A surge protector works by clamping the transient surge voltage to a safe level and directing the excess energy to ground. They are composed of two parts: a switching device or circuit and a good ground connection. Design and choice of a power surge protection device is a challenge because the nature of the event responsible for the surge is unpredictable. A strike might be close or far. The surge can arrive with a wide variation in energy magnitude and temporal behavior. There are three types of devices typically used as the protective switch: Gas tubes, metal oxide varistors (MOV's) and silicon avalanche diodes (SAD's or tranzorbs.) Each has its pros and cons.

1. Gas Tubes can withstand many kilovolts and hundreds of amps. They have been used to suppress high energy lightning surges resulting from direct strikes on telecommunications lines. However, gas tubes have a relatively slow response time. They can knock down big pulses but their slowness can let enough energy by to destroy the typical solid state circuits found in RS-232 equipment.
2. Metal oxide varistors (MOV's) respond much quicker than the gas tubes (< 5 nanoseconds). However, operational life is a drawback. A MOV's protection deteriorates and eventually fails when subjected to repeated overvoltages.
3. Silicon avalanche diodes have proven to be a very effective means of protecting data equipment against power surges. They are able to withstand thousands of high voltage, high current, and transient surges without failure. While they cannot deal with the surge peaks that gas tubes can, SAD's do provide the fastest response time (< 1 nanosecond.)

Surge protectors installed at each end of each RS-232 cable connected to a good earth ground provides excellent defense against damage due to power surges.

There are many manufacturers that sell surge protectors using one or more of the above components. The type you need depends upon several factors including not the least, what your willing to spend for this protection. General purpose devices using SAD's are very effective against most power surges as long as you have a good earth ground and your cable length is relatively short. If you are in a high thunder storm area where close strikes are common, then there are hybrid devices available that have combinations of SAD's, gas tubes and even MOV's that protect against almost anything short of a direct hit.