10
A RCLI M ITE R
TM
A R C FL A SH M I T I G AT I O N S O LU T I O N FO R LV US I N G U FE S
Typically, in industrial plants, the maximum high im-
pedence ground fault current flow value is limited
to 10 A. Phase C could remain energized to the
transformer primary for an extended time and,
since most of the plant’s SUS are delta-wye, all
three transformer phases have the same equally ap-
plied potential. Since there is no potential differ-
ence between the delta windings, there is no cur-
rent flow, therefore, there is no induction to
generate secondary potential. With no supporting
secondary potential, the LV AF collapses in micro-
seconds.
However, having a sustained bolted line-to-ground
phase C 10 A fault will activate the plant’s ground
fault alarm system. Maintenance would be chasing
the wrong problem. To prevent this nuisance alarm,
the MV SWITCH should be installed (or retrofitted)
with a shunt trip activated by one of the QRU1’s trip
out contacts.
Two of the CLFs will still interrupt in about the same
timeframe, about 1-2 ms, Phase C CLF may not in-
terrupt. This prevents the false ground fault alarms.
Upon UFES operation, all three fuses should be re-
placed, since the phase C fuse may be damaged by
microsecond internal arcing, along with the three
PSEs. The UFES to ground bonding jumper can be
small, approximately #2 AWG, thermally, since it
only has to carry current for 5 ms.
—
High resistance/grounded MV
systems
