GE 469 Instruction Manual page 26

2.2 ELECTRICAL 2 INSTALLATION
c) 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 mechanisms 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unit is withdrawn.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 relatively small,
the 1 A or 5 A option is field programmable. Proper selection of this setpoint ensures proper reading of primary phase differ-
ential current. The 1 A / 5 A differential CT chosen must be capable of driving the 469 differential CT burden (see Section
2
1.2: Specifications on page 1–4 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 Multilin
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