GE 369 Motor Management Relay PHASE CURRENT (CT) INPUTS

PHASE CURRENT (CT) INPUTS:

The 369 requires one CT for each of the three motor phase currents to be input into the relay. There are no internal ground connections for the CT inputs. Refer to Chapter 7: APPLICATIONS for a information on two CT connections. The phase CTs should be chosen such that the FLA of the motor being protected is no less than 50% of the rated CT primary. Ideally, to ensure maximum accuracy and resolution, the CTs should be chosen such that the FLA is 100% of CT primary or slightly less. The maximum CT primary is 5000 A. The 369 will measure 0.05 to 20 × CT primary rated current. The CTs chosen must be capable of driving the 369 burden (see specifications) during normal and fault conditions to ensure correct operation. See Section 7.4: CT SPECIFICATION AND SELECTION on page 7–7 for information on calculating total burden and CT rating. For the correct operation of many protective elements, the phase sequence and CT polarity is critical.

GROUND CURRENT INPUTS:

The 369 has an isolating transformer with separate 1 A, 5 A, and sensitive HGF (50:0.025) ground terminals. Only one ground terminal type can be used at a time. There are no internal ground connections on the ground current inputs. The maximum ground CT primary for the 1 A and 5 A taps is 5000 A. Alternatively the sensitive ground input, 50:0.025, can be used to detect ground current on high resistance grounded systems. The ground CT connection can either be a zero sequence (core balance) installation or a residual connection. Note that only 1 A and 5 A secondary CTs may be used for the residual connection. A typical residual connection is illustrated in below. The zero-sequence connection is shown in the typical wiring diagram. The zero-sequence connection is recommended. Unequal saturation of CTs, CT mismatch, size and location of motor, resistance of the power system, motor core saturation density, etc. may cause false readings in the residually connected ground fault circuit.

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