Basic principles and applications of surge protection devices (SPDs)

1 Introduction

Surge Protective Device (SPD), also known as surge protector, is a nonlinear protective device used in live systems to limit transient overvoltage and guide discharge surge current. It is used to protect electrical or electronic systems with low withstand voltage levels from damage caused by lightning strikes, lightning electromagnetic pulses or operating overvoltage. In recent years, electronic information systems (such as television, telephone, communication, computer networks, etc.) have developed rapidly, and a large number of electronic information equipment have emerged and become popular. Such systems and equipment are often expensive and important, and their operating voltage and withstand voltage levels are very low, making them extremely vulnerable to the harm of lightning electromagnetic pulses. For this reason, SPD overvoltage protection is required.

Since different countries follow different standards, product specifications are not unified, and parameter markings have their own emphasis, which is far less than other electrical product specifications, which brings great inconvenience to design and selection. In engineering design, common brands can be divided into domestic products, European products and American products according to their origin. Domestic products have chaotic parameter settings, diverse specifications, and high residual pressure. The model settings of standardized products are some imitations of European products, and some follow national standard parameters. Most products are marked with In and Imax. Since domestic products have lower requirements for application sites, low building grades, and high equipment withstand voltage values, some parameter requirements can be appropriately relaxed.

European products are generally marked with the maximum discharge current, and the product model is also set according to this parameter. For example, a famous European brand XXX65, XXX40, where the values ​​65 and 40 are Imax. However, our country's standards clearly stipulate that the nominal discharge current In should be used for selection, which is an embarrassing situation encountered in engineering design. After checking the product information, the In value of XX65 does not exceed 20 kA, and the In value of XX40 does not exceed 15 kA. If the recommended value of GB50343 is followed, these two products can only be used for the third level of protection at the end of the equipment, but in the actual design, they are installed on the first and second levels, which is obviously inconsistent with the selection parameters of the national standard, and the residual voltage is high. Ordinary models generally exceed 1200 V. Once the wiring environment is not good, it is easy to exceed the equipment withstand voltage value. Generally, the Uc value of European products is small, and the line voltage is marked opportunistically, so it is easy to be misled when selecting.

2 SPD Overview

2.1 Working Principle of SPD

Surge protectors are suitable for 220/380V low-voltage power supply protection. They are nonlinear components. According to IEC standards, surge protectors are devices that mainly suppress overvoltage and overcurrent in the conducted lines. For surge protectors to play a protective role, the basic requirement is that they must withstand the expected lightning current, and through the maximum surge clamping pressure, effectively extinguish the power frequency continuous current generated after the lightning current passes through, limit the instantaneous overvoltage that penetrates into the power line and signal transmission line to the voltage range that the equipment or system can withstand, or discharge the strong lightning current into the ground to protect the protected equipment or system from damage caused by the impact.

The types and structures of surge protectors vary according to their uses, but they contain at least one nonlinear voltage limiting element. Commonly used surge protectors include MOV (Metal Oxide Varistor) and gas discharge tubes. Surges contain strong energy and cannot be blocked. For this reason, the strategy to protect sensitive electrical equipment from surge damage is to divert the surge from the equipment to the ground.

The surge protector MOV consists of three parts: a metal oxide material in the middle, and two semiconductors connecting the power supply and the ground wire. When a surge occurs, the MOV immediately acts, and the response time is 1 to 3 nanoseconds. The "V" in MOV is a variable resistor. At the moment of response, the resistance of the MOV drops from the maximum value to almost zero ohms, and the overcurrent flows into the ground through the MOV. The protected electrical equipment continues to operate at normal operating voltage. Its semiconductor components have the property of changing resistance as the voltage changes. When the voltage is below a certain value, the movement of electrons in the semiconductor produces high resistance. Conversely, when the voltage exceeds the certain value, the movement of electrons changes, and the semiconductor resistance decreases to nearly zero ohms. When the voltage is normal, the surge protector MOV is idle and does not affect the power line.

*价浪涌保护器MOV优劣的指标：（1）箝位电压：表示将导致MOV接通地线的电压值。箝位电压越低，表示保护性能越好。（2）能量吸收/耗散能力：此标称值表示浪涌保护器在烧毁前能够吸收多少能量，单位为焦耳。其数值越高，保护性能就越好。（3）响应时间：浪涌保护器不会立刻断开，它们对电涌做出响应会有略微的延迟。

Another common surge protection device is the gas discharge tube. These tubes work the same way as MOVs, they move excess current from the hot wire to the ground wire, and they do this by using an inert gas as a conductor between the two wires. When the voltage is within a certain range, the composition of the gas makes it a poor conductor. If the voltage surges and exceeds this range, the current is strong enough to ionize the gas, making the tube a very good conductor. It conducts the current to the ground wire until the voltage returns to normal levels, at which point it becomes a poor conductor again.

2.2 Classification of surge protectors

SPD is an indispensable device for lightning protection of electronic equipment. Its function is to limit the instantaneous overvoltage that penetrates into power lines and signal transmission lines to the voltage range that the equipment or system can withstand, or to discharge the strong lightning current into the ground to protect the protected equipment or system from impact.

2.2.1 Classification by working principle

According to their working principles, SPDs can be divided into voltage switching type, voltage limiting type and combination type.

(1) Voltage switch type SPD. It presents high impedance when there is no transient overvoltage. Once it responds to lightning transient overvoltage, its impedance suddenly changes to low impedance, allowing lightning current to pass. It is also called "short-circuit switch type SPD".

(2) Voltage-limiting SPD. When there is no transient overvoltage, it has high impedance, but as the surge current and voltage increase, its impedance will continue to decrease. Its current-voltage characteristics are strongly nonlinear, and it is sometimes called a "voltage-clamping SPD."

(3) Combined SPD. It is composed of voltage switching components and voltage limiting components. It can display the characteristics of voltage switching type or voltage limiting type or both, which depends on the characteristics of the applied voltage.

2.2.2 Classification by purpose

According to their usage, SPDs can be divided into power line SPDs and signal line SPDs.

(1) Power line SPD

Since the energy of lightning strikes is very huge, it is necessary to gradually discharge the lightning energy to the ground through a graded discharge method. In the non-protected area for direct lightning strikes (LPZ0A) or at the junction of the direct lightning strike protection zone (LPZ0B) and the first protection zone (LPZ1), a surge protector or a voltage-limiting surge protector that has passed the Class I classification test is installed as the first-level protection to discharge the direct lightning current, or when the power transmission line is directly struck by lightning, the huge energy conducted is discharged. At the junction of each sub-zone (including the LPZ1 zone) after the first protection zone, a voltage-limiting surge protector is installed as a second, third or higher level of protection. The second-level protector is a protective device for the residual voltage of the previous protector and the induced lightning strike in the zone. When a large amount of lightning energy is absorbed in the previous stage, there is still a part of the energy that is quite huge for the equipment or the third-level protector, which will be transmitted and needs to be further absorbed by the second-level protector. At the same time, the transmission line passing through the first-level lightning arrester will also induce lightning electromagnetic pulse radiation. When the line is long enough, the energy of the induced lightning becomes large enough, and the second-level protector is needed to further discharge the lightning energy. The third-level protector protects the residual lightning energy that passes through the second-level protector. According to the withstand voltage level of the protected equipment, if the two-level lightning protection can limit the voltage to be lower than the withstand voltage level of the equipment, only two levels of protection are needed; if the withstand voltage level of the equipment is low, four or even more levels of protection may be required.

(2) Signal line SPD

With the widespread application of information systems, due to the large number of network lines and the low voltage resistance level of electronic equipment, the harm of lightning to information systems is becoming more and more serious. The harm of lightning to information systems is mainly caused by lightning electromagnetic pulses, including lightning overvoltage waves conducted along the line, high potential counterattacks generated by lightning current in the grounding wire, and electrostatic induction and electromagnetic induction of lightning electromagnetic fields. The protective measures against electromagnetic pulses include interception, shunting, equipotential bonding, shielding, grounding, and reasonable wiring. Installing SPD on the signal line is an important measure for information systems to prevent electromagnetic pulses. It can also play the role of interception, shunting, and equipotential bonding. The signal line SPD should be connected to the signal port of the protected device. Its output end is connected to the port of the protected device, which can be connected in series or in parallel. It is generally installed in series on the signal line. Therefore, when selecting a signal SPD, an SPD with a smaller insertion loss should be selected.

2.3 The importance of SPD in lightning protection

According to the provisions of the "Building Lightning Protection Design Code" GB50057-94 (2000 edition), buildings in the LPZ0B, LPZ1, and LPZn+1 lightning protection zones should take measures to prevent induced lightning, static electricity, or surges. Induced lightning is a lightning strike caused by the strong electromagnetic field changes (electromagnetic pulse induction or electrostatic induction) generated by lightning currents, which induces overvoltage and overcurrent on the conductor. It poses a huge threat to electrical equipment in buildings, especially low-voltage electronic equipment. The focus of lightning protection for equipment inside buildings is to prevent the invasion of induced lightning. In the protection against induced lightning, surge protectors (SPDs) are indispensable devices. They can respond in time to the overvoltage and overcurrent in various lines, discharge the overcurrent in the line, or clamp the overvoltage on the line, so as to achieve the purpose of protecting electrical equipment.

Static electricity, surges and induction lightning have the same nature and can be suppressed by surge protectors (SPDs). Another form of static electricity is that due to friction or high-speed operation of electronic equipment, a large amount of static electricity is generated on the human body and electronic equipment. High-voltage discharge is likely to occur between people and objects, and between objects. After discharge, it is very easy to damage precision electronic equipment. Surges are generated in a wide range of daily situations, such as turning the power on and off, plugging and unplugging the power, starting and stopping elevators, electric gates, motors, electric drills, electric welding, damage to electrical equipment and short circuits. In addition, surges often occur within the power supply system, and may occur in power trunks, branches, generators, transformers, UPS, AC and DC power supplies, and even electrical equipment terminals [2]. Compared with lightning, although the pulse voltage of surges is lower, its pulse width and duration are longer, and the intensity is still not small, but it is enough to interfere with and damage electrical equipment.

3 Application of SPD in Computer Information Systems

Modern computer information systems are mostly composed of large-scale integrated circuits. The insulation strength of microelectronic devices is low, while the working voltage of these sensitive electronic devices is constantly decreasing. Their number and scale are constantly expanding, so the possibility of them being damaged by overvoltage, especially lightning, is greatly increased. The consequences may cause the entire system to be interrupted and cause inestimable economic losses. Lightning and surge voltage have become a major public hazard in the information age.

3.1 Lightning protection design of power supply system

(1) Main distribution panel. Install three MC50-B modules on the three-phase line and add air switches to each of them. Install a MC125-B/NPE module between the N line and the ground line.

(2) UPS protection: Install a V20-C module between each phase line and the neutral line, install an air switch in front of each module, and install a V20-C/NPE module between the N line and the ground line.

(3) Important equipment that needs protection. Use CNS32D to protect its power line.

(4) Power supply protection for weak current equipment: Use V20-C lightning protection module or VF series power supply fine protection module.

The schematic diagram of the power supply lightning arrester is shown in Figure 1, where MC50-B/3+NPE is a combination of three high-energy graphite gap lightning arresters and one MC125-B/NPE module, and V20-C/3+NPE/AS is an enhanced lightning arrester with sound and light functions. The lightning arrester and air switch are installed next to the corresponding MCCB.

Figure 1 Schematic diagram of power supply lightning arrester

3.2 Lightning protection design for computer networks

RJ45S-E100/4-F network lightning arresters are installed at both ends of the 100M twisted pair cables entering and leaving the computer room; optical fiber cables do not need lightning arresters, but their reinforced cores should be grounded at the entrance. RJ45S-E100/4-F network lightning arresters are also installed at the outlet ports of network switches to form multi-level protection.

4 Several issues in SPD application

4.1 Cross-sectional area of ​​SPD connection wire and grounding wire conductor

The connection wire and grounding wire of SPD generally use multi-strand copper wire. The cross-sectional area of ​​the grounding wire should be larger than that of the connection wire and determined as 50% of the cross-sectional area of ​​the main grounding wire of the equipotential bonding bar connected to the SPD. For SPDs with Class II classification tests installed at or near the power supply inlet of electrical equipment, their grounding wire should be a multi-strand copper wire of not less than 4 mm2. For SPDs with Class I classification tests for protection against direct lightning strikes, their grounding wire should be a multi-strand copper wire of not less than 16 mm2. When other materials are used, their cross-sectional area should be equivalent to the cross-sectional area of ​​the above copper wire. The leads at both ends of the SPD should be short and straight to avoid forming too large a loop.

4.2 Accessories of SPD

Most of the existing power system SPD products are installed in a modular rail like miniature circuit breakers. There are single-pole and integrated multi-pole products, and two installation forms: fixed and plug-in with replaceable cores. In order to monitor the aging and operating status of the SPD, a voltage-limiting SPD with metal oxide resistor elements is used, with aging display, overload thermal disconnection device and failure indication function. According to the needs of system operation, a working status monitoring alarm module or a remote monitoring auxiliary contact can also be installed. Gap-type SPD can choose products with operating status indicators and lightning strike counters.

4.3 SPD overcurrent protection

There should be overcurrent protection devices on the SPD installation line, and the configuration should refer to the manufacturer's recommendations. When a circuit breaker is used, a time-delay type release with a C-type tripping curve should be used, and its rated current should not be less than 50 A in the first stage (optional 63 A), and not less than 20 A in the subsequent stages (optional 32 A). When a fuse is used, its configuration principle is the same as that of a circuit breaker. When the SPD is installed on the load side of the residual current protection device (RCD), in order to prevent the RCD from malfunctioning when the surge current passes through, an S-type residual current protector with a delay can be used. For particularly important load equipment, an SI-type residual current protector that is insensitive to atmospheric overvoltage can be used, and it should have a surge current anti-interference capability of not less than 3 kA (8/20μs).

5 Conclusion

Surge protector (SPD) is an important part of the comprehensive lightning protection system and plays an irreplaceable role. The connection circuit of surge protector (SPD) has different forms according to different needs. After shielding (line, room, equipment), equipotential connection and establishment of joint common grounding system, the protection should be done in depth, detail and well, and a line of defense that cannot be crossed by lightning, static electricity and surge should be built to truly ensure the safety of information system and electrical equipment.