What happens inside a surge protector during a lightning strike?

What Happens Inside a Surge Protective Device During a Lightning Strike?

During a lightning strike, surge protective devices (SPDs) play a crucial role in protecting electrical equipment from transient overvoltages (i.e., surges). Below are the main processes and mechanisms that occur inside an SPD during such events:

1. Surge Detection and Response

When a surge caused by a lightning strike enters the power system, the surge protective device quickly detects this abnormal voltage. Typically, SPDs have a threshold voltage set; once the detected voltage exceeds this threshold, the protector activates its protection mechanism.

2. Energy Absorption and Dissipation

SPDs absorb and dissipate surge energy to prevent it from reaching connected electrical equipment. Common absorption and dissipation mechanisms include:

a. Metal Oxide Varistors (MOVs)

• Working Principle: MOVs are nonlinear resistive materials whose resistance changes with applied voltage. Under normal operating voltages, MOVs exhibit high resistance; when the voltage exceeds a certain threshold, their resistance drops sharply, allowing current to pass through.

• Energy Dissipation: MOVs convert excess electrical energy into heat and dissipate it. While MOVs have self-recovery characteristics and can continue to function after multiple small surges, they may fail after large or frequent surges.

b. Gas Discharge Tubes (GDTs)

• Working Principle: GDTs are sealed tubes filled with inert gas. When the voltage across the two ends exceeds a certain value, the gas inside ionizes, creating a conductive path for current.

• Energy Dissipation: GDTs dissipate surge energy through the plasma created by gas ionization and automatically extinguish the plasma once the voltage returns to normal, restoring insulation.

c. Transient Voltage Suppression (TVS) Diodes

• Working Principle: TVS diodes remain in a high-resistance state under normal operating voltages. When the voltage exceeds their breakdown voltage, the diode rapidly switches to a low-resistance state, allowing current to flow.

• Energy Dissipation: TVS diodes dissipate surge energy through the avalanche effect within their internal PN junctions and are suitable for fast-response small surges.

3. Energy Diversion and Grounding

SPDs not only absorb surge energy but also divert some of it to ground lines to further reduce impact on equipment. Specific mechanisms include:

• Diversion Circuits: SPDs are designed with specialized diversion circuits to guide overvoltage to the ground line, preventing it from directly entering load devices.

• Grounding System: A good grounding system is key to ensuring effective SPD operation. The grounding system should provide a low-impedance path to quickly dissipate surge energy into the earth.

4. Post-Surge Recovery

After the surge event, the SPD needs to return to its normal operating state. Different types of protectors have different recovery mechanisms:

• MOVs: If the surge does not cause permanent damage to the MOV, it will automatically return to a high-resistance state once the voltage normalizes.

• GDTs: Once the voltage returns to normal, the plasma inside the GDT automatically extinguishes, restoring the insulating state.

• TVS Diodes: After the voltage normalizes, TVS diodes also automatically return to a high-resistance state.

5. Failure Modes and Protection

Although SPDs are designed to handle surges, they can still fail in extreme cases. To ensure safety, many SPDs include additional features:

• Thermal Disconnect Devices: When an MOV or other component overheats and fails, the thermal disconnect device will break the circuit to prevent fires and other hazards.

• Indicator Lights/Alarms: Some SPDs come equipped with indicator lights or alarms to notify users if the protector is still functioning correctly.

Conclusion

During a lightning strike, surge protective devices protect electrical equipment through the following steps:

• Surge Detection: Identify situations where voltage exceeds normal ranges.

• Absorption and Dissipation of Energy: Utilize components like MOVs, GDTs, and TVS diodes to convert surge energy into heat or other forms of energy.

• Diversion to Ground Lines: Guide overvoltage to ground lines to minimize impact on equipment.

• Return to Normal State: After the surge, the protector returns to its normal operating state.

• Fault Protection: Provide additional safety measures in extreme cases to prevent further damage.