Short circuit. Causes of occurrence and how to avoid it. The essence of a short circuit in an electrical circuit. Voltage (EMF) and current during short circuit

What is a short circuit? Most often, this phrase can be heard from electricians, as well as people who do not understand electronics and electrical engineering at all. When asked why there was smoke coming from any appliance or device, everyone unanimously says: “A short circuit has occurred.” A very universal excuse for those who want to seem like a smart dunno).

Nature of 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 light bulb will glow.

Let's assume that our wires that lead to the light bulb are completely bare. Suddenly, by some miracle, another similar 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 a short path for electric current to flow through a circuit where there is the least resistance.

Now current flows through both the light bulb and the wiring. But our wiring is much less than the resistance of the light bulb, and almost all the current will flow where there is less resistance - that is, through the wiring. And since our wire has very little resistance, the current will therefore 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 flow 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 catch fire. This case is called short circuit.

You've probably heard more than once in news reports that the fire occurred due to a short circuit. In this case, the bare phase wire touched the bare zero wire in some place, or the phase touched the ground. A short circuit occurred, and the wires began to heat up to such an extent that their heating ignited nearby objects. Hence the fire.

Basically, short circuits occur in old houses from old cables that are cracking at the seams and can short circuit together. Therefore, the first thing you need to do when buying an apartment or house on the secondary market is to look at the condition of the wiring.

Typical signs of a short circuit

burnt fuses in electronic equipment (REA)
heating the circuit in which the short circuit current flows
voltage source low voltage
high current
smoke
charred wires
burnt out PCB tracks
black soot in the place where the short circuit occurred

How to deal with short circuits? This, of course, involves installing fuses, circuit breakers and trying to do neat wiring installation.

We often hear “There is a short circuit”, “There is a short in the circuit”. It is immediately clear that something unplanned and bad happened. But why is the circuit short and not long? Let's put an end to the uncertainty and figure out what exactly happens when there is a short circuit in the electrical circuit.

What is a short circuit (SC)

An electric stingray swims in the ocean and is not happy short circuit, completely dispensing with knowledge of Ohm's law. For us, to understand the nature and causes of a short circuit, this law is simply necessary. So, if you haven’t already, let’s read about Ohm’s law, current, voltage, resistance and other wonderful physical concepts.

Now that you know all this, you can give the definition of a short circuit from physics and electrical engineering:

Short circuit- this is a connection of two points of an electrical circuit with different potentials, which is not provided for by the normal operating mode of the circuit and leads to a critical increase in current strength at the junction.

A short circuit leads to the formation of destructive currents that exceed permissible values, failure of devices and damage to wiring. Why is this happening? Let's look in detail at what happens in the circuit during a short circuit.

Let's take the simplest chain. It contains a current source, resistance and wires. Moreover, the resistance of the wires can be neglected. Such a diagram is quite sufficient to understand the essence of the short circuit.

In a closed circuit, Ohm's law applies: current is directly proportional to voltage and inversely proportional to resistance. In other words, the lower the resistance, the greater the current .

More precisely, for our circuit Ohm’s law will be written in the following form:

Here r is the internal resistance of the current source, and the Greek letter epsilon denotes the emf of the source.

What is meant by short circuit current? If resistance R in our circuit will not be, or it will be very small, then the current strength will increase, and a short circuit current will flow in the circuit:

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Types of short circuits and their causes

In everyday life, short circuits occur:

single-phase– when the phase wire is shorted to zero. Such short circuits happen most often;
two-phase– when one phase closes to another;
three-phase– when three phases are closed at once. This is the most problematic type of short circuit.

For example, on Sunday morning, your neighbor behind the wall connects phase and neutral in the socket by plugging in a hammer drill. This means that the circuit is closed and the current flows through the load, that is, through the device plugged into the outlet.

If a neighbor connects the phase and neutral wires in the socket without connecting the load, then a short circuit will occur in the circuit, but you will be able to sleep longer.

For those who do not know, for a better understanding it will be useful to read what phase and zero are in electricity.

A short circuit is called a short circuit, since the current in such a circuit closure seems to follow a short path, bypassing the load. A controlled or long circuit is the usual, familiar to everyone, plugging in devices into a socket.

Short circuit protection

First, about what consequences a short circuit can cause:

Damage to a person by electric shock and generated heat.
Fire.
Failure of devices.
Power outage and lack of internet at home. As a result, there is a forced need to read books and dine by candlelight.

As you can see, a short circuit is an enemy and a pest that needs to be fought. What are the methods of short circuit protection?

Almost all of them are based on quickly opening the circuit when a fault is detected. This can be done using various short circuit protection devices.

Almost all modern electrical appliances have fuses. High current simply melts the fuse and the circuit breaks.

The apartments use short circuit protection circuit breakers. These are circuit breakers designed for a specific operating current. When the current increases, the machine is triggered, breaking the circuit.

To protect industrial electric motors from short circuits, special relays are used.

Now you can easily define a short circuit, at the same time you know about Ohm’s law, as well as phase and zero in electricity. We wish everyone not to cause short circuits! And if you are stuck in your head and have absolutely no energy for any work, our student service will always help you cope with it.

And finally, a video about how NOT to handle electric current.

Let's consider a special case of parallel connection of conductors - the so-called short circuit. It is called the parallel connection of a conductor with very low resistance in a circuit. Let's look at an example.
Let the lamps and the switch be connected as shown in the diagrams. Note that the switch and the second lamp are connected in parallel, in addition, the closed switch in the right diagram is a conductor with very little resistance. Therefore, according to the definition, In the right diagram there is a short circuit in the lamp.

Suppose, for example, that the voltage of the current source is selected so that when the switch is open, both lamps do not glow very brightly - at half intensity (that is why in the first diagram they are half shaded). If the switch is closed, the left lamp will burn brightly, and the right lamp will go out altogether. Thus, increasing the brightness of the left lamp indicates to us that When there is a short circuit in the circuit, the current increases sharply. According to the Joule-Lenz law, an increase in current strength can lead to overheating of the wires and a fire.
Let us explain why the left lamp lights up brighter. Let us remember that when conductors are connected in parallel, their total resistance becomes less than the smaller one, that is, even less than the resistance of the switch (for which it is already almost zero). According to Ohm's law, a decrease in resistance leads to an increase in current. And an increase in current, according to the Joule-Lenz law, leads to a stronger heating of the spiral of the left lamp.
Let us now explain why the right lamp goes out. Since when the conductors are connected in parallel, the voltage on each of them is the same, the voltages on the right lamp and on the switch are the same. According to Ohm's law U=I·R. As we found out in the previous paragraph, the resistance of this connection is almost zero, that is, R»0. Substituting zero into the formula, we get: U=I·0=0. That is, the voltage on the switch and lamp is zero (more precisely, very small). This voltage is clearly not enough to keep the lamp glowing, so it goes out.

To protect electrical appliances from short circuits they are used circuit breakers. Their purpose is to turn off the power if the current increases above the permissible value. In the picture on the right you see auto fuse with a screw base like a lamp. Such fuses (in common parlance “plugs”) are screwed into special sockets that are mounted on the wall.
There are also fuses. The main part in them is a thin (about 0.1 mm in diameter) wire made of tin or lead (see figure below). In the event of a strong increase in current, it melts almost instantly and the circuit opens, interrupting the current. Unlike "reusable" fuses, fuses are disposable electrical devices.

If we assume that the wires supplying current to the apartment wiring are made of aluminum and have a diameter of 1 mm, then the cross-sectional area of the lead wire will be 100 times smaller. In addition, looking at the table, we see that the resistivity of lead is about 10 times greater than that of aluminum. Therefore, the resistance of the wire is approximately 1000 times greater than the resistance of an aluminum wire of the same length.
Since the wire and the fuse (that is, the wire inside it) are connected in series, the current in them is the same. Since, according to the Joule-Lenz law, Q = I2Rt, therefore, the amount of heat released in the wire at each moment of time is 1000 times greater than in the wire. That is why the wire melts, but the electrical wiring remains intact. Currently, fuses are practically not used in technology, giving way to automatic ones.

Every day, whether at home or at work, we close the electrical circuit, and nothing explosive happens. By completing a circuit using the plug of an electrical appliance, electricity is converted into:

- into mechanical energy - engines of pumps, vacuum cleaners and various electrical devices.
- into thermal energy - hot air from a hairdryer, boiling water from an electric kettle, thermal radiation from an electric convector.

This is a good closure, let’s call it conditionally as opposed to a short, “long” closure of the electrical circuit.

A short circuit has a negative result, that is, the energy positions itself in the form of sparks, bangs, often ignition of wiring and easily flammable materials - a fire.

What is a short circuit?

Example: A locomotive must deliver cargo, say, from the city of Nizhny Novgorod to a metropolis such as Moscow. The journey of the train must be long. A locomotive, pulling 50 wagons of coal behind it, picks up speed. But suddenly, in the city of Vladimir, the dispatcher makes a fatal mistake, switching the arrow to the track where another train is located - an accident cannot be avoided.

A train that has gained high speed cannot be stopped quickly. A clear example may seem primitive, but I would like to show the underlying principle - this is strength, power, used for other purposes, bringing destruction. The route of the locomotive with many cars turned out to be short, incomplete, and did not reach the goal.

It is the POWER of current that produces destruction; during a short circuit, the current increases by 20 times, the amount of heat increases by about 400 times.

Here is another clear explanation of what a short circuit is.

It is known that faulty electrical wiring leads to a short circuit, which most often causes a fire. This is often mentioned in fire reports. What is a short circuit, and why is it dangerous?

In normal operation, the current in the wiring between the phase and neutral wires flows through the load, which limits this current to a level that is safe for the wiring. When the insulation is destroyed, current flows, bypassing the load, directly between the wires. Such contact is called short because it occurs in addition to the electrical appliance.

Let's remember Ohm's law: I = U/R, which is usually pronounced like this: “The current in a circuit is directly proportional to the voltage, and inversely proportional to the RESISTANCE.” It is RESISTANCE that is worth paying attention to here.

The resistance of electrical wiring is, as a rule, small, so it can be neglected and considered equal to zero. According to the laws of mathematics, division by zero is impossible, and the result will tend to infinity. In the event of a short circuit, the current in the circuit will tend to this same infinity.

Of course, this is not entirely true, the wires have some kind of finite resistance, so the current, of course, will not reach infinity, but it will be strong enough to produce a destructive effect, a fairly powerful explosion. A voltaic arc occurs, the temperature of which reaches 5000 degrees Celsius.

Causes of short circuit

Errors of personnel servicing electrical networks.
Due to wear and tear of (outdated) electrical wiring.
Incorrect wiring installation.
Poor contact in wiring connections and electrical appliances
Due to electrical circuit overload.
May occur due to mechanical damage to the wires.
Shortcomings can be caused by rodents.

How to prevent a short circuit?

To prevent short circuit it is necessary.

Properly install and operate electrical installations.
Select electrical wiring in accordance with the current value.
Carry out regularly scheduled inspections and measurements of insulation resistance;
Choose the right automatic protection devices that are designed to disconnect the damaged area.
Before working with the wiring, it must be de-energized.

Benefit of short circuit

Arc welding, which is used in production, was born on the basis of a short circuit. The contact point of the rod and the metal surface is heated to the melting point, the metal structure is connected into a single whole. For example, modern car bodies are fastened precisely through a short circuit - arc welding.

As we have seen, a short circuit can cause destruction if the current is used inappropriately. If energy is managed correctly, great technological advances can be achieved.

One of the main causes of fire is a short circuit. This phrase is constantly heard, but what does it mean?

This is a connection of a ground wire or a neutral wire with a phase wire or two phase wires. This results in the interaction of two conductors with different potentials. It is called a short contact because it occurred without an electrical appliance.

When such wires are connected, a small explosion occurs. This is explained by a sharp jump in current strength, reaching an unacceptable value. Such a rapid increase in current leads to overheating of the wires and the formation of an electric arc between them, the temperature of which reaches 5000 degrees C.

Particularly spectacular is the short circuit of phase wires in a three-phase electrical network. If a person shorts the phases with a screwdriver, he may be thrown several meters away and may receive serious injuries and burns. The screwdriver will simply evaporate. In everyday conditions, there may not be a big explosion, but melting of the wire and insulation is guaranteed, and this is a direct path to the ignition of objects that are nearby.

It is important to remember that when a power line (power line) breaks due to a short circuit, a real explosion with an electromagnetic shock can occur. Therefore, under no circumstances should you approach the place where the line breaks.

The causes of a short circuit are known: old or damaged electrical wiring, incorrect installation (this is typical for amateurs who have little knowledge of electricity), defective insulation, electrical appliances that do not meet electrical safety conditions (again, old or damaged), loosening of wire connections, random line breaks.

All of the above reasons can be successfully combated if you follow some rules:

1. Do not use old wires with inadequate insulation.

2. Be careful when carrying out electrical installation work. Do not drill, tap, or cut walls in places where the power cable is laid.

3. Remove insulation during installation very carefully, do not cut the wire with a knife along the cores.

4. Make sure that the network is turned off when working with it. You need to post a sign on the sign saying “work in progress, do not turn on the electricity” or leave a person on duty.

5. Install protective shutdown devices - automatic ical switches, residual current devices, automatic devices.

6. Regularly monitor the condition of electrical points - sockets and switches. If necessary, replace them immediately.

7. Do not operate damaged electrical appliances from which sparks fly, with the exception of some tools, for example, which have carbon brushes - they spark a little during operation (this happens in a drill, jigsaw and other tools).

8. When installing wiring, do not run the wires in one large bundle; it is better to run them parallel next to each other or use special boxes.

Following these simple rules will significantly reduce the risk of short circuits and fires. And it is important to remember that it is better to entrust work with electricity to a professional electrician. Then life will be calmer and safer!