Short circuit.
Short circuit (short circuit) This mode of operation of an electrical energy source is called when its terminals are closed by a conductor whose resistance can be considered zero. A short circuit occurs when wires connecting a source of electrical energy to a receiver are connected to each other, since these wires usually have little resistance and can be taken equal to zero. A short circuit can also occur if the wire insulation is damaged.
Possible short circuit circuits
Due to the fact that the internal resistance of the source r 0 is usually very small, and the resistance of the ammeter is almost equal to 0, the current in the circuit increases to very large values.
Short circuit is an emergency mode, because the large current that arises can render unusable both the source itself and the devices, devices and wires included in the circuit. Only for some special types generators, for example welding, short circuit does not pose a hazard and is normal operating mode. This is done in cases where it is extremely important to obtain the greatest possible current from the generator. The high internal resistance of the generator limits the current, and it does not reach values dangerous for the generator
Example:
An electrical receiver with a resistance of 109 Ohms is connected to a circuit with a voltage of 220V. Electrical resistance wires 1 Ohm. Find the current strength in this circuit in operating mode and in short-circuit mode.
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Short circuit. A short circuit (short circuit) is a mode of operation of an electrical energy source when its terminals are closed by a conductor whose resistance can be considered zero. A short circuit occurs when wires are connected...
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 = 240 Ohm, and the mains voltage U= 120 V, then according to Ohm’s law the current in the lamp circuit will be:
Figure 1. Short circuit diagram at the switch terminals
Let's look at the case when the wires going to the incandescent lamp are short-circuited through a very small resistance, for example, a thick metal rod with resistance r= 0.01 Ohm, accidentally falling on two wires. In this case, the network current passing to the point A, will branch along two paths: one large part of it will go along the 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.
An emergency mode of operation of a network, when, due to a decrease in its resistance, the current in it sharply increases compared to normal, is called short circuit.
Let us determine 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 current 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 consumed previously an incandescent lamp.
Short circuit power at current I short circuit = 12,000 A will be:
P kz = U × I short circuit = 120 × 12,000 = 1,440,000 W = 1,440 kW.
Current passing through a 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 an enormous amount of heat, which will certainly lead to charring and burning of the wire insulation , melting of wire material, damage to electrical measuring instruments, melting of switch contacts, knife switches, and so on. The source of electrical energy powering such a circuit may also be damaged. Overheating of the wires may cause a fire.
Each electrical network is designed for its own normal current.
Due to the dangerous, destructive and sometimes irreparable consequences of a short circuit, certain conditions must be observed during installation and operation electrical installations to eliminate the causes of a short circuit. The main ones are the following:
1) the insulation of the wires must correspond to its purpose (network voltage and operating conditions);
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) connections and branches must be as reliably insulated as the wires themselves;
5) crossing 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 dampness, mechanical and chemical damage and are well insulated.
Short circuit protection
To avoid a sudden, dangerous increase in current in an electrical circuit during a short circuit, the circuit is protected with fusible or automatic switches.
Fuses are a low-fusible wire connected in series to the network. When the current increases above 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 repeated operations to protect circuits during emergency operating conditions. Structurally, the circuit breaker is made in a dielectric housing with a tripping mechanism built inside. The release mechanism has fixed and moving contacts. The moving contact is spring-loaded; the spring provides force for quick release of the contacts. The release mechanism is activated by one of two releases: thermal or magnetic.
The thermal release is a bimetallic plate heated by flowing current. When a current flows above the permissible value, the bimetallic plate bends and activates the tripping mechanism. The response time depends on the current (time-current characteristic) and can vary from seconds to an hour. Unlike a fuse, a circuit breaker is ready for next use once the plate has cooled.
An electromagnetic release is an instantaneous release, which is a coil made of a 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 specified current threshold is exceeded. An instantaneous release, unlike a thermal release, operates very quickly (fractions of a second), but at a much higher current: 2 ÷ 14 times the rated current.
Video 1. Short circuit
Electrical energy carries a fairly high danger, from which neither the workers of individual substations nor household appliances. Short circuit current is one of the most dangerous species electricity, but there are methods to control, calculate and measure it.
What is it
Short circuit current (SCC) is a sharply increasing shock electrical impulse. Its main danger is that, according to the Joule-Lenz law, such energy has a very high rate heat release. As a result of a short circuit, wires may melt or certain electrical appliances may burn out.
Photo - timing diagramIt consists of two main components - the aperiodic current component and the forced periodic component.
Formula – periodic
Formula – aperiodic According to the principle, the most difficult thing to measure is the energy of aperiodic occurrence, which is capacitive, pre-emergency. After all, it is at the moment of the accident that the difference between the phases has the greatest amplitude. Also, its peculiarity is the non-typical occurrence of this current in networks. The diagram of its formation will help show the principle of operation of this flow.
The resistance of the sources due to the high voltage during a short circuit is short-circuited over a short distance or “short-circuited” - that’s why this phenomenon got its name. There is a short circuit current of three-phase, two-phase and single-phase - here the classification occurs according to the number of closed phases. In some cases, the short circuit may be shorted between phases and to ground. Then, to determine it, you will need to separately take into account grounding.
Photo – result of short circuit You can also distribute short circuits according to the type of electrical equipment connection:
- With grounding;
- Without him.
To fully explain this phenomenon, we suggest considering an example. Let's say there is a specific current consumer that is connected to a local power line using a tap. At correct scheme the total voltage in the network is equal to the difference in EMF at the power source and the voltage reduction in local electrical networks. Based on this, Ohm’s formula can be used to determine the short-circuit current:
R = 0; Ikz = Ɛ/r
Here r is the short-circuit resistance.
If you substitute certain values, you can determine the fault current at any point along the entire power line. There is no need to check the short circuit multiplicity here.
Calculation methods
Let's assume that a short circuit has already occurred in a three-phase network, for example, at a substation or on the windings of a transformer, how then the short circuit currents are calculated:
Formula - three-phase fault current Here U20 is the voltage of the transformer windings, and Z T is the resistance of a certain phase (which was damaged in the short circuit). If the voltage in the networks is a known parameter, resistance must be calculated.
Each electrical source, be it a transformer, a battery terminal, or electrical wires, has its own nominal resistance level. In other words, everyone has their own Z. But they are characterized by a combination of active resistances and inductive ones. There are also capacitive ones, but they do not matter when calculating currents high strength. Therefore, many electricians use a simplified method for calculating this data: an arithmetic calculation of the direct current resistance in series-connected sections. When these characteristics are known, it will not be difficult to calculate the impedance for a section or an entire network using the formula below:
Full grounding formula Where ε is the emf, and r is the resistance value.
Considering that during overloads the resistance is zero, the solution takes the following form:
I = ε/r = 12 / 10 -2
Based on this, the short circuit strength of this battery is 1200 Amperes.
In this way, it is also possible to calculate the short-circuit current for a motor, generator and other installations. But in production it is not always possible to calculate acceptable parameters for each individual electrical device. In addition, it should be taken into account that in case of asymmetrical short circuits, the loads have a different sequence, which requires knowing cos φ and resistance to take into account. For the calculation, a special table GOST 27514-87 is used, where these parameters are indicated:
There is also the concept of a one-second short circuit, here the formula for the current strength during a short circuit is determined using a special coefficient:
Formula – short circuit coefficient It is believed that, depending on the cross-section of the cable, a short circuit can pass unnoticed by the wiring. The optimal short circuit duration is up to 5 seconds. Taken from Nebrat’s book “Calculation of short circuits in networks”:
| Section, mm 2 | Short circuit duration permissible for a specific type of wire | |||
| PVC insulation | Polyethylene | |||
| Copper veins | Aluminum | Copper | Aluminum | |
| 1,5 | 0,17 | No | 0,21 | No |
| 2,5 | 0,3 | 0,18 | 0,34 | 0,2 |
| 4 | 0,4 | 0,3 | 0,54 | 0,36 |
| 6 | 0,7 | 0,4 | 0,8 | 0,5 |
| 10 | 1,1 | 0,7 | 1,37 | 0,9 |
| 16 | 1,8 | 1,1 | 2,16 | 1,4 |
| 25 | 2,8 | 1,8 | 3,46 | 2,2 |
| 35 | 3,9 | 2,5 | 4,8 | 3,09 |
| 50 | 5,2 | 3 | 6,5 | 4,18 |
| 70 | 7,5 | 5 | 9,4 | 6,12 |
| 95 | 10,5 | 6,9 | 13,03 | 8,48 |
| 120 | 13,2 | 8,7 | 16,4 | 10,7 |
| 150 | 16,3 | 10,6 | 20,3 | 13,2 |
| 185 | 20,4 | 13,4 | 25,4 | 16,5 |
| 240 | 26,8 | 17,5 | 33,3 | 21,7 |
This table will help you find out the expected conditional duration of a short circuit in normal operation, the amperage on the busbars and various types of wires.
If there is no time to calculate data using formulas, then special equipment is used. For example, the Shch41160 indicator is very popular among professional electricians - this is a 380/220V phase-to-zero short circuit current meter. The digital device allows you to determine and calculate the short-circuit strength in household and industrial networks. Such a meter can be purchased at special electrical stores. This technique is good if you need to quickly and accurately determine the current level of a loop or section of circuit.
The “Emergency Emergency” program is also used, which can quickly determine the thermal effect of a short circuit, loss rate and current strength. The check is carried out automatically, known parameters are entered and it calculates all the data itself. This is a paid project, the license costs about a thousand rubles.
Video: protecting the electrical network from short circuits
Protection and equipment selection guidelines
Despite the danger of this phenomenon, there is still a way to limit or minimize the likelihood of emergency situations. It is very convenient to use an electrical apparatus to limit short circuits; this can be a current-limiting reactor, which significantly reduces the thermal effect of high electrical impulses. But this option is not suitable for domestic use.
Photo - diagram of the short-circuit protection unit At home, you can often find the use of automatic circuit breakers and relay protection. These releases have certain restrictions (maximum and minimum network current), if exceeded, the power is turned off. The machine allows you to determine permissible level amperes, which helps improve safety. The choice is made among equipment with a higher protection class than necessary. For example, in a 21 amp network, it is recommended to use a 25 A circuit breaker.
Content:
The movement of charged particles in a conductor in electrical engineering is called electric current. Electric current is not characterized only by the amount of electrical energy passing through the conductor, since in 60 minutes electricity equal to 1 Coulomb can pass through it, but the same amount of electricity can be passed through the conductor in one second.
What is current strength
When the amount of electricity flowing through a conductor over different time intervals is considered, it is clear that over a shorter period of time the current flows more intensely, so another definition is introduced into the characteristics of electric current - this is the current strength, which is characterized by the current flowing in the conductor per second of time. The unit of measurement for the magnitude of passing current in electrical engineering is the ampere.
In other words, strength electric current in a conductor - this is the amount of electricity that has passed through its cross-section in a second of time, marked with the letter I. The current strength is measured in amperes - this is a unit of measurement that is equal to the strength of a constant current passing through endless parallel wires with the smallest circular cross-section, distant from each other per 100 cm and located in a vacuum, which causes an interaction on a meter of length of the conductor with a force = 2 * 10 minus 7 degrees of Newton for every 100 cm of length.
Experts often determine the magnitude of the passing current; in Ukraine (strum power) it is equal to 1 ampere, when 1 coulomb of electricity passes through the cross-section of the conductor every second.
In electrical engineering, you can see the frequent use of other quantities in determining the value of the passing current: 1 milliampere, which is equal to one / Ampere, 10 to the minus third power of Ampere, one microampere is ten to the minus sixth power of Ampere.
Knowing the amount of electricity passing through a conductor over a certain period of time, you can calculate the current strength (as they say in Ukraine - strumu force) using the formula:
When an electrical circuit is closed and has no branches, then the same amount of electricity flows per second at each place in its cross-section. Theoretically, this is explained by the impossibility of accumulating electrical charges in any place in the circuit; for this reason, the current strength is the same everywhere.
This rule is also true in complex circuits when there are branches, but it applies to some sections of a complex circuit that can be considered as a simple electrical circuit.
How is current measured?
The magnitude of the current is measured with a device called an ammeter, and also for small values - a milliammeter and a microammeter, which can be seen in the photo below:
There is an opinion among people that when the current strength in a conductor is measured before the load (consumer), the value will be higher than after it. This is an erroneous opinion based on the fact that supposedly some amount of force will be expended to bring the consumer into action. Electric current in a conductor is an electromagnetic process in which charged electrons participate; they move in a direction, but it is not the electrons that transmit energy, but the electromagnetic field that surrounds the conductor.
The number of electrons leaving the beginning of the chain will be equal to the number of electrons after the consumer at the end of the chain, they cannot be used up.
What types of conductors are there? Experts define the concept of “conductor” as a material in which particles with a charge can move freely. Almost all metals, acids and saline solutions have such properties in practice. A material or substance in which the movement of charged particles is difficult or even impossible is called insulators (dielectrics). Common dielectric materials are quartz or ebonite, an artificial insulator.
Conclusion
In practice, modern equipment works with large current values, up to hundreds or even thousands of amperes, as well as with small values. An example in everyday life of the current value in different devices can be an electric stove, where it reaches a value of 5 A, and a simple incandescent lamp can have a value of 0.4 A; in a photocell, the value of the passing current is measured in microamps. In the lines of the city public transport(trolleybus, tram) the value of the passing current reaches 1000 A.
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