There are three important steps in surge arrestor selection.
Deciding the Surge Arrestor Voltage.
This is decided by the nature of the earthing i.e. whether we use grounded or ungrounded systems.
Selecting the class of arrestor.
Surge Arrestors are classified into three types Station Class, Intermediate Class and Distribution Class. The class determines the degree of protection, capacity and, ultimately, the price. The Station Class Surge Arrestor is the most expensive.
Deciding on the location of the surge arrestor.
The location of the arrestor is crucial. For an equipment to be properly protected, the arrestor should be placed as close to the equipment as possible.