short circuit current) occurs during an accident in the operation of an electrical installation. It most often appears due to damage to the insulation of live parts.

As a result of a short circuit, the uninterrupted power supply to consumers is disrupted, and leads to malfunctions and failure of equipment. As a result, when selecting current-carrying elements and devices, it is necessary to calculate them not only for normal operation, but also to check them under the conditions of the expected emergency mode, which can be caused by a short circuit.

Causes of insulation damage

Impact on insulation mechanically.
Electrical breakdown of live parts due to excessive loads or overvoltage.
Similar to insulation failure, the whipping of uninsulated overhead line wires from strong winds can be considered a cause of damage.
Throwing metal objects onto a line.
Impact of animals on live conductors.
Errors in the work of maintenance personnel in electrical installations.
Failure in the functioning of protection and automation.
Technical aging of equipment.
A deliberate action aimed at damaging insulation.

Consequences of a short circuit

The short circuit current is many times higher than the current during normal operation of the equipment. Possible consequences of such a short circuit may be:

Overheating of live parts.
Excessive dynamic loads.
Stopping the supply of electrical energy to consumers.
Disruption of the normal functioning of other interconnected receivers that are connected to healthy sections of the circuit due to a sharp decrease in voltage.
Disorder of the power supply system.

Types of short circuits

The concept of a short circuit implies an electrical connection that is not provided for by the operating conditions of the equipment between points of different phases, either a neutral conductor with a phase or earth with a phase (if there is a neutral grounding circuit of the power source).

When operating consumers, the supply voltage can be connected in various ways:

According to the three-phase network diagram, 0.4 kilovolts.
Single-phase network (phase and zero) 220 V.
A constant voltage source with positive and negative potential terminals.

In each individual case, insulation failure may occur at certain points, resulting in a short circuit current.

For a 3-phase AC network, there are types of short circuit:

Three-phase circuit.
Two-phase circuit.
Single-phase ground fault.
Single-phase earth fault (Isolated neutral).
Two-phase ground fault.
Three-phase ground fault.

When implementing a project for supplying electrical energy to an enterprise or equipment, such modes require certain calculations.

Operating principle of short circuit

Before the onset of a short circuit, the current value in the electrical circuit had a steady value i p. During a sharp short circuit in this circuit, due to a strong decrease in the total resistance of the circuit, the electric current significantly increases to a value i p. Initially, when time t is zero, the electric current cannot change sharply to another steady-state value, since in a closed circuit, in addition to active resistance R, there is also inductive resistance L. This increases the process of current increase over time when switching to a new mode.

As a result, in the initial period of a short circuit, the electric current retains its original value i K= i no. It takes some time for the current to change. In the first moments of this time, the current rises to a maximum value, then decreases slightly, and then after a certain period of time it assumes a steady state.

The time period from the beginning of the fault to the steady state is considered a transient process. The short circuit current can be calculated for any moment during the transient process.

The short-circuit current in the transition mode is best considered as a sum of components: periodic current i pt with the largest periodic component I pt and aperiodic current i at (its largest value is I am).

The aperiodic component of the short-circuit current during a fault gradually attenuates to zero. In this case, its change occurs according to an exponential dependence.

The possible maximum short-circuit current is considered shock current i y. When there is no attenuation at the initial moment of the circuit, the surge current is determined:

I y – i p m+ i a t=0 ', Where i p m is the amplitude of the periodic current component.

Useful short circuit

It is believed that a short circuit is a negative and undesirable phenomenon, which causes destructive consequences in electrical installations. It can create conditions for a fire, shutdown of protective equipment, blackout of objects and other consequences.

However, short circuit current can be of real benefit in practice. There are many devices that operate in high current mode. For example, you can consider. The most striking example of this is electric arc welding, during which the welding electrode is short-circuited with a ground loop.

Such short circuit modes operate for a short time. The power of the welding transformer ensures operation under such significant overloads. During welding, a very large current is generated at the point of contact of the electrode. As a result, a significant amount of heat is released, sufficient to melt the metal at the point of contact and form a weld of sufficient strength.

Methods of protection

Even at the beginning of the development of electrical engineering, the problem of protecting electrical devices from excessive current loads, including short circuits, arose. The simplest solution was the installation, which burned out due to their heating due to the current exceeding a certain value.

Such fuse links are still in operation today. Their main advantage is reliability, simplicity and low cost. However, there are also disadvantages. The simple design of the fuse encourages a person, after the combustion of the fuse element, to replace it with improvised materials in the form of paper clips, wires and even nails.

Such protection is not capable of providing the necessary short circuit protection, since it is not designed for a specific load. In production, they use it to disconnect circuits in which a short circuit has occurred. They are much more convenient than conventional fuses and do not require replacement of the burnt element. After eliminating the cause of the short circuit and cooling of the thermal elements, the machine can simply be turned on, thereby applying voltage to the circuit.

There are also more complex protection systems in the form. They have a high cost. Such devices turn off the circuit voltage in the event of minimal current leakage. Such a leak can occur if a worker is electrocuted.

Another method of short circuit protection is a current-limiting reactor. It is used to protect circuits in high-voltage networks, where the magnitude of the short-circuit current can reach such a size that it is impossible to select protective devices that can withstand large electrodynamic forces.

The reactor is a coil with inductive reactance. It is connected in a circuit in a series circuit. During normal operation, the reactor has a voltage drop of about 4%. In the event of a short circuit, the main part of the voltage falls on the reactor. There are several types of reactors: concrete, oil. Each of them has its own characteristics.

Ohm's law for short circuit

The basis for calculating circuit closures is the principle that determines the calculation of current from voltage by dividing it by the connected resistance. The same principle works when determining rated loads. The difference is this:

When an emergency occurs, the process proceeds randomly and spontaneously. However, it lends itself to some calculations using methods developed by specialists.
During normal operation of an electrical circuit, resistance and voltage are in a balanced mode and may vary slightly within operating ranges within normal limits.

Power supply power

Based on this power, an assessment is made of the energy power potential of the destructive action that a short circuit current can carry out, and an analysis of the flow time and size is carried out.

For example, consider that a piece of copper conductor with a cross-sectional area of 1.5 mm 2 and a length of 50 cm was first connected directly to the Krona battery. And in another case, the same piece of wire was inserted into a household outlet.

In the case of the Krona, a short-circuit current will flow through the conductor, which will heat up this battery until it fails, since the battery power is not enough to heat and melt the connected conductor to break the circuit.

In the case of a household outlet, protective devices will operate. Let's imagine that these protections failed and did not work. In this case, the short circuit current will flow through the household wiring, then through the wiring of the entire entrance, house, and then through the overhead line or cable. So he will reach the substation.

As a result, a long circuit with many cables, wires, and various connections is connected to the transformer. They will greatly increase the electrical resistance of our experimental piece of wire. However, even in this case, there is a high probability that this piece of wire will melt and burn.

Circuit resistance

The section of the power line from the power source to the short circuit has some electrical resistance. Its value affects the magnitude of the short circuit current. The windings of transformers, coils, chokes, and capacitor plates contribute to the total resistance of the circuit in the form of capacitive and inductive reactances. In this case, aperiodic components are created that distort the symmetry of the main forms of harmonic oscillations.

There are many different methods used to calculate short-circuit current. They allow you to calculate the short-circuit current with the required accuracy using the available information. In practice, it is possible to measure the resistance of an existing circuit using the phase-zero method. This resistance makes the calculation more accurate and makes appropriate adjustments when selecting short circuit protection.

In this article we will look at the main headache of any electrician - short circuit. At the same time, we will explain what short-circuit current is and dispel the myth about what short-circuit voltage is, at the same time discussing that short circuit (aka short circuit) means for the electrical network. But first, a little physics, which will help you remember that electricity is the transfer of charge by electrons from one point to another. Consistent and orderly process. But sometimes an accident interferes with this strict sequence, and this is where you have to remember these two words “short circuit”.

Why is there a short circuit, and who is to blame for this?

Any electrical circuit diagram is represented by “plus” and “minus”, just like in any battery. If you place a light bulb between them, it will start to burn when the circuit is closed. A correctly assembled circuit will allow the light bulb to burn for quite a long time, which is successfully demonstrated by any flashlight. But let's see what happens if we simply connect the plus and minus batteries. Without a light bulb and without any resistance at all. Yes, in this model we get a pure electrical circuit. The wire between the battery contacts will heat up, the charge will be depleted almost instantly, and after a couple of seconds this battery will not light a single light bulb. All the energy of the battery will be spent on maximizing the short circuit current, heating the wire and completely depleting the resource. Such an experiment is safe for the experimenter, since the currents are small.

However, approximately the same thing will happen if you stick scissors into an outlet to see what happens. The current, having discovered the shortest path (scissors), will rush into the socket precisely through this short path from “plus” to “minus” (), forgetting about the other paths on which the circuit resistance awaits it. Hence the name of this trouble - “short circuit”. In fact, a short circuit is an opportunity for current to reach from “plus” to “minus” as quickly as possible and with maximum effect. In this case, the current becomes indiscriminate in its means, which is what short circuit protection is based on, and the basic rules on how to avoid this scourge.

So, a short circuit is an emergency situation in an electrical network, where the passage of current receives the shortest and most direct path to eliminate the potential (the potential difference between “plus” and “minus”), leading to an avalanche-like increase in current strength and strong heating of the circuit section, in in which the short circuit occurred.

Note that permanent (continuous short circuit) also occurs in networks that use power wires with an insufficient level of insulation (low insulation resistance), numerous unnecessary switchings (twists in distribution boxes, lines, etc.), as well as in wet areas.

It turns out that anyone is to blame for the short circuit, but not the electrician who did the wiring? Not certainly in that way. It is the electrician who is obliged, when laying the line or, including the terminal (pass-through) device, to ensure the impossibility of a short circuit. Otherwise, any short circuit protection will be useless. Most often, the protection fails precisely in shields assembled with violations, which leads to catastrophic consequences:

A little more about the causes of short circuits

Improperly insulated wires or physical movement of contacts in terminal devices (shifting, turning, other actions that can connect two wires).
Damage to cable insulation when laying (including hidden) power lines or during repair and finishing work.
The use of faulty devices (from the lamp socket to the terminal block and socket), in which there is a direct possibility of a short circuit.
Ignoring short circuits in electrical wiring during work (the most common mistake of novice electricians), since the short circuit effect is not repeated.
“Floating”, “sporadic” wiring faults, which are not given enough attention due to their rare occurrences.

This is a list of the most common causes of short circuits, failure of apartment and home electrical networks, as well as fires that are difficult to extinguish due to the constant feeding of fire from burning cables. Obviously, no one needs such troubles.

A few more words about the physics of short circuits.

Let's go back to our desk and remember that when current passes, you can observe how the current strength drops as the resistance of the conductor increases. This is the very factor due to which the short circuit current significantly exceeds the permissible parameters. This is how short circuit protection works - it monitors sudden surges in current, de-energizing the “suspicious” line.

Not everyone will remember that when the resistance in the conductor is removed, another parameter will also change. We are talking about the fact that the short circuit voltage will become very suspicious. And in the presence of an inductive factor (for example, a person with a hairdryer fell into a bathtub of water) it is completely nonlinear and not sinusoidal. In this case, there may not be a direct short circuit, but short circuit protection works in this case too - these are RCD circuit breakers. A protective shutdown device, the operating principle of which excludes response to changes in current strength only.

What do protective devices evaluate, and what should we know about short circuits if we don’t want to be saved only?

Any electrical network has points of instability. These are contacts, terminals, light switches and other automatic switches that operate on the basis of programs (for example, a light tracking sensor). Each of these points is a potential source of short circuit. It is to them that the electrician must pay maximum attention during work and installation;
Availability of grounding in the network. You will be surprised, but a ground fault (zero) is the safest short circuit. Yes, it will also cause a lot of trouble and trouble, but at least it won’t kill anyone. In addition, grounding devices allows you to assess the presence of insulation breakdown and leakage BEFORE a short circuit occurs.

Microwave ovens, dishwashers, washing machines, freezers and electric ovens must be grounded. Look at the back of the microwave. You will see a copper contact screwed on. This is grounding. You should not rely on a plug with zero contacts. Find a professional to ground this furnace. You will find the same contact on the back wall of the electric oven. On a freezer, this contact will most likely be in the area of the cooling coil. This is done for a reason, so don’t think that a fork can protect you. Find a way to really nullify this technique!

In addition to the above, the machines also determine the constant “network balance”, monitoring overloads and peak differences in both short-circuit currents (or similar in value) and voltages. But automatic machines will not become a panacea if a short circuit occurs on a section of your network that is laid in violation of the requirements and rules. For example, a wire running under a sheet of plywood or other flammable finishing material. Below is what will happen during a short circuit in such a place.

The process of occurrence of a short circuit. Shutdown time, process development, consequences

Despite the apparent “instantaneousness”, the short circuit process has well-described stages when it occurs.

The appearance of an unauthorized bridge between two conductors;
Breakdown of the “insulation barrier” by current and the emergence of a new, short circuit in the electrical circuit;
Redirection of energy and the occurrence of short circuit current in a new area;
A sharp increase in current strength, a drop in voltage and rapid heating of a new section of “resistance” - the wires in which a short circuit occurs;
Melting of wires (heating does not stop on its own, and the heating temperatures significantly exceed the melting temperatures of alloys and metals) with simultaneous ignition of the insulation;
Triggering of circuit breakers attempting to de-energize the problem area;
Relieve voltage and de-energize the line;
Continued heating of the damaged section of the network (even after de-energization, since heating is a much longer process) with fire of insulation or wires if the short circuit protection did not work as expected;
Failure of the network section in which the short circuit occurred.

All this takes about 2-4 seconds. Sufficient time for the wire to heat up to 1100 degrees and the insulation flares up like a match. In this case, it will not be possible to prevent a short circuit, only to minimize the damage. Despite the time, even with visual observation of the process of closing the electrical wiring, the occurrence of a short circuit, you simply will not have time to do anything. Therefore, here are some recommendations on how to avoid such a disaster:

If you can’t prevent it, lead it!

This phrase of the great politician perfectly describes the situation with the power grid, to which we trust a lot. And your life, and comfort, and almost all your property. Therefore, a list of simple recommendations will not be superfluous.

Test new electrical networks and communications with excess currents, simulating an overload. Such a test must be carried out with a specialist; doing it yourself is dangerous.

Do not neglect measuring the insulation resistance in the finished network. Yes, it costs money and takes time, but such a measurement will eliminate ground faults typical of long cables, and will also show the most dangerous areas, which may be better to replace.

The image shows that an arc (breakdown) can occur without physical contact of the conductors. That is why, when assembling sockets and switches, strip the wire insulation only in the area that is completely retracted into the terminal! Do not allow even a few millimeters of exposed wires, otherwise what can happen in the photo is an electric arc inside the device. Let us remind you that in such an incident, short circuit protection is almost guaranteed to be late in disconnecting the line!

Ill-considered expansion and addition of lines without protective measures is a direct path to short circuits and fire. This is a good example of what you should never do.