GE AKR-30 Series Maintenance Manual page 224

SECTION 1 0-Type SST Overcurrent Trip Device (Cont.)
M O U N T I N G
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BRACKET
FIG. 58 - SST P ROG RAMMER
46
FIG. 59 - AKR-58-30
1 0.2 C U R R ENT S ENSORS
The SST system uses two types of cu rrent 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 i n puts
necessary to operate the trip syste m . Each sensor has four
taps which provide field adj ustment 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 cond uctors (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 tran sformer
mounted in the programmer unit. Its secondary output is
zero so long as there is not ground cu rrent.
Application of the G round Fault e le ment 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" ne utral
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 mou nted in
the bus or cable compartment of the switchgear. I n draw­
out construction, its output is automatically con nected to
the breaker via secondary disconnect blocks. See Fig.
62.