I.L. 17562
Page 49
PR 0.3 Effective 8/99
300 × 5/400 = 3.75 amperes, an ideal value.
For a 1 ampere Ct set:
For optimum metering accuracy at low loads, the Ct should deliver between 0.7 and 0.8 amperes
at 100 percent FLA. Attempt to achieve 0.75 A.
For reliable motor protection, the Ct should deliver between 0.5 and 0.8 amperes at
100 percent FLA.
For example, if the motor has an FLA of 300A, a 400:1 Ct will supply:
300 × 1/400 = 0.75 amperes, an ideal value.
Note that PCT/FLA must lie between 0.5 and 1.5 for successful programming - you will not be
able to exit Program Mode until one or the other value is adjusted. The following are the display
messages which appear when exit is blocked:
Display
Complete Help Message
FLA CT L
THE RATIO OF FLA TO CT RATIO IS BELOW .5 -
INCREASE FLA OR DECREASE CT RATIO - PRESS
PROGRAM TO CONTINUE
FLA CT H
THE RATIO OF FLA TO CT RATIO IS ABOVE 1.5 -
DECREASE FLA OR INCREASE CT RATIO - PRESS
PROGRAM TO CONTINUE
IOC OUT
IOC SETPOINT IS OUT OF RANGE - DECREASE IOC OR
DECREASE FLA OR INCREASE CT RATIO - PRESS
PROGRAM TO CONTINUE'
JAMT OUT
JAM TRIP SETPOINT IS OUT OF RANGE - DECREASE JAM
TRIP OR DECREASE FLA OR INCREASE CT RATIO -
PRESS PROGRAM TO CONTINUE
JAMA OUT
JAM ALARM SETPOINT IS OUT OF RANGE - DECREASE
JAM ALARM OR DECREASE FLA OR INCREASE CT RATIO
- PRESS PROGRAM TO CONTINUE
FLA is a fixed property of the motor. Select a Ct whose ratio is suitable for this motor.
CAUTION
Be careful when determining Ct turns ratio. An improper value can cause the MP-3000 to
receive incorrect motor current data. Motor damage could result.
5.1.6 Setpoint P1L6, Ground Fault Ct Ratio (GCT) — Specifies the numerator of the turns
ratio of the single ground current transformer used only for ground fault protection and ground
current metering. Obtain from the Ct nameplate or from the installation drawings, watching for
the use of taps on cts which have tapped windings.
Typically, the three phase power conductors pass through the single window of a ground fault Ct,
whose ratio is usually much lower than that of the phase Cts for good ground fault sensitivity. A
very common type of ground fault Ct has a ratio of 50:5 or 50:1, but check the Ct in use.
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