Phase Current Inputs; Ground Current Input - GE MOTOR MANAGEMENT RELAY 469 Instruction Manual

2.2 ELECTRICAL
The 469 has three channels for phase current inputs, each with an isolating transformer. There are no internal
ground connections on the current inputs. If the unit is withdrawn, each phase CT circuit is shorted by auto-
matic mechanisms on the 469 case. The phase CTs should be chosen so the FLA is no less than 50% of the
rated phase CT primary. Ideally, the phase CT primary should be chosen such that the FLA is 100% of the
phase CT primary or slightly less, never more. This will ensure maximum accuracy for the current measure-
2
ments. The maximum phase CT primary current is 5000 A.
The 469 correctly measures up to 20 times the phase current nominal rating. Since the conversion range is
large, 1 A or 5 A CT secondaries must be specified at the time of order to ensure the appropriate interposing
CT is installed in the unit. The chosen CTs must be capable of driving the 469 phase CT burden (see Section
1.2: SPECIFICATIONS on page 1–5 for ratings).
Verify that the 469 nominal phase current of 1 A or 5 A matches the secondary rating and con-
nections of the connected CTs. Unmatched CTs may result in equipment damage or inade-
quate protection. Polarity of the phase CTs is critical for Negative Sequence Unbalance
CAUTION
calculation, power measurement, and residual ground current detection (if used).
See Appendix B.1: TWO-PHASE CT CONFIGURATION on page B–1 for 2-phase CT information.
The 469 has a dual primary isolating transformer for ground CT connection. There are no internal ground con-
nections on the ground current inputs. The ground CT circuits are shorted by automatic mechanisms on the
469 case if the unit is withdrawn. The 1 A / 5 A tap is used either for zero-sequence/core balance applications
or residual ground connections where the summation of the three phase current CTs is passed through the
ground current input (see the figure below). The maximum ground CT primary current is 5000 A for the 1 A /
5 A tap. Alternatively, the 50:0.025 ground CT input has been designed for sensitive ground current detection
on high resistance grounded systems where the GE Power Management 50:0.025 core balance CT is to be
used. For example, in mining applications where earth leakage current must be measured for personnel safety,
primary ground current as low as 0.25 A may be detected with the GE Power Management 50:0.025 CT. Only
one ground CT input tap should be used on a given unit.
2-10
Figure 2–12: RESIDUAL GROUND CT CONNECTION
469 Motor Management Relay
2 INSTALLATION

2.2.4 PHASE CURRENT INPUTS

2.2.5 GROUND CURRENT INPUT

GE Power Management