Current Inputs - GE 469 Instruction Manual

CHAPTER 3: INSTALLATION
3.2.4

Current Inputs

469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
Phase Current Inputs
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 If the unit is withdrawn,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 automatic 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
measurements. 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 Specifications on page 2–6 for ratings).
Verify that the 469 nominal phase current of 1 A or 5 A matches the secondary rating
and connections of the connected CTs. Unmatched CTs may result in equipment
damage or inadequate protection. Polarity of the phase CTs is critical for Negative
Sequence Unbalance calculation, power measurement, and residual ground current
detection (if used).
See Two-Phase CT Configuration on page A–1 for 2-phase CT information.
Ground Current Input
The 469 has a dual primary isolating transformer for ground CT connection. There are no
internal ground connections 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shorted by automatic mechanisms on the 469 case if the unit is withdrawn.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
Multilin 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 Multilin 50:0.025 CT. Only one ground CT input
tap should be used on a given unit.
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