**Preface**
The role of a lightning arrester is to prevent damage caused by lightning currents, which is essential for protecting electronic equipment and devices. As the use of electronic products becomes more widespread in daily life, surge protectors have become increasingly familiar to people. However, the field of lightning protection for electronic systems is still relatively new, and there are many unresolved issues regarding the selection and application of these devices. This article aims to explain the response time of surge arresters and the sequential operation of multi-stage lightning protection systems.
When it comes to the operation of multi-stage lightning arresters, a single-stage device may not be sufficient to suppress overvoltage to a predetermined level. Therefore, surge protection should employ two, three, or more stages of non-linear components. These components can include varistors (RV1 and RV2), gas discharge tubes, surge tubes, or TVS diodes. The isolation element between the stages, such as an inductor (Ls) or resistor (Rs), plays a crucial role in determining how the surge current is distributed.
Some people believe that when a surge wave enters the system, the first stage (RV1) will always activate first, followed by the second stage. In reality, the sequence depends on several factors: the waveform of the incoming surge, particularly the rate of current rise (di/dt); the relative on-voltages of the nonlinear elements (Un1 and Un2); and the nature and value of the isolation impedance (Zs).
If Zs is a resistance (Rs), the second stage typically activates first. Once the second stage turns on, if the surge current rises to a level where iRs + Un2 ≥ Un1, the first stage will also activate. Since the equivalent impedance of the first stage is much lower 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 after conduction is usually lower than Un2, causing the second stage to turn off, and the remaining current to be discharged through the first stage.
If Zs is an inductance (Ls), and the surge current rises rapidly, the condition Ls(di/dt) + Un2 > Un1 may be met, allowing the first stage to activate first. As the current increases, the limiting voltage of the first stage may decrease until the condition UC1(1) ≥ Ls(di/dt) + Un2 is satisfied, at which point the second stage activates. After this, the output voltage is suppressed to a lower level.
**SPD Response Time**
Many people mistakenly believe that the response time is a key indicator of a surge protector’s performance. Manufacturers often highlight this in their technical specifications, but many do not fully understand its meaning or measure it accurately. A common misconception is that during the response time, the surge arrester has no effect on the incoming surge, allowing the full voltage to reach the protected equipment. This is incorrect, as it contradicts the fundamental working principle of surge protection devices.
Surge arresters contain non-linear components that can be categorized into “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 previous technical documents, the term “response time†was used in the context of IEEE C62.33-1982, referring 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 loop. This overshoot is not an intrinsic property of the device but rather a result of the circuit layout and wire length.
Recent international standards like IEC 61643-1 and IEC 6163-21 no longer include the response time parameter. Similarly, IEEE C62.62-2000 explicitly states that the wavefront response requirement is not necessary for typical applications and could lead to misleading specifications. Therefore, unless specifically required, response time is not tested, measured, or certified.
For power surge protectors, three key technical parameters are important: 1) Limiting voltage (protection level), 2) Current handling capacity (impact current rating), and 3) Continuous operating voltage life. Other parameters, such as operating voltage, discharge current, and clamping voltage, are also relevant but may not be well understood by outsiders. It is crucial to choose reliable and suitable surge protection products when selecting a lightning arrester.
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