**Preface**
The role of a lightning arrester is to prevent damage caused by lightning currents, which is essential for protecting electronic equipment. As more and more electronic devices are integrated into daily life, surge protectors have become increasingly familiar to the general public. However, the field of electronic system lightning protection is still relatively new, with many unresolved issues in the selection and application of these devices. This article aims to clarify key concepts such as the response time of surge protectors and the sequential operation of multi-stage systems.
When dealing with high-energy surges, a single-stage lightning arrester may not be sufficient to suppress overvoltage to a safe level. In such cases, it is necessary to use multiple stages of surge protection components—typically two, three, or more. These components often include nonlinear elements like varistors (RV1 and RV2), which can also be gas discharge tubes, surge tubes, or TVS diodes. The isolation element between stages, such as an inductor (Ls) or resistor (Rs), plays a crucial role in determining how the surge is distributed across the stages.
Some people mistakenly believe that the first stage (RV1) always activates first when a surge enters the system. In reality, whether the first or second stage conducts depends on several factors:
1. The waveform of the incoming surge, particularly the rate of current rise (di/dt).
2. The on-voltages of the nonlinear components (Un1 for RV1 and Un2 for RV2).
3. The nature and value of the isolation impedance (Zs), whether it's resistive or inductive.
If Zs is a resistor (Rs), the second stage usually conducts first. Once the second stage activates, the voltage across the first stage increases, eventually triggering it. Since the first stage has much lower impedance at high current levels, most of the surge current flows through it, leaving only a small portion for the second stage. If the first stage is a gas discharge tube, its residual voltage is typically lower than the second stage’s conduction voltage, causing the second stage to turn off.
If Zs is an inductor (Ls), and the surge current rises rapidly, the condition Ls(di/dt) + Un2 > Un1 may trigger the first stage first. Afterward, as the current increases, the voltage across the first stage may drop below the threshold required to activate the second stage, leading to a more effective suppression of the surge.
**SPD Response Time**
Many people mistakenly believe that the response time of a surge protector is a critical performance indicator. Manufacturers often highlight this in their technical specifications, but few truly understand its meaning or measure it accurately. A common misconception is that during the response time, the surge protector is ineffective, allowing the full surge voltage to pass through to the protected device. This is incorrect and does not align with the working principle of surge protection devices.
Surge arresters use nonlinear components, which can be classified into two types: "limited voltage type" (e.g., varistors, Zener diodes) and "switch type" (e.g., gas discharge tubes, thyristors). Zinc oxide varistors, for example, respond very quickly to voltage changes.
In earlier technical documents, the term "response time" was used in the context of IEEE C62.33-1982, where it referred to the "overshoot" phenomenon. Overshoot occurs when the measured limit voltage of a varistor exceeds the standard 8/20 μs waveform due to magnetic coupling in the lead loops. This overshoot is not an intrinsic property of the device but rather a result of external factors like lead length and loop coupling.
Recent international standards, such as IEC 61643-1 and IEC 6163-21, no longer include the response time parameter. Similarly, IEEE C62.62-2000 clearly states that wavefront response time is not a necessary requirement for typical applications and may lead to misleading specifications.
For power surge protectors, three key parameters are important: 1) Limiting voltage (protection level), 2) Discharge current capacity (impact current withstand), and 3) Continuous operating voltage life. Other parameters, such as operating voltage and discharge current, are also relevant but may be less well understood by non-experts.
When selecting a surge protection device, it is crucial to pay attention to these key indicators and choose reliable, suitable products to ensure proper protection against lightning-induced surges.
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