Lightning is arguably the most spectacular electrical phenomenon in nature—a single bolt flashing across the sky carries instantaneous voltages of millions of volts and currents of tens of thousands of amperes. But you don’t need to be directly struck to suffer damage. In fact, the vast majority of lightning‑related equipment failures are caused by “induced surges”—when lightning strikes a transmission line kilometers away, the electromagnetic pulse propagates along the conductors, enters your distribution cabinet, and creates a transient overvoltage spike within milliseconds. That spike might only be a few thousand volts, but for electronic devices designed for 220V or 380V operation, it’s more than enough to puncture power supplies, burn out circuit boards, and destroy variable frequency drives. The surge protective device (SPD) is specifically designed to handle these transient overvoltages—it remains in a high‑impedance state during normal operation, consuming virtually no power; the moment it detects an overvoltage spike on the line, it switches into a low‑impedance state within nanoseconds, diverting the massive surge current to ground and clamping the voltage to a safe level that connected equipment can withstand. The entire process happens far faster than you can perceive, yet every day it’s taking the bullet for all the sensitive electronic components inside your equipment. The core component of most SPDs is the metal oxide varistor (MOV). The MOV is a nonlinear resistor whose resistance changes dramatically with applied voltage: under normal operating voltage, the MOV exhibits high impedance (mega‑ohm range), behaving like an open circuit; when the voltage exceeds its “varistor voltage” threshold, the MOV instantly switches to low impedance (ohm range), shunting the surge current to ground; after the surge passes and voltage returns to normal, the MOV reverts to high impedance. This “conduction‑recovery” cycle takes just a few nanoseconds and can theoretically repeat hundreds of thousands of times. However, each conduction causes microscopic damage to the MOV grain boundaries, and over cumulative stress, the MOV’s leakage current gradually increases until thermal aging leads to failure—that’s why SPDs have a finite service life. SPDs are classified into three types based on installation location and protection level: Type 1 (Class I) is installed at the main incoming panel, directly facing direct lightning strikes or step voltages, tested with a 10/350μs waveform representing the energy characteristics of a direct lightning strike—extremely high energy with relatively long duration—and its surge current capacity typically ranges from 25kA to 100kA; Type 2 (Class II) is installed in sub‑distribution boards or floor distribution boxes, protecting against induced lightning and switching overvoltages, tested with an 8/20μs waveform, with capacities typically from 20kA to 80kA—this is the most common and widely used type in industrial applications; and Type 3 (Class III) is installed directly in front of terminal equipment (such as UPS inputs or precision instrument power sockets) for additional fine protection, with smaller capacities (usually 5kA‑20kA). A complete protection system should implement all three levels in combination—dissipating energy step by step, progressively clamping the voltage from thousands of volts down to hundreds or even tens of volts. Installing only Type 1 or only Type 3 is scientifically inadequate. When selecting an SPD, many users focus solely on the “kA” rating, assuming higher is always better. But the real determinant of protection effectiveness is the residual voltage (Up)—the voltage that remains across the SPD terminals after it conducts. The lower the Up, the better the protection, but typically the higher the cost. For example, if your equipment’s power supply has a maximum withstand voltage of 1.5kV, the SPD you select must have a Up lower than 1.5kV—otherwise, even though the SPD conducts during a surge, the residual voltage still exceeds the equipment’s tolerance, and the equipment will be damaged anyway. That’s why the Up must be matched to the equipment’s impulse withstand voltage (Uw) during selection. IEC 60364 provides recommended Up values for different equipment categories: general industrial equipment ≤2.5kV, precision electronic equipment ≤1.5kV, and medical devices and sensitive instruments ≤1.0kV. In installation, there is an extremely important principle regarding SPD wiring: the connecting conductors must be as short as possible, especially the grounding conductor. Why? Because conductors themselves have inductance, and surge currents flowing through them create inductive voltage drops. If the grounding conductor is too long, this voltage drop adds to the SPD’s residual voltage, resulting in a voltage actually applied to the equipment that is much higher than the SPD’s claimed Up—this is known in the industry as the “Kevin wiring” issue. Standards require that the phase conductor to the SPD should not exceed 0.5 meters, and the grounding conductor should not exceed 0.5 meters. If the physical installation makes this impossible, you should use larger cross‑section conductors (copper wire at least 6mm²) to reduce inductance. A more scientific approach is to use “Kevin (V‑shape)” wiring, where the SPD conductors are tapped off from the main circuit with the shortest possible leads. Under normal conditions, the SPD consumes almo no power, but it does have failure modes—the most common being the MOV degrading after prolonged overvoltage stress or accumulated lightning strikes, causing leakage current to increase continuously until it overheats, smokes, or even catches fire. Although most SPDs come with internal thermal disconnectors, these thermal disconnectors are not equivalent to short‑circuit protection—if the SPD fails internally as a short circuit, the thermal disconnector may not operate fast enough, causing a phase‑to‑ground fault. Therefore, a dedicated backup fuse or circuit breaker must be installed upstream of the SPD to quickly切断 the circuit in case of SPD short‑circuit failure. During selection, note that the backup protector’s rated current shouldn’t be too high (typically recommended between 16A and 63A)—otherwise it won’t trip even when the SPD shorts, still posing a fire hazard. An SPD is not a “install once, forget forever” device—it has a finite life. Most SPDs feature a status indicator window on the front or side: green/normal means it’s still functional; red/failed means the internal MOV is damaged and the protection function is lost  requiring immediate replacement. Some advanced models include remote signaling contacts that transmit failure signals to a supervisory system, convenient for unattended installations. Additionally, if your area experiences frequent thunderstorms, even if the indicator hasn’t turned red, preventive replacement every two years is recommended—because cumulative MOV damage isn’t always visually apparent. Before thunderstorm season, using a surge arrester tester to check the varistor voltage and leakage current is a more scientific approach than visual inspection alone; if the varistor voltage deviates by more than ±10% from the rated value, or if leakage current exceeds 20μA, replacement is due. There is also a crucial piece of common sense often overlooked: SPDs only suppress transient overvoltages (microsecond to millisecond duration); they cannot eliminate sustained overvoltages or undervoltages. If the system experiences continuous issues like a broken neutral line or severe three‑phase voltage imbalance, the SPD will not operate—that’s the job of over/under voltage protectors or voltage relays. Additionally, SPDs are powerless against direct lightning strikes themselves—if lightning directly strikes the distribution lines inside a building, no SPD can withstand it. The true first line of defense is always the external lightning protection system (air terminals, down conductors, and grounding systems). The SPD stays silent and inconspicuous, lurking in the corner of your distribution cabinet, but with every lightning strike and every surge caused by starting high‑power equipment, it takes the hit for you. Don’t wait until equipment fails en masse before checking your SPDs—a quick glance at the status window and a periodic preventive test cost far less than replacing a single variable frequency drive or repairing a circuit board.