UFES
MV “E” RATED FUSE
MV BUS
OIL or DT
XFMR
LV EQUIPMENT
MV SWITCH
(normally closed)
REA
BONDING JUMPER
6
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
On a solidly grounded MV system, shown in Figure
2, the ArcLimiter application for LV AF mitigation is
proven very effective by test. The UFES closes all
three phases to ground simultaneously with some
minor contact bounce. At this point, the LV AF is
over.
Depending upon UFES closing time and phase se-
quence, phases A and B fuses melt in 1 ms (MV sys-
tem voltage recovery starts) with full CLF interrup-
tion by 1.5-2 ms. The actual CLF interrupting time is
dependent upon the MV system fault availability
(many times referred to as SCC). The higher the
SCC, the faster the CLF interruption, the shorter du-
ration of the fault induced voltage dip, the less im-
pact on other on-going operations.
When phases A and B CLF fully interrupt, the entire
phase C current briefly appears as a zero sequence
current on the traces. The only path for fault current
is through the phase C fuse, to UFES phase C PSE,
to transformer grounded neutral. Since there is no
impedance in that path, the fault current flow is
high, only limited by the transformer’s short circuit
impedance.
Since phase C is delayed by 120 degrees, about 5
ms (assuming a 60 Hz system), in order for the
phase C fuse to melt, those amps will not flow via
phases A and B fuses, which are already open, but
back to the source neutral. The UFES to ground
bonding jumper can be small thermally, approxi-
mately #2 AWG, since it only has to carry current for
5 ms.
Upon UFES operation, all three fuses should be re-
placed, since the phase C fuse may be damaged by
microsecond internal arcing, along with all three
PSEs.
—
Solidly grounded MV systems
On a solidly grounded MV system, the ArcLimiter application for LV arc
flash mitigation is proven very effective by test.
—
02 solidly grounded
MV system
—
01
