8
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, that ground fault
value can be limited to 200-600 A and in power
plants up to 1200 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 evenly 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, a sustained bolted line-to-ground phase C
fault on a low resistance grounded system will be
detected and tripped by the upstream 51G-1 or
51G-2 protection relay “x” seconds later. Review the
ground return path in Figure 4. This action shuts
down the entire MV BUS resulting in poor reliability
for customers’ plants. The 51G-1 or 51G-2 will be
faster than a sustained high ground fault value (say
1200 A) through the CLF.
To prevent this reliability issue, the MV SWITCH
should be installed (or retrofitted) with a shunt trip
activated by one of the QRU1’s trip out contacts
Phase A and B CLF will still interrupt in about the
same timeframe, about 1-2 ms, since they are in se-
ries through the closed PSEs. Phase C may not in-
terrupt. The upstream 51G-1 or 51G-2 will not see
ground current within its pick up range.
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.
—
Low resistance/grounded MV
systems
