APPENDIX G
G-1
Ground Fault and Short Circuit Instan-
taneous Elements
The 269 has two programmable instantaneous ele-
ments, for Short Circuit and Ground Fault protection.
When the Short Circuit instantaneous element is pro-
grammed, care must be taken not to set the trip level
too sensitively, to minimize nuisance tripping, espe-
cially on start. It is a known fact that on motor starts,
and for the first cycle or more there is an asymmetrical
component associated with the motor starting current
that can reach as much as twice the starting current
level. Please consult appendix E for more details on
"Asymmetrical Starting Current".
Also, care must be taken when the instantaneous
Ground Fault level is set, especially in applications
where motors are fairly large (2000 HP or more) and/or
several of these motors are being fed from the same
line. At start a large motor may induce a large amount
of current in its zero sequence CT. It may induce
ground current in adjacent motors as well, causing their
zero sequence CTs and relays to pick up and possibly
trip. This nuisance trip may occur if the ground fault
instantaneous element for any of those motors is set.
This phenomenon has been seen and identified to last
a cycle or more depending on many factors, such as
the size of the motors, the trip levels set, the sensitivity
of the relay as well as the number and proximity of
motors to each other. For the above reasons, and to
eliminate nuisance tripping on start, the 269 has been
equipped with a 2 cycle delay built in both the Short
Circuit and Ground Fault instantaneous elements to
ride through such phenomena.
In addition, to accommodate for even larger motors
with ground currents persisting for longer than 2 cycles
induced at start, users are urged to contact the factory
to get instructions on programming the 269 instantan-
eous ground fault feature to ride through an additional
cycle, bringing the total delay on instantaneous ground
fault tripping to approximately 48 ms or 3 cycles, still
meeting the NEMA standards for instantaneous ele-
ments of less than 50 ms.
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