There is a misconception that optical isolators and surge protectors are the same thing. That is no so. Whereas surge protectors re-direct excess energy to ground, optical isolators block unwanted currents originating on one side from reaching the other side. Optical isolators are designed to eliminate ground loops. A ground loop is a current across the cable created by a difference in potential between two grounded points. This happens if you have a long cable run of several hundred feet, or cables run between two buildings. When two devices are connected and their ground potentials are different, current flows from high to low by traveling through the data cable even the ground wire. If the voltage difference is large enough one of the RS-232 ports can be damaged. Even a small potential difference can cause trouble even though it does not cause circuit failure. Small ground loop voltages cause transmission errors with data signals riding on top the ground loop current. Optical isolators block this damaging or interfering current.

So, if optical isolators provide a barrier against ground loops, won't it provide a barrier against surges as well? This belief fails to account for the fundamental difference between ground loops and surge transients. Ground loops tend to be of long duration and relatively low voltage. Surges, on the other hand, tend to be of short duration and very high voltage. Consequently, the amount of current instantly presented by a surge must be directed safely to ground. A high voltage exceeding its rating will destroy an opto-isolator. It is true that transients less than about 2500 volts will not get past the barrier and components on the other side of the opto-isolator will be spared, however, components on the side receiving the "hit" can be damaged. In any case the opto-isolator will no longer pass data after the "hit."

Optical isolators will also block power surges caused by a catastrophic failure in the equipment on one side of the cable from getting to the other side. It will not protect the communication circuits on the side where the failure occurred. A surge protector is need to do that; that is direct the catastrophic currents to ground.

For long cable runs, the best protection is to use a pair of surge protectors at the ends of the cable with one optical isolator in between.