Surge vs. lightning protection

A surge is a brief rise in voltage that's caused when power is switched on or off to a high-powered device. Surges can produce high-transient voltages, which damage LAN and serial interfaces that typically use less than 12 volts to transmit.

Surges can be protected by using avalanche diodes. These normally have a clamping voltage in the range of 5 to 16 volts and divert the current through the diode to ground when the overvoltage occurs. These diodes can handle instantaneous currents of up to 1000 amps.

Lightning strikes can discharge currents up to 200 kA, but 50% of these are under 28 kA. A lightning protector, therefore, requires additional current-handling capacity. This is achieved using a gas discharge tube (GDT). A GDT can divert thousands of amps but doesn't respond as quickly as avalanche diodes (usually less than 10ns), so a hybrid circuit using GDTs and avalanche diodes is commonly used.