As the core equipment for internal lightning protection, Surge Protective Devices (SPDs) play a crucial role in safeguarding electronic equipment from damage caused by induced lightning. This article focuses on the working mechanism, key selection parameters, and application guidelines of SPDs, assisting readers in the scientific selection and proper use of SPDs.

1. The Working Mechanism of SPDs

An SPD functions as a "voltage safety valve" in an electrical circuit, with a core working mechanism of "detection-conduction-discharge-recovery". When an overvoltage exceeding the safe threshold occurs in the circuit, the nonlinear components (such as varistors and discharge tubes) inside the SPD rapidly conduct, diverting the excess voltage and current to the ground through the grounding loop. Once the overvoltage dissipates, the components quickly return to a high-resistance state, ensuring no interference with the normal power supply of the circuit.

2. Types of SPDs by Application Scenarios

•Power SPDs: Suitable for 220V/380V and other power circuits. Classified by protection level into Class I (protecting against direct lightning induction, impulse current ≥10kA), Class II (protecting against induced lightning, impulse current ≥5kA), and Class III (providing fine protection for the front end of equipment), which need to be used in a multi-level configuration.
•Signal SPDs: Designed for network (RJ45), monitoring (BNC), communication (RS485), and other signal circuits. It is necessary to match the circuit transmission frequency and interface type to avoid affecting signal transmission quality.
•RF SPDs: Used for radio frequency circuits such as antennas and satellite receiving equipment. The key requirement is low insertion loss to ensure communication quality.

3. Key Selection Parameters of SPDs

The core of SPD selection is to match the protection scenario and equipment requirements, as key parameters directly determine the protection effect and service life:
•Maximum Continuous Operating Voltage (Uc): The maximum AC/DC voltage that an SPD can withstand for an extended period. Selection criterion: Uc must be higher than the maximum operating voltage of the circuit (e.g., Uc=275V for 220V civil circuits, Uc=420V for 380V industrial circuits).
•Nominal Discharge Current (In): The peak value of the 8/20μs lightning current waveform that an SPD can withstand multiple times (usually 20 times). Selection criterion: Match the protection zone and lightning activity intensity (10-20kA for Class I SPDs, 5-10kA for Class II SPDs, 2.5-5kA for Class III SPDs).
•Maximum Discharge Current (Imax): The maximum single peak value of the 8/20μs lightning current that an SPD can withstand under specified conditions, reflecting its extreme current discharge capacity. Selection criterion: Imax should be ≥1.5-2 times In and match the regional lightning level (Imax ≥40kA for high lightning activity areas).
•Protective Voltage (Ucpv): The maximum voltage across the SPD when the nominal discharge current In passes through it, directly determining the protection effect on downstream equipment. Selection criterion: Ucpv must be lower than the impulse voltage withstand limit of the protected equipment (≤1.2kV for precision electronic equipment).
•Response Time (tA): The time from when the SPD detects an overvoltage to when it is fully conducting. Selection criterion: ≤25ns for power SPDs, ≤1ns for signal SPDs (faster response is required for high-frequency signals).
•Protection Level (IP): Indicates the SPD enclosure's ability to protect against solid foreign objects and liquid ingress. Selection criterion: IP65 or higher for outdoor installation, IP20 for indoor dry environments.
•Interface and Frequency (for signal/RF SPDs): Match the circuit interface type and operating frequency range to ensure no impact on signal transmission (insertion loss ≤0.5dB for RF SPDs).

4. Principles of Multi-Level Protection Application for SPDs

The installation of SPDs must follow the "multi-level protection" principle based on the division of LPZs:
•Install Class I power SPDs at the junction of LPZ 0 and LPZ 1 (e.g., the main distribution box of a building) for primary discharge;
•Install Class II power SPDs at the junction of LPZ 1 and LPZ 2 (e.g., floor distribution boxes) to further attenuate overvoltages;
•Install Class III power SPDs at the front end of equipment (e.g., server power ports) for fine protection;
•Install corresponding signal SPDs at the interfaces where signal lines enter and exit the protection zone to ensure full-link protection without blind spots.

5. Common Misconceptions About SPDs

•Misconception 1: "Installing a lightning rod is sufficient" — Lightning rods only protect against direct lightning strikes; SPDs and equipotential bonding are required for induced lightning protection.
•Misconception 2: "The more SPDs installed, the better" — Scientific configuration based on protection levels is necessary; excessive installation can cause mutual interference and malfunctions.
•Misconception 3: "Indoor equipment does not require lightning protection" — Induced lightning mainly endangers indoor electronic equipment; internal lightning protection measures are essential.

6. Development Trends of SPDs

Current lightning protection systems are evolving towards intelligence and integration. Intelligent SPDs (equipped with status monitoring, fault alarm, and remote communication functions) have become an industry trend. By real-time monitoring the operating status of SPDs, the operation and maintenance efficiency of lightning protection systems can be significantly improved, which is one of the core development directions of future lightning protection technology.