Lightning strikes are essentially huge spark discharge that occur when charge buildup to a certain point between a thunderstorm cloud and the ground or between them, disrupting the insulation of the air. lightning strikes are triggered mainly by:
1.Direct lightning strike
In the worst cases, lightning strikes directly at buildings, equipment or people.
Trigger conditions: Objects on the ground (e.g., a tall building, tree, transmission tower, trees, towers, isolated people) form the strongest electric field along with thunderclouds, making it the shortest path for charge discharge.
2.Indirect Lightning Effects (More Common, Wider Range)
Even if lightning does not strike your device directly, it can cause a destructive surge (transient) by:
· Induced Lightning (divided into Electrostatic Induction and Electromagnetic Induction):
• Electrostatic induction: When a charged thundercloud passes overhead, it induces a large amount of opposite charges on overhead wires below. When the cloud discharges to another target, these binding charges are immediately released, creating high-voltage surges that travel along the line.
Electromagnetic induction: Strong and transient currents in lightning channel can cause very high voltage in surrounding metal loops (e.g., power line loops, signal lines).
· Conduction (Ground Potential Rise):
• When lightning strikes a a building's lightning protection system or nearby ground, a huge current of lightning passes through the grounding resistance, causing the electrical potential at the point of contact to rise to hundreds of thousands or even millions of volts in an instant. This high potential can be "flashback" to the internal device via the device's ground wire, causing a destructive potential difference across the device and destroying it.
· Lightning Surge Intrusion:
• Lightning strikes distant wires, telephone lines, cable lines, etc. Huge lightning currents travel along these metal conductors in the form of waves, invading and damaging connected devices.
Part 2: How to use SPD for protection
The purpose of Surge Protective Device is not to prevent lightning strikes, but to provide a a safe, low-impedance path lightning current and surge energy during lightning strike strikes, thus limiting the voltage on the device to a safe level.
1. Core Working Principle of the SPD
Think of it as a "smart switch" or "voltage sensitive valve":
During normal operation: SPD high impedance and is essentially an open circuit with little effect on the circuit.
When a Surge/Overvoltage Occurs: The occur: SPD becomes a low impedance state (very quickly) in nanoseconds, rapidly conducting and directing large surges of current to the ground.
After the surge subsided: SPD automatically reset to its high impedance state and the circuit returns to normal.
2.Correct SPD Protection System: Multi-Stage Protection
In order to provide comprehensive protection, the concept of hierarchical protection is often used, such as multiple lines of defence:
Category 1 / I (Formerly B Class):
Installation location: main distribution board (service entrance).
Function: Releases most of lightning strikes high energy from direct or induced lightning strikes (usually 10/350 μM waveform). SPD with high discharge capacity is commonly used based on spark gaps.
Objective: To protect the electrical system of the whole building against the largest thunderstorm.
Category 2 / II (Formerly C Class):
Location: Distribution board (e.g., floor panels) or key equipment distribution board (e.g., server rooms, air conditioning, elevator).
Function: Further limits residual voltage and absorbs residual energy through Type 1 SPD. Solid state drives (SSDs) based on Metal Oxide Varistor (MOV) are commonly used.
Objective: To protect distribution circuits and equipment in specific areas.
Category 3 / III (formerly D):
Location: The front end of a device, such as a plug strips for a sensitive instrument or a surge protection module inside the device.
Function: Provides fine protection by fixing the voltage to a level that the device can safely withstand (usually less than 1.5 kV).
Objective: To provide a final line of defense for the most sensitive and expensive electronic equipment (e.g., servers, medical equipment, industrial PLCs).
Signal SPDs: lines such as network cables, telephone lines and surveillance video cables also require specialized signal processing systems. They operate in the same way, but they are designed for the voltage levels and interface type of signal line.
3.Key points for SPD Installation
• Good grounding is a prerequisite: SPD SPD efficiently transfer energy without a a low-impedance grounding system. Grounding is the foundation of lightning protection.
• Short and thick connection wires: SPD connection and ground wires must be as short and thick as possible to minimize lead inductance. Excessively long leads can lead to a noticeable induction voltage, greatly reducing protection effectiveness.
Coordination: different stages of SSD require energy coordination to ensure gradual discharge as designed to prevent overloading of any single SPD.