What is a single phase short circuit. Short circuit in electrical wiring. Short circuit current and voltage

Short circuit - this is a phenomenon in electrical engineering, which is accompanied by a short circuit (electrical connection) between two or three phases, a phase to a neutral conductor, a short circuit of a phase conductor to earth in networks with dead-earthed, and also effectively grounded neutral V three-phase network. In addition, a short circuit is an interturn circuit in electrical machines.

Characteristics this process is a significant increase in current and a drop in voltage. The current rises to values exceeding the nominal value by several times.

The generally accepted letter abbreviation of this phenomenon is KZ. Depending on the number of closed phases, several types of short circuits are distinguished. For clarity, we will depict diagrams that illustrate one or another type of short circuit in a three-phase electrical network.

The probability of occurrence of single-phase short circuits is the highest and is more than 60% of the total number of short circuits. Two-phase short circuits, including those to earth, occur less frequently, the probability of this emergency occurring is 20%. Three-phase short circuits are quite rare, the probability of their occurrence is 10%.

Causes of a short circuit

The main reason for the occurrence short circuit- violation of the insulation of electrical equipment, including cable and overhead power lines. Here are a few examples of the occurrence of a short circuit due to insulation failure.

When conducting earthworks the high-voltage cable was damaged, which led to the occurrence of a phase-to-phase short circuit. In this case, insulation damage occurred as a result of mechanical impact on the cable line.

in the open switchgear substation arose single-phase short circuit to the ground as a result of a breakdown of the support insulator due to the aging of its insulating coating.

Another fairly common example is the fall of a branch or tree on the wires of an overhead power line, which leads to whipping or breaking of the wires.

Methods for protecting equipment from short circuits in electrical installations

As mentioned above, short circuits are accompanied by a significant increase in current, which leads to damage to electrical equipment. Therefore, the protection of electrical installation equipment from this emergency mode is the main task of the power industry.

For protection against short circuits, as an emergency operation of equipment, in electrical installations distribution substations use different protective devices.

The main purpose of all relay protection devices is the section of the network where a short circuit has occurred.

In electrical installations with a voltage of 6-35 kV, overcurrent protection (MTP) is used to protect power lines from short circuits. To protect 110 kV lines from short circuits, differential-phase protection is used as the main line protection. In addition, for the protection of 110 kV transmission lines, distance protection and earth protection (TZNP) are used as backup protection.

In low-voltage networks, to protect circuits from short circuits, circuit breakers.

The short circuit is electrical connection different phases that are not typical for normal operation. As a result, the current strength increases sharply in the conductor, which leads to adverse consequences. Consider what a short circuit is, the classification of the phenomenon, potential threats and ways to prevent short circuit.

Short circuit is divided depending on the phase of the network. In a single-phase system, the following classification is distinguished:

phase and zero- the most common type in everyday life. The closure happens if you use electrical devices, which are not designed for standard value currents or if there is a bad contact in the socket. As a result of this, overheating is observed, and the insulation of the wires is broken;
phase and ground- a situation in which phase wire comes into contact with the grounded chassis of other equipment.

A short circuit can occur in a three-phase system:

single-phase- discussed above;
two-phase There are two systems involved in the process. A similar situation often happens with overhead power lines. Most often this happens during strong wind when the lines of wires intersect each other and form a short circuit;
three-phase and earth– simultaneous contact of three systems with the ground;
three-phase- simultaneous contact of three systems, provoked by the connection between a conductive object.

The main reasons that provoke the occurrence of short circuit:

violation of the integrity of the insulation, which may occur due to wear and tear of electrical equipment, due to contamination of the surface of devices, as well as mechanical damage;
mechanical violation of the integrity of the network elements (for example, a break in the transmission line);
voltage surges - breakdown of the conductor insulation, which leads to the development of current leakage and the creation of a short-term arc discharge;
lightning strike;
hit of animals and birds on current-carrying parts;
human factor - personnel errors during switching operations;
intentional short circuit using short circuiters - used to save circuit breakers. Today this technology does not apply and is prohibited.

What could be the consequences?

During short circuits, a sharp increase in current strength is observed, which leads to the melting of metals. "Splashes" can spread in all directions, leading to the ignition of objects around and fires. This is especially dangerous for home environments, as a short circuit can cause loss of property and housing. The impact on businesses is emergency situation, damage to equipment and the risk that people may be injured.

A short circuit, depending on the place of its formation, can lead to an accident in the system, the consequences of which will be economic and technical damage. Equipment that has been subjected to amplified current is out of order or is seriously damaged.

Another consequence of the closure is the deterioration of working conditions for personnel and consumers - a sharp drop in pressure leads to a shutdown of production facilities and economic damage. The greatest damage is done to the place in which the closure directly occurred.

Protection methods

The most reliable and in an efficient way short circuit prevention is the installation of circuit breakers. Fuses are an alternative. The machine timely catches the occurrence of a short circuit and turns off the power, so that the occurrence of an emergency situation is impossible.

Other Precautions:

regular revision of electrically conductive channels - visual definition weak points of the cable, where the insulation wears out and timely elimination of the problem;
the use of electrical reactors that regulate the supply of current;
the use of special electrical circuits, which, if necessary, turn off sectional switches;
the use of step-down transformers, which are equipped with a split low voltage winding.

Advice: For home use it is recommended to install circuit breakers. They are designed for a certain current, after exceeding the value of which, the circuit breaks. Other measures are mainly indicated for industrial use.

What is the risk of short circuit?

Closing first of all poses a threat to human health and life. It's connected with fire hazard: ignition of wire insulation, ignition of surrounding objects, the ability of insulation to spread combustion. Also, a change in current strength can be detrimental to the devices and devices used, leading to catastrophic consequences. A short circuit can cause economic loss. Therefore, it is important to use preventive measures for the occurrence of the phenomenon and resort to the installation of protection methods.

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

Why is the circuit short, and who is to blame?

Any scheme electrical circuit represents "plus" and "minus", as in any battery. If a light bulb is placed between them, it will start to burn when the circuit is closed. A properly assembled circuit will allow the light bulb to burn for quite a long time, which any flashlight successfully demonstrates. But let's see what happens if we just connect the plus and minus of the batteries. Without a light bulb and without any resistance at all. Yes, in this model we will get a circuit of the electrical wiring in its purest form. The wire between the contacts of the battery will heat up, the charge will be depleted almost instantly and after a couple of seconds this battery will not light a single bulb. All the energy of the battery will go to the maximum rise in 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, about the same thing will happen if you put scissors into the outlet to understand what will happen. The current, having found the shortest path (scissors), will rush into the outlet through this short path from “plus” to “minus” (), forgetting about the rest of the paths on which the resistance of the circuit awaits it. Hence the name of this trouble - "short circuit". In fact, a short circuit is an opportunity for the current to reach from “plus” to “minus” as quickly as possible and with maximum effect. At the same time, the current becomes illegible in means, on which the short circuit protection is built, and the basic rules of how to avoid this scourge.

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

Note that a permanent (continuous short circuit) also takes place in networks that use power wires With insufficient level insulation (low insulation resistance), numerous unnecessary connections (twisting in junction 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, protection does not cope precisely in shields assembled with violations, which leads to disastrous consequences:

A little more about the causes of a short circuit

Wrong insulated wires or physical movement of contacts in terminal devices (shift, rotation, other actions that can connect two wires).
Damage to cable insulation during laying (including hidden) power lines or during repair and decoration work.
The use of faulty devices in operation (from the cartridge to the lamp to the terminal block and socket), in which there is a direct possibility of a short circuit.
Ignoring electrical wiring short circuits during work (the most common mistake novice electricians), since the short circuit effect is not repeated.
"Floating", "sporadic" wiring faults that have not been given enough attention due to rare manifestations.

This is a list of the most common causes short circuits, failure of apartment and home electrical networks, as well as fires that are difficult to extinguish due to the constant supply 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 the desk, and remember that when the current passes, you can observe how the current strength drops with an increase in the resistance of the conductor. This is the very factor due to which the short-circuit current significantly exceeds valid parameters. This is how short circuit protection works - it monitors sudden current surges, 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 hair dryer fell into a bath of water), it is completely non-linear and not sinusoidal. In this case, there may not be a direct short circuit, but the short circuit protection works in this case too - these are RCD disconnectors. Device protective shutdown, the principle of operation of which excludes the response to a change in current strength only.

What do protective devices evaluate, and what should we know about short circuit if we do not want to be saved only?

Any electrical network has points of instability. These are contacts, terminals, light switches and other automatic switches that work 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 is obliged to pay maximum attention during work and installation;
The presence 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 will not kill anyone. In addition, grounding devices allows you to assess the presence of insulation breakdown and leakage BEFORE a short circuit occurs.

It is mandatory to ground the microwave oven, dishwasher and washing machine, freezer and electric oven. Look at the back of the microwave. You will see a screwed copper contact. This is grounding. Do not rely on a plug with "zero" contacts. Find a professional to ground this furnace. You will find the same contact on back wall electric oven. On freezer this contact will most likely be in the area of the cooling coil. This is done for a reason, so do not think that the fork can protect you. Find a way to “zero” such a technique for real!

In addition to the above, the machines also determine the constant “balance of the network”, tracking overloads and peak drops of both short-circuit currents (or close in value) and voltages. But automatic machines will not become a panacea if a short circuit occurs in a section of your network, which is laid with violations of requirements and rules. For example, a wire passing under a sheet of plywood or other combustible finishing material. About what happens during a short circuit in such a place below.

short circuit process. Shutdown time, process development, consequences

Despite the apparent "instantaneous", the process of a short circuit has well-described stages when it occurs.

The occurrence of an unauthorized bridge between two conductors;
Breakdown of the "isolation barrier" current and the emergence of a new, short, circuit in wiring diagram;
Redirection of energy, and the occurrence of a short circuit current in a new section;
A sharp increase current strength, voltage drop and rapid heating of a new section of "resistance" - wires in which a short circuit occurs;
Melting of wires (heating does not stop by itself, and heating temperatures significantly exceed the melting temperatures of alloys and metals) with simultaneous ignition of the insulation;
The operation of circuit breakers trying to de-energize the problem area;
Removal of voltage and de-energization of the line;
Continued heating of the damaged section of the network (even after a blackout, since heating is a much longer process) with ignition of insulation or wires if the protection against short circuit did not work as it should;
Failure of a section of the network in which a short circuit occurred.

All this takes about 2-4 seconds. Enough time for the wire to warm up to 1100 degrees and the insulation flared up like a match. In this case, it will not work to prevent a short circuit, only to minimize the damage. Despite the time, even with visual observation of the process of closing the wiring, the occurrence of a short circuit, you simply will not have time to do anything. Therefore, a few recommendations on how to avoid such a disaster

If you can't prevent it, take the lead!

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

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

Do not neglect the measurement of insulation resistance in the finished network. Yes, it costs money and takes time, but such a measurement will eliminate the ground fault inherent in long cables, and will also show the most dangerous areas that may be more correct to replace.

The image shows that the 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 bare wires, otherwise it may happen that in the photo - an electric arc inside the device. Recall that in such an event, the short-circuit protection is almost guaranteed to be late with the disconnection of the line!

An ill-conceived extension and the addition of lines without protective measures is a direct road to a short circuit and a fire. This good example what should never be done.

Short circuit occurs when connecting two circuit wires connected to different terminals (for example, in circuits direct current these are the "+" and "-") of the source through a very small resistance, which is comparable to the resistance of the wires themselves.

Short circuit current may exceed rated current in the chain many times. In such cases, the circuit must be broken before the temperature of the wires reaches dangerous values.

To protect the wires from overheating and to prevent the ignition of surrounding objects, protective devices, such as fuses, are included in the circuit.

Causes of short circuits

The main reason for the occurrence of short circuits is a violation of the insulation of electrical equipment.

Insulation faults are caused by:

1. Overvoltages (especially in networks with isolated neutrals),

2. Direct lightning strikes,

3. Aging insulation,

4. Mechanical damage insulation, passing under the lines of oversized mechanisms,

5. Poor maintenance of the equipment.

Often the cause of damage in the electrical part of electrical installations are unskilled actions. service personnel.

Intentional short circuits

When implementing simplified connection diagrams of step-down substations, use special devices- short circuiters that create deliberate short circuits in order to quickly disconnect the damage that has occurred. Thus, along with random short circuits in power supply systems, there are also intentional short circuits caused by the action of short circuits.

Consequences of short circuits

When short circuits occur in the power supply system, its total resistance decreases, which leads to an increase in currents in its branches compared to normal mode currents, and this causes a decrease in the voltage of individual points of the power supply system, which is especially large near the short circuit.

Depending on the location and duration of the damage, its consequences may be local or affect the entire power supply system.

If the short circuit is far away, the value of the short circuit current can be only a small part of the rated current of the supply generators, and the occurrence of such a short circuit is perceived by them as a slight increase in load. A strong voltage drop is obtained only near the short circuit, while at other points in the power supply system this reduction is less noticeable. Consequently, under the conditions under consideration, the dangerous consequences of a short circuit appear only in the parts of the power supply system closest to the accident site.

The short circuit current, even though small compared to the rated current of the generators, is usually many times higher than the rated current of the branch where the short circuit has occurred. Therefore, even with a short-term short-circuit current, it can cause additional and conductors above the allowable one.

Short-circuit currents cause large mechanical forces between the conductors, which are especially large at the beginning of the short-circuit process, when the current reaches its maximum value. With insufficient strength of the conductors and their fastenings, mechanical failures can occur.

A sudden deep voltage drop during a short circuit affects the work of consumers. First of all, this applies to motors, since even with a short-term voltage drop of 30-40%, they can stop (the motors tip over). Overturning engines seriously affects the work industrial enterprise, since in order to restore normal production process a long time is required and an unexpected stop of the engines can cause the rejection of the company's products.

With a small distance and a sufficient duration of a short circuit, it is possible for parallel operating stations to fall out of synchronism, i.e. violation normal operation the entire electrical system, which is the most dangerous consequence of a short circuit.

The unbalanced current systems arising from ground faults are capable of creating magnetic fluxes sufficient to induce significant EMF in neighboring circuits (communication lines, pipelines), dangerous for the operating personnel and equipment of these circuits.

Thus, the consequences of short circuits are as follows:

1. Mechanical and thermal damage to electrical equipment.

2. Fires in electrical installations.

3. Reducing the voltage level in the network, leading to a decrease in the torque of the electric motors, their braking, reduced performance, or even their overturning.

4. Loss of synchronization of individual generators, power plants and parts of the electrical system and the occurrence of accidents, including system failures.

5. Electromagnetic influence on communication lines, communications, etc.