41-337.3L
5
2.5.
Indicating Instantaneous Trip Unit (IIT)
2.5.1. Fault Detector (FD) –
when used
The instantaneous trip unit is a small ac operated
clapper type device. A magnetic armature, to which
leaf-spring mounted contacts are attached, is
attracted to the magnetic core upon energization of
the switch. When the switch closes, the moving con-
tacts bridge two stationary contacts, completing the
trip circuit. Also during this operation, (for the IIT
only) two fingers on the armature deflect a spring
located on the front of the switch which allows the
operation indicator target to drop. The target is reset
from the outside of the case by a push rod located at
the bottom of the cover.
A core screw accessible from the top of the switch
provides the adjustable pick-up range.
3. OPERATION
The types CA-16 and CA-26 relays are induction disc
relays with four electromagnets mounted on two
discs that are fastened on a common shaft. One of
the electromagnets is the operating element while
the other three are restraint elements. The restraint
elements are energized from the secondaries of cur-
rent transformers connected to the bus, and the oper-
ating circuit is energized in accordance with the
current flowing in the differential connection of the
current transformers.
A current of 5 amperes in a terminal 18 and out of
terminal 19 will produce a definite amount of restrain-
ing torque (see Figure 3). Similarly, a current of 5
amperes flowing in at terminal 16 and out of terminal
17 will produce an equal amount of torque. If both of
these currents flow at the same time with the polarity
as indicated above, their effect will be additive and
they will produce the same torque as through 10
amperes were flowing in terminal 16 and out of termi-
nal 17. Conversely, if equal currents flow in these two
coils, but in opposite directions, their ampere turns
will cancel and no torque will be produced. The same
relationship applies for the paired coils of the other
two restraining units of the relay. The restraint effect
will always be additive if currents flow in the coils
which belong to different restraint elements.
Curve 537957
Figure 7: Typical Burden Characteristic of the Operating
Circuit of the CA-16 and CA-26 Differential
Relays.
Sub 5
187A424
Figure 8: External Schematic of the Type CA-16 Relay for
Three and Four Circuit Bus Protection.
