SECTION 1 0-Type SST Overcurrent Trip Device (Cont.)
M O U N T I N G
BRACKET
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FIG. 58 - SST P ROGRAMMER
46
FIG. 59 - AKR-58-30
1 0.2 CURRENT SENSORS
The SST system uses two types of current sensors, a
phase sensor and a neutral sensor. Fig . 60 shows a phase
sensor. Fig.
61
shows the neutral sensors available. The
cu rrent sensor supplies the power and sig nal inputs
necessary to operate the trip system . Each sensor has four
taps which provide field adjustment of the trip device's
continuous ampere rating.
The SST Ground Fault trip element operates on the
principle that the instantaneous values of current in the
three conductors (four on 4-wire systems) add to zero
unless ground current exists. On SST's equipped with
Ground Fault, the ground trip signal is developed by con
necting each phase sensor in series with a companion
pri mary winding on a ground differential transformer
mounted in the programmer unit. Its secondary output is
zero so long as there is not ground current.
Application of the G round Fault element on 4-wire sys
tems with neutral grounded at the transformer requires the
additional, separately mounted neutral sensor (Fig.
6 1 )
inserted in the neutral conductor; its secondary is con
nected to a fourth primary winding on the ground differen
tial transformer. See Fig .
70.
This "fourth-wire" neutral
sensor is an electrical duplicate of the phase sensor, in
cluding taps. Therefore, when taps are changed on the
phase sensors, those on the neutral sensor must be cor
respondingly positioned .
When used, the neutral sensor is separately mounted in
the bus or cable compartment of the switchgear. I n draw
out construction, its output is automatically connected to
the breaker via secondary disconnect blocks. See Fig.
62.
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