Feature
When is a Fuse Not a Fuse
by Rick Youngblood
American Electrical Testing Co.
M
any utilities still use the S&C power fuse as
their distribution transformer primary protection for faults. Most technical people are
aware that this protection only works if the fuse opens the
source during the fault incident. The problem is many of
our maintenance technicians and engineers are not aware
of a serious potential issue with S&C type SMD-1A, -2B,
-2C and -3 power fuses. Most maintenance personnel do
not regularly receive factory service advisories and seldom
have opportunity to attend seminars providing the safety
and reliability information needed to make sound decisions
pertaining to the systems they are tasked to maintain.
A. Stainless-steel drive spring provides stored energy to drive
arcing rod upward through arcextinguishing medium during fuse operation
B. Solid-material arc-extinguishing
medium
C. Silver-clad copper arcing rod
D. Filament-wound glass-epoxy
tube
E. Fusible element — helically coiled silver or pretensioned nickel-chrome — provide precise
melting characteristics and nondamageable performance
www.netaworld.org Most of us who have been using the S&C power fuse
for more than 40 years trust its ability to protect our
valuable assets during a fault condition. All older S&C
fuses and many newer designs are susceptible to center
bore close down as a result of moisture ingress into the
boric acid core used to provide the arc interruption gas
during fault. The resulting water ingress causes the boric
acid to grow and adhere to the fuse link over a period of
time. When this happens the link cannot be pulled away
by the link spring. The link spring is used to stretch the
arc beyond its ionization limit as the fuse mechanically
opens, disrupting the arc and protecting the transformer.
During a fault the link coated with boric acid burns in
two, and gas is formed in the fuse barrel, but the arc
will continue due to the close proximity of the mating
parts. Additionally, the fuse will fail to fall open and
the transformer will continue to see the fault until the
fuse burns physically in two or the fault is interrupted
elsewhere.
Even with the rain shield end cap provided by S&C,
water ingress happens naturally because of rain and
high humidity. Improper storage outdoors will cause the
condition to be worsened if the fuses are hung upside
down or laid on the wet ground. All removed or spare
fuses should be stored indoors or inside a weatherproof
cabinet to prevent this condition from beginning.
S&C provided information in the maintenance section
of Instruction Sheet 212-501 on a method to airflow
test their SMD-1A, -2B, and -2C fuses to determine
if bore close down has begun. The test is relatively easy
and fast to perform by measuring a specific flow rate at
a given pressure for each model of fuse, and provides
the vital information needed to determine fuse health
and reliably protect the equipment. An airflow test instrument is available from S&C. The SMD-3 fuses are
not suitable for airflow testing but can be returned to
S&C for a disassembly inspection. S&C recommends
checking the fuse-unit bore periodically, whenever the
protected equipment is taken out of service for routine
maintenance.
Winter 2006-2007 NETA WORLD
An additional problem not related to weather occurs
when a fuse is dropped on its end cap. There is an internal
spring used to pull the calibrated link apart when the link is
melted by a fault. The fuse uses a tensioning wire to prevent
the link from being pulled apart under normal operating
conditions. This wire also melts during a fault permitting
the spring to tension the link. Premature fuse failure can
occur with rough handling due to failure of the tensioning
wire. Precautions should be taken to prevent fuse damage
and ultimate failure creating a false (no fault) outage of the
transformer at an inopportune time after maintenance to
the substation has been recently performed.
For more information contact S&C directly or Rick
Youngblood of American Electrical Testing Company
1-800-992-3826
Rick Youngblood graduated from Indiana State University in 1973
after leaving active duty in the Air Force. Rick joined Cinergy Corporation in 1982 then known as Public Service of Indiana. Rick was promoted
to Project Engineer after receiving his BSEE from Purdue University in
1985. In 1987 Rick became the Manager of Technical Services in their
Northern Division. Rick was responsible for implementing Cinergy’s
CMM System and creation of its Predictive and Preventive Maintenance Programs. In 2000 Rick was promoted to Supervising Engineer
for Substation Services where he remained until taking early retirement
in 2004.
Rick joined American Electrical Testing Company in August of 2004
as Regional Manager heading up the Midwest office located in Indiana.
Rick holds a Level III NETA test technician certification.
NETA WORLD Winter 2006-2007
www.netaworld.org