For 220 V or opposite phases between themselves or with zero, not provided for by the design of the electrical circuit or electrical appliances, which disrupts the normal operation of the electrical network.
A short circuit occurs due to a violation of the insulation of electrical wires, cables or current-carrying elements in electrical appliances, as well as when mechanically touching non-insulated elements, therefore it is important to always insulate the bare ends of the electrical wiring separately from each other using electrical tape or electrical ones with an electrically insulating case, i.e. not conductive electric current.
In the event of a short circuit in the electrical circuit, the value of the current increases instantly and repeatedly, leading to high heat generation, as a result of which the electrical wires melt with the ignition of the electrical wiring and the spread of fire in the room where the short circuit occurred.
As a result of a short circuit, normal functioning is disrupted not only in your apartment, but also in your neighbors, due to a drop in the supply voltage, which often leads to breakdown of electrical appliances and household appliances.
In apartments with 220 V, only a single-phase short circuit occurs (short circuit of a phase to a neutral conductor or to), and in some private houses or garages with a three-phase input of 380 Volts, a much more dangerous two-phase (short circuit of two phases to each other + to "Earth" ) or three-phase (short circuit of three phases between themselves + to "Earth")
In electric motors and devices, in the event of a breakdown, internal short circuits are also possible:
For example, interturn, which occur when the turns of the windings in the stator or rotor of the electric motor are closed to each other, or between the turns in the winding of the transformer.
And if the electrical appliance has a metal case, then insulation breakdown and a short circuit to the metal case are possible. In this case, only the body will protect a person from electric shock.
Attention wires in a polyethylene and, especially in a rubber sheath are more prone to fire. Therefore, I, as a professional electrician for many years, engaged in electrical installation in Minsk, strongly recommend using in apartments, houses, garages, etc. for laying a VVG Ng brand cable hidden under plaster, with non-combustible insulation, but openly on a fireproof base more expensive cable - VVG Ng Ls, which does not even smoke during a short circuit.
An overload of the electrical network in a garage or apartment is often found in everyday life and is also very dangerous and is an emergency. And as practice has shown, it is more dangerous than short-circuit currents. Because the wiring is reliably protected or.
The cause of the overload is the connection, the inclusion of a large number of electrical appliances in one group of electrical outlets or damage to electricity consumers, in which the total current passing through the electrical cable or wires exceeds the nominal value for which they are designed. For a house or apartment where cables or wires with a cross section of 1.5 square millimeters are mainly laid, the rated current should not be higher than 16 Amp or no more 3.5 kilowatts.
It is important to know and use in practice only switches or sockets for connecting electric lighting or electrical equipment with at least the voltage and current values indicated on the body of the electrical socket or switch. For example, the socket says “10 A; 250 V ”, which means it is designed for a single-phase network of 220 Volts, and the maximum value of the current passing through the outlet should not be higher than 10 Amperes or, approximately in terms of power, no more than 2 Kilowatts. A powerful electrical appliance, for example, with a power of 2.5-3 kilowatts, cannot be connected to such an outlet, which will lead to burnout of the socket contacts.
Topic: what is a short circuit in an electrical circuit, what are the consequences of a short circuit.
Many have heard about an electrical short circuit, but not everyone knows the essence of this phenomenon. Let's deal with this. So, if you delve into the phrase “short circuit” itself, then you can understand that some kind of process is taking place in which something closes along a short, namely the shortest path for the flow of electric current (electric charges in a conductor). Simply put, there is a path through which electricity flows, its current of charges. These are various electrical circuits, conductors of electricity. The longer this path, the more obstacles the charges need to overcome, the greater the electrical resistance of this path. And from Ohm's law, it is known that the greater the resistance of the circuit, the less current will be in it (at a certain voltage value). Therefore, on the shortest path, there will be the maximum possible current, and this path will be short if the ends of the power source itself are shorted.
In general, we have, for example, an ordinary car battery (in a charged state). If you connect a light bulb designed for battery voltage (12 volts) to it, then as a result of the passage of a current of a certain value through this lamp, we will receive light and heat radiation. The lamp has a certain electrical resistance, which limits the current flowing through this circuit. To intentionally short circuit, we just need to take a piece of wire and connect it to the ends of the battery leads (parallel to the lamp). This wire has very little resistance compared to a lamp. Therefore, there is no special restriction that would prevent the movement of charged particles. And as soon as we close such a circuit, we get our short circuit. A large current will immediately flow through the wire, which can simply heat and melt this piece of wire.
As a result of such a short circuit, the conductor (its insulation) will ignite, up to a fire, if this conductor, by its ignition, transfers fire to flammable things that are nearby. In addition, such a sharp, spasmodic current flow can be harmful to the battery itself. It also starts to heat up at this time. And as you know, batteries really do not like excessive heat. At a minimum, their service life is significantly reduced after this, and as a maximum, they fail and even catch fire and explode. If such a short circuit occurs, for example, with a lithium battery in a phone (which has no electronic protection inside), strong heating occurs for several seconds, then a flame and an explosion form.
There are some batteries that are originally designed to deliver high currents (traction batteries), but even with them a complete short circuit can lead to big trouble. Well, what happens to the voltage during a short circuit? From school physics it should be known that the greater the current, the greater the voltage drop in this section of the circuit. Therefore, when no load is connected to the power supply, the maximum voltage value can be seen on it (this is the EMF of the power supply, its electromotive force). As soon as we load this power supply, a certain voltage drop immediately appears. And the greater the load, the greater the voltage drop. Since, in the event of a short circuit, the resistance of the circuit is practically zero, and the current strength will be the maximum possible, the voltage drop across the power source will also be maximum (near zero).
This we considered the option of a complete short circuit, which occurs directly at the terminals of the power source. Yes, here's what else to add about it. In the case of a battery, there will be a large current load on the internal parts and chemicals of the battery itself (electrolyte, plates, leads). In the event of a short circuit on such power sources as electric generators, the current load falls on the windings of these generators, which leads to its excessive heating and damage (well, those circuits that operate in the generator after this winding). A short circuit at the terminals of various power supplies leads to overheating and failure of the electrical circuits of the current sources themselves and the secondary winding of the transformer.
A short circuit can occur in the electrical wiring circuit itself, the circuit. In this case, the consequences are also extremely negative. But at the same time, the current strength will already be, as a rule, slightly less than in the case of a short circuit at the output of the power source. For example, there is a sound amplifier circuit. Suddenly, due to poor insulation of the speakers themselves, a short circuit occurs at the sound output of this amplifier. As a result, the output transistors, the microcircuit in the last stages of sound amplification, will most likely burn out. The power supply itself in this case may not even be affected, since an excessive current load may not reach it. I think you got the point of the short circuit.
P.S. In any case, the phenomenon of an electrical short circuit leads to disastrous consequences. Normal fuses, circuit breakers, protection circuits, etc. are usually used to protect against this. Their task is to quickly break the electrical circuit with a sharp increase in current strength. That is, an ordinary fuse is, as it were, the weakest link in all electrical circuits. As soon as the current strength has increased sharply, the fusible link simply melts and breaks the circuit. This in most cases results in other circuits in the circuit remaining undamaged.
What is a short circuit? Most often, this phrase can be heard from electricians, as well as people who do not understand electronics and electrics at all. To any question why smoke came from any device or device, they all say as one: "There was a short circuit." A very universal excuse for those who want to seem like a smart dunno).
The nature of the short circuit
Let's look at a simple circuit consisting of a light bulb and a car battery:
IN this case current will flow through the circuit and the bulb will glow.
Let's assume that our wires that lead to the light bulb are completely bare. Suddenly, by some miracle, another bare wire falls on these wires. This wiring closes our two bare wires and the most interesting thing begins - a short circuit (short circuit). A short circuit is the shortest path for electric current to flow through a circuit where there is least resistance.
Now the current flows through both the light bulb and the wiring. But our wiring is much smaller than the resistance of the light bulb, and almost all the current will flow to where there is less resistance - that is, through the wiring. And since the resistance of our wire is very small, then the current, therefore, will flow very large, according to Ohm's Law. And if a large current flows, therefore, the amount of heat generated by the wiring will be very large, according to the Joule-Lenz Law. In the end, a large circuit will flow along the circuit, which is highlighted in red, and this circuit will heat up very much. Heating the wires can cause them to burn out or even ignite. This case is called short circuit.
You have probably heard more than once in the news bulletin that the fire was caused by a short circuit. In this case, the bare wire of the phase in some place touched the bare wire of zero, or the phase touched the ground. A short circuit occurred, and the wires began to heat up to such an extent that nearby objects ignited with their heat. Hence the fire.
Basically, a short circuit occurs in old houses from an old cable that is bursting at the seams and can short-circuit with each other. Therefore, the first thing to do when buying an apartment or house in the secondary market is to look at the condition of the wiring.
Typical signs of a short circuit
- blown fuses in electronic equipment (REA)
- heating of a circuit in which a short-circuit current flows
- low voltage source voltage
- high current
- smoke
- charred wires
- burnt PCB tracks
- black soot in the place where the short circuit occurred
How to deal with a short circuit? This, of course, is to install fuses, circuit breakers and try to do a neat wiring.
Short circuit current
Figure 1 shows a diagram of connecting an electric incandescent lamp to an electrical network. If the resistance of this lamp r l \u003d 240 Ohm, and the mains voltage U\u003d 120 V, then according to Ohm's law, the current in the lamp circuit will be:
Figure 1. Schematic diagram of a short circuit at the breaker terminals
Let us analyze the case when the wires going to the incandescent lamp turned out to be closed through a very small resistance, for example, a thick metal rod with resistance r\u003d 0.01 Ohm, accidentally falling on two wires. In this case, the network current, passing to the point A, will branch out in two ways: one large part of it will go along a metal rod - a path with low resistance, and the other, a small part of the current, will pass along a path with high resistance - an incandescent lamp.
The emergency mode of the network, when, due to a decrease in its resistance, the current in it increases sharply against the normal one, is called short circuit.
Let's determine what is the strength of the short circuit current flowing through the metal rod:
In fact, in the event of a short circuit, the network voltage will be less than 120 V, since a large current will create a large voltage drop in the network and therefore the current flowing through the metal rod will be less than 12,000 A. But still this current will be many times higher than the current previously consumed by an incandescent lamp.
Short circuit power at current I kz \u003d 12,000 A will be:
P kz = U × I kz \u003d 120 × 12,000 \u003d 1,440,000 W \u003d 1,440 kW.
The current passing through the conductor generates heat, and the conductor heats up. In our example, the cross section of the wires of the electrical circuit was designed for a small current - 0.5 A. When the wires are closed, a very large current will flow through the circuit - 12,000 A. Such a current will cause the release of a huge amount of heat, which will certainly lead to charring and burning of the wire insulation , melting of the wire material, damage to electrical measuring instruments, melting of the contacts of switches, knife switches, and so on. The source of electrical energy supplying such a circuit may also be damaged. Overheating wires can cause a fire.
Each electrical network is calculated for its own, normal current for it.
Due to the dangerous, destructive and sometimes irreparable consequences of a short circuit, certain conditions must be observed during the installation and operation of electrical installations in order to eliminate the causes of a short circuit. The main ones are the following:
1) insulation of wires must correspond to its purpose (mains voltage and conditions of its operation);
2) the cross section of the wires must be such that their heating under existing operating conditions does not reach a dangerous value;
3) laid wires must be reliably protected from mechanical damage;
4) junctions and branches must be as securely insulated as the wires themselves;
5) the crossing of wires must be done so that the wires do not touch each other;
6) wires must be laid through walls, ceilings and floors so that they are protected from moisture, mechanical and chemical damage and well insulated.
Short circuit protection
To avoid a sudden, dangerous increase in current in an electrical circuit when it is short-circuited, the circuit is protected by fuses or circuit breakers.
Fuses are low-melting wire connected in series to the network. When the current increases beyond a certain value, the fuse wire heats up and melts, as a result of which the electrical circuit automatically breaks and the current in it stops.
A circuit breaker is a more complex and expensive protection device than a fuse. However, unlike a fuse, it is designed for multiple trips when protecting circuits during emergency operation. Structurally, the circuit breaker is made in a dielectric case with a trip mechanism built into it. The release mechanism has fixed and moving contacts. The moving contact is spring-loaded, the spring provides force for quick disengagement of the contacts. The release mechanism is actuated by one of two releases: thermal or magnetic.
The thermal release is a bimetallic plate heated by a flowing current. When current flows above the allowable value, the bimetallic plate bends and activates the trip mechanism. The operating time depends on the current (time-current characteristic) and can vary from seconds to hours. Unlike a fuse, a circuit breaker is ready for the next use after the plate has cooled down.
An electromagnetic release is an instantaneous release, which is a solenoid (coil made of a copper conductor), the movable core of which can also actuate the release mechanism. The current passing through the switch flows through the solenoid winding and causes the core to retract when the current threshold is exceeded. The instantaneous release, in contrast to the thermal release, operates very quickly (fractions of a second), but with a much higher current excess: 2 ÷ 14 times the rated current.
Video 1. Short circuit
Occurs when connecting two circuit wires connected to different terminals (for example, in DC circuits these are "+" and "-") of the source through a very low resistance, which is comparable to the resistance of the wires themselves.
The short circuit current can exceed the rated current in the circuit many times over. In such cases, the circuit must be broken before the temperature of the wires reaches dangerous values.
To protect the wires from overheating and prevent the ignition of surrounding objects, protection devices are included in the circuit - or.
Short circuits can also occur due to overvoltage as a result of thunderstorms, direct lightning strikes, mechanical damage to insulating parts, and erroneous actions of maintenance personnel.
During short circuits, the currents in the short-circuited circuit increase sharply and the voltage decreases, which poses a great danger to electrical equipment and can cause interruptions in the power supply to consumers.
Short circuits are:
three-phase (symmetrical), in which all three phases are short-circuited;
two-phase (asymmetric), in which only two phases are short-circuited;
two-phase to ground in systems with deafly grounded neutrals;
single-phase unbalanced to earth grounded neutrals.
The current reaches its greatest value with a single-phase short circuit. As a result of the application of special artificial measures (for example, grounding neutrals through, grounding only part of the neutrals), the largest value of the single-phase short-circuit current can be reduced to the value of the three-phase short-circuit current, for which calculations are most often carried out.
Causes of short circuits
The main reason for the occurrence of short circuits is a violation of the insulation of electrical equipment.
Insulation faults are caused by:
1. Overvoltages (especially in networks with isolated neutrals),
2. Direct lightning strikes,
3. Aging insulation,
4. Mechanical damage to the insulation, passage under the lines of oversized mechanisms,
5. Poor maintenance of the equipment.
Often the cause of damage in the electrical part of electrical installations are unskilled actions of maintenance personnel.
Intentional short circuits
When implementing simplified connection diagrams for step-down substations, special devices are used - which create deliberate short circuits in order to quickly turn off the damage that has occurred. Thus, along with random short circuits in power supply systems, there are also intentional short circuits caused by the action of short circuits.
Consequences of short circuits
As a result of a short circuit, the current-carrying parts overheat greatly, which can lead to insulation failure, as well as the occurrence of large mechanical forces that contribute to the destruction of parts of electrical installations.
In this case, the normal power supply to consumers in undamaged sections of the network is disrupted, since the emergency mode of a short circuit in one line leads to a general decrease in voltage. At the place of the short circuit, the conjugation becomes equal to zero, and at all points up to the place of the short circuit, the voltage drops sharply, and the normal supply of undamaged lines becomes impossible.
When short circuits occur in the power supply system, its total resistance decreases, which leads to an increase in currents in its branches compared to normal mode currents, and this causes a decrease in the voltage of individual points of the power supply system, which is especially large near the short circuit. The degree of voltage reduction depends on the work and the distance from the fault.
Depending on the location and duration of the damage, its consequences may be local or affect the entire power supply system.
If the short circuit is far away, the value 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 slight increase in load.
A strong decrease in voltage is obtained only near the place of a 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.
The short circuit current, even though small compared to the rated current of the generators, is usually many times higher than the rated current of the branch where the short circuit has occurred. Therefore, even with a short-term short-circuit current, it can cause additional and conductors above the allowable one.
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 the conductors and their fastenings, mechanical failures can occur.
A sudden deep voltage drop during a short circuit affects the work of consumers. First of all, this applies to motors, since even with a short-term voltage drop of 30-40%, they can stop (the motors tip over).
The overturning of engines has a serious impact on the operation of an industrial enterprise, since it takes a long time to restore the normal production process and an unexpected stop of the engines can cause rejection of the enterprise's products.
With a small distance and a sufficient duration of a short circuit, it is possible for parallel operating stations to fall out of synchronism, i.e. disruption of the normal operation of the entire electrical system, which is the most dangerous consequence of a short circuit.
The unbalanced current systems arising from ground faults are capable of creating magnetic fluxes sufficient to induce significant EMF in neighboring circuits (communication lines, pipelines), dangerous for the operating personnel and equipment of these circuits.
Thus, the consequences of short circuits are as follows:
1. Mechanical and thermal damage to electrical equipment.
2. Fires in electrical installations.
3. Reducing the voltage level in the electrical network, leading to a decrease in the torque of the electric motors, their braking, reduced performance, or even overturning them.
4. Loss of synchronization of individual generators, power plants and parts of the electrical system and the occurrence of accidents, including system failures.
5. Electromagnetic influence on communication lines, communications, etc.
What is the calculation of short-circuit currents for?
A short circuit of the circuit causes a transient process in it, during which the current can be considered as the sum of two components: forced harmonic (periodic, sinusoidal) ip and free (aperiodic, exponential) ia. The free component decreases with the time constant Tk = Lk/rk = xk/ωrk as the transient process decays. The maximum instantaneous value iу of the total current i is called the shock current, and the ratio of the latter to the amplitude Ipm is the shock coefficient.
The calculation of short-circuit currents is necessary for the correct choice of electrical equipment, the design of relay protection and automation, and the choice of means for limiting short-circuit currents.
Short circuits (short circuits) usually occur through transient resistances - electric arcs, foreign objects at the point of damage, supports and their grounding, as well as resistance between the phase wires and the ground (for example, when the wires fall to the ground). To simplify the calculations, individual transient resistances, depending on the type of damage, are taken equal to each other or equal to zero (“metallic” or “deaf” short circuit).
