UFES
MV “E” RATED FUSE
MV BUS
OIL or DT
XFMR
LV EQUIPMENT
MV SWITCH
(normally closed)
REA
BONDING JUMPER
UFES: 3 PSEs
(CLOSED)
MV “E” RATED FUSES
MV BUS (3 PHASE)
OIL or DT
XFMR
LV EQUIPMENT
(3 PHASE)
MV SWITCH
REA
BONDING
JUMPER
A
C
B
T EC H N I C A L A N D A P P L I C AT I O N G U I D E
11
If the MV system is delta ungrounded (Figure 7) the
application is very similar to high resistance
grounded systems. These types of systems are
typically applied in older industrial plants feeding a
process.
A single line to ground fault does not cause any
increase in ground current to flow except from
inter-electrode capacitance. The MV switchgear
breakers do not have ground fault protection.
Indications of a ground fault are usually via
installed voltmeters. The operating process
continues as a plant benefit.
When triggered, the UFES closes all three phases to
ground simultaneously. At this point, the LV AF is
essentially over. Depending upon the UFES closing
time and phase sequence, phase A and B fuses melt
in 1 ms (MV voltage recovery starts) with full CLF
interruption by 1.5-2 ms (SCC dependent). After
phases A and B CLF clear, there is no phase C
current since the ground return path is open. See
Figure 8.
—
Ungrounded Delta MV systems
If the MV system is delta ungrounded, the application is very similar to
high resistance grounded systems.
—
07 MV system is
delta ungrounded
—
08 After phases A and
B CLF clear, there is
no phase C current
since the ground
return path is open
—
07
—
08
