Every time you plug an electrical appliance into a socket, you close the electrical network, and an electric current begins to flow through it.
The simplest electrical circuit consists of a current source, a consumer of electricity
and a switch connected by wires.
Consumer electric current transforms electrical energy, which enters it, into other types of energy - mechanical (for example, in electric motors), thermal (in irons, heating appliances), light ( lighting) etc.
The current in this circuit is
,that is, it is directly proportional to the voltage in the network, and inversely proportional to the resistance that the appliance creates (Ohm's law).
And what happens if the circuit closes not as provided for by the design of the circuit and the electrical appliance, but directly, bypassing the electrical appliance?
Schematically it looks like this:
In this case, the resistance of the network drops sharply, and as a result, the current strength in the circuit increases sharply. And, as you know, the amount of heat released in a circuit section is proportional to the square of the current strength in this section (Joule-Lenz law). So, if during a short circuit the current increases 20 times, then the amount of heat released in this case will increase by about 400 times! This is why a short circuit can cause the wires to melt, the insulation to ignite and eventually to ignite combustible objects around the site. short circuit, and to the fire.
A short circuit also occurs if the load resistance becomes less than the internal resistance of the power supply.
What can cause a short circuit? Most often, its cause is a violation of the insulation of the wires (due to their wear, improper operation, etc.). Also, the cause of a short circuit can be mechanical damage in electrical circuit or in an electrical appliance, as well as network overload.
How can a short circuit be prevented?
To do this, special fuses are installed in the circuits.
The simplest fuses are made of fusible material. In the case of a strong increase in current, this material melts or burns and opens the circuit long before the circuits occur more than serious consequences. The little hero who sacrificed himself is replaced by a new one.
Automatic fuses are designed so that in the event of a high current short circuit, an electromagnetic release is instantly activated, which disconnects the electrical circuit without harming itself. In order to turn on the electricity again after the short circuit has been eliminated, you just need to press the white button (the red one is used to turn it off) or throw up the lever that has fallen when the fuse is triggered.
Any person whose work is related to the maintenance of electrical engineering is very well aware of the troubles that a short circuit (short circuit) is fraught with. Sometimes it is considered to be damage. This is wrong. Short circuit is a process, or if you like, emergency mode operation of any section of the electrical installation. But the consequences of it really lead to damage. The generally accepted definition reads: “A short circuit is a direct connection of two or more points with different potential. It is an abnormal (unintended) mode of operation."
To understand what exactly happens in the circuit at the moment when a short circuit occurs there, it is necessary to recall the principles of the functioning of the circuit elements. Imagine a simple circuit consisting of two conductors and a load (for example, a light bulb). IN normal conditions in the conductor there is a directed movement of charged elementary particles, due to the constant influence of the source. They move from one pole of the source to the other through two sections of wire and a lamp. Accordingly, the lamp emits light, since the particles do a certain amount of work in it.
When the direction of movement is constantly changing, but in this case it is not important. The number of electrons passing through a certain section of the circuit per unit of time is limited by the resistance of the lamp, conductors, EMF source. In other words, the current does not grow indefinitely, but corresponds to a steady state.
But for some reason, the insulation in the circuit section is damaged. For example, the lamp was flooded with water. In this case, it decreases. As a result, the current flowing through the circuit is limited by the total resistance of the power source, wires and water "isthmus" on the lamp. Usually this amount is so insignificant that it is not taken into account in the calculations (with the exception of specialized calculations).
The result is an almost infinite increase in current, determined by the classical Ohm's law. In this case, short circuit power is often mentioned. It is determined by the limiting value of the electric current that the power source is capable of delivering before failure. By the way, that is why it is forbidden to wire (short) the opposite contacts of the batteries.
Although in the example we are considering removing the resistance of the lamp from the circuit due to the ingress of water on it, there are many reasons for a short circuit. For example, if we talk about the same scheme, then k.z. It can also occur if the insulation of at least one wire is broken and it comes into contact with the ground. In this case, the current from the power source will follow the path of least resistance, that is, into the ground, which has a huge capacity. Damage to the insulation of two wires at once and their contact will lead to the same result.
The above can be summarized: short circuits can be with and without earth. It does not affect ongoing processes.
What kind of damage was discussed at the beginning of the article? As you know, the higher the value of the current flowing through the sections of the circuit, the greater their heating. With sufficient source power at short circuit. some sections of the circuit simply burn out, turning into copper dust (for copper elements).
Short circuit protection is quite simple and effective. Messages about damage due to a short circuit arise, first of all, due to incorrectly selected parameters of protection devices, incorrect selectivity. If we are talking about a 220 V household circuit, then they use it with an excessive increase in current, the electromagnetic release inside breaks the circuit.
Hi all. I am very glad that you have visited my site. And today, we will talk about what a short circuit is and what kind of circuits are.
A short circuit is a connection (contact) of two or more points (conductors) of an electrical circuit with different potential values.
Different potentials - this is when the phase and zero in the network alternating current, or plus and minus in a DC network.
Now let's look at what types of short circuits are.
IN single-phase network There can only be two types of short circuit:
1. phase and zero - this type of closure is very common in simple living conditions. For example, with the onset of winter it becomes cold, and many people try to keep warm with the help of electric heaters.
But few people pay attention to the sockets that include these same heaters. It often happens that the sockets are not designed for the currents that the heaters consume, or often there may be poor contact in the sockets.
Because of this, sockets and plugs begin to warm up. As a result of prolonged heating, the insulation of the wires is destroyed. And at one fine moment, two, already bare, conductors can touch, and a short circuit will result.
2. phase and grounding - this is when phase wire, somehow comes into contact with the grounded body of any electrical equipment. Either electric water heater, lamp, machine tool and so on.
It also happens that the case can be zeroed, then such a short circuit can be attributed to the first case.
But in situations in which a short circuit occurs, there can be much more:
1. single-phase short circuit- phase and zero. I have already described this view above, so let's move on to the next one.
2. two-phase - this is when two phases are connected to each other. Happens often on overhead lines power lines. Such a phenomenon, probably, every person has seen in his life. When on the street strong wind and starts loosening the wires, and gets a little salute. In industrial enterprises, such a short circuit often occurs in power circuits.
3. two-phase and earth - this, of course, happens less often, but it still happens. An example when two phases can be connected to each other, and at the same time also contact the ground.
4. three-phase - this is when all three phases are somehow closed to each other. Such a short circuit will occur when a conductive object falls or touches all three phases at the same time.
What are the consequences of short circuit currents.
In the event of a short circuit, the current instantly increases, which leads to strong heating and melting of the metals. Splashes of this metal scatter in all directions, and all this is accompanied by a bright flash and fire. Which can easily lead to a fire and very serious consequences.
In normal home conditions, if you do not choose the right protection against short circuits, then you can really lose a lot. Starting from housing and furniture, and ending with my life and the lives of people living with you under the same roof.
In enterprises, short-circuit currents can lead to emergencies, damage to equipment, and people can also suffer from this. But enterprises usually use several protections at once, which practically eliminates the occurrence of short circuits.
That's all I wanted to say. If you have any questions, then ask them in the comments. If the article was useful to you, then share it with your friends in in social networks and subscribe for updates. See you again.
Sincerely, Alexander!
A short circuit occurs when current-carrying parts of different potentials or phases are connected to each other. A short circuit can also form on the equipment case that is connected to the ground. This phenomenon is also typical for electrical networks and electrical receivers.
Causes and effects of short circuit current
The causes of a short circuit can be very different. This is facilitated by wet or aggressive environment, which deteriorates significantly. A closure can result mechanical influences or human error during repairs and maintenance.
The essence of the phenomenon lies in its name and is a shortening of the path along which the current passes. As a result, current flows past the resistive load. At the same time, it increases to unacceptable limits if the protective shutdown does not work.
However, power outage may not occur even if there are protective equipment. This situation occurs when the short circuit is very far away and significant resistance makes the current insufficient to trip. protective devices. However, this current is enough to ignite the wires and start a fire.
In such situations great importance have the so-called time-current characteristics inherent in circuit breakers. Here, current cutoff and thermal releases that protect against overloads play an important role. These systems are completely different time operation, therefore, the slow action of thermal protection can lead to the formation of a burning arc and damage to conductors located nearby.
Short-circuit currents have an electrodynamic and thermal effect on equipment and electrical installations, which ultimately leads to their significant deformation and overheating. In this regard, it is necessary to pre-calculate the short-circuit currents.
How to calculate the short circuit current using the formula
The calculation of these currents, as a rule, is made if it is necessary to check the operation of the equipment in extreme situations. The main purpose is to determine the suitability of protective automatic devices. In order to correctly calculate the short-circuit current, first of all, it is necessary to know exactly the metal from which the conductor is made. For calculations, you will also need the length of the wire and its cross section.
For determining resistivity it is necessary to know the indicator of active resistance Rp, the value of which consists of the resistivity of the wire multiplied by its length. The value of the inductive resistance Xp is calculated from the specific inductive resistance, taken as 0.6 Ohm / km.
The Zt exponent is full resistance phase winding installed in the transformer from the side low voltage. Thus, timely preliminary calculations will help to avoid serious damage to electrical equipment caused by a short circuit.
Calculations make it possible to accurately determine which circuit breaker provide the most effective protection from closures. However, all the necessary measurements can be made using a special device, which is just designed to determine these quantities. To carry out the measurement, the device is connected to the network and switched to the required mode.
Network short circuit protection
A short circuit is the connection to the poles of an EMF source of resistance commensurate with its internal resistance. In most cases, this mode of operation is implemented by accident and is extremely harmful to the EMF source. However, there are electrical devices (, arc melting devices) specially designed for long-term operation in short-circuit mode.
Short circuit hazard
For a person, a short circuit is dangerous primarily by splashes of molten metal, which are formed at the places of accidental contact of conductors and ultraviolet radiation from electric arc that occurs when they break. Besides, high temperatures at the point of contact, in most cases, they can ignite the insulation and cause a fire.
In the first approximation, the maximum possible short circuit current is equal to the ratio of the EMF of the source to its internal resistance. The more power is removed from the source in the design mode of operation, the less it internal resistance and the more dangerous for him is the closure mode. However, the specificity of the effect in different EMF sources different.
The principle of the short circuit
Randomly closed non-recoverable galvanic cells (batteries, etc.), despite the low EMF, not exceeding a few volts, are able to give a current that heats the element to a significant temperature. If you leave such a battery unattended, then it is able to melt the plastic case of the device into which it is inserted. Therefore, when replacing batteries, you should always check them for heating after about a minute. After such a process, the resistance of the element increases unacceptably and it should be replaced. 
Batteries, and even more so, have a much higher EMF of 6 - 48 volts, and with a capacity of tens and hundreds of ampere-hours, their short-circuit current can be measured in hundreds and thousands of amperes. A shorted battery that has no fuses in its circuit can easily become a source of fire, so they must be handled very carefully.
In the vast majority of cases, residential electrical wiring is equipped with protection means in the form of automatic input switches that operate when the current in it exceeds a certain value. This prevents the wires from melting.
Similar protective measures are used in industrial electrical engineering, where excessive current is interrupted either by an electromagnetic circuit breaker or by melting a fusible link, a section much smaller than that of the main conductors of the network.