Gas Discharge Tube – These are Voltage Driven.
In a circuit the GDT will be in Off State until a voltage level is reached. At that time the Gas in the GDT will be ionized and will conduct allowing the High voltage level to be grounded. After the high voltage has gone down the GDT will go back to the Off state until the next surge.
2-Electrode GDT - Used is a Line to Line or line to Ground circuit.
3-Electrode GDT - This can be Line to Line or line to ground all in one part.
Fuse - A Fuse will be made up of a metal strip or wire fuse element which is mounted between two electrical terminals.
When current is passed through the metal strip it will heat up. If it heats up to much the metal strip will melt and break causing the circuit to be open.
When the Fuse is open it will need to be replaced in order for the circuit to work.
Circuit Breaker - A circuit breaker will protect an electrical circuit from excessive current similar to a fuse, but can be reset.
Varistor - This is a Voltage driven part. Varistors have a high resistance and as voltage levels increase the resistance will decrease. When a High voltage comes through the Varistor, and the breakdown voltage is exceeded, it will clamp the voltage to a safe level. This will be partially conducted and absorbed to protect the circuit.
After the voltage drops, and the part is not damaged, it will go back to a high resistance and back to normal use.