GE 469 Instruction Manual page 80

CHAPTER 3: INSTALLATION
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withdrawn.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 settings ensures proper reading of primary phase differential current. The
1 A / 5 A differential CT chosen must be capable of driving the 469 differential CT burden
(see Specifications on page 2–6 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 3–17: Core Balance Method
3–16 469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL