Most station and intermediate class arrestors are of unit design, where gap
and valve elements are enclosed in a single porcelain housing. These employ
a series of gap elements with shunting resistors to shield the gaps and to provide
uniform voltage distribution across the individual gaps and unit. The
arrestor has electrical characteristics, such as AC grading current and dielectric
loss, which is measurable. Failure of the arrestors can be attributed to several
causes, such as damaged, defective or contaminated units, lightning
strokes, long-duration surges due to switching and misapplication. Arrestors
have low capacitance and experience has shown that the measurement of
dielectric loss is effective in detecting defective, contaminated, and deteriorated
arrestors. The dielectric loss indicates the mechanical condition and
insulating qualities of the arrestor. Arrestors may consists of single units or
assemblies (stacked units) depending on application for a given equipment
rating. Arrestors are tested as single units to assure that the arrestor’s mechanical
and insulation integrity is intact so that it can perform its intended function.
Before conducting any tests on arrestors, the line connected to the arrestor
should be fi rst de-energized and grounded, then disconnected from the arrestors.
Arrestor assemblies consisting of single units are fi rst disconnected from
the bus and then tested by the GST method as shown in Figure 3.16a. Assemblies
consisting of two or more units are tested using a combination of GST method
and UST method as shown in Figure 3.16b. In case of assemblies of three or
more units per phase, it is only necessary to de-energize the line and ground
the top of the arrestor stack. In this case the bus need not be disconnected
from the arrestor stack