The boundaries between these levels of protection may not be sharply defined and the devices chosen will vary accordingly. In practice, the following components have proven themselves in these applications:

Gas filled surge arrester

The gas filled surge arrester is comprised of two electrodes in a ceramic or glass tube filled with a pressurized inert gas. A spark gap is simply two electrodes in air, spaced a predetermined distance apart. Once the ignition voltage is reached, the resistance drops due to ionization and current begins to flow. The resistance of the device drops from high to low as it conducts. The voltage across the device after the arc is struck is typically 10 V- 30 V. Therefore, the current will continue to flow until the voltage drops below the level. Since this cannot be guaranteed in typical power situations, the device must be preceded by a fuse to ensure disconnection from the supply.

The advantage of a gas filled arrester is its high current carrying capacity for its size. However, the key disadvantages are the slow speed of response and in AC applications, the crowbar action across the supply.

Varistor

A varistor is a voltage dependent resistor. A common type of varistor is referred to as an MOV (Metal Oxide Varistor) due to its method of construction. Varistors have a very high resistance until their threshold voltage is exceeded. Then they drop to a low resistance and are capable of carrying high currents for a short period of time.
They have a fast response time, and have a current carrying capacity dependant on the surface area of metal oxide material.   MOVs can be sized to handle currents in excess of 100 kA. Undersized varistors can age with continued surge conduction, resulting in lower impedance and higher leakage.   When subjected to continuous (AC or DC) voltages in excess to the device rating, MOVs can overheat and even explode in certain circumstances. For this reason a quality surge protection device will incorporate measures to disconnect faulty components before safety is compromised.

Suppressor diode

These devices have electrical characteristics similar to Zener diodes, but are rated for surge currents.
Once the rated breakdown voltage is exceeded (in the non-conductive direction) the diode becomes conducting. The suppressor diode differs from a Zener in its higher current carrying capability and faster response time (in the picosecond range).