Date Code 20010719 701 Motor Protection Relay
Installation
Relay Connections
2.11
Ground Current Transformer Input
The relay IN input can be connected in either of two ways, as shown in Figure 2.6
on page 2.7 and Figure 2.7 on page 2.8. The preferred method is the ground CT
method in Figure 2.6. Connecting the IN input residually, as in Figure 2.7, requires you
to select a relatively high overcurrent element pickup setting to avoid tripping due to
false residual current caused by CT saturation during high starting current. The IN
connection shown in Figure 2.6 is preferred and provides for a lower ratio flux-balance
CT that avoids saturation and provides greater ground fault sensitivity.
When you use a ground CT, its placement is critical and depends on the type of
cable used to connect the motor to the source. As Figure 2.10 on page 2.12 shows,
using unshielded cable requires that the CT be placed between the neutral connection
to ground and the motor, with the neutral lead included in the CT window. With
shielded cable, the shield connection to ground must pass through the CT window.
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