Lightning arrestors are used to protect equipment from lightning strikes and high-impulse switching surges. Because of the relatively low Basic Insulation Level (BIL) of a transformer, a lightning arrestor should be installed to protect every transformer bushing connected to a line or a high-voltage bus. A lightning strike or switching surge initiating an arc-over inside a costly transformer is clearly undesirable. On some older installations, rod gaps have been used in place of arrestors. Rod gaps are not the most desirable protective device because of their wide variance in break-over
voltage, but they are certainly cheaper.
Each arrestor, and each individual section of stacked arrestors, should be Meggered at 2500 VDC. In the past, most arrestors were checked at BPA's laboratory facility prior to shipment. The Laboratory's present policy requires testing of arrestors having nameplate ratings of 172 kV and above, all new types not previously used on the BPA system (regardless of their rating), and all new maintenance spares (minimum emergency stock). BPA Laboratories should be contacted if problems or special tests are required for lightning arrestors or Metal Oxide Voltage Limiters (MOVLs).
The nameplate kV rating for each arrestor should be verified to be sure it is the proper size for the application. Mismatching can occur, resulting in equipment failure and outages. When stacked units are installed, the size and order of stacking should be checked and compared with manufacturer's recommendations. Be sure to record nameplate data from the installed arrestors.
Verify that hardware is properly provided or connected. For example, grading rings should be installed for arrestors 115 kV and above. All 500-kV and some 230-kV arrestors must be mounted on proper standoff insulators, and ground conductor connections must be present that assure electrical discharges will pass through the pulse counter (if outfitted with one). Eliminate extra ground path connections that would prevent the counter from operating; arrestors have been installed with counters shorted past. The standoff and ground side of the counter must be connected to the
station ground mat. This is usually done with 4/0 copper cable. Newer MOVLs are not being installed with counters, in part because the MOVL clips electrical surges at speeds that the counters cannot respond to predictably.