Current Sensors - GE AKR-30 Series Maintenance Manual

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 PROGRAMMER
46
FIG. 59 - AKR-58-30
1 0.2 C U R R ENT S ENSORS
The SST system uses two types of current sensors, a
phase sensor and a ne utral sensor. Fig . 60 shows a phase
sensor. Fig. 61 shows the ne utral sensors avai lable. The
cu rrent sensor s u pplies the power and sig nal i n puts
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 G round Fault trip element operates on the
principle that the instantaneous values of cu rrent i n the
three conductors (four on 4-wire systems) add to zero
unless ground current exists. On SST's equipped with
G round Fault, the ground trip signal is developed by con­
necting each phase sensor in series with a companion
primary winding on a ground d iffe rential 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" ne utral
sensor is an electrical du plicate of the phase sensor, in­
cluding taps. Therefore, when taps are changed on the
phase sensors , those on the neutral sensor m ust 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 connected to
the breaker via secondary discon nect blocks . See Fig. 62.