Differential Current Inputs - GE 469 Manual

2.2 ELECTRICAL 2 INSTALLATION

2.2.6 DIFFERENTIAL CURRENT INPUTS

The 469 has three channels for differential current inputs, each with an isolating transformer. There are no
internal ground connections on the current inputs. Each differential CT circuit is shorted by automatic mecha-
nisms on the 469 case if the unit is withdrawn. The maximum differential CT primary current is 5000 A.
The 469 measures up to 5 A secondary current for the differential CT inputs. Since the conversion range is rel-
2
atively small, the 1 A or 5 A option is field programmable. Proper selection of this setpoint ensures proper read-
ing of primary phase differential current. The 1 A / 5 A differential CT chosen must be capable of driving the
469 differential CT burden (see Section 1.2: SPECIFICATIONS on page 1–5 for ratings).
The differential CTs may be core balance as shown in the first figure below. Alternatively, the summation of two
CTs per phase into the differential input will provide a larger zone of protection. If the summation of two CTs is
used, observation of CT polarity is important. The summation method may also be implemented using the
phase CTs as shown below. They will have to have the same CT ratio.
Figure 2–14: CORE BALANCE METHOD
WITHOUT PHASE CTs WITH PHASE CTs
Figure 2–15: SUMMATION METHOD
2-12 469 Motor Management Relay GE Power Management