What is a phase-to-phase short circuit - concept and reasons

During the operation of high-voltage electrical circuits, there is often a phenomenon defined by regulatory documents as an interphase short circuit. Such a deviation from the normal operation of power supply systems is associated with faults in the supply lines, the consequences of which are unpredictable. The especially dangerous nature of the possible damage forces us to deal with a number of questions concerning what this phenomenon is, what troubles it leads to and how to avoid them.

The concept and reasons for closures

Interphase short circuit of electricity in multiphase circuits is an unintentional connection between insulated conductors with a damaged protective coating.

In some cases, it manifests itself as a single-phase earth fault or the body of operating electrical equipment.

This state of the electrical network is a violation of the normal operation of the system and is interpreted as an emergency. In this case, at the points of closure of two conductors or at the points of their contact with the ground, the current increases significantly. Its maximum value sometimes reaches several thousand Amperes. Uncontrolled flows of electricity can lead to devastating consequences.

The causes of emergencies in high-voltage electrical networks are:

• Damage to the protective insulation of each of the phase conductors due to violations of the rules for the operation of cable lines.
• Accidental breakage of one of the cores of the air cable and its short circuit to another wire or ground.
• Shorting a wire with damaged insulation to the body of an existing electrical installation.

Each of the cases of a short circuit is the result of a gross violation of the rules for the operation of electrical equipment and, in accordance with the requirements of regulatory documents, needs a thorough investigation.

Types of emergency closures

According to the type of power supply, all short circuits are divided into damages that occurred in single-phase or three-phase circuits, and according to their number - into single and double short circuits. The simplest case is single-phase lines, in which only a single phase fault to neutral or ground is possible. A three-phase short circuit is distinguished by a large variety of possibilities, since the number of wires in a cable increases to 3. In this case, the following damage options are possible:

• Shorting two high-voltage wires to each other.
• Short circuit of one wire to neutral or ground (single-phase short circuits).
• Contact of two conductors at once with the ground surface.

In each of these cases, including two-phase earth faults, the considered fault manifests itself in a special way, characterized by spreading currents and distributions of emergency potentials. In addition to these factors, the current process is described by such an indicator as touch voltage. The specified parameter is the voltage applied to the human body between two points of contact on the bare wire.

The same type of hazardous effects include a potential difference that appears between parts of the body in contact with a bare wire shorted to ground. With single-phase short circuits, of particular interest is the question of what value the touch voltage reaches when the phase is closed.According to the provisions of the PUE, this indicator depends on the distance between the contact zones and increases with its increase.

In some cases, when the resistance to current spreading to the ground is too high, the contact voltage reaches a value that is dangerous for humans.

Consequences of short circuit

The dangerous manifestations of an interphase short circuit of a three-phase circuit (as well as a single-phase one) include the consequences associated with the flow of currents in the line of extremely high values. They naturally cause the following emergency situations:

• Fire outbreak due to melting and strong heating of the insulation of the phase conductors.
• Failure of the power equipment connected to the damaged line.
• Electric shock to a person who accidentally finds himself in an emergency circuit.

When moving in this area, it is important to take into account the so-called "step voltage", which is formed due to the spreading of the leakage current into the soil between the legs of a person. This indicator is measured between his feet when moving around a cable that has fallen to the ground. It can also reach a dangerous value, especially in case of accidents in high-voltage overhead lines of 6.3-10 kW. Therefore, PUE is prescribed to move in these zones with a characteristic goose-step: the foot is close to the foot.

The main condition for reliable protection against single-phase and two-phase faults in 220/380 Volt power lines is high-quality insulation capable of withstanding test voltages up to 1000 Volts. The value of its resistance, according to the PUE, must be at least 0.5 Mohm for each of the phases. To prevent fires and equipment breakdowns, special protective devices are installed in the power circuits, which provide instant disconnection of the line when a short circuit occurs. Such devices include:

• Linear automatic fuses.
• Current breakers and high voltage relays.
• Current protection devices and others.

With their help, it is possible to prevent the destructive consequences of phase closures, which sometimes occur for reasons beyond the control of a person.

Thanks to the timely adoption of appropriate measures, it is possible to preserve the integrity of material resources, as well as to protect the personnel from electric shock.

Phase-to-phase high-voltage line closure: protection methods

It is not allowed to use automatic disconnectors in supply circuits with an operating voltage of more than 1000 Volts, since when their power contacts are opened, a high-power arc is formed. In this case, oil, vacuum or gas switches are used to switch the lines.

Relay circuits are also used to protect high-voltage networks. They differ in simple design and are converting devices operating according to Faraday's law of induction - guidance of the e / m field. The relay equipment, which provides protection of high-voltage lines from overvoltage, is based on a current transformer. With its help, it is possible to control the magnitude of the current in the emergency line and, when it reaches the limit value, generate a signal that goes to the winding of a powerful electromagnet. This protective device, after being triggered, disconnects the entire supply circuit from the power supply.

Regardless of the availability of switching equipment, the main method of protection against phase-to-phase and three-phase short-circuits is the use of cable products with high-quality insulation. If this condition is met, any high-voltage line is capable of withstanding short-circuit currents that are many times higher than the permissible rate.

Preventive measures

The most effective and reliable way to prevent short circuits is a professional approach to solving the following technical and organizational issues:

• Selecting a suitable power cable that can withstand high overcurrents.
• Strict observance of the rules for the installation and operation of electrical networks, as well as the machines and devices connected to them.
• Availability of acts of acceptance of the power supply system upon its commissioning.
• The use of modern types of protective equipment, which guarantees instant shutdown of the line in the event of an emergency.

Particular attention is paid to preventive measures carried out in strict accordance with the requirements of the current regulations. According to the provisions regarding the maintenance of electrical networks, prevention is carried out according to a pre-drawn up plan approved by the head of a particular department. When implementing it, it is necessary to distinguish between the following types of preventive maintenance:

• Visual inspections.
• Routine and preventive maintenance.
• Test tests of electrical equipment during its acceptance and during operation.

Shorting electrical wires to the ground is a very dangerous phenomenon that can lead to a fire and subsequent fire. In addition, it is fraught with the possibility of hitting people serving the installations with high voltage. All this ultimately forces us to take special protection measures to ensure the normal operation of networks in the absence of critical modes.