12
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
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 applied potential.
Since there is no difference in potential between
the delta windings, there is no current flow, there-
fore there is no induction to generate secondary
potential.
However, having a sustained bolted line-to-ground
phase C 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 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 thermally, approximately #2 AWG, since it only
has to carry current for 5 ms.
—
Ungrounded Delta MV systems
