Manual and automatic switches are electrical devices that are used
to open and close circuits. They are designed to carry their
rated current continuously without overheating and must have
clearances and insulation for the normal voltage of the circuit.
Fuses and circuit breakers provide a simple, comparatively
inexpensive method of automatic overcurrent protection, as well as a
means of controlling the location of breakdowns.
DISCONNECT SWITCHES
Disconnect switches are generally used
in a primary circuit where opening the circuit is necessary under
voltage with little or no load current. They must interrupt only the
charging or exiting current of lines or apparatus connected. These
switches are ordinarily used to disconnect branch lines, off-circuit
breakers, and transformers where the load current may otherwise be
broken.
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| Hook Type Disconnect |
TYPES
Disconnect switches are available in
various types, ratings, and classes. If the mounting height is not
too great, the switch can be operated from the ground with a
longhandled switch stick.
Sectionalizing Swithches are generally
installed on the crossarms carrying the primary circuit and are
operated by a switch stick that can be fastened to the lineman's
belt.
BYPASS SWITCHES
Bypass switches may be used at booster
and regulator installations to provide a quick, reliable means of
taking such apparatus in and out of service without de-energizing the
feeder circuits and to prevent winding burnouts from open-circuit
windings.
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| Bypass Switch |
GANG-OPERATED DISCONNECTS
Gang-operated disconnects are used
where more than one phase of a circuit must be opened simultaneously.
The most common gang-operated disconnect switches are the air-brake
type manufactured in 200-, 300-, and 400-ampere ratings in all
voltage classes from 5,000 volts up. They are used at substations,
switching structures, and on the lines for energizing and
de-energizing transformer banks and other apparatus.
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| Gang Operated Disconnect Switch |
They are also used for sectionalizing.
Although they can be motor-operated, they are more commonly provided
with a switch handle for hand operation. This type of switch is ideal
because it lends itself to operation from the ground, often
permitting service to be restored to sections of the network without
pole climbing.
MAINTENANCE
Contacts of disconnects must stay
smooth and covered with a thin film of nonoxide grease. The bearing
must be well lubricated, and the blades should move freely yet be
rigid enough for proper alignment with contacts. Locate broken or
defective insulators during inspections and replace them immediately.
Ensure that all bolts and nuts are tight.
OIL CIRCUIT BREAKERS
Oil circuit breakers open a circuit
automatically under load. They are generally designed and connected
for one or more automatic reclosings to restore service quickly when
a fault has cleared itself. Their use is generally confined to
substations or switching stations where either high interrupting
capacity or high-grade service is required.
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| Oil Circuit Breaker |
Pole-mounted oil circuit breakers are
also called reclosers, sectionalizing oil circuit breakers, or
interrupters. They are adaptable for use on the low side of step-down
substations, on branch circuits that are connected to important
feeders, and for protecting important loads and isolating line
trouble. Reclosers are available with ratings up to 50 amperes and
15,000 volts; the 50-ampere breaker has an
interrupting rate of about 1,200
amperes.
The recloser is connected in the line
and is normally closed. The trip coil is in series with the contacts
and derives energy from fault current, which may lift the armature
and the movable contact of the interrupting element by magnetic
attraction. When a fault occurs, the circuit promptly opens and then
automatically recloses in about three seconds. If the fault is not
cleared on the first interruption, the recloser opens the circuit a
second and possibly a third time.
If the fault is cleared after the
second or third interruption, the recloser mechanism automatically
resets. If the fault persists after the third interruption, the
recloser opens a fourth time and locks open. It must
then be reset manually.
The recloser contains one pair of
contacts--the lower (stationary) contact is in the bottom of the
unit, and the upper (movable) contact is connected to one end of the
operating or trip coil. The contacts are normally held in the closed
position by positive pressure, but when a short circuit occurs, the
movable contact rises rapidly, drawing an arc in the oil.
The heat of the arc forms a gas bubble,
which sets up pressure in the oil-blast chamber. This pressure in the
chamber forces a blast of cool air between the contacts, preventing
the arc from reestablishing itself after an early current zero. The
butt-type contacts automatically compensate for burning caused by
repeated operation. They can be replaced if renewal eventually
becomes necessary. The use of bypass switches is optional. Some types
of breakers include internal lightning protective devices so external
lightning arresters are not required.
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