Figure 4-36.Three-phase oil switch recloser.
In high-voltage circuits or in circuits carrying
large currents, a current transformer is connected into
the line and the secondary leads from this transformer
supply the current to the trip coil of the oil switch (fig.
4-37). Since there is a fixed ratio between primary and
secondary currents of the current transformer, the coil
can be adjusted to trip at any predetermined value of
current in the line.
The use of the current transformer on such circuits
serves the dual purpose of providing a small current for
operating the tripping coil and insulating the coil from
the high voltage of the line.
Figure 4-37.Current transformer used to supply current to
the trip coil of an oil switch.
An oil circuit recloser is a type of oil switch
designed to automatically interrupt and reclose an
alternating-current circuit. It can be made to repeat this
cycle several times. Reclosers are designed for use on
single-phase circuits or on three-phase circuits.
A recloser opens the circuit in case of fault as
would a fuse or circuit breaker. The recloser, however,
recloses the circuit after a predetermined time (for
hydraulically controlled reclosers about 2 seconds). If
the fault persists, the recloser operates a predetermined
number of times (1 to 4) and locks out, after which it
must be manually reset before it can be closed again. If,
however, the fault was of a temporary nature and
cleared before lockout, the recloser would reset itself
and be ready for another full sequence of operations.
Temporary faults arise from wires swinging
together when improperly sagged, from tree branches
falling into the line, from lightning surges causing
temporary flashover of line insulators, and from
animals on the conductors short circuiting the
A recloser is unlike a fuse link because it
distinguishes a temporary from a permanent fault. A
fuse link interrupts temporary and permanent faults
alike. Reclosers give temporary faults repeated
chances, usually four, to clear or be cleared by a
subordinate device, like a fuse or sectionalizer. If the
fault is not cleared after four operations, the recloser
recognizes it as a permanent fault and operates to lock
out and leave the line open.
A recloser can be magnetically operated by a
solenoid connected in series with the line. Minimum
trip current is usually twice the normal load current
rating of the recloser coil. The operations are
performed by a hydraulic mechanism and a
mechanical linkage system. When the fault current
reaches twice the normal line current, the increased
magnetic field pulls the plunger down into the coil. As
the plunger moves downward, the lower end trips the
contact assembly to open the contacts and break the
circuit. As soon as the contacts are open, there is no
more current in the coil to hold them open, so a spring
closes the mechanism and reenergizes the line.
Protection to the lineman is most important when a
transmission or distribution line or a portion of a line is
removed from service to be worked on using de-
energized procedures. Precautions must be taken to be
sure the line is de-energized before the work is started