GE MULTILIN 269 MOTOR MANAGEMENT RELAY Series Instruction Manual page 125

APPENDIX H
exciting current. These curves may be obtained from
the CT manufacturer, or by experimentation (see
ANSI/IEEE C57.13.1 for procedures). The curves illus-
trate the values of secondary volts for which the output
of the CT will be linear. The desired operating secon-
dary voltage is below the knee point (A or B on the
graph (ANSI or IEC respectively) or just slightly above
it, staying within 10% CT ratio error correction at 20
times rating. Using this information, it is important to
recognize that the secondary exciting voltage is the
total voltage that the CT can develop at the secondary.
In this case, that voltage will drop across the secondary
winding resistance as well as any load that is applied to
the unit. Therefore, the secondary winding resistance
must always be included with the excitation curves, or
the information is incomplete. A curve with a knee at
100 V for example could drive a total burden of
100V/(20*5A) or 1Ω.
Evaluation of CT performance is best determined
from the excitation curves. They present the com-
plete story and eliminate any guess work. Most CT
manufacturers will provide excitation curves upon
request.
H-2