What is short circuit current and phase-zero loop

What is a short circuit?

Many people know such a stable expression - “short circuit”. In addition to the name of a famous blockbuster from the 90s, the average person associates these words with a common cause of fires. There are many myths and cliches floating around on this topic. I decided to figure out what was what and why all this was needed.

A short circuit (SC) is a mode of operation of the electrical network, or a phenomenon in which the maximum possible current flows in the circuit at the point of the circuit. This event is difficult to predict and emergency, and the sooner it stops, the better. When a short circuit occurs, all the energy of the power source is spent only on heating the wires. In addition, dynamic (mechanical) consequences are possible. This process is usually very fleeting and explosive, since the thermal energy released is colossal. If you do not stop this disgrace as quickly as possible (we will look into how this is done below), then short circuit can lead to large material and human losses.

The shutdown time of household circuit breakers during a ground fault must be less than 0.4 s (PUE 1.7.79, 7.1.72). If the speed is not ensured due to low short-circuit current, shutdown should occur through devices that respond to differential current (RCD, RCBO), the response time will be (according to GOST) less than 0.04 s.

A short circuit can occur between any points in an electrical circuit that have different potentials. Here's what it looks like in three-phase form:

Short circuits in the power supply system with the TN-S grounding system. Who will see the error in the diagram?

The figure conventionally shows the secondary winding of a step-down transformer installed in a transformer substation (TS), a five-wire power line and a three-phase electrical installation. The electrical installation can be a private or apartment building, or maybe something industrial.

Closures can come in different forms:

• two- and three-phase (phase-to-phase),
• one-, two- or three-phase to neutral N or protective PE conductor.

If we consider the safest grounding system TN-S with a solidly grounded neutral of the transformer, then most often (in practice - about 90%) a single-phase short circuit occurs between the phase wire and the neutral N (or the protective conductor PE). Therefore, further we will consider a simpler, single-phase option:

Short circuit to neutral and protective conductors

I recommend my article: How three-phase voltage differs from single-phase. And linear from phase.

A short circuit can happen anywhere - even near a transformer substation (TS) due to the inattention of an excavator operator, or in an apartment because of a cat that dropped a Christmas tree. In any case, the protection must work clearly, minimizing the consequences of a short circuit.

By the way, our cat once dropped our Christmas tree. They threw her out of the 5th floor.

Single-phase short circuits

In practice, single-phase short circuits occur in most cases. In networks with an isolated neutral, when one phase is connected to ground, the mode is not a short circuit and the uninterrupted power supply is not disrupted, but it must be turned off, since it corresponds to an emergency condition. When one phase is short-circuited to ground in a given network, the voltages on the other two phases increase by 1.73 times, and the voltage at the zero point becomes equal to the phase voltage relative to ground.

It will be interesting➡ What is a thermocouple: about the device in simple words

In networks with a solidly grounded neutral, when the wire is connected to the ground, a fuse burns or a circuit breaker trips, which disrupts the power supply, and if the fuse burns, the motor windings can be damaged when operating on two phases.

If in any part of the electrical wiring or electrical appliance (light bulb, iron, etc.) the insulation is broken and the phase wire touches the neutral wire, a short circuit will occur.

Since there is no load between the shorted wires, in other words, the electrical resistance of the contact point is practically zero, the current through the contact will begin to increase until the wires melt, which, in particular, can lead to a fire. Fuses are used to protect against short circuits.

A simple (in the form of a “plug”) fuse is a low-fusible insert included in the phase wire, which, when the current increases, will burn out and open the circuit long before more serious troubles occur. Structurally, the fuse is designed in such a way that this micro-disaster does not lead to damage to the fuse block. The little heroine who sacrificed herself is thrown out and replaced by the next one.

Causes of short circuit

A short circuit can occur for various reasons, the main one of which is a violation of the insulation or the relative position of live parts.
Very often, human or natural factors are to blame for the occurrence of short circuits. An example that women will appreciate (a miracle if they read this article) - due to constant kinks, the insulation deteriorates, and at one “wonderful” moment the hair dryer or iron “bangs” at the input or near the plug.

Another example is that due to mechanical failure or external influence, current-carrying parts for some reason end up too close to each other, even to the point of complete contact. This can happen due to natural phenomena (a tree fell on the wires), shocks, or falling electrical appliances.

Well, a classic example is a short circuit due to interference in the electrical wiring of home “jacks of all trades”. According to the laws of the genre, after this incident the master’s hair must stand on end and his face must be black. Such pictures don’t make me laugh – everything happens differently.

How does a short circuit differ from an overload?

If the phase and neutral of the electrical network are connected under voltage to each other not through the consumer, but directly, then a short circuit will occur, abbreviated as short circuit. A short circuit is the connection of conductors of individual phases to each other or to the ground through a relatively low resistance, taken equal to zero in the case of a solid metal short circuit.

No network is designed to operate in this mode for long periods of time. However, this emergency mode sometimes occurs. Thus, a short circuit can occur due to a violation of the insulation of the electrical wiring or due to the accidental short circuit of opposite conductors with conductive parts of electrical equipment. The normal operation of the electrical network will be disrupted. To prevent this undesirable phenomenon, electricians use terminal blocks or simply isolate the connections.

It will be interesting➡ What is electrolysis and where is it used in practice?

The problem with the short circuit mode is that at the moment of its occurrence, the current in the network increases many times (up to 20 times the nominal), which leads to the release of a huge amount of Joule heat (up to 400 times the norm), since the amount of heat released is proportional to the square of the current and resistance consumer.

Now imagine: the consumer resistance here is a fraction of an ohm of the wiring, and the current, as is known, the higher the lower the resistance. As a result, if the protective device does not immediately operate, excessive overheating of the wiring will occur, the wires will melt, the insulation will ignite, and a fire may occur in the room. In neighboring rooms supplied by the same network, the voltage will drop, and some electrical appliances may fail.

Important on the topic: How to ring a transistor.

A typical type of short circuit for residential apartments is a single-phase short circuit, when a phase closes to zero. For three-phase networks, for example in a workshop or garage, a three-phase or two-phase short circuit is possible (two phases between each other, three phases between each other, or several phases to zero). Three-phase equipment, such as an asynchronous motor or a three-phase transformer, is characterized by an interturn short circuit, when the turns are short-circuited inside the stator winding or inside the transformer winding, shunting the remaining working turns and thus disabling the device.

Or a short circuit may occur through the conductive body of the device. In general, conductive housings should be grounded in order to protect personnel from accidental electric shock, and wires in apartments should be those with non-flammable insulation. There is another type of emergency load mode of the electrical network associated with excess of normal current.

This is the so-called overload. Overloads sometimes occur in apartments, houses, and businesses. This is a dangerous mode, sometimes more dangerous than a short circuit. After all, a short circuit in an apartment can be stopped in the bud by an instantly triggered circuit breaker in the panel. But current overload is a more tricky case.

Switches for short circuit protection.

Imagine that you decided to plug a lot of electrical appliances into one single outlet through a tee and extension cords. What undesirable things could happen in this case? If the wiring core connected to the outlet is not designed for a current of more than 16 amperes, then when a load of more than 3500 watts is connected to such an outlet, overheating of the electrical wiring will begin, fraught with fire.

In general, the thermal effect on wire insulation sharply reduces its mechanical and dielectric properties. For example, if the conductivity of electrical cardboard (as an insulating material) at 20°C is taken as unity, then at temperatures of 30, 40 and 50°C it will increase by 4, 13 and 37 times, respectively.

And thermal aging of insulation most often occurs precisely because of overload of electrical networks with currents exceeding the long-term permissible for a given type and cross-section of conductors. It is also impossible to connect consumers of more than 2500 W to a socket on which 250 V 10 A is indicated, because the contacts may begin to overheat, leading to their accelerated oxidation. To protect against overloads in the apartment, as well as to instantly stop the short circuit mode, use circuit breakers.

How to avoid short circuit?

It is clear that it is impossible to completely avoid this unpleasant phenomenon - there is a great element of chance here. However, we are able to significantly reduce the risk of short circuits. And here regular inspection and maintenance of electrical networks becomes of enormous importance.

Examples of preventive measures:

• cleaning live parts, contacts and insulators from dust and dirt,
• checking moisture protection,
• checking the integrity of installation and installation,
• fencing and additional protection of dangerous areas,
• hanging and sticking warning signs and inscriptions,
• checking and pulling contacts,
• pruning trees and eliminating other hazardous factors.

What do you think are the necessary preventive measures to protect against short circuits in the photo below?

Drainpipe, electrical panels and corrugation that goes under the tiles. Installation in the old part of Batumi

Serious organizations regularly check cables and contacts with a thermal imager, as well as measure insulation resistance and test insulation with high voltage voltage.

What to do if it shorts: emergency actions

If there is a power outage in your home, you should make sure that there is an emergency at home.
Smoke and the acrid smell of melted plastic will tell you about problems. It is necessary to turn off the machines, unscrew the plugs on the electrical panel, and disconnect electrical appliances from the sockets. It is better to put out an open fire by covering it with a blanket or thick cloth.

If you have a powder (car) or carbon dioxide fire extinguisher, be sure to use it to extinguish the fire. Do not pour water on the fire!

The causes of serious electrical faults are varied, but in most cases are known. The occurrence of a short circuit is associated with improper operation and violation of clause 2.1.21 of the PEU rules during installation of electrical wiring.

Short circuit and overload

What is the difference between these two phenomena - short circuit and overload?

In an electrical circuit, 4 fundamentally different modes can be distinguished, which differ in current consumption:

1. Idle mode. The current is zero, the voltage is rated, there are no losses on the wires. An outlet to which nothing is connected acts as a voltage source in idle mode.
2. Nominal mode. Otherwise - normal mode, when the load power does not exceed the design one. In this mode, everything is fine, we are quietly enjoying the benefit of electrification of the country. If there is a voltage drop, it will be insignificant - a few percent.
3. Overload mode. In this mode, the current can slightly (tens of percent) or several times (hundreds of percent) exceed the rated current. Overload can occur due to partial deterioration of insulation, excess of the total power of connected consumers, or due to a malfunction inside a separate electrical appliance (for example, an interturn short circuit or a jammed electric motor, or a short circuit inside the heating element).
4. Short circuit mode. This is the most severe, destructive mode with large heat release. The current at the fault point is the maximum possible for the given conditions. Other side effects of a short circuit are a decrease in voltage for other consumers (how new German refrigerators burned out in the regional Magnit warehouse due to low voltage) and phase asymmetry (what phase asymmetry (misalignment) leads to and how to protect against it).

That is, an overload differs from a short circuit in the magnitude of the overcurrent. During a short circuit, the current becomes the maximum possible at a given point in the circuit, and during an overload, the current value is greater than the rated value, but less than the short circuit current.

Any currents higher than the rated current are called overcurrent .

Due to an overload, a short circuit can easily occur - the wires heat up, the insulation melts, and so on, with all the ensuing, shooting and exploding consequences.

Do not confuse overload, short circuit and sparking (arc breakdown). If the first two concepts differ in the value of the overcurrent, then in the event of a sequential arc breakdown (for example, the tightening of a terminal in a socket is loose), the effective value of the current can be quite insignificant (units of amperes), which will not trigger either the circuit breaker or the RCD. Only an anti-sparking device (against arc breakdown), which is still relatively rare, can save the situation from a fire.

I have several articles on such devices on my blog, here is the last one for today.

Application of short circuit

Besides its negative characteristics, this phenomenon is widely used in some electrical equipment. Short circuits, which are high-speed drives, operate on this principle.

They are used to create a deliberate short circuit in order to cause a protective shutdown. Such devices are used in emergency situations in high-voltage lines. When a power transformer breaks down, the device causes a short circuit between phases in electrical lines up to 35 kV or phase and ground at voltages from 110 kV.

You might be interested in Calculation and tables for selecting cable cross-sections based on power and current

The device turns on both automatically and manually, if necessary. Electric arc welding works on the basis of closure, which makes it possible to obtain strong metal connections. Most often, such a device is used to connect car body parts.

What determines the voltage and current during a short circuit?

Above I said that a short circuit can occur at any point on the line. Let's figure out how the current and voltage will depend on the location of the short circuit.

A short circuit is a physical phenomenon. Short circuit current is a parameter of the power supply network, measured in amperes or kiloamperes (kA).

The German physicist Ohm has taught us since school years that voltage and current are determined through the resistance of the circuit:

Short circuit current, like any current, is also calculated according to Ohm's law and depends on the voltage and resistance in a given section of the circuit. Since the resistance of wires in real life is not only what the multimeter shows, but also the inductive component, we will write Ohm’s law for short-circuit current in a more general form:

In the numerator U is the rated voltage in the network (no-load voltage at the output of the transformer at the transformer substation). The number that is obtained in the calculations in the denominator is the total resistance of the circuit Z, on which the short-circuit current depends. Let's consider a single-phase power supply circuit for an apartment and a real case of a short circuit with a shorted hair dryer:

Short circuit at the end of the supply line (minimum short-circuit current)

The diagram shows the impedances of various sections of the supply network:

• Z1 – internal resistance of the transformer at the substation, taking into account the recalculated resistance of the high-voltage part,
• Z2 – cable line from the transformer substation to the distribution point (DP) of the apartment building,
• Z3 – cable line from the distribution point to the apartment panel,
• Z4 – cable from the panel to the socket in one of the rooms,
• Z5 – carrying from an outlet to a closed hair dryer.

The hair dryer burned out and caused a short circuit

Here's what a voltage level graph might look like in different areas - from the terminals of a transformer at a substation to a shorted hair dryer plug:

Voltage drop to zero as a result of a short circuit at the end of the line

The voltage drop is accompanied by the release of heat in all parts of the supply line. In powerful sections with a large cross-section of wires, the share of the “indoor” short-circuit current is negligible, so the drop there is small (sections with resistance Z1, Z2).

Article about voltage drop. Calculation in low-voltage circuits and in DC circuits, without taking into account the reactive component.

In connection with the voltage drop as a result of a short circuit, it can be noted that this will be noticeable on parallel loads connected, for example, to the same RP. In the event of a short circuit or severe overload of one of the consumers, the light bulbs in neighboring houses and entrances will begin to burn dimmer. Has it happened?

And here’s what the change in short-circuit current from the source to the fault point might look like:

Decrease in current when moving away from the power source

The typical value of the short-circuit current at the terminals of a transformer with a power of up to 1000 kVA, which is used to power urban consumers, is about 10 kA. But in the sockets of our apartments, the short-circuit current can be about 1000 A. In the private sector and rural areas, the value of the short-circuit current can be much less - up to 100 A.

Transformer at a 10000/0.4 kV substation with a power of 1000 kVA with a solidly grounded neutral of the secondary winding. This is roughly what our “districts, neighborhoods, residential areas” are powered by.

Types and reasons

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 is closed 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.

An electrical circuit is, as a rule, two conductors with opposite potentials and a connected current consumer. Each end consumer has its own internal resistance, which resists and limits the current, thereby dosing its quantity and density in the conductor, forcing it to produce work.

Estimated value of short-circuit current

How to find out the short-circuit current? It would seem – what’s difficult? Substitute the values ​​into the formula and count!

However, the full calculation of the short-circuit current is very complex, and it can be devoted to a coursework, or even a diploma project. In this case, you need to know a lot of initial data (for example, the power of the transformer at the TP and the inductive reactance of all sections of cable lines), and still the result will be theoretical, not taking into account reality - for example, contact resistances. It is also important to take into account that during a short circuit there are two current components: aperiodic (the shock part, the most powerful and unpredictable), which acts only at the initial moment during the transition process, and periodic, which practically does not change its value from the beginning to the end of the incident.

Therefore, calculations are usually left to graduate students and designers, but in practice the actual short-circuit current is measured using special instruments. For a more accurate calculation, you can use the books posted at the end of the article or calculation programs.

Short circuit protection

As we found out, short-circuit currents are very dangerous, primarily from the point of view of fire safety. Therefore, it is necessary to build protection against short-circuit currents, that is, install circuit breakers in the switchboard. Automatic fuses are designed so that in the event of a short circuit, an increase in the short-circuit current leads to the operation of an instantaneous electromagnetic release, which disconnects the electrical circuit without harming itself.

In order to turn on the electricity again after eliminating the short circuit, you just need to press the white button (the red one is used to turn it off) or flip up the lever that dropped when the fuse was activated.

The rules for installing electrical wiring provide for the calculation of the load and currents passing through the circuit breakers. It is clear that the fuse must operate at current values ​​selected with a significant margin. Otherwise, random small fluctuations in the voltage in the network (and therefore the current) will lead to constant false operation of the protection. On the other hand, the margin should not be too large so that the action of the current does not cause harm to the network before the cutoff occurs.

Cable insulation fire.

Automatic fuse protects internal and external networks

Note that automatic fuses installed at the beginning of each house line (working group) protect not only the house network, but also the external one, from short circuits. In fact, if they were not there, then an emergency short circuit would lead to the failure of the transformer substation, or rather, the electrical power panel of a higher level, so that a significant number of users would lose electricity, and even without calling the emergency service there would be not enough. And if you have a “machine”, it is enough to turn it on after operation (removing, of course, the cause of the short circuit).

The need for several lines in the house also becomes clear: if one line fails, there are others in stock. By the way, here is the conclusion: it is convenient if each work group supplies power to an emergency light bulb in the area of ​​the meter or an emergency socket into which a portable lamp can be plugged in.

What to do if the measured short-circuit current is too low?

Let's say we measured it with a device and got the value of the short-circuit current in the socket (as a rule, the measurement is carried out at the most remote point). How can you tell if this current is too low? This is assessed by the criterion of guaranteed operation of the electromagnetic release of the circuit breaker in the measured circuit. It is logical that for this the short-circuit current must be greater than the upper limit of the tripping range. Let me remind you that for characteristic “B” the spread is 3…5 In, for “C” – 5…10 In, for “D” – 10…20 In. To be more precise, let’s turn to the PUE (clause 7.3.139):

7.3.139. In electrical installations up to 1 kV with a solidly grounded neutral, in order to ensure automatic shutdown of the emergency section, the conductivity of the neutral protective conductors must be selected such that, in the event of a short circuit to the housing or neutral protective conductor, a short-circuit current occurs that is at least 4 times greater than the rated current of the fuse link of the nearest fuse and at least 6 times the current of the circuit breaker release, which has an inverse current characteristic.

When protecting networks with automatic circuit breakers that have only an electromagnetic release (without a time delay), you should be guided by the requirements regarding the short-circuit current multiplicity and given in 1.7.79.

As I understand it, the first part of 7.3.139 only talks about the thermal release - its rated current must be at least 6 times less than the short-circuit current. The second part of this paragraph, as well as paragraph 1.7.79, talks about the maximum shutdown time during a short circuit (0.4 s), which must be provided only by an electromagnetic release. At the same time, it is not clearly indicated on the choice of AV taking into account its shutdown characteristics.

Because of this vagueness of the wording, they use the rule set out in PTEEP (checking the operation of protection in a power system with a grounded neutral, clause 28.4), which states that when a short circuit to the neutral protective conductor occurs, the short-circuit current must be at least “1.1 the upper value of the operating current of the instantaneous release.”

That is, for a B10 circuit breaker, the short-circuit current at the end of the line that it protects must be at least 10x5x1.1 = 55 A. If a C25 circuit breaker is installed, the short-circuit current must be at least 25x10x1.1 = 275 A.

If the short-circuit current is less, the permissible response time is by no means guaranteed. What to do? There are two ways out:

1. increase the short-circuit current, this requires the cost of laying a new supply line (at least its weakest link),
2. reduce the rating of the machine (for example, 25 A to 16) and the letter of the shutdown characteristic (from “C” to “B”) to the detriment of the maximum load power.

Read in more detail why it is always preferable to put “B” rather than “C” for group machines.

Short circuit currents: accurate calculation required

Hence the need arises to calculate the short-circuit current - short-circuit current. The magnitude of short-circuit currents may change if other electrical receivers in more remote locations are connected to the power supply network of your home. In such cases, the short-circuit current is calculated again at the installation site of new electrical receivers. Short-circuit currents also produce an electrodynamic effect on devices and conductors, when their parts can be deformed under the influence of mechanical forces that arise at high currents.

In the event of a short circuit, overheating of devices and wires occurs. The thermal effect of short-circuit currents is the overheating of devices and wires. Therefore, when choosing devices, they need to be checked for short-circuit conditions so that they can withstand short-circuit currents at the place of their installation. As is known, along with networks with a solidly grounded neutral, there are networks with an isolated neutral. Let us consider the characteristic differences of these networks during a short circuit.

This is interesting! All about semiconductor diodes.

Why do you need to know the values ​​of the short-circuit current and the resistance of the “Phase-zero” loop?

I have already said a lot of things in the article. But what good is it to us to know these parameters of the power grid?

Knowing the short-circuit current (or the resistance of the “Phase-zero” loop) and the load power allows us to correctly and optimally (in terms of safety/functionality/reliability/price ) select the main elements of the power system - protection devices and cable cross-sections. A little more detail below.

Safety

I have already spoken about this, but I will repeat it. Electrical networks must be safe in all areas and in all modes. For this, in addition to insulation, circuit breakers and devices controlled by differential current (RCD) are used. Together with protective grounding, these devices protect equipment from short circuits and overloads, and people from the danger of direct or indirect contact.

Functionality

Knowing the short-circuit current, you can issue a conclusion about the need to install a stabilizer, or replace the cable line with a new one. In addition, we can draw a conclusion about selectivity - can it be ensured at least partially?

Reliability

In case of high short-circuit current, it is necessary to use switches with high breaking capacity for reliable operation at the moment of short-circuit. In addition, high demands must be placed on the quality of installation and components.

Price

It’s clear here - fulfilling the previous points significantly affects the price of the entire electrical network.

Consequences of an electrical short circuit

When bare wires come into contact with different potentials, a short circuit occurs, the current increases sharply, the temperature rises and the insulation melts, and sometimes even an electric arc occurs.

The consequences of a short circuit in the home electrical network can be extremely unpleasant:

• the temperature at the short-circuit site, especially when an electric arc occurs, can reach the melting point of the wiring strands, and the process itself will become explosive;
• a fire may occur as a result of overheating of the wires and ignition of the insulation;
• Some electrical appliances may fail.

Is high short-circuit current good or bad?

As I showed in the graph earlier, the further the fault location is from the power source, the lower the short circuit current will be because the line resistance will be greater. High short-circuit current usually occurs in those places of the power network that are located closest to the substation, and cable lines have a large cross-section of wires. In supply networks with a voltage of 0.4 kV, short-circuit currents of more than 6 kA are considered relatively high, and short-circuit currents above 15 kA are practically never encountered. So what we have:

Disadvantages of low short circuit current

• large voltage drop under sufficiently powerful load;
• As a rule, low voltage on electrical appliances. In this case, the stabilizer will not always help;
• instability of voltage on electrical appliances depending on the time of day or season. I conducted an investigation according to the voltage standards and its tolerances;
• high (up to infinity) response time of circuit breakers during a ground fault (only the thermal release works);
• the need to install circuit breakers with a shutdown characteristic of “B” in order to more likely trigger the electromagnetic release during a short circuit. This controversial issue is discussed in my article on Zen Why install machines with the “B” characteristic;
• mandatory installation of an RCD - in addition to its “main” responsibilities (switching off power at high leakage current, as well as to protect people during direct and indirect contact), the RCD performs the function of protection against ground faults (PUE 1.7.59, 7.1.72 ).

Pros of low short circuit current

• you can install cheap circuit breakers with a low rated maximum breaking capacity (Icn = 4500 A);
• It is relatively easy to ensure selectivity between the input and subordinate automata. But we need to calculate and measure the exact value of the short-circuit current,
• low starting current of electric motors and other inertial loads. Article What is inrush current, how to measure and calculate it.

Disadvantages of high short circuit current

• impossibility of ensuring selectivity between higher and lower-level machines. The solution is to install a switch or a time-selective circuit breaker;
• the need to install an AV with a high rated maximum breaking capacity (Icn = 6000, 10000 A, etc.). The breaking capacity must be higher than the short-circuit current at the beginning of the protected section (PUE clause 3.1.3);
• large negative consequences when a short circuit occurs.

Pros of high short circuit current

• it is easy to guarantee stable voltage at the load and the quality of electricity in general;
• there is a prospect of connecting new consumers and increasing the load;
• guaranteed line shutdown in case of short circuit.

Selectivity of circuit breakers and RCDs is a separate big topic, and there are plans.

Summarizing the pros and cons, we can say that the value of the short-circuit current is a double-edged sword. In the domestic sector, the short-circuit current is often low, and they try to increase it by laying new lines with a high cross-section of wires and installing new transformer substations. In serious energy, on the contrary, methods are used to reduce short-circuit current.

Causes

It is believed that a short circuit (SC) is a random phenomenon that can occur at any time. There are a number of direct and indirect reasons leading to this negative event. These include:

1. During long-term operation, there is great wear and tear on energy systems or the household electrical network. Wires lose their insulation quality over time, leading to unintended connections. This situation is checked at the junctions of the electrical wiring according to the degree of its heating. If there is a lot of heating of the conductors, it means that there is an insulation failure somewhere.
2. Often the cause of a short circuit is considered to be a lightning strike on a high-voltage line. A short-term network overvoltage occurs, followed by a short circuit. Even if lightning strikes close to the line, it still causes ionization of the air, which leads to an increase in electrical conductivity. As a result, an arc is formed connecting the electrical transmission lines.
3. In domestic conditions, mechanical damage to the insulation occurs. This situation occurs especially often during renovations.
4. It is possible that foreign metal objects may come into contact with current-carrying elements. This situation indicates unsatisfactory care of electrical equipment.
5. Connecting faulty devices with low internal resistance to the network.

You may be interested in the principle of operation of electronic and mechanical time relays

In addition, human actions are of great importance, which can sometimes lead to closure. Especially such moments often occur when electrical wiring is installed incorrectly.

Download

The same article, beautifully laid out and published in the paper magazine “Electrical Technical Market”:
• Short-circuit current: size matters / Article about short-circuit current, published in the magazine Elek.ru, pdf, 4.45 MB, downloaded: 626 times./

Respect and respect if you have read this far and intend to download books on this topic!
You are a serious person! • Shabad_M.A._Calculations_of_relay_protection_and_automation / Shabad M.A. Calculations of relay protection and automation. A good book from 1985, which tells about the design of electrical networks - from the equipment of substations to the selectivity of protective circuit breakers, pdf, 38.87 MB, downloaded: 1071 times./ • Belyaev A.V. Selection of equipment, protection and cables 0.4 kV / Belyaev A.V. Selection of equipment, protection and cables 0.4 kV - a book for the theoretical calculation of short circuit current. St. Petersburg 2008, pdf, 17.39 MB, downloaded: 839 times./ • RD 153-34.0-20.527-98 / Guidelines for calculating short circuit currents and choosing electrical equipment RD 153-34.0-20.527-98. The guidelines are intended for use by energy engineers when performing calculations of short circuit currents (SC) and testing electrical equipment (conductors and electrical apparatus) under the SC mode. MPEI, 1998, pdf, 3.61 MB, downloaded: 791 times./ • Electrical part of power plants and transformer substations / Electrical part of power plants and substations. Detailed description of circuits and calculations with examples. Tutorial. N.V. Kolomiets, Tomsk Polytechnic, 2007, pdf, 1.37 MB, downloaded: 735 times./ • Selection of electrical equipment and calculations of transformer substations / Selection of electrical equipment and calculations of medium and low voltage transformer substations. ABB, educational manual, pdf, 9.16 MB, downloaded: 681 times./ • Kharechko V.N., Kharechko Yu.V. Automatic switches of modular design / Kharechko V.N., Kharechko Yu.V.
Automatic circuit breakers of modular design: Reference manual. The reference manual sets out the requirements of GOST R 50345-99 (IEC 60898-95) for household circuit breakers intended for overcurrent protection, examines the design of circuit breakers, gives characteristics and their classification. Errors are analyzed that are partially corrected in the new version of GOST R 50345-2010, pdf, 7.17 MB, downloaded: 1103 times./ I look forward to questions and comments in the comments!

What are the types

Short circuit. Everyone has heard this phrase. Many have seen the inscription “Do not short-circuit!” Often, when an electrical appliance breaks down, they say: “It’s short!” And despite the negative connotation of these words, professionals know that a short circuit is not a sad death sentence. Sometimes it is pointless to fight a short circuit (SC), and sometimes it is fundamentally impossible. This article will answer the most important questions: what is a short circuit and what types of short circuits are found in technology.

It will be interesting➡ Electric circuit and its elements

Let's begin to look at these issues from an unusual angle - we will find out in which cases short circuits are inevitable and where they do not play the role of damage. Let's take an ordinary metal wire at both ends. Let's connect the ends together. The wire is short-circuited - a short circuit has occurred. But since there are no sources of electrical energy or load in the circuit, such a short circuit does not cause any harm. In some areas of electrical engineering, the short circuit that we have considered plays into our hands, for example, in electrical devices and electrical machines.

Related material: How to connect a capacitor

Let's take a look at a single-phase relay or starter, the design of which includes a magnetic system with moving parts - an electromagnet that attracts an armature. Due to the constantly changing polarity of the current flowing in the windings of the electromagnet, its magnetic flux periodically becomes zero, which causes the armature to rattle, vibrations and a characteristic hum, familiar to all electricians, appear. To get rid of this phenomenon, a short-circuited turn - a ring or rectangle made of copper or aluminum - is attached to the end of the electromagnet core or armature.

Due to the phenomenon of electromagnetic induction, a current is created in the coil, creating its own magnetic flux, compensating for the loss of the main magnetic flux created by the electromagnet, which leads to a decrease or disappearance of vibrations that destroy the structure.

A short circuit in the rotor of an asynchronous electric motor also plays a role. Due to the interaction of the magnetic field created by the stator windings with the squirrel-cage rotor, according to the already mentioned law, currents appear in the rotor, creating their own field, which causes the rotor to rotate. Of course, it is important to properly design an electric motor or electrical apparatus so that the currents flowing in short-circuited elements do not lead to overheating and damage to the insulation of the main windings.

Socket fire

Similarly, the concept of "short circuit" is used in relation to transformers. People who are in one way or another connected with the energy industry know that one of the most important characteristics of a transformer is the short-circuit voltage, VSC, measured as a percentage. Let's take a transformer. We short-circuit one of its windings, say, low voltage (LV), with an ammeter, the resistance of which, as is known, is taken equal to zero. We connect the high voltage winding (HV) to a voltage source. We increase the voltage on the HV winding until the current in the LV winding becomes equal to the rated one, and fix this voltage.

We divide it by the rated voltage of the higher side, multiply by 100%, and get USC. This value characterizes the power loss in the transformer and its resistance, which determines the short circuit current leading to damage. Let's finally talk about short circuits that have negative consequences. Such short circuits occur when current from the power source does not flow through the load, but only through wires that have negligible resistance. For example, a three-phase cable is powered by a transformer, and with one careless movement of the excavator bucket it is damaged - two phases are short-circuited through the bucket. Such a short circuit is called two-phase. Other short circuits are called similarly based on the number of closed phases.

A single-phase earth fault in networks with an isolated neutral is not short, but can pose a threat to the lives of living beings. A short circuit in which the transition resistance is zero is called metallic - for example, with a bolted or welded connection. Short-circuit currents, depending on the voltage and type of damage, can reach thousands and hundreds of thousands of amperes, leading to fires and colossal electrodynamic forces that “twist” tires and wires. Protection against short circuits can be carried out by circuit breakers or fuses, and in high-voltage networks - by means of relay protection and automation.

Protection of the power supply from short circuit.

What is an RCD

RCD systems that have recently appeared on the market of protective equipment (outwardly similar to circuit breakers) are reliable devices for protection against current leakage due to insulation damage, which is detected due to current imbalance in the circuit. This system does an excellent job of protecting against human contact with voltage, but not against short circuits. Thus, the installation of an RCD increases the safety of the circuit section and the equipment being serviced, but this does not mean that the installation of circuit breakers with current releases is not required in this circuit.

Attention! There is a misconception that a residual current device (RCD) will protect the circuit from a short circuit. The RCD performs a protective function when a person comes under dangerous network voltage (potential), and also responds to deterioration of insulation resistance, which can subsequently lead to a short circuit to the ground or to the neutral conductor.

Prevention of short circuits

Taking preventive actions is safer, more reliable and cheaper than restoring wiring after a short circuit. Sockets need to be checked periodically. If they start to spark, they need to be repaired or replaced.

If the wiring has been partially replaced, the reliability of the connection points and the integrity of the insulating layer should be checked.

Light sources, lighting network and power cables should be checked every few months. A short circuit may occur over time. It can be detected by changes in the color of devices or their melting. The apartment must have automatic switches. Powerful electrical appliances are equipped with separate protective equipment that should operate in an emergency.

When installing electrical wiring yourself, it is important to correctly calculate the cable cross-section. If it is not able to withstand the power of all connected devices, an overload will occur, leading to a short circuit. Cables should not be laid closely together - this may damage the protective layer. Also, when connecting, you need to choose the right way to create a contact and purchase the necessary equipment in advance. Do not connect wires by twisting.

If you need to drill into a wall, you should check the place with a homemade metal detector or study the electrical wiring diagram. This way you can detect a hidden wiring cable that a technician could accidentally damage.

Measures to prevent short circuits in the home electrical network

To reduce the likelihood of a short circuit, the homeowner should periodically inspect his lighting network. Of course, an inspection will not give a 100% guarantee, but it will help eliminate faults that lead to the appearance of a short circuit in the network.

During the inspection, the following actions must be performed:

• if the socket starts to heat up, sparks, or has a plastic smell, then it should be replaced with a new one or repaired;
• an audit of the entire lighting network and power group of wires is carried out every six months. You need to pay attention to the color of the insulation. Possible places of dangerous heating are determined by the color of the wire insulation;
• installation of a circuit breaker with RCD. The circuit breaker will turn off the network in the event of a short circuit, and the RCD (residual current device) reacts to a person touching exposed wires. The use of this device can save lives;
• the cross-section of the electrical wiring wires is calculated based on the total power of all electrical appliances;
• During installation, do not lay the wiring cables too tightly;
• If you need to do any work, for example, drill a wall, you should make sure that there is no electrical wiring under the plaster in this place.

Do not turn on partially damaged appliances

If the cable in an iron or refrigerator is frayed and the inner shell is visible, do not turn it on until you have repaired it. First, carefully remove the top layer of insulation in the damaged area and inspect the external condition of the insulation. Wrap all damage and cracks tightly with electrical tape. Then put the top shell back and rewind it too.

The plug often needs to be replaced, for example if it is very loose or the housing is damaged. It is sold in any transition or store, so don’t delay your purchase.

Damage can occur not only on the power cord, but also inside. For example, if you turn on something and hear sparking inside. This already indicates a serious malfunction, even if the electrical equipment works, at first glance, normally. In this case, unplug it from the outlet and take it to a service center (or repair it yourself).

Remember that turning on a faulty electrical appliance often leads to a short circuit, which will destroy all wiring in the house and is likely to cause a fire. If you remain nearby, you risk serious injury.

Even if you have checked all the wiring and turn on only new, working equipment, this does not provide a 100% guarantee that an accident will not occur in your network. Therefore, always install high-quality circuit breakers and RCDs in the panel.

When laying parallel, separate the lines

If you have several power lines laid in parallel, try to maintain a distance of at least 10 cm between them. The fact is that when the cable is laid tightly, it cools worse, because of this the sheath heats up more, and its insulating properties are lost. As a result, it may melt or a breakdown may occur and the contacts will close.

For parallel installation, special cable channels with a partition in the middle are also used. It isolates the lines of force from each other.

Protect the cable when installing

Try to make hidden wiring in walls or plaster, where the insulation will last much longer and there is less risk of damage. When wiring is open, try to use protective equipment: cable ducts, plastic pipes, corrugation. By protecting against external factors, they increase the service life of the wiring several times.

Replace aluminum wiring with copper

With a smaller wire cross-section, copper conducts electricity better and can withstand greater loads. In addition, it withstands more mechanical bending and does not oxidize as quickly as aluminum.

New PUEs generally prohibit the installation of aluminum wiring in household networks, since it is potentially dangerous and less efficient in operation than copper.

In Soviet times, aluminum wiring was often used in residential buildings. If your apartment still enjoys such a “Soviet legacy”, think about it; its service life has probably expired long ago.

Don't ignore dust and moisture protection

When placing sockets, switches or electrical appliances in places of high humidity, take care of a high level of dust and moisture protection. For example, outdoors where precipitation, dew and fog are possible, it should be at least IP67. The minimum level for a bathroom is IP44, if there is a possibility of direct water splashes, then IP56 is better.

If water gets inside, the socket will begin to spark, the plastic casing will melt, and eventually a short circuit will occur. Therefore, always choose the optimal level of dust and moisture protection.

Choose a cable of sufficient cross-section

Before purchasing, be sure to calculate the likely maximum load on the line. The cross-section must be sufficient to safely pass current during peak load hours, for example, in winter when the heating is on or on weekends, when the maximum number of electrical consumers is working at home.

The optimal cross-section for socket groups is 2.5mm² and above, and for lighting 1.5mm² or 0.5mm² for LED. But it’s better to make accurate calculations of the maximum power and select the cross-section based on them.

How to fix

Replacing an outlet after a short circuit
When a place is found where the wiring is shorted, it needs to be replaced and repaired. First, you need to de-energize the room and only then change the problem area. The junction of two conductors must be reliably insulated. It is also important to choose the right way to create a contact. If a switch or socket burns out due to a short circuit current, it is recommended to purchase and install a new product. It is cheaper and more reliable than repairing a broken device.

In older houses, electrical wiring was made of aluminum. This is an unreliable outdated method of laying networks, so it needs to be completely replaced. It is the aluminum wiring that can short out.

When a household appliance shorts out, it should be repaired by a professional. Repairing a device yourself without professional tools, skills and abilities is dangerous to your health.

How to prevent short circuit, protection against it

Since short circuit is an emergency mode, there are ways to protect against this dangerous process and prevent it:

• Fast-acting electromagnetic or electronic protection against an instantaneous increase in current in a load or line, which will quickly disconnect the emergency section of the circuit from voltage. For this purpose, circuit breakers, fuses, and differential circuit breakers are used. At home, to protect against short circuits, it is enough to install a correctly designed circuit breaker (AB) on a group of devices.
• For high-voltage lines and power circuits of substations, oil (vacuum and other) switching devices are used with configured and tested protection against sudden increases in current on outgoing lines.

The way to prevent a short circuit at the moment when this process has already occurred is simple: it consists in immediately automatically disconnecting a section of the circuit from voltage. In principle, any circuit breaker has an electromagnetic release inside its design, which, when the rated current is exceeded, breaks the load circuit quite efficiently and quickly.

Important! Short circuit protection must be reliable and fast; these are two basic rules for the safe operation of electrical circuits.

Possible consequences

Consequences of a short circuit in an electrical panel
A short circuit is a negative phenomenon that can lead to a fire and breakdown of household appliances. But other problems caused by the defect can also be identified:

• Melting veins. If they explode, they can cause harm to human health.
• Ignition of insulation. Relevant for low-quality insulating layers or made of flammable materials.
• Equipment breakdown.

To prevent all negative consequences, you need to find the short circuit in time and eliminate it.

Preventive measures to prevent short circuits

Most short circuits and the fires they cause can be avoided. To do this, you should follow simple rules:

1. Do not overload the electrical wiring with power (current). Select the cable cross-section and lay it in accordance with the requirements of the PUE.
2. Periodically inspect the electrical panel, sockets and switches. The smell of burning, smoke, sparking and crackling are harbingers of a short circuit and fire.
3. Replace old circuit breakers with new ones. Especially if a revision of the shield has not been carried out since Soviet times.
4. If you use fusible plugs at the entrance to the apartment, then under no circumstances place so-called bugs on them. Even if an electrician I know “did this 100 times and everything was fine.” It is best to replace traffic jams with automatic machines.

If you notice that there is a burning smell coming from the electrical panel or switchgear, immediately notify the management company. The actions of the emergency service will be aimed at preventing short circuits and inspecting equipment.

A short circuit is one of those problems that are easier to prevent than to correct the consequences. Any wiring, various devices and panels require periodic inspection by a specialist. Compliance with this rule will ensure long and safe operation of electrical equipment.

If it was not possible to avoid a short circuit, then you need to think about its causes. Wiring that is old, too thin or damaged by animals must be replaced. Damp - dried and further tested by a professional electrician using a megger or more advanced instruments.

Search for damaged area

If calling and waiting for a professional technician is not an option, then the owners themselves have to inspect the entire electrical network - open sections of wiring, as well as connected household and special appliances, including network extension cords. Before the operation, the electrical circuit is de-energized, turning off the machines that did not work, then the plugs of all household appliances are pulled out of the sockets.

How to find a short circuit in hidden wiring? The ideal option is to have an electrical wiring plan, but homeowners often do not have these documents on hand. Sometimes they are completely useless, since a drawing and a real diagram are “two big differences.” The reason is the entrepreneurial spirit of electricians during the construction of the facility. Therefore, more often than not, owners are forced to carry out “research” in rather thorny ways.

The first symptoms are a burning smell and blackening (burnout) of the area where the short circuit occurred. When inspection of visible wiring, junction boxes and sockets with switches does not bring results, they move on to checking household and lighting fixtures. If in this case the searches are not successful, then the research continues. It includes several stages.

Preparing to search for damage during a short circuit

How to find a short circuit in hidden wiring? First of all, provide a condition under which the search is possible at all. If in the event of a network break the operation sometimes does not promise any difficulties, then in the event of a short circuit you have to act differently, since when voltage is applied the machine simply turns off.

The exception is the burning out of the wires going to the switch or socket. It is easy to check whether this has happened with an indicator screwdriver: just make sure that there is a phase in the device. If it is, then we can say that at least one conductor is in order. Finding a neutral break is very difficult if you have no experience in such work. In this case, it is recommended to remove the section completely and then replace it with new wiring.

To prevent a short circuit that prevents power from being restored, it must be prudently excluded from the “scenario”. Since a short circuit is most often a contact between the neutral and the phase, one of the conductors is disconnected. Usually it becomes the neutral wire, the insulation of which is blue or light blue. It is disconnected, isolated, and then taken aside.

We must remind you once again that before this operation all electrical appliances must be unplugged from the outlets. If there are no more “injured” sections in the network, after eliminating the zero from the circuit, the machine will not operate.

Finding the short circuit

The first step is to determine the problem area, since a short circuit in hidden wiring can only be found after the technician determines exactly where it occurred.

In houses or apartments, the wiring principle is the same: from the distribution box, the wiring radiates to the sockets, and separate cables are provided for switches. The work is much simpler if the owners have a wiring diagram at their disposal. But more often it is absent.

First, the junction box is opened, then the resistance and voltage are measured on each line. If a line is found where there are no readings, then this is the area that needs to be checked. The next stage is searching for a specific location of the short circuit.

Help from measuring instruments

The best option is to check the resistance on the “suspected” section of the circuit (or insulation) with a megohmmeter, since the multimeter has one serious limitation. Due to the low voltage, it is only suitable for examining short sections of electrical circuits - up to 3 m, but no more.

The megohmmeter is connected with one wire to the phase conductor, the other to zero, then to the phase and ground. If the display shows a value that is less than one (0.5), then we can state that everything is in order with the wiring. When a different number (1) appears on it, or the indicators change, this means that the bare conductors are touching in some place.

Finding the culprit among household appliances

It is not uncommon for short circuits to occur in electrical appliances. To accurately determine its source, the exclusion method is used. First, absolutely all household appliances are disconnected from the sockets, then the operation of the machine is restored. All devices are connected one at a time, one at a time. The culprit will be found when the machine is triggered.

Folk method

In this case, it is important for the researcher to have good hearing, since in the place where the short circuit occurred, a sound should be detected - a quiet crackling sound. However, this option refers to the “old-fashioned methods”, so you can hope for a result, but you don’t need to rely heavily on the fact that it will happen.

How to find a short circuit in hidden wiring?

More often than not, the source of the problem is visible, but there are exceptions to the rule. In this case, the simplest option is to remove the plaster along the entire area from the junction box to the point of damage. But this option can be considered in the case when the master has a diagram.

However, such a large-scale operation has a disadvantage: few people are attracted by the opening of the gates, the noise and dust from this type of activity. And all for the sake of one small plot. The work will be somewhat simplified by the main rules for installing electrical wiring: cables should be laid only vertically or horizontally. But this condition does not exclude protracted searches and a large amount of garbage.

Sometimes difficulties of a different kind arise. Electricians are “people too,” so the desire to save money and get the job done quickly often leads to the fact that the route is not laid as prescribed by the installation rules and GOST, but “bypassing” - the shortest route. In this case, you will need to remove all the plaster from the walls. No one will like this prospect.

The simplest devices

More often, the cables are hidden under a not very thick layer of plaster, so you can find them in the walls yourself, and it’s relatively easy and not too expensive. To avoid hard and fruitless work, electricians use special devices to detect the network or the exact location of the fault - non-contact voltage indicators. There are several types of detectors that electricians use when searching for hidden wiring.

1. MEET MS-58M. This tool is a professional device, but its price is low: it is less than 500 rubles. Using this indicator, faults are looked for in both open and hidden sections of the wiring. This non-contact probe tester can determine line voltage through concrete, wood, brick and other building materials.
2. MS-48NS. This is the simplest device - an indicator screwdriver, which allows you to do a tensile test, adjusting the sensitivity level of the tester, and determine the voltage on sections of the route. The average price of such an indicator is 300 rubles.

Both electrical devices quite accurately determine the location of the fault. The error is 50-100 mm. The help of inexpensive devices makes it possible to avoid the most unpleasant work - completely “ripping out” the walls.

After finding the “emergency section of the route,” the plaster is opened, the damaged electrical cable is removed and repaired (replaced). After the work is completed, it is returned to its place and the wall is restored. The ideal solution is to lay a new line, but temporary repairs to the damaged area are also allowed.

Hidden wiring locators

They are called professional identifiers, meters, cable detectors, or simply hidden wiring finders. This option is not the best, since the price of powerful devices will not suit everyone, and purchasing them for one-time work does not make any sense.

The range of this search equipment cannot be called “narrow”, however, high-quality, efficient, sensitive devices cost several thousand, and such expenses are hardly justified for amateur electricians. The second reason for refusing this purchase is the difficulty of working with locators: finding a fault requires not only a lot of practice, but also theoretical knowledge.