Electric Power Generation, Transmission, and Distribution

10.4 Insulator Failure Mechanism

10.4.1 Porcelain Insulators

Cap-and-pin porcelain insulators are occasionally destroyed by direct lightning strikes, which generate a
very steep wave front. Steep-front waves break down the porcelain in the cap, cracking the porcelain. The
penetration of moisture results in leakage currents and short circuits of the unit.
Mechanical failures also crack the insulator and produce short circuits. The most common cause is
water absorption by the Portland cement used to attach the cap to the porcelain. Water absorption
expands the cement, which in turn cracks the porcelain. This reduces the mechanical strength, which
may cause separation and line dropping.
Short circuits of the units in an insulator string reduce the electrical strength of the string, which may
cause flashover in polluted conditions.
Glass insulators use alumina cement, which reduces water penetration and the head-cracking prob-
lem. A great impact, such as a bullet, can shatter the shell, but will not reduce the mechanical strength
of the unit.
The major problem with the porcelain insulators is pollution, which may reduce the flashover voltage
under the rated voltages. Fortunately, most areas of the U.S. are lightly polluted. However, some areas
with heavy pollution experience flashover regularly.

10.4.2 Insulator Pollution

Insulation pollution is a major cause of flashovers and of long-term service interruptions. Lightning-
caused flashovers produce short circuits. The short circuit current is interrupted by the circuit breaker
and the line is reclosed successfully. The line cannot be successfully reclosed after pollution-caused
flashover because the contamination reduces the insulation’s strength for a long time. Actually, the
insulator must dry before the line can be reclosed.

Injection molded

EPDM Rubber

covering and

weathersheds

Fiberglass

reinforced resin

rod

End fitting joined

to rod by

compression

process

Malleable iron end

fitting; outer surfaces

galvanized for

corrosion protection

FIGURE 10.11 Post-type composite insulator. (FromToughened Glass Insulators. Sediver, Inc., Nanterre Cedex,
France. With permission.)