Surge and surge protection equipment

        To write this text, I was encouraged by the feeling of many people not knowing the principles of operation, using (or even not knowing about the existence) of parallel protection against surge surges in the network, including those caused by lightning discharges.
        Impulse noise in the network is quite common, they can occur during a thunderstorm, switching on / off powerful loads (since the network is an RLC circuit, then vibrations causing voltage surges occur in it) and many other factors. In low-current, including digital circuits, this is even more relevant, since switching interference penetrates through power sources quite well (flyback converters are most protected - in them the transformer energy is transferred to the load when the primary winding is disconnected from the mains).
        In Europe, it has long been de facto almost mandatory to install surge protection modules (hereinafter, for simplicity, I will call lightning protection or SPD), although they have better networks than ours, and there are less storm areas.
        The use of SPDs over the past 20 years has become especially relevant, when scientists began to develop more and more options for field MOSFET transistors, which are very afraid of overvoltage. And such transistors are used in almost all switching power supplies up to 1 kVA, as keys on the primary (network) side.
        Another aspect of the application of SPDs is to provide voltage limits between the neutral and earth conductors. Overvoltage on a neutral conductor in the network can occur, for example, when switching the Automatic Transfer Switch with a divided neutral. During the switching, the neutral conductor will be “in the air” and anything can be on it.

    Characteristics of overvoltage impulses


    Surge 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 has often happened recently, they adopted European standards and GOST R 51992-2002 appeared. The numbers in the designation of the pulse shape mean the following:
       - the first is the time (in microseconds) of the rise of the current pulse from 10% to 90% of the maximum current value;
       - the second - the time (in microseconds) of the decay of the current pulse to 50% of the maximum current value;

        Protective devices are divided into classes depending on the power of the pulse that they can dissipate:
    1) Class 0 (A) - external lightning protection (we will not consider this post);
    2) Class I (B) - protection against overvoltages characterized by surge currents with an amplitude of 25 to 100 kA waveform 10/350 μs (protection in the input and distribution panels of the building);
    3) Class II (C) - protection against overvoltages characterized by surge currents with an amplitude of 10 to 40 kA waveform 8/20 μs (protection in floorboards, electrical panels of premises, inputs of power supply equipment);
    3) Class III (D) - protection against overvoltages characterized by surge currents with an amplitude of up to 10 kA waveform of 8/20 μs (in most cases, protection is built into the equipment if it is manufactured in accordance with GOST);

    Surge protection devices


        The main two surge arresters are surge arresters and varistors of various designs.

    Arrester
        Arrester - an electric device of open (air) or closed (filled with inert gases) type, containing in the simplest case two electrodes. When the voltage at the electrodes of the spark gap is exceeded, a certain value, it "breaks", thereby limiting the voltage at the electrodes at a certain level. During the breakdown of the arrester, a significant current flows through it (from hundreds of amperes to tens of kilo amperes) in a short time (up to hundreds of microseconds). After removal of the overvoltage pulse, if the power that the arrester is capable of dissipating has not been exceeded, it switches to the initial closed state until the next impulse.

        Main characteristics of arresters:
          1) Protection class (see above);
          2) Rated operating voltage - continuous, recommended by the manufacturer operating voltage of the arrester;
          3) The maximum working alternating voltage is the maximum continuous voltage of the arrester, at which it is guaranteed not to work;
          4) The maximum pulse discharge current (10/350) μs - the maximum value of the amplitude of the current with a waveform (10/350) μs, at which the arrester will not fail and provide voltage limitation at a given level;
          5) Rated impulse discharge current (8/20) μs - the nominal value of the amplitude of the current with a waveform (8/20) μs, at which the arrester will provide voltage limitation at a given level;
          6) Limit 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 opening time of the arrester (for almost all arresters - less than 100 ns);
          8) (a parameter rarely specified by manufacturers) static breakdown voltage of a spark gap - static voltage (slowly changing in time) at which the spark gap will open. Measured by applying a constant voltage. In most cases, it is 20-30% higher than the maximum working alternating voltage reduced to constant (alternating voltage multiplied by the root of 2);

        Choosing a surge arrester is a rather creative process with numerous “spitting into the ceiling” - because we don’t know in advance the value of the current that will appear in the network ...
        When choosing a surge arrester, you can follow the following rules:
          1) When installing protection in the input boards from the overhead power line or in areas where frequent thunderstorms, install arresters with a maximum discharge current (10/350) μs of at least 35 kA;
          2) Choose the maximum continuous voltage slightly higher than the expected maximum mains voltage (otherwise there is a possibility that with a high mains voltage, the arrester will open and fail from overheating);
          3) Select arresters with the lowest possible limiting voltage (in this case, the implementation of rules 1 and 2 is mandatory). Typically, the voltage limiting of class I arresters is from 2.5 to 5 kV;
          4) Install arresters specifically designed for this between N and PE conductors (manufacturers indicate that they are for connecting to N-PE conductors). In addition, these arresters are characterized by lower operating voltages, usually of the order of 250 V AC (there is no voltage at all between neutral and ground in normal mode) and a large discharge current from 50 kA to 100 kA and higher.
          5) Connect the 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 as possible. For example, if a current of 40 kA appears in a 2-meter-long conductor with a cross section of 4 mm2, it will fall (in the ideal case, without taking into account inductance - and it plays a big role) about 350 V. If a spark gap is connected to 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 spark gap and the voltage drop across the conductor at a pulsed current (our 350 V). Thus, the protective properties are significantly impaired.
          6) If possible, install arrester in front of the input circuit breaker and always before the RCD (in this case, it is necessary to install a fuse with a gL characteristic for a current of 80-125 A in series with the arrester to ensure that the arrester is disconnected from the network when it fails). Since no one will allow installing an SPD in front of the input circuit breaker - it is desirable that the circuit breaker be at a current of at least 80A with a trip characteristic D. This will reduce the likelihood of a false trip of the machine when the arrester trips. The installation of an SPD in front of an RCD is caused by the low resistance of the RCD to surge currents, in addition, when the N-PE arrester is triggered, the RCD will falsely trigger. Also, it is advisable to install an SPD in front of electricity meters (which again, power engineers will not allow to do)

    Varistor
        Varistor is a semiconductor device with a “cool” symmetrical current-voltage characteristic.



        In the initial state, the varistor has a high internal resistance (from hundreds of ohms to tens and hundreds of megohms). When the voltage at the contacts of the varistor reaches a certain level, it sharply reduces its resistance and starts to conduct a significant current, while the voltage at the contacts of the varistor changes slightly. Like a spark gap, a varistor is capable of absorbing the energy of an overvoltage pulse lasting up to hundreds of microseconds. But with prolonged overvoltage, the varistor fails with the release of a large amount of heat (explodes).
        All DIN rail-mounted varistors are equipped with thermal protection designed to disconnect the varistor from the mains when it is unacceptable to overheat (in this case, it can be determined from the local mechanical indication that the varistor is out of order).
        In the photo, varistors with a built-in thermal relay after exceeding the operating voltage of different values. With significant overvoltage, such a built-in thermal protection is practically ineffective - varistors explode so that the ears are blocked. However, the built-in thermal protection in the varistor modules on the DIN rail is quite effective for any long-term overvoltage, and manages to disconnect the varistor from the network.

    A small video of naturalistic tests :) (applying a higher voltage to the 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 operational voltage - continuous, recommended by the manufacturer operating voltage of the varistor;
          3) The maximum working alternating voltage is the maximum continuous voltage of the varistor, at which it is not guaranteed to open;
          4) The maximum pulse discharge current (8/20) μs - the maximum value of the amplitude of the current with the waveform (8/20) μs, at which the varistor will not fail and will provide voltage limitation at a given level;
          5) Rated impulse 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) Voltage limiting - the maximum voltage on the varistor when it is opened due to the occurrence of an overvoltage pulse;
          7) Response time - varistor opening time (for almost all varistors - less than 25 ns);
          8) (a parameter rarely indicated by manufacturers) the varistor classification voltage is the static voltage (slowly changing in time) at which the varistor leakage current reaches 1 mA. Measured by applying a constant voltage. In most cases, it is 15-20% higher than the maximum working alternating voltage reduced to constant (alternating voltage multiplied by the root of 2);
          9) (very rarely indicated by the manufacturers parameter) permissible error of the varistor parameters - for almost all varistors ± 10%. This error should be considered when choosing the maximum operating voltage of the varistor.

        The choice of varistors as well as arresters is associated with difficulties associated with the unknown conditions of their work.
        When choosing a varistor protection, you can be guided by the following rules:
          1) Varistors are installed as the second or third stage of protection against surge surges;
          2) When using varistor protection of class II together with protection of class I, it is necessary to take into account the different response speeds of varistors and arresters. Since the arrester is slower than the varistors, if the SPD is not coordinated, the varistors will take on most of the overvoltage pulse and will quickly fail. For matching I and II classes of lightning protection, special matching chokes are used (ultrasound manufacturers have their assortment for such cases), or the cable length between the SPD of I and II classes must be at least 10 meters. The disadvantage of this solution is the need for insertion of chokes into the network or its extension, which increases its inductive component. The one exception is the German manufacturer PhoenixContact, who developed special class I arresters with the so-called "electronic ignition", which are "aligned" with varistor modules of the same manufacturer. These combinations of SPDs can be installed without additional coordination;
          3) Choose the maximum continuous voltage slightly higher than the expected maximum mains voltage (otherwise there is a possibility that with a high mains voltage, the varistor will open and fail from overheating). But here it is impossible to overdo it, since the voltage of the varistor limitation directly depends on the classification (and therefore on the maximum operating voltage). An example of an unsuccessful choice of the maximum operating voltage is the IEK varistor moduleswith 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 borne in mind that varistors tend to “age” (that is, over time, with many varistor operations, its classification voltage begins to decrease). Optimal for Russia will be the use of varistors with a long operating voltage from 320 to 350 V;
          4) You need to choose with the lowest possible voltage limits (the implementation of rules 1 to 3 is mandatory). Typically, the voltage limiting varistors of class II for a mains voltage of 900 V to 2.5 kV;
          5) Do not connect varistors in parallel to increase the total power dissipation. Many manufacturers of SPD protection devices (especially class III (D)) sin by parallel connection of varistors. But, since 100% of the same varistors does not exist (even from the same batch they are different), always one of the varistors will turn out to be the weakest link and will fail with an overvoltage pulse. With subsequent pulses, the remaining 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 the 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 of the smallest possible length (the same reasoning as for the arresters).
          7) If possible, install varistors in front of the opening circuit breaker and always before the RCD. Since no one will allow installing an SPD in front of the input circuit breaker - it is desirable that the circuit breaker be on a current of at least 50A with a trip characteristic D (for class II varistors). This will reduce the likelihood of a false positive when the varistor trips.

    A brief overview of manufacturers of SPDs
        The leading manufacturers specializing in SPD of 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 SPDs of world manufacturers (since Varistor is a fairly simple device, Chinese manufacturers produce fairly high-quality products - for example, TYCOTIU modules ).
        In addition, there are quite a few ready-made surge protection panels on the market, including modules of one or two protection classes, as well as safety fuses in case of failure of protective elements. In this case, the shield is mounted on the wall and connected to the existing wiring in accordance with the manufacturer's recommendations.
    The cost of the SPD varies depending on the manufacturer at times. 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 designs for the required long-term operating voltage of 320 V or 350 V).
        As a remark on quality, I can single out only HAKEL modules (for example, PIIIMT 280 DS) - they have weak contact joints of inserts and are made of combustible plastic, which is prohibited by GOST R 51992-2002. At the moment, HAKEL has updated a number of products - I can’t say anything about it, because I will never use HAKEL again.

        The use of Class III (D) SPDs and the protection of digital circuits of devices will be left for later.
        In conclusion, I can say that if after reading everything you have more questions than after reading the headline, this is good, because the topic has interested, and it is so vast that you can write more than one book.

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