Short circuit- electrical connection of two points of the electrical circuit with different potential values, not provided for by the design of the device and disturbing its normal operation. A short circuit can occur when the insulation of current-carrying elements is violated or due to mechanical contact of elements operating without insulation. Also, a short-circuit is the state where the load resistance is less than the internal resistance of the power source.
Single-phase single-phase short-wave multi-tube conductors, round bars, fixed couplings T and "Tp" or fixed docking switches from electrical transformer stations are manufactured in several versions to provide the most ergonomic use of phase clamps on these types of round conductors.
Types of short circuits
In three-phase electrical networks, the following types of short circuits are distinguished:
- Single-phase (phase-to-ground closure);
- Two-phase (closure of two phases among themselves);
- Two-phase on the ground (2 phases between themselves and simultaneously on the ground);
- Three-phase (3 phases among themselves).
In electrical machines, short circuits are possible:
Single-phase mobile short-circuiters for railway electrical installations are made in two design versions, adapted to the installation conditions. 
Short-circuits to 0, 4 or 6 kV motors are used in the grounding of terminals or motor power cables to prevent accidents by electric shock.
- Interturn - closure between each coil windings of the rotor or stator;
- Shorting of the winding to the metal case.
Methods of protection
To protect against short circuits, special measures are taken:
- Limiting short-circuit current:
- Relay protection devices are used to disconnect the damaged sections of the circuit
Causes of short circuits
The main cause of short circuits is violation of insulation of electrical equipment.
Circuits Limiters are of two types. The regime of short overcurrents. Tasks in the mode of overcurrents. One of them is the so-called protection from lack of voltage or minimum voltage. Unipolar threaded fuses consist of three main elements: base, cover and fuse insert.
A socket made of ceramic material provides a fusible attachment and an electrical connection between them and the circuit. This should allow good contact and easy change of employer, used as a new cartridge. The sockets used are as follows. with links from the back, with links in front and with connection to the industrial type.
Insulation faults are caused by:
1. Overvoltages (especially in networks with isolated neutrals),
2. Direct strokes of lightning,
3. The aging of insulation,
4. Mechanical damages of insulation, passage under the lines of oversized mechanisms,
5. Unsatisfactory maintenance of the equipment.
Often the cause of damage in the electrical part of electrical installations are unqualified actions of maintenance personnel.
The lid consists of a piece of ceramic material on which a metal thread is fixed; The thread of this part is such that it can be screwed onto the threaded part of the socket. The patron consists of a ceramic tubular body that carries two contact pieces at the ends. Between the parts of the lid and the receptacle, the owner with the fuse is crushed. Between the two parts the fusible thread is inserted into the channel inside the casing; the space between the fusible and ceramic body is filled with quartz sand, which plays an active role in the process of extinguishing an electric arc that occurs during the melting of a fusible wire.
Intentional short circuits
When implementing simplified schemes of connecting substation substations, special devices are used - short-circuiters, which create intentional short circuits in order to quickly shut off the faults that have occurred. Thus, along with short-circuits of a random nature in power supply systems, there are also intentional short circuits caused by short-circuit action.
Generalized feature of fuses. Only mount vertically. Electrical relays are devices that, when receiving an electrical command, transmit a fully electric command. When the intensity of the current and the coil of the mechanism reach a certain value, the movable command rotates at a certain angle. At the moment, two contacts are closed, one is fixed, and the other - with a mobile crew.
Closing of these contacts is the date command of the relay, the accepted command consisting in the achievement or exceeding of the regular value of the current in the reels. Devices for protection from broken wrists. Protection against overloads is achieved using thermal relays. They consist of bimetallic, which has electrical resistance. Through resistance, the command current passes; because of the heat produced in the resistance, bimetallic warm and bends. The temperature of a bimetal at a single point depends not only on the current flow at this instant, but also on the previous values of this current; the same can be said for an electrical installation protected by a relay.
Consequences of short circuits
In the event of short circuits in the power supply system, its overall resistance decreases, which leads to an increase in the currents in its branches compared to the currents of the normal mode, and this causes a decrease in the voltage of the individual points of the power supply system, which is particularly large near the short-circuit.
In other words, if the current is equal to the current of the protected installation or is proportional to this intensity, the bimetallic temperature is also proportional to the temperature of the protected installation; states that a bimetallic model is a thermal model of a protected installation.
For each bimetal temperature, a clearly defined arrow corresponds to it. Therefore, the contacts 3 can be placed in such a position that they are actuated bimetallically when the temperature in the protected installation exceeds the allowable temperature. In addition, bimetallic thermal relays can be configured to act on currents that exceed the rated current of the protected installation for some time.
Depending on the place of origin and the duration of the damage, its consequences may be localized or reflected on the entire electricity supply system.
With a large short-circuit remoteness, the magnitude 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 small increase in the load. A strong reduction in voltage is obtained only near the short-circuit, while at other points in the power supply system this decrease 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.
It follows that the thermal relays are synchronized in accordance with the current, with higher current values higher, with their faster action and, conversely, if the currents in the previous period were of low intensity, the bimetal is relatively cold and requires more time to warm up.
These overloaded surgeons, even if they are of high intensity, but are passers-by, do not cause bimetallic action. Thermal relays have a current-dependent function. This function satisfies the following conditions. Protective devices against voltage drop.
The short-circuit current, being even small in comparison with the rated current of the generators, is usually many times higher than the rated current of the branch where a short-circuit occurred. Therefore, in the case of a short-time fault current, it can cause additional heating of current-carrying elements and conductors above the permissible value.
Unlike relays, triggers are devices that receive an electrical command, but the command they give out is not electrical, but mechanical. Typically, this command opens the circuit breaker. The relays and triggers used against the absence of voltage are electromagnetic; their design diagram is shown in Figure 3 of these devices, powered by command voltage; while the voltage above the rated voltage is 0, 7 times, the movable armature 2 is attracted; if the voltage falls below the specified value, the valve will be released and an electrical or mechanical opening command will be given.
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 conductors and their fasteners, mechanical damage can occur.
A sudden deep voltage drop in case of a short circuit affects the operation of the consumers. First of all, this applies to engines, since even with a short-term voltage drop of 30-40%, they can stop (there is a rollover of engines). Tilting engines is seriously affected by the work of an industrial enterprise, since it takes a long time to restore a normal production process and unexpected engine shutdown can cause a marriage of the enterprise's products.
Overvoltage devices. The principle of construction and operation of arresters is shown in the figure. The encoder consists of 2 electrodes arranged at a distance determined by each other in accordance with the magnitude of the operating voltage. During normal operation of the line, the line is isolated from memory by dielectric air from the destructive distance. During an overvoltage, an electric arc appears between the two electrodes, which places the line on the ground, so the electrical loads corresponding to the overvoltage go out, and the overvoltage disappears.
With a small distance and a sufficient duration of a short circuit, it is possible for the parallel stations to be out of sync, i.e. disruption of the normal operation of the entire electrical system, which is the most dangerous consequence of a short circuit.
Unbalanced current systems that arise during ground faults can create magnetic fluxes sufficient to guide significant EMFs in adjacent circuits (communication lines, pipelines), which are dangerous for maintenance personnel and equipment of these circuits.
In the absence of overvoltage, the rated voltage is no longer sufficient to maintain the electric arc, and it is interrupted, so the line is again isolated from memory through a destructive distance. The scraper is often equipped with fire extinguishing horns, which ensure that the spring is extinguished by electromagnetic and thermal blowing.
Resistance 3 plays the role of converting wave energy into heat through the Joule effect of the current flowing during the discharge. The personnel working on electric installations with the electric drive, will use individual means of protection against electric shock and electric shock.
Thus, the consequences of short circuits are as follows:
1. Mechanical and thermal damage to electrical equipment.
2. Ignition in electrical installations.
3. Decrease in the level of voltage in the network, leading to a reduction in the torque of the motors, their braking, a decrease in productivity, or even to overturn them.
Isolating protective devices are designed to protect people by isolating them from living elements. The most frequently used and most important are electrical clamps, electrical insulating pads, any electrically insulated tools, gloves, boots, carpets, electrical insulating platforms, electrical insulating presses.
Mobile voltage indicators; they check for the presence or absence of stress. Panels, screens, signs or other mobile indicators. They are used to delineate work areas and protected areas. Warning labels with the following roles.
4. The loss of synchronism of individual generators, power plants and parts of the electrical system and the occurrence of accidents, including system accidents.
5. Electromagnetic influence on communication lines, communications, etc.
Protection devices in internal networks of residential buildings with a voltage of 380/220 V are fuses and automatic air switches. Power receivers, besides them, are protected against overloads (10.3) by means of thermal relays integrated in magnetic starters; Magnetic starters also provide protection against self-starting. Self-launching is used in residential buildings only in smoke removal systems, which should be taken into account when calculating networks and selecting security devices.
It should be borne in mind that the main contacts of magnetic starters are not designed to trip short-circuit currents (10.2). In addition, the thermal relays of most existing designs of magnetic starters themselves need to be protected against short circuits (10.1), because when the short-circuit currents (10.2) pass, the relay heating element can burn out faster than the relay will have time to shut down the motor. Therefore, when using magnetic starters with thermal relays to protect against overloads (10.3), it is necessary to additionally install fuses (10.4) or circuit breakers (10.5) for short circuit protection (10.1) in these circuits.
It is allowed to consider these relays as thermally stable without checking by calculation if the branch to the electric receiver is protected by one of the following devices: a fuse-link with a rated current not exceeding the longest permissible current of the thermal relay by more than 4 times; automatic circuit-breaker (10.5) with a thermal release, the rated current of which does not exceed the maximum long-time permissible current of the thermal relay by more than 2 times. These conditions predetermine the number and unit power of the electrical receivers, which can be connected to a "chain" with one common protection device on the branch, but with individual control devices and overload protection (10.3) for each electric drive monica.
Fuses (10.4) due to a simple device and low cost have become very popular in networks up to 1000 V, including in the electrical networks of residential buildings. Fuses (10.4) and fuse-links are characterized by rated voltage, rated current and the maximum breaking current.
The rated voltage of the fuse (10.4) is the voltage corresponding to the highest rated voltage of the circuits in which this fuse is allowed to be installed. The rated current of the fuse (10.4) is the highest current that the live parts (cartridge, contact racks) are designed for. The fuse-links can be inserted into the same cartridge for different rated currents.
The rated current of the fuse (10.4) is equal to the largest of the rated currents of the fuse-links intended for installation in this fuse. The rated current of the fuse-link is the largest, the current that the fuse-link withstands for an indefinitely long time. The maximum breaking current of the fuse (10.4) is the greatest melting current, at which arc suppression is still ensured without damaging the fuse holder.
At present, fuses with closed cartridges are used almost exclusively as the safest and having a high switching capacity.
Consider the principle of the most common fuses (10.4) of the series PR2 and PN-2.
A collapsible fuse with a fiber tube of the PR2 series has a cartridge made of a thick-walled fiber tube on the ends of which brass bushings are fitted, protecting the tubes from rupture. Fusible inserts are rolled with screws to the knives, which in turn are fixed with brass clips screwed onto the cartridge hubs. The cartridge assembly with a fuse-link is inserted into the pins, to which suitable and bypassing conductors are attached. Thus, the fuse turns on in series in the line cut. At a current exceeding a certain value, the fuse-link becomes very hot, and then melts. A powerful electric hot air is formed, the temperature inside the fiber cartridge sharply increases. With the appearance of an arc from the surface of the fiber, intensive emission of gases occurs, the pressure inside the cartridge greatly increases, which contributes to the deionization of the space and the effective extinction of the arc. However, the high pressure developed in the cartridge requires its special strength, which limits the switching capacity of fuses of this type. In this regard, fuses with fiber cartridges are much inferior to fuses with fillers. In addition, they are quite expensive and require for the manufacture of scarce materials and are more cumbersome than the fuses with fillers, discussed below. However, the PR2 fuses have the advantage of being easy to replace the fuse-link, whereas in fuses with fillers, the entire cartridge has to be replaced.
Currently, the most common are fuses series PN - 2 with protective cartridges filled with quartz sand. Inside the cartridge there is a fuse-link. In such "backfill" fuses, intense arc suppression is facilitated by the branching of the arc in the finest intervals between the grains of sand. Having a large surface, the filler grains absorb heat well and cool off the evolved gases.
As a result, the pressure in the cartridge decreases sharply when the insert material evaporates. Deionization and arc quenching occur so rapidly that, in the event of a short circuit, the current does not have time to reach its amplitude value. Therefore, such fuses are current-limiting. For example, a PN-2 series fuse with 100 and 250A cartridges transmits a current of no more than 5 kA.
Among the common types of fuses, we note the filling fuses of the NNP series, produced for currents up to 60 A. In principle, they are analogous to the PN-2 series fuses. For protection of apartment group networks, fuse plugs of type Н-20 are still widely used.
Protective characteristics of fuses and circuit breakers.
The melting time of the fuse-link fuse-link (10.4) depends on the strength of the overload current (10.3). The more current, the faster, the melting of the fuse-link comes. The dependence of the total tripping time (the melting time of the fuse-link and arc burning) on the tripping current is called the time of the current or protective characteristic.
Figure (10.1) shows the protective characteristics of the PN-2 series fuses, at which the limiting values of the smallest and largest shut-off time are given for a given current, i.e., the so-called spread of the characteristic. As can be seen from the characteristics, the response time for the same current can fluctuate within a considerable range (up to ± 50%), which depends on the manufacturing tolerances, the insert material, its aging, the state of the contact connections, the influence of the environment,
The considerable variation in the melting times of the inserts is a serious drawback of the fuses (10.4), making it difficult to selectively operate the protection. It is recommended to ensure the selectivity of the fuses, so that each fuse-link inserted into the power supply side is two steps higher than the previous one, if this does not lead to an increase in the cross-section of the wires. A difference of at least one step is mandatory in all cases. For highly responsible buildings, the choice of fuse links for fuses should be made taking into account the variation in protective characteristics.
Benefits circuit breakers (10.5) with prefuse fuses (10.4) are as follows.
1. In case of overload (10.3) or short circuit (10.1), the circuit breaker (10.5) disconnects all three phases of the protected branch to the motor, preventing it from operating in two phases.
2. The circuit breakers (10.5) are soon ready again after operation, while in the fuse (10.4) a replacement of the calibrated insert or even the cartridge is required.
3. Circuit-breakers (10.5) have more accurate protective characteristics than fuses (10.4).
4. Circuit-breakers (10.5), in addition to the protection functions, can be used for infrequent switching of the circuits in which they are installed. Thus, they combine protection and switching functions.
5. Some types of circuit-breakers (10.5) have built-in block contacts used in the interlocking and signaling circuits, as well as independent releases that allow remote control.
6. Circuit-breakers (10.5) exclude the possibility of using uncalibrated elements, which, unfortunately, is often practiced in installations with fuses
The most commonly used circuit breakers (10.5) can be supplied with thermal, electromagnetic or combined releases (the latter are a combination of thermal and electromagnetic releases). The operating time of the thermal releases of the circuit breakers (10.5), as well as the fuses (10.4), decreases with increasing current. that is, they have an inverse current-dependent characteristic. Electromagnetic releases operate almost instantly at the current to which they are adjusted.
The releases are characterized by a rated current, i.e. The current that they can withstand indefinitely, which is guaranteed by the manufacturer.
The lowest current that causes the circuit breaker to trip (10.5) is called the pickup current or the pickup current. Under the setting of the trip unit is meant to adjust it to the selected current value, at which the release trips. The setting of the current of the electromagnetic release for instantaneous tripping is called a cutoff. It is important to emphasize that the rated current of the circuit breaker (10.5) characterizes the throughput of its contact parts and corresponds to the rated current of its largest thermal release.
The circuit breakers (10.5) are divided into unregulated and adjustable. The first include circuit breakers (10.5), the settings of the trip units are adjusted at the factory and no adjustments are required during installation and operation. These include the series AZ-100, AE-1000, AE-2000, AK-63, AB-25. Another group of circuit breakers (10.5) is equipped with devices for changing the installation current by acting on the mechanical system of the machine or on a special device that changes the operating time of the automaton (selective automata). Among the most common adjustable automatic circuit breakers (10.5) we mention the automatic machines of the АП50, А3700, АВ and АВМ series.
Selection of fuse links for fuses and trip units
With these requirements in mind, fusible fuse links (10.4) and releases of circuit breakers (10.5) should be selected according to the relationships given in Table (10.2), which are taken on the basis of catalog data and the current characteristics of protective devices.
When lines are protected by circuit breakers (10.5), which have combined releases (thermal and electromagnetic), electromagnetic releases are verified in terms of the expression for I uS to.a.o. Only at considerable multiplicities of starting currents of 6 and more. The installation of circuit breakers (10.5), which have only electromagnetic releases (cutoffs), in the networks of residential buildings is not recommended.