Abstract: This paper discusses four controversial issues in the application of surge protection devices (SPDs): SPD response time, the operating sequence of multi-stage SPDs, the equivalent transformation of different waveform inrush currents, and the relationship between the residual voltage of the SPD and the peak value of the inrush current. Finally, the interrelationships between various voltages in SPD applications are explained. Keywords: Surge protection device, response time, inrush current, lightning protection. I. Introduction Surge protection devices (SPDs) are essential devices for suppressing inrush voltages generated by lightning, electrical system operation, or static electricity, protecting electronic information technology products. With the increasing penetration of various electronic information technology products into all aspects of society and household life, the application scope of SPDs is expanding, and market demand is growing. Generally speaking, overvoltage protection for electronic information technology products is still a relatively new technical field. The two international standards related to SPDs, IEC 61643-1 and IEC 61643-21, have only been published for a few years, and many issues concerning SPD applications remain controversial. This paper presents the author's personal views on four of these issues in the hope of stimulating discussion. These are: the response time of the SPD, the operating sequence of multi-stage SPDs, the equivalent transformation of different waveform inrush currents, and the relationship between the residual voltage of the SPD and the peak value of the inrush current. Finally, the interrelationships between various voltages in SPD applications are explained. II. SPD Response Time Many people mistakenly believe that response time is an important indicator of SPD protection performance, and manufacturers list this parameter in their technical data. However, many manufacturers do not know its precise meaning and have not measured it. A popular view is that within the response time, the SPD has no suppression effect on intrusion surges, and the surge voltage "passes through" the SPD and acts on downstream equipment. This does not conform to the working condition of the SPD and is incorrect. The nonlinear elements in an SPD that suppress inrush overvoltages can be classified according to their working mechanism into "voltage-limiting type" (such as varistors and Zener diodes) and "switching type" (such as gas discharge tubes and thyristors). A zinc oxide varistor is a compound semiconductor device in which the current responds very quickly to the voltage applied to it. So what about the response time r≤25ns of the SPD made of varistor mentioned in the previous technical data? This is the response time defined in the technical standard IEEE C62.33-1982[2]. It is a physical quantity used to characterize the "overshoot" characteristic, which is a completely different concept from the response time in the usual sense. To explain this, IEEE C62.3 (6.3) Voltage overshoot (UOS). When the surge current wavefront is very steep and the value is very large, the result of measuring the limiting voltage of the leaded varistor is that it is greater than the limiting voltage with the 8/20 standard wave. This voltage increment UOS is called "overshoot". Although the response time of the varistor material itself to the steep surge is different, the difference is not significant. The main reason for overshoot is that a magnetic field is established around the current-carrying lead of the device. This magnetic field induces a voltage in the loop between the device lead and the protected circuit, or in the loop between the lead and the measurement circuit simulating the protected circuit. In typical applications, a certain lead length is unavoidable. This additional voltage will be applied to the protected circuit after the varistor. Therefore, when measuring the limiting voltage under conditions where the impulse wave front is very steep and the value is very large, it is necessary to recognize the dependence of voltage overshoot on lead length and loop coupling, and not to regard overshoot as an inherent characteristic of the device. In recent years, the International Electrotechnical Commission's technical standards for SPDs, IEC61643-1 and IEC6163-21, have not introduced the parameter of response time. The IEEE technical standard C62.62-2000[] further clarifies that the technical requirement of wavefront response is unnecessary for the typical application of SPDs and may cause misleading technical requirements. Therefore, unless otherwise specified, this technical requirement is not specified, nor is it tested, measured, calculated or otherwise certified. This is because: (1) For the purpose of impulse protection, the limiting voltage measured under specified conditions is a very important characteristic. (2) The response characteristics of an SPD to a wavefront are not only related to the internal reactance of the SPD and the conduction mechanism of the nonlinear element that limits the impulse voltage, but also to the rise rate of the intruding impulse wave and the impedance of the impulse source. The length of the connecting wire and the wiring method also have an important influence. The author believes that the following three technical indicators are important for SPDs used for power supply protection: ① limiting voltage (protection level); ② current carrying capacity (impulse current stability); ③ continuous operating voltage life. III. Operation sequence of multi-stage SPDs When a single-stage SPD cannot suppress the intruding impulse overvoltage to below the specified protection level, an SPD containing two, three or more nonlinear suppression elements should be used. The nonlinear elements Rv2 and Rv2 are both varistors. In practice, RV1 can also be a gas discharge tube, and Rv2 can also be a Zener diode or a surge suppression diode (TVS tube). The isolation element Zs between the two poles can be an inductor Ls or a resistor Rs. If the conduction voltages of RV1 and RV2 are Un1 and Un2 respectively, the selected element is always Un2> Un1. Some believe that when an intrusion shock wave is applied to the XE terminal, the first stage RV1 always conducts first, followed by the second stage. In reality, either the first or second stage can conduct first, depending on the following factors: (1) the waveform of the intrusion shock wave, primarily the velocity of sound (di/dt) before the current wavefront; (2) the relative magnitudes of the conduction voltages Un1 and Un2 of the nonlinear elements Rv1 and RV2; and (3) the nature of the isolation impedance Zs—whether it is resistive or inductive—and their magnitudes. When Zs is a resistor Rs, the second stage usually conducts first. After the second stage conducts, the first stage only conducts when the surge current I rises to iRs + Un2 ≥ Un1. After the first stage conducts, because the equivalent impedance of the first stage under high current is much smaller than the sum of Rs and the equivalent impedance of the second stage, most of the surge current is discharged through the first stage, while the current discharged through the second stage is much smaller. If the first stage is a gas discharge tube, its residual voltage after conduction is usually lower than the conduction voltage Un2 of the second stage, so the second stage is cut off, and the remaining inrush current is discharged through the first stage gas discharge tube. If Zs is an inductor Ls, and the initial rise rate of the inrush current is quite fast, the condition Ls(di/dt) + Un2 > Un1 is satisfied, then the first stage conducts first. If the limiting voltage when the first stage conducts is Uc1(1), then as the rise rate of the inrush current (di/dt) decreases, the second stage will only conduct when the condition UC1(1) ≥ Ls(di/dt) + Un2 is satisfied. After the second stage conducts, the voltage at the output terminal Y is suppressed to a lower level. IV. Equivalent Transformation of Inrush Currents with Different Waveforms Inrush current tests on SPDs, different waveforms such as 8/20, 10/350, 10/1000, or 2ms may be encountered. From the perspective of equivalent destructive effect on the SPD, how to convert the peak values ​​of inrush currents with different waveforms? Some advocate conversion based on the principle of equal charge. According to this principle, by integrating the current waves of the two different waveforms over time to obtain the total charge, and setting the two charge values ​​equal, the proportional relationship between the peak values ​​of the two waveforms can be obtained. This transformation method has nothing to do with the components that discharge the inrush current and is obviously impractical. Others advocate conversion based on the principle of equal energy. According to this principle, not only do we need to know the two current waveforms, but also the waveform of the voltage limiting voltage across the voltage suppression component when these two current waves flow into it. Then, multiply the current value and voltage value at each moment to obtain the power wave, and integrate the power wave over time to obtain the energy. Setting the two energy values ​​equal, the proportional relationship between the two peak values ​​of the current can be obtained. This transformation method takes into account specific nonlinear components, but does not consider the thermal effect of the impact current and the electrodynamic effect when the current value is large. In fact, for zinc oxide varistor, the energy of the 8/20 impact current it can withstand is greater than the energy of the 2ms impact current. The figure shows the change of impact current energy that an early varistor sample with a thickness of 1.3mm can withstand with the electrode area. It can be seen that the principle of equal energy does not apply to varistor at least. The following results were obtained from the study of the failure mechanism of zinc oxide varistor under high current [4]: ​​Under the action of high current, there are two failure modes of varistor. When the time width of the large impact current is not greater than 50μs (e.g., 4/10 and 8/20 waves), the resistor body cracks; when the current value is small and the time width is greater than 100μs (e.g., 10/350, 10/1000 and 2ms waves), the resistor body perforates. The two different failure modes can be explained as follows: When a large current flows for a very short time, the heat generated within the resistor cannot be conducted to the surrounding area quickly enough, resulting in an adiabatic process. Combined with the non-uniformity of the resistor, this leads to uneven current distribution, causing significant temperature differences between different parts of the resistor and creating substantial thermal stress that causes cracking. When the impact current lasts for a longer period, the non-uniformity of the resistor causes current concentration, leading to melting of the resistor material and perforation. The relationship between the current density J (A·cm⁻²) and the duration r (μs) of the impact current wave in a logarithmic coordinate system is roughly a straight line with a negative slope, and can be expressed by the following equation: logJ = C - Klogr. In this equation, C and K are two constants related to the specific device and can be calculated from experimental data. Therefore, the peak values ​​of different waveform impact currents that this product can withstand can be calculated. In summary, for SPDs using varistors as nonlinear suppression elements, to achieve equivalent transformation between different waveform inrush currents, the selected varistor should be tested with two different waveforms (e.g., 8/20 and 10/350) of inrush current. The two current peaks that cause sample failure should be obtained, substituted into the above formula, and the specific values ​​of constants C and K should be calculated. The test does not necessarily need to proceed until the sample cracks or perforates; a varistor voltage change of -10% can be used as a failure criterion. It should be noted that even varistors from different manufacturers and batches, despite having the same dimensions and specifications, may have significantly different levels of inrush current (energy) they can withstand. Therefore, each batch of supplies must be sampled and inspected.