It is impossible to prevent voltage surges from either entering your facility or from occurring inside of your facility. When protecting a facility against transients, the best approach is a networked or cascaded approach. As shown in the graphic below, The Institute of Electrical and Electronics Engineers (IEEE) has developed three categories that every facility can be divided into, location Category A, B and C. See IEEE Standard C62.41.1 and C62.41.2 for further reference.


Category A: outlets/receptacles and long branch circuits (indoor) (least severe)
• All outlets at more that 30 ft from Category B
• All outlets at more than 60 ft from Category C

Category B: feeders, short branch circuits and service panels (indoor)
• Distribution panel devices
• Bus and feeder distribution
• Heavy appliance outlets with “short” connections to service entrance
• Lighting systems in large buildings

Category C: outside overhead lines and service entrance (outdoor)
• Service drops from pole to building
• Runs between meter and panel
• Overhead lines to detached building
• Underground lines to well pump

Location Category C devices can be used in place of location Category A or B devices, but generally this is not necessary.

While generally not used, the second variable for classifying the environment of a power disturbance is exposure. Exposure is most often considered when selecting devices for location Category C environments. The IEEE has defined three exposure levels that characterize the rate of surge occurrence versus voltage level at an unprotected site. The three exposure categories include:

Low exposure: applications known for low lightning activity, little load switching
Medium exposure: systems and geographical areas known for medium to high lightning activity or with significant switching transients or both
High exposure: those rare installations that have greater surge exposure than those defined as low or medium

In addition to the location categories mentioned above it is important to also protect facility telecommunications and signaling circuits such as telephone lines, data networks, measurement and control devices and radiofrequency signals.