Insulator are attached to supports and insulate the
conductors from the ground. The insulator plays a vital role in electrical
system. Electrical Insulator is a very high resistive path through which
practically no current can flow. In
transmission and distribution system, the overhead conductors are generally
supported by supporting towers or poles. The towers and poles both are properly
grounded. So there must be insulator between tower or pole body and current
carrying conductors to prevent the flow of current
from conductor to earth through the grounded supporting towers or poles.
The main cause of failure of overhead line insulator,
is flash over, occurs in between line and earth during abnormal over voltage in
the system. During this flash over, the huge heat produced by arcing, causes
puncher in insulator body. Viewing this phenomenon the materials used for
electrical insulator, has to posses some specific properties.