242 Electrical Power Systems Technology
volts and higher. Oilless circuit breakers are ordinarily used for indoor op-
eration.
High-voltage Disconnect Switches
High-voltage disconnect switches are used to disconnect electrical
equipment from the power lines that supply the equipment. Ordinarily,
disconnect switches are not operated when current is flowing through
them. A high-voltage arcing problem would occur if disconnect switches
were opened while current was flowing through them. They are opened
mainly to isolate equipment from power lines for safety purposes. Most
disconnect switches are the “air-break” type, which is similar in construc-
tion to knife switches. These switches are available for indoor or outdoor
use in both manual and motor-operated designs.
Lightning Arresters
The purpose of using lightning arresters on power lines is to cause
the conduction to ground of excessively high voltages that are caused by
lightning strikes or other system problems. Without lightning arresters,
power lines and associated equipment could become inoperable when
struck by lightning. Arresters are designed to operate rapidly and repeat-
edly, if necessary. Their response time must be more rapid than that of the
other protective equipment used on power lines.
Lightning arresters must have a rigid connection to ground on one
side. The other side of the arrester is connected to a power line. Sometimes,
they are connected to transformers or the insides of switchgear. Lightning
is a major cause of power-system failures and equipment damage, so
lightning arresters have a very important function. Lightning arresters are
also used at outdoor substations. The lightning arrester is used to pro-
vide a path to ground for lightning strikes or hits. This path eliminates the
flashover between power lines, which causes short circuits. Valve-type light-
ning arresters are used frequently. They are two-terminal devices in which
one terminal is connected to the power line, and the other is connected to
ground. The path from line to ground is of such high resistance that it is
normally open. However, when lightning, which is a very high voltage,
strikes a power line, it causes conduction from line to ground. Thus, volt-
age surges are conducted to ground before flashover between the lines oc-
curs. After the lightning surge has been conducted to ground, the valve as-
sembly then causes the lightning arrester to become nonconductive once
more.