Due to the widespread use of semiconductor and microprocessor technology in production and in everyday life, the issue of protecting electrical networks up to 1000 V from switching and lightning overvoltages is becoming especially relevant today.
Expensive equipment made using semiconductor elements has weak insulation, and even slight increases in voltage can damage it.
In accordance with the accepted nomenclature, a surge limiter in electrical installations with voltages up to 1 kV is called a surge protection device (SPD).
Operating principle is similar to the operating principle of surge suppressors (OSS) and is based on the nonlinearity of the current-voltage characteristic of the protective element. When designing surge protection in networks up to 1 kV, as a rule, 3 stages of protection are provided, each of which is designed for a certain level of pulse currents and wave front steepness.
SPD I - a 1st class device is installed at the entrance to the building and performs the function of the first stage of surge protection. His working conditions are the most difficult. Such a device is designed to limit pulse currents with a wave front steepness of 10/350 μs. The amplitude of pulse currents 10/350 μs is in the range of 25-100 kA, the duration of the wave front reaches 350 μs.
SPD II - is used as protection against overvoltages caused by transient processes in distribution networks, as well as as a second stage after SPD I. Its protective element is designed for pulse currents with a waveform of 8/20 μs. The current amplitude is in the range of 15-20 kA.
SPD III - used to protect networks from residual overvoltage phenomena after devices of the first and second class. They are installed directly at the protected equipment and are normalized by pulse currents with a waveform of 1.2/50 μs and 8/20 μs.
Device. Devices of all classes have a similar structure, the difference lies in the characteristics of the protective element. Structurally, the device consists of a fixed base and a removable module. The base is attached directly to the distribution cabinet structures on a DIN rail.
The removable module is inserted into the base using blade contacts. This design makes it easy to replace a damaged nonlinear element yourself. Varistors and arresters of various designs are used as a nonlinear element. Their design can be one-, two-, or three-pole; the choice depends on the number of wires of the protected network.
Foreign manufacturers equip their products with device operation indicators, which allows you to visually determine its serviceability. In more expensive models, thermal releases can be installed to prevent overheating of a nonlinear element that is not designed for long-term flow of currents.
Connection diagram. To perform overvoltage protection in electrical installations, current-carrying parts are intentionally connected to the ground loop through elements with a nonlinear current-voltage characteristic.
In electrical installations up to 1000 V, to use an SPD, it is necessary to have a PE grounding conductor with a standardized resistance. Despite the fact that the devices themselves are designed for high pulse currents and voltages, they are not suitable for prolonged voltage increases and the flow of leakage currents.
Many manufacturers recommend protecting surge protectors with fuse links. These recommendations are explained by faster tripping of fuses in surge current areas, as well as frequent damage contact system automatic circuit breakers when currents of this magnitude are interrupted.
When performing three-stage surge protection, the devices must be located at a certain distance from each other along the length of the wire. For example, from SPD I to SPD II the distance must be at least 15 m along the length of the wire connecting them. Compliance with this condition allows you to selectively work on different stages and reliably suppress all disturbances in the network.
The distance between the II and III stages is 5 meters. If it is impossible to separate the devices over the prescribed distances, a matching choke is used, which is an active-inductive resistance equivalent to the resistance of the wires.
Features of choice. The most critical area of lightning surge protection is the entry into the building. The SPD in the first section limits the largest pulse current. Blade contacts for SPDs of the first class represent the greatest vulnerability of the device.
Pulse currents with an amplitude of 25-50 kA are accompanied by significant electrodynamic forces, which can lead to the removable module jumping out of the knife-type contacts and deprive the electrical network of overvoltage protection, therefore, it is better to use an SPD without a removable module as the first stage.
When choosing first class protection, give preference better devices based on arresters. Manufacturing a varistor SPD for a pulse current of more than 20 kA is quite labor-intensive and costly, therefore, their serial production is not justified.
So, if the manufacturer indicates a rated Iimp of more than 20 kA on the varistor device, you should be careful about such a purchase; Perhaps the manufacturer is misleading you.
An SPD using a spark gap with an open chamber is dangerous when triggered, so its use is justified in distribution cabinets where human presence is excluded when the protected area is in operation. The flow of pulsed current through the contacts of the spark gap inevitably leads to ignition of the arc.
When the arc burns, hot gases and splashes of molten metal can harm human health and life. The cabinet in which an SPD of this type is installed must be made of fireproof material, with all holes sealed.
Spark gaps with an igniting electrode circuit can also be used as a nonlinear element. Using an additional electrode, you can regulate the moment of breakdown of the spark gap and opening of the spark gap. The use of an ignition electrode makes it possible to reduce the level of pulse voltage and coordinate the operation of SPDs of different classes.
However, if the ignition electrode control circuit fails, the output will be protection with an unknown characteristic, which may not guarantee not only correct operation, but operation at all.
A modern home contains a considerable amount of household appliances, appliances and electronics. At the same time, most private homes receive energy using overhead power lines (OLTs). In such a situation, it makes sense to have a protection device against surge voltages that occur in the network during lightning strikes.
A lightning strike on a house looks terrible.Causes and nature of overvoltage impulses
Many elderly people, when leaving their home for a long period of time, unplug the cords of all electrical appliances from the sockets in the old fashioned way, fearing lightning. Currently, power lines are relatively weatherproof, and consumer electronics have basic protection against surges of up to several thousand volts.
Thus, in an apartment building, to which electricity is supplied by an underground cable, the problem of protection from thunderstorms is largely solved.
In case of power supply by air, it is necessary to take comprehensive measures to protect against lightning strikes.
Negative effects of atmospheric electricity can occur:
- when lightning strikes directly into a power line near the house, which leads to the occurrence of a pulse of 10/350 μs (the first value is the rise time of the pulse, the second is the fall time);
- when lightning strikes a power line at a long distance and produces a wave with a characteristic of 8/20 μs;
- when there is a lightning discharge in the immediate vicinity and an electromagnetic pulse is directed at the power line.
Options for lightning strike patterns Classification of surge protection
Familiar spark gaps Note that high-voltage pulses in the network can also occur as a result of an accident at an electrical substation or a break in the neutral wire in a three-phase network. As a result of the listed impacts, household appliances, as well as electrical switching devices, fail. If the insulation of the wiring in the house is broken, a short circuit, fire and fire will occur.
Valve arresters at an electrical substation The basis of the surge suppressor is a varistor, that is, a resistor whose resistance varies depending on the applied voltage. Surge arresters are more reliable and smaller in size. In a specific situation, it is possible to install surge suppressors with the most suitable characteristics.
In low-voltage networks that supply power to residential buildings, surge protection devices (SPDs) are used. These small-sized modular devices are divided into three classes and can be used by homeowners in their own houses and apartments.
Modular surge protectors for installation in an electrical panel Class I devices are installed on the input panel of a residential building. They are designed to protect against close lightning strikes (up to 1.5 km) and pass currents from 25 to 100 thousand amperes with a pulse characteristic of 10/350 μs. Class II SPDs are mounted in the distribution board as the second stage of protection against lightning strikes and pass currents of 10-40 thousand amperes with a pulse characteristic of 8/20 μs.
Class III devices suppress pulses with a characteristic of 8/20 µs and are designed for currents up to 10 kA. They are installed directly next to electrical appliances. According to their design, Class III SPDs can be manufactured in the form of modules and mounted on a DIN rail, as well as built into a socket or plug of an energy consumer.
Is installation of an SPD necessary in your case?
Standard electrical diagram for connecting an SPD in a three-phase network The classic SPD connection diagram provides for sequential installation of devices of all three classes. If you limit yourself to only a Class I device, it may not work with relatively weak pulses. On the contrary, the most sensitive class III surge protector will not fulfill its task under powerful influence.
There are standards and methods for calculating the risk of a lightning strike and assessing the consequences. In general, class I SPDs do not need to be installed if the power transmission line supports are grounded, the neutral wire is grounded, a lightning rod is installed, and a potential equalization system is implemented.
However, without special knowledge in the field of power supply, it is much easier to provide a standard surge protection circuit.
In any case, the negative impact of a lightning discharge is greatly reduced when a lightning rod is installed. If you haven't done this yet, read the article
How different types of SPDs work
Surge protection devices use arresters or semiconductor devices – varistors – in their design. The latter heat up when triggered and do not work well when subjected to repeated high-voltage impacts. The varistor must cool down to return to working condition. Modular-type surge protectors often have performance indicators and can be replaced if they fail.
Electrical diagram SPD operation At normal voltage In a network, current flows through conductors to the load. During a voltage surge, the arrester opens and allows current to flow to ground. After the network voltage returns to operating values, the SPD elements are closed again and the power supply flows as usual.
When a protection device is triggered, a current of up to tens of thousands of amperes flows through it. This releases a large amount of energy, that is, heat.
Do-it-yourself surge protection device
An example of installing an SPD in an electrical panel Lightning surge protection can be done with your own hands. A modular type surge protector is installed in an input panel with a metal casing. In this case, you should use a device whose rated operating current is not less than the value limited by the input circuit breaker. Also, the limiting voltage of the SPD should not be lower than permissible in your network.
Class I SPD is connected after the input circuit breaker in a single-phase or three-phase network. Protected power supply lines are connected to the device from above, and grounding from below. Below is a variant of the wiring diagram for connecting a class I surge protector in a single-phase network.
Wiring diagram for connecting an SPD in a single-phase network A class II surge protector is installed in the distribution board inside the house. The third class protection device is installed directly at consumers. If the stages of the protection device are located nearby, chokes must be connected between them for coordination. Otherwise, the SPD will absorb the entire load current with greater sensitivity. If the distance between the protection devices is more than 10 m, the electrical wiring will play the role of chokes.
The topic of selecting and connecting lightning surge protection devices is not easy for non-specialists. In difficult cases, it is better to contact a specialized organization.
The GOST 13109-97 standard does not give any limiting or permissible pulse values, but only gives us the shape of this pulse and its definition. We assume during measurements that pulses should not occur in the network. And if they are, then it will be necessary to sort it out and look for those to blame. In our measurements in 0.4 kV networks, we did not encounter any pulse problems. This is not surprising - measuring on the 0.4 kV side any impulse will be absorbed or cut off by surge suppressors, but this is a topic for another article. But as they say, forewarned is forearmed. Therefore, in the article we will give what we know.
These are the definitions from GOST 13109-97:
voltage pulse - a sharp change in voltage at a point in the electrical network, followed by a restoration of the voltage to the original or close to it level over a period of time of up to several milliseconds;
— pulse amplitude - the maximum instantaneous value of the voltage pulse;
— pulse duration - the time interval between the initial moment of the voltage pulse and the moment of restoration of the instantaneous voltage value to the original or close to it level;
Where do impulses come from?
Pulse voltages are caused by lightning phenomena, as well as transient processes during switching in the power supply system. Lightning and switching voltage pulses differ significantly in characteristics and shape.
Pulse voltage is a sudden change in voltage at a point in the electrical network, followed by a restoration of the voltage to its original or close to it level within 10-15 μs (lightning impulse) and 10-15 ms (switching impulse). And if the duration of the front of a lightning current pulse is an order of magnitude shorter than the switching current pulse, then the amplitude of the lightning pulse can be several orders of magnitude higher. The measured maximum value of the lightning discharge current, depending on its polarity, can vary from 200 to 300 kA, which rarely occurs. Typically this current reaches 30-35 kA.
Figure 1 shows an oscillogram of a voltage pulse, and Figure 2 shows its general view.
Lightning strikes in or near power lines into the ground lead to the appearance of pulse voltages that are dangerous for the insulation of lines and electrical equipment of substations. The main reason for the failure of insulation of electric power facilities, interruptions in power supply and the cost of its restoration is lightning damage to these facilities.
Figure 1 — Voltage pulse oscillogram
Figure 2 — General view of a voltage pulse
Lightning impulses are a common phenomenon. During discharges, lightning enters the lightning protection device of buildings and substations connected by high and low voltage cables, communication and control lines. With one lightning, up to 10 pulses can be observed, following each other with an interval of 10 to 100 ms. When lightning strikes a grounding device, its potential increases relative to distant points and reaches a million volts. This contributes to the fact that in loops equipped with cable and overhead connections, voltages ranging from several tens of volts to many hundreds of kilovolts are induced. When lightning strikes overhead lines, an overvoltage wave propagates along them and reaches the substation busbars. The overvoltage wave is limited either by the strength of the insulation during its breakdown, or by the residual voltage of the protective arresters, while maintaining a residual value reaching tens of kilovolts.
Switching voltage pulses occur when switching inductive (transformers, motors) and capacitive (capacitor banks, cables) loads. They occur during a short circuit and its shutdown. The values of switching voltage pulses depend on the type of network (overhead or cable), the type of switching (on or off), the nature of the load and the type of switching device (fuse, disconnector, circuit breaker). Switching current and voltage pulses have an oscillatory, damped, repeating nature due to arc burning.
The values of switching voltage pulses with a duration at the level of 0.5 pulse amplitude (see Fig. 3.22), equal to 1-5 ms, are given in the table.
The voltage pulse is characterized by the amplitude U imp.a, maximum voltage value U imp, the duration of the leading edge, i.e. time interval from the beginning of the pulse t starting until it reaches its maximum (amplitude) value t amp and voltage pulse duration at a level of 0.5 of its amplitude t amp 0.5. The last two time characteristics are shown as a fraction ∆ t amp/ t imp 0.5 .
Value of switching impulse voltages
List of sources used
1. Kuzhekin I.P. , Larionov V.P., Prokhorov V.N. Lightning and lightning protection. M.: Znak, 2003
2. Kartashev I.I. Electric power quality management / I.I. Kartashev, V.N. Tulsky, R.G. Shamonov et al.: ed. Yu.V. Sharova. – M.: MPEI Publishing House, 2006. – 320 p.: ill.
3. GOST 13109-97. Electric Energy. Compatibility technical means electromagnetic Standards for the quality of electrical energy in power supply systems general purpose. Enter 1999-01-01. Minsk: IPK Standards Publishing House, 1998. 35 p.
I was prompted to write this text by the feeling that many people do not know the principles of operation, the use (or even ignorance of the existence) of parallel protection against surge voltages in the network, including those caused by lightning strikes
Pulse noise in the network is quite common; it can occur during a thunderstorm, when turning on/off powerful loads (since the network is an RLC circuit, oscillations occur in it, causing voltage surges) and many other factors. In low-current circuits, including digital circuits, this is even more relevant, since switching noise penetrates quite well through power supplies (flyback converters are the most protected - in them, the transformer energy is transferred to the load when the primary winding is disconnected from the network).
In Europe, it has long been de facto mandatory to install surge protection modules (hereinafter, for simplicity, I will call lightning protection or SPD), although their networks are better than ours, and there are fewer lightning areas.
The use of SPDs has become particularly relevant over the past 20 years, when scientists began to develop more and more variants of MOSFET field-effect transistors, which are very afraid of exceeding the reverse voltage. And such transistors are used in almost all switching power supplies up to 1 kVA, as switches on the primary (network) side.
Another aspect of the use of SPDs is to provide voltage limitation between the neutral and ground conductors. Overvoltage on the neutral conductor in the network can occur, for example, when switching a transfer switch with a divided neutral. During switching, the neutral conductor will be “in the air” and there could be anything on it.
Characteristics of surge voltages
Overvoltage pulses in the network are characterized by the waveform and current amplitude. The shape of the current pulse is characterized by its rise and fall times - for European standards these are pulses of 10/350 μs and 8/20 μs. In Russia, as often happens recently, European standards were adopted and GOST R 51992-2002 appeared. The numbers in the pulse shape designation mean the following:- first - time (in microseconds) for the rise of the current pulse from 10% to 90% of the maximum current value;
- second - time (in microseconds) for the current pulse to decay to 50% of the maximum current value;
Protective devices are divided into classes depending on the pulse power they can dissipate:
1) Class 0 (A) - external lightning protection (not considered in this post);
2) Class I (B) - protection against overvoltages characterized by pulsed currents with an amplitude from 25 to 100 kA with a waveform of 10/350 μs (protection in the building’s input distribution boards);
3) Class II (C) - protection against overvoltages characterized by pulsed currents with an amplitude of 10 to 40 kA with a waveform of 8/20 μs (protection in floor panels, electrical panels of premises, inputs of power supply equipment);
3) Class III (D) - protection against overvoltages characterized by pulsed currents with an amplitude of up to 10 kA with a waveform of 8/20 μs (in most cases, the protection is built into the equipment - if it is manufactured in accordance with GOST);
Surge protection devices
The main two SPD devices are arresters and varistors of various designs.Arrester
A spark gap is an electrical device of an open (air) or closed (filled with inert gases) type, containing in the simplest case two electrodes. When the voltage on the electrodes of the spark gap exceeds a certain value, it “breaks through”, thereby limiting the voltage on the electrodes to a certain level. When a spark gap breaks down, a significant current flows through it (from hundreds of amperes to tens of kiloamperes) in a short time (up to hundreds of microseconds). After removing the overvoltage pulse, if the power that the arrester is capable of dissipating has not been exceeded, it goes into its original closed state until the next pulse.
Main characteristics of arresters:
1) Protection class (see above);
2) Rated operating voltage - long-term operating voltage of the arrester recommended by the manufacturer;
3) Maximum operating alternating voltage - the maximum long-term voltage of the arrester at which it is guaranteed not to work;
4) Maximum pulse discharge current (10/350) μs - the maximum value of the current amplitude with a waveform (10/350) μs, at which the spark gap will not fail and will ensure voltage limitation at a given level;
5) Nominal pulse discharge current (8/20) μs - the nominal value of the current amplitude with a waveform (8/20) μs, at which the arrester will provide voltage limitation at a given level;
6) Limiting voltage - the maximum voltage on the electrodes of the spark gap during its breakdown due to the occurrence of an overvoltage pulse;
7) Response time - the time of opening the arrester (for almost all arresters - less than 100 ns);
8) (a parameter rarely indicated by manufacturers) static breakdown voltage of the spark gap - static voltage (slowly changing over time) at which the spark gap will open. It is measured by applying a constant voltage. In most cases, it is 20-30% higher than the maximum operating alternating voltage reduced to constant (alternating voltage multiplied by the root of 2);
Choosing a spark gap is quite a creative process with numerous “spitting at the ceiling” - after all, we do not know in advance the value of the current that will arise in the network...
When choosing a spark gap, you can be guided by the following rules:
1) When installing protection in input boards from overhead power lines or in areas where thunderstorms are frequent, install arresters with a maximum discharge current (10/350) μs of at least 35 kA;
2) Choose a maximum continuous voltage slightly higher than the expected maximum mains voltage(otherwise, there is a possibility that at high mains voltage, the spark gap will open and fail due to overheating);
3) Select arresters with the lowest limiting voltage possible (rules 1 and 2 must be followed). Typically the limiting voltage of class I arresters is from 2.5 to 5 kV;
4) Install arresters specifically designed for this purpose between the N and PE conductors (manufacturers indicate that they are for connection to N-PE conductors). In addition, these arresters are characterized by lower operating voltages, usually on the order of 250 V AC (there is no voltage at all between the neutral and the ground in normal mode) and a large discharge current - from 50 kA to 100 kA and higher.
5) Connect arresters to the network with conductors with a cross-section of at least 10 mm2 (even if the network conductors have a smaller cross-section) and as short a length as possible. For example, if a current of 40 kA appears in a conductor 2 meters long with a cross section of 4 mm2, about 350 V will drop on it (in the ideal case, without taking into account inductance - and it plays a big role here). If a spark gap is connected with such a conductor, then at the point of connection to the network the limiting voltage will be equal to the sum of the limiting voltage of the arrester and the voltage drop across the conductor with a pulse current (our 350 V). Thus, the protective properties are significantly deteriorated.
6) If possible, install arresters in front of the input circuit breaker and always in front of the RCD (in this case, it is necessary to install a fuse with gL characteristic for a current of 80-125 A in series with the arrester to ensure that the arrester is disconnected from the network if it fails). Since no one will allow you to install an SPD in front of the input circuit breaker, it is desirable that the circuit breaker has a current of at least 80A with a response characteristic of D. This will reduce the likelihood of a false operation of the circuit breaker when the arrester is triggered. The installation of an SPD in front of the RCD is due to the low resistance of the RCD to pulse currents; in addition, when the N-PE arrester is triggered, the RCD will trigger falsely. Also, it is advisable to install SPDs in front of electricity meters (which, again, power engineers will not allow you to do)
Varistor
A varistor is a semiconductor device with a “steep” symmetrical current-voltage characteristic.
In the initial state, the varistor has a high internal resistance (from hundreds of kOhms to tens and hundreds of MOhms). When the voltage at the varistor contacts reaches a certain level, it sharply reduces its resistance and begins to conduct significant current, while the voltage at the varistor contacts changes slightly. Like a surge arrester, a varistor is capable of absorbing the energy of an overvoltage pulse lasting up to hundreds of microseconds. But with prolonged increased voltage, the varistor fails, releasing a large amount of heat (explodes).
All DIN rail-mounted varistors are equipped with thermal protection designed to disconnect the varistor from the network in case of unacceptable overheating (in this case, it can be determined from the local mechanical indication that the varistor has failed).
The photo shows varistors with a built-in thermal relay after the operating voltage has exceeded different values. If there is a significant overvoltage, such built-in thermal protection is practically ineffective - the varistors explode so that the ears are blocked. However, the built-in thermal protection in varistor modules on a DIN rail is quite effective in case of any prolonged overvoltage, and manages to disconnect the varistor from the network 
A short video of naturalistic tests :) (supplying an increased voltage to a varistor with a diameter of 20 mm - an excess of 50 V)
Main characteristics of varistors:
1) Protection class (see above). Typically, varistors have protection class II (C), III (D);
2) Rated operating voltage - long-term operating voltage of the varistor recommended by the manufacturer;
3) Maximum operating alternating voltage - the maximum long-term voltage of the varistor, at which it is guaranteed not to open;
4) Maximum pulse discharge current (8/20) μs - the maximum value of the current amplitude with a waveform (8/20) μs, at which the varistor will not fail and will ensure voltage limitation at a given level;
5) Rated pulse discharge current (8/20) μs - the nominal value of the current amplitude with a waveform (8/20) μs, at which the varistor will provide voltage limitation at a given level;
6) Limiting voltage - the maximum voltage on the varistor when it opens due to the occurrence of an overvoltage pulse;
7) Response time - the opening time of the varistor (for almost all varistors - less than 25 ns);
8) (a parameter rarely indicated by manufacturers) varistor classification voltage - static voltage (slowly changing over time), at which the varistor leakage current reaches 1 mA. It is measured by applying a constant voltage. In most cases, it is 15-20% higher than the maximum operating alternating voltage reduced to constant (alternating voltage multiplied by the root of 2);
9) (a parameter very rarely indicated by manufacturers) the permissible error of varistor parameters is ±10% for almost all varistors. This error should be taken into account when choosing the maximum operating voltage of the varistor.
The choice of varistors, as well as arresters, is fraught with difficulties associated with the unknown conditions of their operation.
When choosing varistor protection, you can be guided by the following rules:
1) Varistors are installed as the second or third stage of protection against surge voltages;
2) When using class II varistor protection together with class I protection, it is necessary to take into account the different response speeds of varistors and arresters. Since arresters are slower than varistors, if the SPD is not matched, the varistors will take over most overvoltage impulse and will quickly fail. To coordinate lightning protection classes I and II, special matching chokes are used (ultrasound manufacturers have an assortment of them for such cases), or the cable length between SPDs of classes I and II must be at least 10 meters. The disadvantage of this solution is the need to embed chokes into the network or extend it, which increases its inductive component. The only exception is the German manufacturer PhoenixContact, which has developed special Class I arresters with so-called “electronic ignition”, which are “matched” with varistor modules from the same manufacturer. These SPD combinations can be installed without additional approval;
3) Select the maximum continuous voltage slightly higher than the expected maximum mains voltage (otherwise, there is a possibility that at high mains voltage, the varistor will open and fail due to overheating). But you can’t overdo it here, since the varistor limiting voltage directly depends on the classification voltage (and therefore on the maximum operating voltage). An example of an unsuccessful choice of the maximum operating voltage is IEK varistor modules with a maximum continuous voltage of 440 V. If they are installed in a network with a rated voltage of 220 V, then its operation will be extremely inefficient. In addition, it should be taken into account that varistors tend to “age” (that is, over time, with many operations of the varistor, its classification voltage begins to decrease). Optimal for Russia would be the use of varistors with a long-term operating voltage of 320 to 350 V;
4) You need to select one with the lowest limit voltage possible (in this case, rules 1 - 3 must be followed). Typically, the limiting voltage of class II varistors for line voltages is from 900 V to 2.5 kV;
5) Do not connect varistors in parallel to increase the total power dissipation. Many manufacturers of surge protection devices (especially class III (D)) sin parallel connection varistors. But, since 100% identical varistors do not exist (even from the same batch they are different), one of the varistors will always turn out to be the weakest link and will fail during an overvoltage pulse. With subsequent pulses, the remaining chain varistors will fail, since they will no longer provide the required dissipation power (this is the same as connecting diodes in parallel to increase the total current - this cannot be done)
6) Connect varistors to the network with conductors with a cross-section of at least 10 mm2 (even if the network conductors have a smaller cross-section) and as short a length as possible (the reasoning is the same as for arresters).
7) If possible, install varistors in front of the input circuit breaker and always in front of the RCD. Since no one will allow you to install an SPD in front of the input circuit breaker, it is desirable that the circuit breaker has a current of at least 50A with response characteristic D (for class II varistors). This will reduce the likelihood of false operation of the machine when the varistor is triggered.
Brief overview of SPD manufacturers
Leading manufacturers specializing in surge protection devices for low-voltage networks are: Phoenix Contact; Dehn ; OBO Bettermann; CITEL; Hakel. Also, many manufacturers of low-voltage equipment have SPD modules in their products (ABB, Schneider Electric, etc.). In addition, China successfully copies surge protectors from global manufacturers (since the Varistor is a fairly simple device, Chinese manufacturers produce quite high-quality products - for example, TYCOTIU modules).In addition, there are quite a few ready-made surge protection panels on the market, which include modules of one or two protection classes, as well as fuses to ensure safety in the event of failure of the protective elements. In this case, the shield is fixed to the wall and connected to the existing electrical wiring in accordance with the manufacturer's recommendations.
The cost of surge protectors varies significantly depending on the manufacturer. At one time (several years ago), I conducted a market analysis and selected a number of manufacturers of protection class II (some were not included in the list due to the lack of module versions for the required long-term operating voltage of 320 V or 350 V).
As a note on quality, I can only highlight HAKEL modules (for example PIIIMT 280 DS) - they have weak contact connections of the inserts and are made of flammable plastic, which is prohibited by GOST R 51992-2002. On this moment HAKEL have updated a number of products - I can’t say anything about them, because... I will never use HAKEL again
We will leave the use of class III (D) surge protectors and the protection of digital circuits of devices for later.
In conclusion, I can say that if after reading everything you have more questions than after reading the title, this is good, because the topic interested you, and it is so vast that you could write more than one book.
Tags:
- lightning protection
- SPD
- overvoltage protection
Surge protection is a blocking device against excessive voltage in the form of current pulses. It is installed in apartments and houses and has such advantages as high efficiency, low cost, and perfect design.
This type of equipment protection for power distribution lines up to 1000 volts serves to protect against elevated voltages associated with surges.
Sources of impulses can be:
- Lightning discharges into the power supply circuit or into the lightning rod of an object near the power input to the object.
- Lightning discharges at a distance of up to several thousand meters near the facility's communications.
- Connections of sufficiently powerful loads, short circuits in power distribution lines.
- Interference from electromagnetic waves, from electronic devices and equipment.
Offices and apartments have a lot of household appliances, computers and other expensive equipment that consume electricity. Therefore, in order to avoid the risk of damage and failure from equipment surges, it is better to purchase and install a protective device.
One sudden voltage drop is enough to cause several household devices to fail at once. This issue is especially relevant in country houses and country houses, in which the power supply, heating, and water supply systems are connected to autonomous power networks. Electrical safety requirements must not be neglected.
Surge protection is used to limit voltage in the form of pulses from lightning strikes, connections of a powerful inductive load (This can be large electric motors, a transformer), etc.
Types and classes of protection against voltage surges
- Type 1. Class B . The devices are used in case of a possible direct lightning strike into the power circuit or near an object into the ground. If the power supply is carried out via an overhead line, and also if there is a lightning rod, then the installation impulse protection is strictly required. The equipment is mounted in an iron casing, next to the power input to the building, or in a distribution panel.
- Type 2. Class C. It has reduced protection against voltage surges and is mounted at the entrance to the electrical installation and into the room as the 2nd level of protection. Mounted in distribution panels.
- Type 3. ClassD. Protects electrical equipment from residual overvoltage, unbalanced currents, and high-frequency interference. Mounted near electrical appliances. It is recommended to install impulse protection near the consumer, no more than five meters from it, and if there is a lightning rod, then directly at the consumer’s power input, since the current in the lightning rod provokes a significant impulse in the electrical wiring.
Operating principle
The effect of protection against voltage surges can be easily explained, since it simple circuit overvoltage output. A shunt is built into the device circuit, through which current is supplied to the load of the consumer connected to the power supply. A jumper is connected from the shunt to the ground, which consists of a spark gap or a varistor.
At normal network voltage, the varistor has a resistance of several mOhms. When an overvoltage appears on the line, the varistor begins to pass current through itself, which then flows into the ground. This is how impulse protection works simply. When the supply voltage is normalized, the varistor ceases to be a current conductor, and power is supplied to the consumer via the built-in shunt.
Protection device
Surge protection is based on varistors or arresters. There are also indication devices that give signals about failure of the protection. The disadvantages of varistor protection include the fact that when the protection is triggered, the varistors heat up, and it takes time to cool down to operate again. This adversely affects operation in stormy weather and multiple lightning strikes.
Often, protection on varistors is made with a device for mounting on. The varistor is easily changed by simply removing it from the protection housing and installing a new varistor.
Practical use
To reliably protect the energy consumer from overvoltage, you must first install a good one. For this purpose, circuits with a protective and separated neutral conductor are used.
Next, protective devices are installed in such a way that the distance from adjacent protective devices is at least 10 meters along the power line wire. This rule is important for the correct order of protection activation.
If an overhead line is used for power, then the best option The application will be pulse protection based on fuses and arresters. In the main panel of the house, protection is installed on class 1 and 2 varistors, in floor panels - class 3. To further protect electrical consumers, portable impulse protection in the form of extension cords with fuses is plugged into sockets.
Such protective measures reduce the likelihood of exposure to increased voltage, but do not provide a complete guarantee. Therefore, during thunderstorms, it is best to turn off sensitive devices and equipment if possible.
How to protect the protection device itself
The protection device itself also needs to be protected from damage. They can arise due to the destruction of parts when absorbing overvoltage pulses. There have been cases where the protection devices themselves caught fire and caused a fire.
- Class 1 devices are protected by 160 amp inserts.
- Class 2 is protected by 125 amp inserts.
If the fuse rating is higher than recommended, then you need to install an auxiliary insert that protects the panel parts from malfunctions. When high voltage is applied to the protection for a long time, the varistors become very hot. The thermal release turns off power protection if the varistor reaches a critical temperature.
Surge protection can be equipped. Class 1 protection can only be protected by inserts, since the inserts interrupt short circuit currents at high voltages.
It can be concluded that the correct use of surge protection makes it possible to effectively protect equipment from malfunctions caused by excessive power line voltage.
Impulse protection -how to choose
by lightning current
Electricity can be supplied to a building through an overhead line with the following properties:
- Insulated wires, self-supporting.
- Simple wires without insulation.
If the wires of the overhead line and its elements are insulated, this affects the effective protection and connection circuits, and also reduces the effect of a lightning strike.
SPD in the TN-C-S system When connecting a house from an isolated line, grounding is carried out according to the diagram shown in the figure. Surge protection is installed between phases and PEN. The point of disconnection of PEN to PE and N conductors at a distance of 30 m from the house requires auxiliary protection.
If the house has installed lightning protection, there are metal communications, then this affects the circuit and choice of connection of impulse protection, and also negatively affects the electrical safety of the house.
Options for proposed schemes
Option 1. Conditions.
Electricity is supplied through an insulated overhead line.
- No lightning protection.
- There are no metal structures outside the house. The grounding circuit is made according to the TN – C – S scheme.
Solution
In this case, it is unlikely that there will be a direct lightning strike to the house, due to:
- Availability of insulation of overhead line wires.
- Lack of lightning rod and external metal communications on the house.
As a result, protection against high voltage pulses, which have a shape of 8/20 μs for current, will be sufficient. Suitable for impulse protection with a mixed protection class in one housing.
The current range from voltage pulses is selected from the range from 5 to 20 kiloamperes. It is better to choose the largest value.
Option 2. Conditions.
The electric current flows through an insulated overhead line.
- There is no lightning protection.
- Outside the house there are metal communications for gas or water supply. The grounding system is made according to the TN-C-S scheme.
Solution
If we compare it with the previous option, here there can be a lightning strike on a pipe with a current of up to 100 kiloamperes. Inside the pipe, this current will be divided into two ends of 50 kiloamperes. On our side of the building, this part will be divided by 25 kiloamperes into the building and grounding.
The PEN wire will take over a portion of 12.5 kiloamperes, and the rest of the pulse of the same magnitude will pass through the protection device into the phase conductor. The same protection device can be used as before.
Option 3. Conditions.
Electricity is supplied through an overhead line without insulation.
Solution
There is a high probability of lightning discharge into the wires; the building uses a CT grounding scheme.
SPD in the TT system Pulse protection must be provided, both from the phase wires relative to the ground and from the neutral wire. Protection from the neutral wire to ground is rarely used due to local conditions.
When installing wires to an open line without insulation, the safety of the home is influenced by the shape of the branch, which can be made:
- By cable.
- Wires with insulation, like an insulated overhead line.
- Exposed wires.
When branching over the air, less risks are created by insulated wires with a cross-section of at least 16 mm square. The likelihood of a lightning strike on such wires is very low. A lightning discharge is possible into the wire cutting unit near the insulators at the input. In this case, half the voltage from the lightning discharge will appear on the phase.
