What is a short circuit: a simple definition, types and causes of a short circuit

Often, when discussing any topics related to electricity, a short circuit (SC) is called the cause of equipment breakdown or a fire. From school physics lessons, people know that it is troublesome, but many do not even realize its benefits. Having learned what KZ is, and having understood the nature of its appearance, one can understand when it becomes destructive, and in what cases it helps.

Physical definition

In physics textbooks, a short circuit is usually called the occurrence of an electrical connection between two points in an electrical circuits that are at different potential values ​​in cases not provided for by the design of the device and leading to violations in its work.

In order to understand this definition, one should understand what potential is. But it would be more correct to say "electrostatic potential", because it can be biological, chemical and mathematical. Charge is a physical quantity that determines the property of bodies or particles to interact in an electromagnetic field. The concept of electric charge is primary in electrodynamics; on its basis, definitions are made for other physical processes and phenomena, including for potential.

Potential refers to the energy possessed by a charge placed in an electric field. The unit of its measurement is the volt, which is equal to the ratio of the joule to the coulomb. That is, the electric potential is characterized by the ratio of the interaction energy of the charge with the field surrounding it to the value of its magnitude.

The work that electricity or energy does is determined by the potential difference (voltage). The higher it is, the more work must be done by the electrical force to move the charge from one point to another. Movement requires energy, while power is released during movement. If the movement of the charge occurs in an orderly manner, then a current arises.

In this case, the work is considered useful when potential energy from different ends of the electrical circuit overcomes a certain resistance. But if it does not exist, then from one part of the circuit the current will flow without loss to the other, while the power corresponding to the current strength will be released.

To explain in a simple way what a short circuit is, you can as follows: if in a conductor with a current that is energized, resistance will decrease to an insignificant value, then all the energy will begin to be released at the place of electrical connection of two values potentials. The formation of this power is called the occurrence of a short circuit or breakdown.

Types of phenomenon

The process of the appearance of a short circuit implies the formation of an electrical contact between two potentials. Depending on the area, the closure can be point and solid. If it occurs in electrical installations, then it is divided into turn-to-turn or hull. The first type is typical for a rotor or stator. In this case, breakdown can occur through the ground or air. Depending on the type of wiring and the number of closed phases, the phenomenon is divided into the following types:

• three-phase - all phase wires are connected to one point;
• two-phase - an electrical contact occurs between two wires at different potentials;
• single-phase - a phase closure to a neutral wire or ground loop.

Short circuit current

To understand the definition of a short circuit, you should remember the fundamental principle in electricity - Ohm's law. Its formulation is as follows: the current in a section of the circuit is directly proportional to the potential difference and inversely proportional to the electrical resistance of this section. The formula can express this as I = U / R, where:

• I - current strength, A;
• U is the voltage value, V;
• R - resistance (impedance), Ohm.

Thus, the higher the impedance value, the lower the current will become. For example, when an electrical line with a voltage of 220 volts is bridged with a resistance of 1 kΩ, then a current flows through the element equal to I = 220/1000 = 0.220 A.

If the resistor is of a suitable power, then nothing will happen, a certain amount of voltage will drop across it, but the circuit will continue to work. But if, instead of a resistor, a conductor with an infinitesimal internal resistance is used, then the force the current will begin to grow rapidly until the closed parts burn out and break the electrical contact. The current at which the contact is broken is called breakdown.

By the way, this is why the short circuit got its name, because the current tends to go in the direction with the least resistance for its flow. This line turns out to be the shortest on his way.

When calculating the breakdown accurately, a formula is used that takes into account the internal resistance of the power source: Ikz = E / (R + r). In this case, the appearing thermal power will be determined using the expression P = E2 * R / (R + r).

The calculation of the short-circuit current is important because it is used in the selection of electrical equipment, protection, automation devices and current limiters. The breakdown phenomenon occurs most often due to the appearance of transient resistances through which current begins to flow. These include electric arcs, foreign objects in the place of damage to insulations, supports and their grounding, as well as contact with the ground or between phase conductors. But to simplify calculations, these resistances are taken equal to zero and are not taken into account.

Reasons for the appearance

When a contact is formed between two flows of charges, a huge current strength arises, which is characterized by a short circuit. Since this sharp increase in energy occurs due to the appearance of an electrical shock pulse, then, according to the Joule-Lenz law, the resulting power has a high degree of heat release.

The reason for this phenomenon is the violation of the insulating properties of the conductors. This can happen due to natural wear and tear or emergencies in the operation of electrical equipment. In practice, this means direct contact between the neutral and phase conductors.

But the wires don't have to touch each other. An instantaneous increase in current strength also occurs if a conductive body or medium with negligible resistance appears between them, for example, a conductive object, water or moist air. This situation is called a breakdown. The following reasons for the occurrence of short circuit can be distinguished:

1. Overvoltage. When the physical parameters of the power line are exceeded, when a large current strength passes through the conductor, its resistance turns out to be large for it, therefore, an intense heat release occurs. The conductor heats up by transferring heat to the insulating layer, which cracks and collapses. The appearance of high voltage can be caused both by natural phenomena (lightning strike), and errors in the operation of electrical appliances or energy sources.
2. Natural aging of insulation. Any insulation has its own resource, which is reduced when in an aggressive environment, for example, with temperature drops.
3. Mechanical damage: chafing, damage during construction work, the result of the action of rodents.
4. Installation error. Careless handling of live parts of equipment, violations in cable laying (bending at an acute angle, crossing wires).
5. Incorrect wiring of electrical equipment or cables. Occurs due to human actions. This may be due to unsuccessful repairs, an error in the connection of wires or power lines, the use of broken devices.

Consequences and protection

The biggest nuisance that can arise during a short circuit is the occurrence of a fire. The resulting excess heat causes the insulation and nearby combustible substances to ignite. In addition, during a breakdown, an electric arc occurs, which is an open source of fire. In doing so, she can ignite nearby objects.

If a person falls into the coverage area of ​​the circuit, then he is subjected to a shock of high amperage. This leads to the release of heat by the body and the subsequent combustion of the body.

Less significant consequences include the occurrence of a large electric field and electromagnetic shock, which negatively affect electronic components and radio equipment, as well as electric motors. The appearance of a short circuit leads to a phase imbalance in the power grid, which causes a voltage surge in it, and this leads to breakdowns of the power supplies of electrical devices connected to it.

To prevent consequences in the design of electrical systems, special devices and protective measures are used. These measures include:

• periodic testing of insulation of equipment and power lines;
• the use of equipment or electrical wiring with an increased class of protection in potentially hazardous locations such as high quality dielectrics or flame retardant dual isolation;
• installation of fuses that burn out when the current reaches a dangerous value;
• use of lightning protection, automatic and differential switches, voltage control devices.

It is very important to use the correct materials and protective equipment when constructing a transmission system. To a greater extent, this concerns the cross-section of the conductive cable, the calculation of the parameters of the used protective modules and the competent construction of the grounding circuit.

Intentional use

The unintentional occurrence of a short circuit carries with it dangers and troubles, but by controlling it, you can benefit. Thanks to this, electric arc welding appeared. One winding of the transformer is connected to the workpiece to be welded, forming permanent contact with it, and the second is connected to the electrode. When it touches the part, a short circuit occurs with the formation of an arc and the release of heat. It is this arc that is used to melt metals. But if the electrode "sticks" to the metal, then all the power used will begin to be released on the converter, which will lead to its turn-to-turn closure.

Another application of the phenomenon is used in short-circuits - devices equipped with circuit breakers. If it becomes necessary to quickly disconnect the power line, it closes, to which an electromechanical device reacts, turning off the section.

Induction vibrometers and seismic receivers use SC in the basis of their work, which allows damping mechanical vibrations. Breakdown mode is often used when connecting amplifier stages in electronics, such as cascode amplifiers. The power circuits of the electronic boards also operate in short-circuit mode for alternating current. Bypass blocking capacitors are installed on their power lines. They are designed to reduce self-excitation of amplifying stages, interference and code failures in digital devices.

Summing up, important aspects of electrical processes can be noted. In order for the current to arise, a potential difference must appear, and then their physical connection. Then the charge will begin to be transferred from one point to another, meeting resistance on its way. Energy will be released on it, which is used for any processes. The generated power is characterized by useful work. But if the current does not meet obstacles on its way, then when the two potentials are connected, all the power will be released along this path. As a result, it is converted into thermal energy and partly into light.

This is the principle of the occurrence of a short circuit, as well as the phenomena accompanying it - excessive heating and a light flash. When this mode occurs, the current in the circuit is much higher than the nominal values ​​(due to the lack of resistance), which leads to the failure of the energy source and its receiver in an electrical circuit.