An electric arc can be extremely destructive for equipment and, more importantly, endanger people. An alarming number of accidents occur due to it occur annually, often resulting in severe burns or death. Fortunately, significant progress has been made in the electrical industry in terms of creating tools and methods for protection against the effects of an arc.
Causes and locations
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 an arc flash, however, can actually be greater and occur more often at lower voltages with the same devastating 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 happens, the resulting short-circuit current melts the wires, ionizes the air and creates a fiery conductive plasma channel of a characteristic arc-shaped form (hence the name), and the temperature of the electric arc in its core can reach in excess of 20,000 ° C.
What is an electric arc?
In fact, this is how commonly referred to in the everyday life as an arc discharge, well-known in physics and electrical engineering, a type of an independent electrical discharge in a gas. What are the physical properties of an electric arc? It burns in a wide range of gas pressure, at a constant or alternating (up to 1000 Hz) voltage between the electrodes in the range from a few 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 the cathode spot, which is very bright and small in size. It randomly and continuously moves over the entire area of the electrode. Its temperature is such that the material of the cathode in it boils. Therefore, ideal conditions arise for the thermionic emission of electrons into the cathode space. A small layer is formed above it, which is positively charged and provides acceleration of the emitted electrons up to speeds at which they impactfully ionize the atoms and molecules of the medium in the interelectrode gap.
The same spot, but somewhat larger and less mobile, is also formed at the anode. The temperature in it is close to the cathode spot.
If the arc current is of the order of several tens of amperes, then plasma electrodes or torches normally flow out from both electrodes with high speed normally to their surfaces (see the photo below).
At high currents (100-300 A), additional plasma jets arise, and the arc becomes similar to a beam of plasma filaments (see the photo below).
How does the arc manifest itself in electrical equipment
Electric Arc Effect
Severe injuries, and even fatal ones, in case of its occurrence can be received not only by people working on electrical equipment, but also by people nearby. Arc injuries can include external skin burns, internal burns from inhalation of hot gases and evaporated metal, damage to hearing, vision, such as blindness from ultraviolet light flash, as well as many other damaging damage.
When a particularly powerful arc can also occur such a phenomenon as its explosion, creating a pressure of more than 100 kilopascals (kPa) with the release of debris particles, such as shrapnel, at speeds up to 300 meters per second.
Persons who have experienced the effects of electric current of 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 an electric arc is often overlooked because most people do not know how to evaluate and effectively manage this danger. Protection from the effects of an electric arc involves the use of a whole range of tools, including the use of special electrical protective equipment, working clothes, as well as the equipment itself, primarily high-voltage switching devices, designed with the use of arc quenching equipment when working with electrical equipment under voltage.
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 the switch, which is not equipped with special devices to prevent the arc, are opened, then it will surely ignite between them.
At the moment when the contacts begin to separate, the area of the latter decreases rapidly, which leads to an increase in current density and, consequently, to an increase in temperature. The heat generated in the gap between the contacts (the usual medium is oil or air) is sufficient for air ionization or evaporation and oil ionization. Ionized air or steam acts as a conductor for the arc current between the contacts. The potential difference between them is quite small, but it is enough 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 process of interrupting the current, but also generates a tremendous amount of heat, which can damage the switch itself. Thus, the main problem in the circuit breaker (primarily high voltage) is the extinguishing of the electric arc in the shortest possible time so that the heat generated in it cannot reach a dangerous value.
Factors maintain the arc between the contacts of switches
1. Voltage of electric arc, equal to the potential difference between the contacts.
2. Ionized particles between them.
Taking this, we note in addition:
- 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 against an electric arc in switches is carried out:
- high resistance method;
- method of zero current.
Arc extinction 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 disadvantage of this method is that the quenching time is large enough, and huge energy is dispersed in the arc.
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 extinction by zero current method
This method is used only in AC circuits. In it, the arc resistance is kept low until the current drops to zero, where it is extinguished naturally. Its re-ignition is prevented despite the increase in voltage at the contacts. All modern switches of high alternating currents use this method of arc extinction.
In the AC system, the latter drops to zero after each half period. In each such zeroing, the arc is extinguished for a short time. At the same time, the medium between the contacts contains ions and electrons, so that its dielectric strength is small and can be easily destroyed by the growing voltage at the contacts.
If this happens, the arc will burn during the next half cycle of current. If immediately after its zeroing the dielectric strength of the medium between the contacts grows faster than the voltage on them, then the arc will not ignite 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 the 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 deionization of the medium between the contacts as soon as the current becomes zero.
Ways to deionize the medium between contacts
1. The elongation of the gap: the dielectric strength of the medium is proportional to the length of the gap between the contacts. Thus, by quickly opening 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, increases, the density of the particles that make up the channel of the arc discharge, also increases. The increased density of particles leads to a high level of their 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 with non-ionized, then the dielectric strength of the medium can be increased. This can be achieved by using a gas explosion directed into the discharge zone, or by injecting oil into the contact 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). Gas captures free electrons in the arc and forms an excess of sedentary negative ions. The number of electrons in the arc quickly decreases, and it goes out.