GEI-86134, Power Circuit Breakers
TYPE SST OVERCURRENT TRIP DEVICE
The SST is a solid-state, direct-acting, self
powered trip device system. Referring to the block
diagram of Fig. 16 and the photograph of Fig. 15,
it comprises the following individual components:
Unit - provides the comparison
basis
overcurrent detection and delivers the
energy necessary to trip the breaker. Contains the
electronic circuitry for the various trip elements.
Their associated pickup and time delay adjustments
(set-points) are located on the face plate. Depend
ing on the application, programmer units may be
equipped with various combinations of Long Time,
Short Time, Instantaneous and Ground Fault trip
elements. See Table 4 for available ratings,
settings and trip characteristics.
The SST programmer units can be optionally
equipped with trip indicators (targets). These are
pop-out, mechanically-resettable plungers located
across the top of the programmer's front. Units
with a ground fault element employ three targets:
from left to right, the first is for overload, the
second for short circuit (actuated by the short time
and instantaneous elements) and the third for ground
fault. The latter is omitted on units without ground
fault.
Each target pops out when its associated trip
element operates to trip the breaker. After a trip,
the popped target must be reset by hand. However,
neglecting to reset does not affect normal operation
of any trip element or prevent the breaker from
being reclosed.
Current
- each pole mounts a single
sensor coil
which supplies the power and sig
nal inputs necessary to operate the trip system.
Each sensor has four taps which provide field ad
justment of the trip device's continuous ampere
rating. See Fig.
18.
The SST Ground Fault trip element operates on
the principle that the instantaneous values of cur
rent in the three conductors (four on 4-wire sys
tems) add to zero unless ground current exists.
On SST's equipped with Ground Fault, the ground
trip signal is developed by connecting each phase
sensor in series with a companion primary winding
on a ground differential transformer mounted in the
programmer unit. Its secondary output is zero so
long as there is no ground current.
14
Fig. 15.
SST Programmer unit
8
t
�- -----
Fig. 16.
SST Block diagram
OVERlOAD
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TARGET
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SHORT
lciRCUIT
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TARGET
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