Gas Discharge Tube – These are voltage-driven parts.
In a circuit the GDT will be in Off State until a specific 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 gas de-ionizes and the GDT will go back into Off state until the next surge. GDTs can absorb very large energy spikes as compared to other circuit protection components.
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 this happens, the fuse is destroyed and will need to be replaced in order for the circuit to work again. Fuses are the cheapest type of circuit protection, and often the easiest to use.
Circuit Breaker - A circuit breaker will protect an electrical circuit from excessive current similar to a fuse, but unlike fuses they don’t destroy themselves when they open a circuit and can be reset.
Varistor - This is a voltage-driven part. Varistors normally have a high resistance when no voltage is passing through them. As voltage levels increase the varistor’s resistance will decrease. When a High voltage comes through the Varistor and its breakdown voltage is exceeded, it will rapidly decrease in resistance and clamp the voltage to a safe level. This will be partially conducted and absorbed to protect the circuit.
After the voltage drops, if the part is not damaged, it will go back to a high resistance and back to normal use.
For more information see: