It is impossible to prevent voltage surges from either entering your facility or from occurring inside your facility. When protecting a facility against transients, the best approach is a networked approach. The Institute of Electrical and Electronics Engineers has developed three categories that every facility can be divided into. See IEEE Std. C62.41.1 and C62.41.2
Category A: outlets and long branch circuits
• All outlets at more that 10m (30 ft) from Category B
• All outlets at more than 20m (60 ft) from Category C
Category B: feeders and short branch circuits
• Distribution panel devices
• Bus and feeder distribution
• Heavy appliance outlets with “short” connections to service entrance
• Lighting systems in large buildings
Category C: outside and service entrances
• Service drops from pole to building
• Runs between meter and panel
• Overhead lines to detached building
• Underground lines to well pump
The Category C surges can enter the building at the service entrance. SPD’s must be sized to withstand these types of surges when installed at switchgear or service entrance switchboard. The second variable used to classify the environment of a power disturbance is exposure. 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
The graphics below demonstrate several areas in and around your industrial facility that you should think about protecting, as well as suggesting types of SPDs to use.
Click on the image to view the larger size:
