An electric arc can be extremely damaging to the equipment and, more importantly, a danger to people. An alarming amount of accidents caused by it occurs annually, often leading to serious burns or death. Fortunately, significant progress has been made in the electrical industry in the creation of means and methods of protection against arc effects.
Causes and places of origin
The electric arc is one of the most deadly and least understood hazards of electricity and prevalent in most industries. It is widely recognized that the higher voltage electrical system, the greater the risk for people working on or near wiring and equipment energized.
The thermal energy from the flash of the arc, however, can actually be larger and arise more often at lower voltages with the same destructive consequences.
Electrical arcing usually occurs in case of accidental contact between the live conductor such as the contact wire trolleybus or tram lines with another conductor or a grounded surface. When this occurs, the resulting short-circuit current melts the wires, ionizes the air and creates a fiery channel of the conducting plasma of a characteristic arc-shaped shape (hence the name), where the temperature of the electric arc in its core can reach over 20,000 ° C.
What is an electric arc?
In fact, so in everyday life is called the well-known in physics and electrical engineering arc discharge - the kind of self-discharge in gas. What are the physical properties of the electric arc? It burns in a wide range of gas pressure, with a constant or variable (up to 1000 Hz) voltage between the electrodes in the range of several volts (welding arc) to tens of kilovolts. The maximum current density of the arc is observed at the cathode (10 2 -10 8 A / cm 2), where it is pulled into a cathode spot, very bright and small in size. It randomly and continuously moves across the entire area of the electrode. Its temperature is such that the cathode material in it boils. Therefore, ideal conditions arise for the thermionic electron emission to the cathode space. A small layer is charged above it, positively charged and ensuring the acceleration of the emitted electrons to velocities at which they ionically ionize atoms and molecules of the medium in the interelectrode gap.
The same spot, but somewhat larger and less mobile, is also formed on the anode. The temperature in it is close to the cathode spot.
If the current of the arc is of the order of several tens of amperes, plasma jets or torches follow from both electrodes at a high speed normally to their surfaces (see the photo below).
At high currents (100-300 A), additional plasma jets appear, and the arc becomes similar to a beam of plasma filaments (see in the photo below).
How arc manifests itself in electrical equipment
Exposure to the electric arc
Severe injuries, and even fatalities, when it occurs, can not only get people working for electrical equipment, but also people in the vicinity. Arc trauma can include external skin burns, internal burns from inhaling hot gases and vaporized metal, hearing damage, sight, such as blindness from ultraviolet flash light, and many other damaging injuries.
A particularly powerful arc can also cause a phenomenon such as an explosion that creates a pressure of more than 100 kilopascals (kPa) with the emission of debris like shrapnel at a speed of up to 300 meters per second.
Persons who have suffered electrical shock from an electric arc may need serious treatment and rehabilitation, and the price of their injuries can be extreme - physically, emotionally and financially. Although legislation requires enterprises to conduct risk assessments for all types of work, the risk of electric arc damage is often overlooked, because most people do not know how to assess and effectively manage this hazard. Protection from the impact of the electric arc involves the use of a whole range of means, including the use of special electrical protective equipment, work clothes, as well as equipment, especially high-voltage switchgear designed with arc suppression means, when working with live electrical equipment.
Arc in electrical apparatus
In this class of electrical devices (circuit breakers, contactors, magnetic starters), the fight against this phenomenon is of particular importance. When the contacts of a switch that is not equipped with special devices to prevent an arc open, it must be ignited between them.
At the moment when the contacts begin to separate, the area of the latter decreases rapidly, which leads to an increase in the current density and, consequently, to an increase in temperature. The heat generated in the gap between the contacts (the usual oil or air medium) is sufficient to ionize the air or evaporate and ionize the oil. Ionized air or steam acts as a conductor for the arc current between the contacts. The potential difference between them is very small, but it is sufficient to maintain the arc. Consequently, the current in the circuit remains continuous as long as the arc is not eliminated. It not only delays the current interruption process, but also generates a huge amount of heat, which can damage the switch itself. Thus, the main problem in the circuit-breaker (primarily high voltage) is the damping of the electric arc in the shortest time so that the heat released in it can not reach a dangerous value.
The factors of maintaining the arc between the contacts of the switches
1. Electric arc voltage equal to the potential difference between the contacts.
2. Ionized particles between them.
Accepting this, we note additionally:
- When between the contacts there is a small window, even a small potential difference sufficient to maintain the arc. One of the ways of damping is the separation of the contacts at such a distance that the potential difference becomes inadequate to maintain the arc. However, this method is impracticable in high voltage equipment, which may require the separation of many meters.
- Ionized particles between the contacts tend to support the arc. If her path is deionized, the process of extinction will be facilitated. This can be achieved by cooling the arc or removal of ionized particles from the space between the contacts.
- There are two ways by which protection from the electric arc in the switches is carried out:
- high resistance method;
- zero current method.
Extinguishing an arc by increasing its resistance
In this method, the resistance in the arc path increases over time so that the current decreases to a value insufficient to maintain it. Consequently, it is interrupted, and the electric arc goes out. The main drawback of this method is that the quenching time is sufficiently long, and huge energy dissipates in the arc.
The arc resistance can be increased by:
- Lengthening of the arc – the arc resistance is directly proportional to its length. The arc length can be increased by changing the gap between the contacts.
- Cooling of the arc, or rather the medium between the contacts. Efficient cooling air should be directed along the arc.
- The placement of contacts in trudnovospituemyh gaseous medium (gas switches) or in a vacuum chamber (vacuum circuit breakers).
- Reduction of the cross section of the arc by passing through a narrow aperture, or decrease area of the contacts.
- Division of arc resistance can be increased by division into a number of small arcs connected in series. Each of them experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts.
Arc suppression by zero current method
This method is only used in AC circuits. In it, the resistance of the arc is kept low until the current decreases to zero, where it goes out naturally. Its re-ignition is prevented despite the increase in voltage on the contacts. All modern switches of large alternating currents use this method of arc extinction.
In an alternating current system, the latter drops to zero after each half-cycle. In each such zeroing, the arc goes out for a short time. The medium between the contacts contains ions and electrons, so that its dielectric strength is small and can easily be destroyed by the growing voltage at the contacts.
If this occurs, the electric arc will burn during the next half-cycle of the current. If immediately after it is reset, the dielectric strength of the medium between the contacts increases faster than the voltage on them, the arc will not light and the current will be interrupted. A rapid increase in the dielectric strength of the medium near zero current can be achieved by:
- recombination of ionized particles in the space between contacts into neutral molecules;
- removing the ionized particles away and replacing them with neutral particles.
Thus, the real problem in interrupting the alternating current of the arc is the rapid de-ionization of the medium between the contacts as soon as the current becomes zero.
Methods of de-ionizing the medium between contacts
1. Elongation of the gap: the dielectric strength of the medium is proportional to the length of the gap between the contacts. Thus, with a rapid opening of the contacts, a higher dielectric strength of the medium can be achieved.
2. High pressure. If it is in the immediate vicinity of the arc, the density of the particles that make up the arc discharge channel also increases. The increased density of particles leads to a high level of deionization and, consequently, the dielectric strength of the medium between the contacts increases.
3. Cooling. Natural recombination of ionized particles occurs more quickly when they cool down. Thus, the dielectric strength of the medium between the contacts can be increased by cooling the arc.
4. The effect of the explosion. If the ionized particles between the contacts are swept away and replaced by non-ionized particles, the dielectric strength of the medium can be increased. This can be achieved by means of a gas explosion directed into the discharge zone, or by injecting oil into the intercontact space.
In such switches as the medium of arc extinction is used the gas sulfur hexafluoride (SF6). It has a strong tendency to absorb free electrons. The switch contacts are opened in the high pressure flow of SF6) between them (see figure below). The gas captures free electrons in the arc and forms an excess of inactive negative ions. The number of electrons in the arc rapidly decreases, and it goes out.