UFES
MV “E” RATED FUSE
MV BUS
OIL or DT
XFMR
LV EQUIPMENT
MV SWITCH
(normally closed)
REA
BONDING JUMPER
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
5
LV starters can drop out while we are waiting for full
fuse interruption. The CLF interruption time deter-
mines when the MV BUS returns to nominal operat-
ing voltage.
Consider the following two examples for illustration
(in either case, the LV AF problem is solved equally
well):
• System SCC= 10,000A; CLF is a 150E; ratio of the
SCC to the fuse FLA rating is 67. Fuse will inter-
rupt in ¼ cycle. Operations should not be im-
pacted.
• System SCC= 4,600A; CLF is a 200E; ratio is 23.
Fuse will interrupt in cycles. Operations could be
impacted and the plant needs to be informed up-
front.
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01 Internal distribution
system configuration
Post testing analysis revealed that MV system
grounding has an impact on overall system behavior
in case of an AF and applying a UFES. This applica-
tion note addresses system grounding configura-
tions and how to technically apply the UFES, mini-
mizing any reliability issues for customers’ plants.
Each MV grounding method is addressed.
