6.8.1Method of operation#
Overvoltage protection devices fall into two categories depending on their method of operation:
6.8.1.1Voltage limiting#
These have high impedance when no surge is present but reduce impedance with increased voltage and current, clamping the voltage to a defined upper limit. They are sometimes referred to as clamping devices and include varistors, and avalanche or suppressor diodes.
6.8.1.2Voltage switching#
These have high impedance when no surge is present but suddenly change to low impedance when surge arrives. They are sometimes referred to as crowbar devices and include air gaps, gas discharge tubes, triacs and silicon controlled rectifiers.
