What does an uninterruptible power supply for a computer consist of? An uninterruptible power supply for a computer how it works. Additional UPS functionality

Source uninterruptible power supply or UPS (UPS) is a device that allows you to receive high-quality electrical current without interruption. In this article we will explain what UPS is and what it is used for.

There is always a risk of losing unsaved work results due to voltage surges. Sometimes a failure like this can cost many hours of work. Moreover, if you are unlucky, such a drop can damage your computer. In order to avoid such problems, a so-called UPS is used.

Connected to the network via an uninterruptible power supply, you will have a few minutes autonomous work in case of loss of voltage in the network. This time will be enough to save the work results and shut down the computer correctly.

But not all peripherals worth connecting to UPS. So, and "copiers" are not recommended to be connected to an uninterruptible power supply. These devices generate too much peak power at some point.

A modern UPS performs the following functions:

• Removal of small and short-term voltage surges
• Voltage filtering, noise reduction
• Ensuring uninterruptible power supply in the event of a power failure
• Short circuit and overload protection

Main characteristics of UPS:

• Power. The higher the power, the more powerful equipment the UPS can support.
• Working hours. This is the battery life that UPS can provide.
• Number of sockets
• A type. Modern UPSs are of the following types:
  • Reserve.
  • Line-interactive
  • Continuous action

Main types of UPS

Redundant or Off-Line are the simplest devices. When there is voltage in the network, they simply supply it to their outputs, passing it through a filter. Such UPSs switch to battery use only in the event of a sag or loss of voltage in the network.

Line-Interactive or Line-Interactive these are already more advanced and complex devices. Line-interactive uninterruptible power supplies are able to regulate the voltage depending on how the voltage in the network changes. At the same time, the transition to battery operation occurs only when the drops in the network exceed the levels allowed for the given UPS.

Continuous or On-Line these are the most advanced and expensive uninterruptible power supplies. In these models, the batteries are constantly used, and act as a "buffer". The disadvantage of such UPS is the dissipation of energy in the form of heat and rapid wear of the device.

Uninterruptible power system ( UPS or UPS) is a device that allows a computer to work autonomously by maintaining a constant voltage for some time after a power outage or a power surge. Using a UPS will make it possible to save data. In addition, you need a margin of time to properly shut down your computer and avoid damage to it.

The uninterruptible power supply operates from its own battery - a battery that produces a voltage of 220 volts. It is for this voltage that modern computers and household appliances are designed. Don't skimp on a device like UPS. Keeping your computer securely protected can help prevent power surges. It is much better to buy an uninterruptible power supply than to buy a new one later. household appliances for much more money.

How to provide uninterrupted power for personal computers

If you are purchasing an uninterruptible power supply, then provide several inputs for electrical plugs. It makes sense to connect not only the processor to the UPS, but all other components personal computer (monitor, printer). This system connection will ensure reliable and trouble-free operation of all components.

For the autonomous operation of a personal computer, the following types of UPS are allocated:

• outdoor
• built into the device

Built-in UPS can be found in any mobile device - from smartphone to laptop. The UPS built into the device assumes autonomous uninterrupted protection of equipment in case of voltage surges by switching loads. If you constantly use various gadgets for work, then you do not need to look for additional uninterruptible power supplies. Built-in systems let you keep your devices secure.

Types of uninterruptible power supplies

According to the accepted standard IES (International Electrotechnical Commission) all UPS are divided into 3 classes:

• line-interactive
• passive standby
• double conversion

Double conversion UPS or UPSdoubleconversion

To ensure the operation of server stations and network server equipment, more powerful uninterruptible power supplies are used. Their work is based on double conversion of mains voltage. The inverter conversion here also goes in the form of a sinusoidal voltage. Regardless of the frequency difference, the double-conversion UPS does not allow noise to pass through and does not return emissions back through the surge protector. The load is supplied directly from the mains. For this, a special bypass line is connected to work. This gives a significant advantage during autonomous operation, since maintaining the voltage remains at the desired level even if the inverter fails.

Line-interactive power supplies or line-interactive

UPSs of this type perform voltage stabilization in the same way as passivestandby. In this case, the operation of the linear-interactive UPS is based on a transformer with a step stabilizer.

The most common problem with power grids in Russia is power surges. The Line-interactiveUPS device provides normal power in this situation without going into stand-alone mode. A significant disadvantage of a line-interactive UPS is considered to be a short transition time from conventional to battery power (about four ms)

The advantage of Line-interactiveUPS is its relatively low cost and usability for protecting computers.

Line-interactiveUPS devices with an approximated sine wave are purchased for personal computers, where pulses are used as power sources. This view UPS is the most common.

Sinusoidal voltage devices are used for more powerful electrical appliances.

How long does the UPS work in offline

When choosing an uninterruptible power supply important aspect is the battery life. The ideal time is about five minutes. This time will be enough for an ordinary user to save documents and turn off the computer.

There are UPS devices that are capable of supporting run times of up to twenty minutes. Such devices are appropriate for office work or local network computers.

How long one or another UPS will operate autonomously will depend on the level of battery capacity. This data can be easily found in the UPS passport. The power of the UPS is also indicated there. More than or equal to 1 KV - the running time is about five minutes. By choosing the right power, you can provide yourself with the necessary amount of time to complete the work.

One solution to extend the battery life of your UPS is to install a new autonomous battery whether the battery. This option is rather unsafe, since you can simply damage the UPS's native battery. But if your UPS has for additional devices special slots, then this method will work with a 100% guarantee. But when connecting, be sure to look at what the power circuit of your system unit is.

How to choose an uninterruptible power supply

An uninterruptible power supply is installed:

• for one workplace
• for office
• for server station

When choosing an uninterruptible power supply (UPS), you need to consider how long you need to properly shutdown your personal computer. If you intend to install the UPS on only one workstation, then give preference to line-interactive power supplies. In this situation, you will be provided with sufficient time to complete the work and save your computer from damage.

If you choose an uninterruptible power supply to provide constant voltage in the office or for the server station, choose the doubleconversion UPS. These power supplies will be able to provide a fairly long battery life. Even if the inverter fails, such a UPS will continue to work and will make it possible not to lose important data.

When choosing uninterrupted source power supply, carefully study the passport of the device, choose the desired power level depending on how much time is needed for autonomous operation. Do not attempt to install additional batteries if the UPS is not designed for this.

Your best bet when choosing UPS is to rely on the expert opinion, who can be ordered quickly and easily at youdo.com. The order can be placed quickly, and the services of the performers are inexpensive.

The basis of this type of uninterruptible power supply is a special line-interactive technology. A distinctive feature that allows you to achieve greater efficiency in comparison with off-line sources is the presence of a transformer, which provides a stable output voltage even with quite different input voltages. That is, this type of UPS allows you to cope with much larger voltage surges in the network than conventional off-line sources can do.

Interactive sources are able to protect electrical equipment from short-term disappearance or surges, surges or sags. These devices are not capable of controlling the voltage waveform and effectively filtering noise and surges. Thus, all the interference that can be created by the loads will simply go back to the mains.

Conclusion: Line-interactive uninterruptible power supplies do an excellent job of supporting electrical equipment for which the magnitude of the input voltage is important, but the spectral parameters are not important. In this regard, such sources are widely used in the protection of network equipment (routers, switches and other devices).

The device and workflow of this type is significantly different from that in the two previous versions. Online systems use double conversion voltage. This allows the incoming AC voltage to be rectified and converted to DC current. Then this voltage is fed to the battery, after which the inverter again changes the current to AC and outputs it to power the devices. A distinctive feature is that the battery is constantly connected to the network, and this allows you to instantly (without any delay) level any voltage surges.

DC is free from the disadvantages that can be associated with frequency, phase or noise, which are inherent in AC. The current is supplied with strictly limited parameters, and then converted into current with strictly defined frames of shape, frequency and phase.

Conclusion: On-line UPSs are very expensive, so they are used only to ensure the safe operation of equipment, which is very demanding. Modern high-performance servers can be examples of such equipment. Disruptions in their work can sometimes lead to losses in the tens of thousands of dollars or a threat to the safety of important business data. Therefore, their high cost justifies itself in full, and the use of an online uninterruptible power supply will maximize the protection of large business projects from unforeseen failures.

For clarity, you can see our products from the section Online UPS \u003e\u003e\u003e

UPS Offline

The operation of an off-line uninterruptible power supply has a fairly simple algorithm and structure. The main elements of this type of source are: an automatic switch, an autonomous power supply (battery) and outputs for external power supply. In normal mode, the battery is positively charged by external charging.

In normal cases, the switch allows you to use electricity from external sources, but even with a short jump or power failure, it connects an autonomous energy source (battery). In this case, the entire switching process takes about 4 milliseconds, which does not affect the performance of the connected equipment in any way. The electric current leaves the battery in the form of direct current, so special inverters are used. They convert direct current into alternating current, which is already used to power electrical equipment.

The battery included in the structure of this type of UPS is capable of providing electricity needs for a fairly short period of time. therefore given type rather suitable for short-term insurance in case of problems with the power supply from the central network.

Conclusion: Off-line UPS is one of the simplest, cheapest and most portable sources. But they can only cope with short-term power outages.

The article discusses the types of UPS, the principles of UPS operation, and also provides real oscillograms of the output voltages.

To begin with, some general terminology. Uninterruptible power supplies (UPS) are also called UPS, from the English abbreviation Uninterruptable Power Supply (uninterrupted power supply). Therefore, they say both UPS and UPS, whoever is more convenient. In the article I will call it this way and that.

Why do you need a UPS (UPS)

The principle of operation of the UPS is revealed in the name - it is such a source at the output of which there is always tension... But we have gathered here realist techies, and we understand that nothing is eternal, so below we will understand the principle of action.

UPSs are mainly used where power outages can cause negative consequences. For example, power supply of computers and servers, power supply of communication and signal distribution devices (routers), power supply of devices, automatic reboot (restart) of which is impossible without human intervention.

How my reader modified a UPS for a strategically important system (2 servers, etc.). In addition, he improved the circuit, and added the ability to use a conventional car battery.

For household items, these are primarily computers and heating systems.

It should be understood that UPSs are selected for a load operating time of 10-15 minutes, rarely up to half an hour. It is assumed that during this time, power will appear, or a person (operator) will take the necessary actions (save the data, call the energy service of the enterprise, complete the technological process).

The UPS cannot be considered as a backup power supply. It is only an emergency source, and at best it is used very rarely, for a total of no more than 10 minutes a year (several times, for no more than a minute). If this time is longer, then you should think about improving the quality of power supply.

A backup power source can be considered such sources that can completely replace the main power supply for a long time, from several hours to several days. It may be another line (see article about), a wind generator. Theoretically, a UPS can serve for these purposes, but this requires batteries of enormous capacity, which will significantly affect the price of such a system.

Types of uninterruptible power supplies

The types (types) of UPS have many names, but there are still exactly three of them. Let's figure it out.

So, there are three main types of UPS:

Back UPS

Other equivalent names are Off-line UPS, Standby UPS, and standby UPS. The most common OOIs are used for most types of household and computer equipment.

Back simply switches the load to battery power when the input voltage goes out of range. Lower limit y different models - about 180V, top - about 250V. Transitions to the battery and vice versa - with hysteresis. That is, for example, when lowering, the transition to the battery will take place at 180 V or less, and vice versa - at 185 or more. The same principle applies to all types of UPS.

Something like that disconnects load, and Back UPS does not turn off, but switches on the battery, which allows her to work for some time.

Smart UPS

Other names - Line-Interactive, interactive type UPS. We are not far from Back.

Smart UPS are smarter, as the name suggests. They additionally switch the internal autotransformer, in a sense, stabilizing the input voltage. And only as a last resort they switch to the battery.

Thus, the output voltage norm is maintained at large input deviations (150 ... 300V). The autotransformer has several switching stages, so the Smart UPS switches the autotransformer outputs until the last moment, including the battery only at the last moment. This saves the battery by turning it on only when the power is completely lost.

This device resembles a stepwise switching of the autotransformer windings. The only difference is that when it goes beyond the operating limits, the stabilizer will be powerless, and our “clever girl” will put the battery into operation, and the power will not be lost.

Online UPS

Other names are online, double conversion uninterruptible power supply, inverter. A completely different principle of action, for lovers of pure sinus. The energy from the input is converted into DC voltage and fed to the inverter, which generates a pure sine wave. And at the same time it keeps the battery 100% ready. If necessary, the inverter continues to operate in the same way, only it receives power from the battery.

Used for emergency power supply of equipment that is sensitive to the shape of the output voltage - for example, gas boilers, servers, professional audio-video equipment and other strategically important equipment.

There are two disadvantages of online UPS - price and efficiency. The efficiency is low because such a UPS is always on, as the name implies. Unlike the other two types.

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There are varieties of online UPS, which use the so-called "pass-through zero" for the correct operation of gas-fired electric boilers. This is due to the fact that such boilers are sensitive to the presence of a real zero, for correct ignition.

Examining the UPS with an Oscilloscope

Now comes the fun part.

Back UPS Output Voltage

Conducted a study using a Fluke 124 oscilloscope. Oscillograms (shape of pulses and oscillations at the output of ups) are given and commented on below.

What does this timeline show? Period 20ms, frequency 50Hz, amplitude 315V. It is worth noting that the phase of the sine and the generated pulses are the same, which is good. When the mains voltage is lost, the UPS hesitates for 5-7 ms, and then there are pulses, which are called "quasi-sine". Here they are:

Back UPS. Output voltage when powered by batteries.

The oscilloscope has measured the RMS voltage (root mean square), it is correct. However, when I measured the same voltage with a multimeter, I got a reading of 155 V. Why is the UPS output low?

The fact is that the multimeter measures only the first harmonic with a frequency of 50 Hz. For the sine, everything is smooth. But if you measure the voltage of such pulses, you need to measure exactly RMS, rms, otherwise the following harmonics will not be taken into account - 100, 150, 200 Hz. And they make up a significant part of the energy, up to 30%. UPS manufacturers know this feature, and in order not to bother (and not to raise the price of their products), they issue such pulses to our devices with an amplitude of about 370V.

Read more about measuring the RMS non-sinusoidal voltage in the video:

Here is an enlarged graph, where you can see that the voltage after switching first rises for half a second to 400V, and then stabilizes:

Back UPS. Exit, duration 2 seconds

And here is how the voltage waveform at the Back-UPS output changes at the moment of switching from battery to mains supply:

Back UPS, - UPS output voltage when transferring from battery to utility. Pulse shape at the ups output

The phase does not change either, everything is fine. I connected the UPS to the output, switched power modes back and forth - the starter is pulled in securely, no problems.

The subject was an APC Back-500-RS UPS, the parameters are in the photo below:

Back UPS Options - Rear Panel

Smart UPS Output Voltage

Now I will give for completeness a picture of the voltage oscillogram at the output of Smart UPS. The UPS Ippon Smart Power Pro 1000 was tested.

Smart UPS_Grid-Battery

The switching time is also insignificant for all modern equipment - less than 7 ms.

I did not smoothly change the voltage at the input, since there was no such goal. I believe that in this case, the Smart UPS behaves in the same way as a relay voltage stabilizer.

These studies were carried out within the framework of the industrial refrigerator project.

As civilization develops, it begins to consume more and more energy, in particular, electric - machine tools, factories, electric pumps, street lamps, lamps in apartments ... The appearance of radio, televisions, telephones, computers gave mankind the opportunity to speed up the exchange of information, however, it made it even tighter it to the sources of electricity, because now, in many cases, the loss of electricity is tantamount to the loss of the delivery channel information flow... This situation is most critical for a number of the most modern industries, in particular, where the main production tool is computer networks.

It has long been calculated that after a couple of months of work, the cost of information stored on a computer exceeds the cost of the PC itself. For a long time, information has become a kind of product - it is created, evaluated, sold, bought, accumulated, transformed ... and sometimes lost for a variety of reasons. Of course, up to half of information loss problems arise from software or hardware failures in computers. In all other cases, as a rule, problems are associated with poor-quality computer power supply.

Ensuring high-quality power supply of PC components is a guarantee stable work any computer system... The fate of whole months of work sometimes depends on the shape and quality characteristics of the mains supply, on the successful choice of power components. Based on these considerations, the research methodology outlined below was developed, designed to further become the basis for testing the quality characteristics of uninterruptible power supplies.

1. GOST provisions
2. UPS classification (description, diagram)
   • Offline
   • Line-interactive
   • Online
   • Basic types by capacity
3. Physics
   • a. Power types, calculation formulas:
     • Instant
     • Active
     • Reactive
     • Full
4. Testing:
   • Test purpose
   • General plan of the
   • Parameters to check
5. Equipment used in testing
6. Bibliography

GOST provisions

Everything related to electrical networks, in Russia it is regulated by the provisions of GOST 13109-97 (adopted by the Interstate Council for Standardization, Metrology and Certification instead of GOST 13109-87). The regulations of this document are fully consistent with international standards IEC 861, IEC 1000-3-2, IEC 1000-3-3, IEC 1000-4-1 and publications IEC 1000-2-1, IEC 1000-2-2 in terms of electromagnetic compatibility levels in power supply systems and methods for measuring electromagnetic interference.

The standard indicators for power grids in Russia, established by GOST, are the following characteristics:

• supply voltage - 220 V ± 10%
• frequency - 50 ± 1 Hz
• harmonic distortion of the voltage waveform - less than 8% for a long time and 12% - for a short time

Typical power supply problems are also discussed in the document. Most often we have to deal with the following of them:

• Complete loss of voltage in the network (no voltage in the network for more than 40 seconds due to violations in the power supply lines)
• Sagging (short-term decrease in the voltage in the network to less than 80% of the nominal value for more than 1 period (1/50 of a second) are the result of switching on powerful loads, externally manifests itself as flickering of lighting lamps) and bursts (short-term increases in voltage in the network by more than 110% of the nominal for more than 1 period (1/50 of a second); appear when a large load is disconnected, externally appear as flickering of lighting lamps) voltages of different duration (typical for large cities)
• High-frequency noise - radio frequency interference of electromagnetic or other origin, the result of the operation of powerful high-frequency devices, communication devices
• Frequency deviation out of range
• High-voltage surges - short-term voltage pulses up to 6000V and duration up to 10 ms; appear during thunderstorms, as a result of static electricity, due to arcing of switches, they have no external manifestations
• Frequency run-out - a change in frequency by 3 or more Hz from the nominal (50 Hz), appear when the power source is unstable, may not appear externally.

All these factors can lead to failure of rather "thin" electronics, and, as is often the case, to data loss. However, people have long learned to defend themselves: mains voltage filters, "damping" surges, diesel generators that provide electricity to systems in case of power outages on a "global scale", finally, uninterruptible power supplies - the main tool for protecting personal PCs, servers, mini-automatic telephone exchanges and others. Just about the last category of devices and will be discussed.
UPS classification

The UPS can be "divided" according to various criteria, in particular, by power (or application) and by the type of operation (architecture / device). Both of these methods are closely related to each other. By capacity, UPSs are divided into

1. Uninterruptible Power Supplies low power(with full power 300, 450, 700, 1000, 1500 VA, up to 3000 VA - including on-line)
2. Small and medium power (with full power 3-5 kVA)
3. Medium power (with full power 5-10 kVA)
4. High power (with full power 10-1000 kVA)

Based on the principle of operation of devices, two types of classification of uninterruptible power supplies are currently used in the literature. According to the first type, UPSs are divided into two categories: on-line and off-linewhich, in turn, are divided by reserve and line-interactive.

According to the second type, UPSs are divided into three categories: reserve (off-line or standby), line-interactive (line-interactive) and Double-conversion UPS (on-line).

We will use the second type of classification.

Let's first consider the difference between UPS types. Sources of the reserve type are made according to the scheme with a switching device, which in normal operation provides connection of the load directly to the external power supply network, and in emergency mode - transfers it to power supply from storage batteries. The advantage of this type of UPS is its simplicity, the disadvantage is the non-zero switching time to battery power (about 4 ms).

Line Interactive UPS are made according to the scheme with a switching device, supplemented by an input voltage stabilizer based on an autotransformer with switched windings. The main advantage of such devices is load protection against overvoltage or undervoltage without going into emergency mode. The disadvantage of such devices is also a non-zero (about 4 ms) switching time to batteries.

Double conversion UPS voltage differs in that in it the AC voltage entering the input is first converted by a rectifier into a DC, and then - with the help of an inverter - again into an AC. The battery is permanently connected to the output of the rectifier and the input of the inverter and supplies it in emergency mode. Thus, a sufficiently high stability of the output voltage is achieved regardless of voltage fluctuations at the input. In addition, interference and disturbances that are abundant in the supply network are effectively suppressed.

In practice, a UPS of this class when connected to the network alternating current behave like a linear load. The advantage of this design is zero switching time to battery power, the disadvantage is the decrease in efficiency due to losses during double voltage conversion.

Physics

All reference books on electrical engineering distinguish four types of power: instant, active, reactive and complete. Instant power is calculated as the product of the instantaneous voltage value and the instantaneous current value for an arbitrarily chosen moment in time, that is

Since in a circuit with resistance r u \u003d ir, then

The period average power P of the circuit under consideration is equal to the constant component of the instantaneous power

The average AC power over the period is called active ... The unit of active power, volt-ampere, is called watt (W).

Accordingly, the resistance r is called active. Since U \u003d Ir, then

Usually, it is the active power that is understood as the power consumption of the device.

Reactive power - a value characterizing the loads created in electrical devices by fluctuations in the energy of the electromagnetic field. For a sinusoidal current, it is equal to the product of the effective current and voltage by the sine of the phase angle between them.

Full power - the total power consumed by the load (both active and reactive components are taken into account). Calculated as the product rms values input current and voltage. The unit of measurement is VA (Volt-Ampere). For sinusoidal current is

On almost any electrical appliance there is a label indicating either full power device, or active power.
Testing

The main purpose of testing - to demonstrate the behavior of the tested UPS in real conditions, to give an idea of additional characteristics, which are not reflected in the general documentation for the device, in practice, determine the influence of various factors on the operation of the UPS and, possibly, help determine the choice of one or another uninterruptible power supply.

Despite the fact that there are currently a great many recommendations for choosing a UPS, during testing we expect, firstly, to consider a number of additional parameters that should be inquired before purchasing equipment, and secondly, if necessary, adjust the set of selected methods and parameters testing and develop a basis for future analysis of the entire power path of systems.

The general plan for testing is as follows:

• Specifying the device class
• Indication of the manufacturer's declared characteristics
• Description of the scope of delivery (availability of a manual, additional cords, software)
• Short description appearance UPS (functions located on the control panel and list of connectors)
• Battery type (indicating the capacity of the batteries, serviced / non-serviced, name, possibly - interchangeability, the ability to connect additional battery blocks)
• "Energy" component of the tests

During testing, it is planned to check the following parameters:

• The input voltage range at which the UPS operates on utility power without transferring to batteries. Larger input voltage range reduces UPS transfers to battery and extends battery life
• Time to switch to battery power. The shorter the transfer time, the lower the risk of failure of the load (device connected via the UPS). The duration and nature of the switching process largely determine the possibility of the normal continuation of the equipment operation. For a computer load, the permissible power interruption time is 20-40 ms.
• Oscillogram of switching to battery
• Switching time from battery to external power supply
• Oscillogram of switching from battery to external power supply
• Battery life. This parameter is solely determined by the capacity of the batteries installed in the UPS, which in turn increases as the maximum output power of the UPS increases. To provide self-powered two modern SOHO computers with a typical configuration for 15-20 minutes, the maximum output power of the UPS should be about 600-700 VA.
• Output voltage parameters for battery operation
• Pulse shape at the beginning of battery discharge
• Pulse shape at the end of battery discharge
• UPS output voltage range when input voltage changes. The narrower this range, the less the effect of changes in the input voltage on the supplied load.
• Output voltage stabilization
• Output voltage filtering (if any)
• UPS behavior during output overload
• UPS behavior in the event of a load failure
• Calculation of UPS efficiency. Defined as the ratio of the device's output power to the power consumption from the power source
• Total Harmonic Distortion, which characterizes the degree to which a voltage or current waveform differs from a sinusoidal one
  • 0% - sinusoid
  • 3% - distortion is not noticeable to the eye
  • 5% - distortion is visible to the eye
  • up to 21% - trapezoidal or stepped waveform
  • 43% - the signal is rectangular

Equipment

During testing, we will use not real workstations and servers, but equivalent loads, which have a stable consumption pattern and a power utilization factor close to 1. The following set is currently being considered as the main equipment that will be used during testing:

Bibliography

1. GOST 721-77 Power supply systems, networks, sources, converters and receivers of electrical energy. Rated voltages over 1000 V
2. GOST 19431-84 Energy and Electrification. Terms and Definitions
3. GOST 21128-83 Power supply systems, networks, sources, converters and receivers of electrical energy. Rated voltages up to 1000 V
4. GOST 30372-95 Electromagnetic compatibility of technical equipment. Terms and Definitions
5. Theoretical Electrical Engineering, ed. 9th, revised, M.-L., Energiya publishing house, 1965
6. Company promotional materials
7. Internet resource