GE 239 Instruction Manual page 22

2 INSTALLATION
c) GROUND CT INPUT (10/11/12)
Ground sensing terminals are labeled 5A, 50:0.025, and COM. Connection depend on the grounding
system and sensitivity required. For high resistance grounded systems that limit the ground current
or in mines where low levels of ground leakage must be detected, use a separate CT to sense
ground current. In this configuration, referred to as zero sequence or core balance detection, all
three phase conductors must pass through the CT window. If the phase conductors are bundled in a
2
cable with a ground, the ground wire must either pass outside the ground CT window or be routed
back through the window if it passes through as part of the cable. Shielded and unshielded cable
installations are illustrated in the TYPICAL WIRING DIAGRAM. A ground CT with a ratio of 50:0.025
for sensing primary ground currents from 0.05 to 15 A is available from GE. Connect this CT to termi-
nals 50:0.025 and COM. If a conventional 5 A secondary CT is used for zero sequence ground sens-
ing, connect it to the 5A and COM terminals. A 1 A secondary CT can also be used; however, to
prevent readings from being off by a factor of 5, the ground CT primary setpoint must be adjusted.
See Section 4.3a) CT INPUTS on page 4–10 under *5281' &7 35,0$5< for suitable settings in this
situation. Due to the low secondary currents, it is recommended that the ground CT secondary leads
be twisted together and routed to the 239 away from high current carrying conductors. NOTE: The
50:0.025 input is only recommended for resistance grounded systems. Where the system is
solidly grounded or high levels of current are to be detected use the 5A ground input.
For low resistance or solidly grounded systems where higher ground fault currents will flow, the
phase CTs can be residually connected to provide ground sensing levels as low as 20% of the phase
CT primary rating. For example, 100:5 CTs connected in the residual configuration can sense ground
currents as low as 20 A (primary) without requiring a separate ground CT. This saves the expense of
an extra CT however 3 phase CTs are required. If this connection is used on a high resistance
grounded system verify that the ground fault alarm and trip current setpoints are below the maximum
ground current that can flow due to limiting by the system ground resistance. Sensing levels below
20% of the phase CT primary rating are not recommended for reliable operation.
2-6
Courtesy of NationalSwitchgear.com
UNSHIELDED
CABLE
Figure 2–4: CORE BALANCE GROUND CT INSTALLATION
239 Motor Protection Relay
2 INSTALLATION
SHIELDED
CABLE
GE Multilin