Created by: Glen Zhu | Updated Date: January 27th, 2024
Surge protection and grounding of protected equipment
Connect the grounding wire or shell of the protected equipment directly to the grounding wire of the surge protector, and make the connecting wire as short as possible. Grounding at a single point on the ground terminal of the surge protector. This can prevent high voltage between the surge protector and the ground wire of the protected equipment, effectively providing protection.
Installation and wiring of surge protection for security type
When using the intrinsically safe surge protective device (SPD) to protect the safety grid and connected equipment, the surge protector should be installed separately from the safety grid (as shown in the figure below) to meet the requirement of a 50mm spacing between the wiring terminals on the hazardous side and safety side, while also making wiring more organized.
When the surge voltage is applied to the input terminal of the protection circuit, the transient suppression diode TVS with the fastest response time first operates. By selecting appropriate coupling component (inductor or resistor) parameters, the circuit is designed to allow for an increase in discharge current before potential damage to the suppression diode occurs, so that the voltage drop generated on L2 plus the voltage drop on TVS reaches the breakdown voltage of MOV, at which point MOV starts to discharge. Similarly, as the discharge current further increases, it causes an increase in voltage drop on L1 plus MOV’s breakdown voltage to reach GDT’s operating voltage, ultimately releasing a larger surge current through GDT as shown in Figure 8.
For example: When a surge voltage rises at a standard rate of 1KV/µs and a peak pulse voltage of 6KV is applied to a 24V combination protection circuit, after passing through gas discharge tubes, the voltage is approximately limited to 700V. This voltage is then limited to around 150V through attenuation by coupling components (inductors or resistors) and suppression by varistors. Furthermore, by clamping with suppression diodes, output voltage is limited to around 40V. In this way, protected electronic devices only need to withstand momentary overvoltage up to their rated value multiplied by 1.5 times.
The basic requirements of Surge Protective Devices (SPD) are fast response time, high discharge current, low residual output voltage, and long service life. To meet these requirements, different protective components need to be used to form a multi-stage protection circuit. The commonly used protective components include three types: Gas Discharge Tube (GDT), Metal Oxide Varistor (MOV), and Transient Voltage Suppressor Diode (TVS).
Gas Discharge Tube
Its structure involves filling inert gas such as argon or neon inside a ceramic shell with metal electrodes at both ends. When the external voltage between the two poles increases to exceed the insulation strength of the gas, a gap breakdown occurs between the two poles. The resistance between the two poles becomes low. The breakdown voltage of the gas gap is related to the rate of overvoltage rise (as shown in Figure 5).
From Figure 5, it can be seen that when the voltage rise rate is slow, such as 100V/µs, then the operating voltage Uz2 for protection is equal to the rated breakdown voltage of the gas discharge tube. When there is a rapid transient overvoltage rise rate like 1000V/µs, then response time decreases and operating voltage can reach up to ten times its rated breakdown voltage. For example, if the DC breakdown voltage of the gas discharge tube is 90V, under a transient overvoltage rise rate of 1KV/µs it can reach up to 900V. The main feature of gas discharge tubes is their large discharge current capacity which can go up to 100KA maximum. Its drawbacks include an output residual pressure of around 700V (at 1KV/µs) and a relatively slow response time ranging from nanoseconds to seconds.
Another drawback is the potential follow-on current issue – after breakdown occurs in the discharge tube causing low voltages at both ends of the electric arc; only when external voltages drop below this value will the arc extinguish; otherwise gas discharge tube absorb energy from the line until burnt out completely which should be given special consideration in power supply protection circuits rather than signal line protection circuits.
Varistor
A varistor is a metal oxide semiconductor with zinc oxide as its main component. It is a nonlinear resistor. When the voltage applied to both ends exceeds its rated voltage, its resistance will rapidly decrease and approach a short circuit (as shown in Figure 6).
Surge voltage is discharged to the ground through the varistor. It serves as an intermediate level of multi-stage protection circuits to further clamp overvoltage. The varistor has moderate discharge capacity, and the discharge capacity of the varistors used in signal line protection is below 5KA, and its response time is also moderate at about 25ns. Its residual pressure is much smaller than that of gas discharge tubes and there are no problems with continued flow. However, it has two disadvantages: it is prone to aging and has a large capacitance of about 1000-10000PF. Aging results from leakage current. Therefore, varistors are not suitable for use in measurement circuits with higher sensitivity or communication circuits with higher frequencies (above 100KHz). In particular, they should not be used in intrinsically safe explosion-proof SPDs.
Transient Voltage Suppressor Diode
Transient voltage suppressor diodes come in unipolar and bipolar types. Their main feature is a very short response time, in the picosecond range, with low clamping voltage. However, their leakage current capability is not large enough, generally less than 1500W (10/1000μs waveform). They also have relatively large capacitance ranging from 100pF to 10000pF; the lower the clamping voltage, the larger the junction capacitance. They are typically used as precision clamps in the final stage, as shown in Figure 7 of the varistor characteristics curve.
The impact of lightning strikes on the electronic information system of buildings is multifaceted, including direct lightning strikes, lightning electromagnetic pulses invading through power lines and signal lines (capacitive coupling), electromagnetic field induction formed by lightning strikes near the building (inductive coupling), and ground potential backlash caused by grounding devices after surge arresters are triggered (resistive coupling). Therefore, external and internal combined lightning protection measures should be adopted for the comprehensive protection of electronic information systems. External lightning protection uses traditional lightning rods, lightning protection nets, and other protective equipment to guide the lightning current to the ground. However, usually only 50% of the energy from a thunderstorm directly enters the ground; the remaining 50% will enter the electronic information system in various ways within buildings. To achieve internal protection, all metal pipes and equipment grounds inside buildings must achieve equipotential grounding. Additionally, surge protectors compatible with external communication transmission line ports should be installed in various parts of the electronic information system as shown in Figure 4.
Voltage Surge
Refers to the instantaneous increase in rated operating voltage of electronic systems, with amplitudes reaching several times to hundreds of times the rated operating voltage. Voltage surges may cause data distortion and loss in communication systems, and even damage electronic equipment. The causes of voltage surges include lightning strikes, equipment switching, electrostatic discharge, and line faults. Although lightning strikes occur less frequently compared to other reasons, they pose the greatest threat.
Thunderstorm
Is a common natural phenomenon where charges accumulate in thunderclouds to a certain extent, leading to air breakdown between clouds or between clouds and the ground causing static discharge. When this type of direct discharge occurs through buildings on the ground, overhead power lines, or communication cables resulting in voltage surges it is called a direct lightning strike, with discharge currents reaching 3-200kA. Lightning-induced static discharges occurring near buildings, overhead power lines, or communication cables due to changes in ground potential or electromagnetic coupling are known as indirect lightning strikes. A lightning strike at a distance of 100m from data cables or buildings can induce a voltage surge of 6kkV and 3kA on those data cables.
Hazards of Lightning Strikes and Voltage Surges
China has vast territory with most regions being prone to frequent thunderstorms. Except for fewer thunderstorm days in northwest China, other regions such as northeast China have around 30 days per year with thunderstorms on average; north China has 40-50 days; most areas south of the Yangtze River experience between 40-80 days while Fujian, Guangdong, Guangxi, Yunnan, Hainan provinces have more than 80 days.
Major economic regions like the Yangtze River Delta Economic Belt area (Changjiang Economic Zone), and Pearl River Delta region are all located in high-incidence areas for thunderstorms – whether it’s direct or indirect lightning strikes – not only pose risks to personal safety but also cause significant damage to electronic devices. In these areas where computer control systems and electronic signal products are widely used encounters with damages caused by lightning strikes are severe too. With advancements in science & technology increasing integration levels within electronic information systems their ability to withstand surge impacts is decreasing over time leading to losses due to data loss or damage from surges induced by lighting increasing annually.
Direct lightning strikes can cause massive damage. The induced voltage surges on power lines and data lines near direct lightning strikes can also cause harm. The secondary effect of induced voltage surges from lightning strikes is called the secondary effect of lightning strikes. There are three main coupling pathways: 1) resistance coupling, 2) inductance coupling, 3) capacitance coupling.
Resistance Coupling
When a lightning strike occurs on or near building A’s air terminal or nearby ground, it will cause a sharp increase in the ground potential near the building. Due to the presence of ground resistance, there will be a huge potential difference between points A and B. This potential difference will generate large surge currents through the electronic grounding system, causing damage to electronic equipment connected by wires inside points A and B (as shown in Figure 1).
Inductive coupling
When lightning strikes the external lightning protection system of a building (such as a lightning rod) discharges, and the huge electromagnetic field generated by the lightning current will induce destructive voltage surges on the cable lines connected to electronic devices inside the building (as shown in Figure 2).
Capacitive coupling
When lightning strikes an overhead power line, a strong electric field is generated between the nearby cable lines. Due to its high-frequency characteristics, capacitive coupling through the distributed capacitance between cable lines can cause voltage surges on low-potential cable lines, thereby damaging electronic devices (as shown in Figure 3).
LSP’s reliable surge protection devices (SPDs) are designed to meet the protection needs of installations against lightning and surges. Contact our Experts!
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