September 1993
Cooper Power Systems
Application Solutions
Provided With The
ELF™ Current-Limiting Dropout Fuse
PRESENTED AT THE
SOUTHEASTERN ELECTRICAL EXCHANGE OVERHEAD DISTRIBUTION COMMITTEE
ATLANTA, GEORGIA
SEPTEMBER 30, 1993
Author
Karen L. Leix, Overcurrent Equipment Manager,
Cooper Industries, Inc., Cooper Power Systems Division,
Components and Protective Equipment, Pewaukee, WI
Bulletin 93033
September 1993 © Cooper
1 Power Systems, Inc.
Printed in U.S.A.
Cooper Power Systems
GENERAL
• Line tap laterals
Fuses have been used on distribution systems for a long
time. They protect the distribution equipment from
damaging overcurrents and protect the system from failed
equipment.
• Underground taps
An important consideration to utilities in fuse selection and
application relates to the ability of the fuse to be physically
integrated with the utility's existing system. It must also be
easy to install and service and be economical.
• Locations where an external back-up current limiting
fuse is used in series with an expulsion fuse
At the present time, two types of fuses are employed:
expulsion and current-limiting. Each employs a fusible
element designed to melt when a current of a large enough
magnitude and duration passes through the element.
However, expulsion and current-limiting fuses are very
different in their means of clearing high currents. An
expulsion fuse interrupts overcurrents at a current zero
through the de-ionizing action of gases that are expelled
when the fusible element melts. When a current-limiting
fusible element melts, the resulting arc rapidly loses its
heat energy to the surrounding sand. This energy melts or
"fuses" the sand into a glass-like tunnel structure called a
fulgurite. The rapid loss of heat energy and the confinement
of the arc by the molten glass fulgurite literally chokes off
the current, thus clearing the high current fault in less than
one-half cycle.
• Safety tool for re-fusing
• Sensitive environments (sensitive flashover areas
and/or fire hazard areas)
• Poletype transformer protection
• Station service transformer protection
• Enclosure mounting
EXPULSION CUTOUT FUSE
OPERATION
There are inherent disadvantages and/or limitations to
expulsion fuses mounted in cutouts. These include:
• Loud noise level and release of gases during operation
• Requires at least one-half cycle of current before
clearing and allows high energy levels to be
dissipated into protected equipment
The most commonly used expulsion fuse is a fuse link
mounted in a fuseholder installed in an overhead cutout. A
cutout consists of a mounting with an insulating support
designed to be mounted on an utility pole or crossarm and
a pair of contacts spaced apart which are designed to
receive and electrically engage a fuseholder. The industry's
dimensional standard for fuseholders and mounting
commonly referred to as "interchangeable" cutouts.
Interchangeable refers to one manufacturer's fuseholder
properly fitting into the mounting of another manufacturer's
cutout. Interchangeable cutouts are extensively utilized
throughout electrical distribution systems in this country.
There are limitations, however, to the application of
expulsion fuses in cutouts which until now the industry has
accepted.
• Voltage dips
• Tight coordination
• Limited interrupting capability
The expulsion fuse link is mounted in the fuseholder of the
cutout. The fuseholder is lined with an organic material,
such as bone fiber, to aid in the interruption. Interruption
of an overcurrent takes place within the fuseholder by the
deionizing and explosive action of the gases which are
released when the liner is exposed to the heat of the arc.
The clearing operation of the expulsion-type fuse is
characterized by loud noise and violent emission of gases,
flame, and burning debris, all of which pose a potential
safety concern to personnel who may be in close proximity
to the fuse when it operates. Because of its violent mode
of operation, this type of fuse has generally been restricted
to outdoor usage only. Even when installed outdoors, the
expulsion fuse must be mounted well away from the
Cooper Power Systems has developed the ELF™ currentlimiting dropout fuse to fit into the industry's standard
interchangeable cutout. The ELF fuse offers solutions to
various applications on the distribution system including:
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September 1993
that the current exists, current-limiting fuses reduce the
energy dissipated in equipment. When an expulsion fuse
operates for a high current fault, the fuse allows the
available current to flow until the current has cyclically
returned to zero amps. The first loop of current can have
durations that approach 0.0167 seconds (1 cycle on a 60 Hz
system), with peak values that approach two times the
symmetrical peak.
equipment it is intended to protect, as well as other
equipment. In addition, expulsion fuses mounted on
distribution system poles have been known to initiate grass
fires resulting from the flaming debris which may be
expelled.
An expulsion type fuse requires at least one-half cycle of
current before it clears a high current fault. During this
time, the equipment the fuse is designed to protect must
endure the full available fault current that is allowed to pass
though the fuse to the equipment. Potentially damaging
energy is dissipated in the equipment as a result of the high
current. If the energy level is high enough it can cause a
disruptive failure of the equipment.
The high current that the expulsion fuse allows to flow,
prior to its interruption at a system current zero, can cause
voltage dips on the system. This causes lights to flicker and
sensitive computers and electronic equipment to suffer.
In addition, in situations of tight coordination, expulsion
fuses may not clear the overcurrent condition soon enough
to prevent sectionalizing fuses, reclosers or other protective
relays and circuit devices from also sensing the overcurrent.
They may respond by de-energizing other portions of
the system.
Figure 1. Energy Released for a 10,000A Symmetric
Fault Current
Interrupting capability is a limitation that exists with
expulsion fuses. A typical cutout with an expulsion fuse
link has a maximum interrupting rating of 1200-13200A
RMS symmetrical. There has been an increased demand
for electrical service in recent years which has led to lower
impedance distribution systems and the need for greater
interrupting capabilities. Sometimes these requirements
exceed the capabilities of expulsion fuses.
Figure 1 illustrates the effect of using a current-limiting
fuse instead of an expulsion fuse to limit the momentary
duty from a high current symmetric fault. This is a plot of
current squared-time for a 5.5kV system, 10,000 amps
RMS fault current. The area under a curve is I2t. The peak
current available from this symmetric fault is 14.1kA. The
I2t available from the fault is 3.26 x 105 A2s. If an expulsion
fuse was used, all of this energy per unit resistance would
be let through to the fault. If a current-limiting fuse was
used, the I2t released to the system would be 3.6 x 104 A2 s,
a reduction by a factor of 9 in the I2t level when compared
to the I2t when an expulsion fuse is used.1
CURRENT-LIMITING FUSE
OPERATION
The most common reasons for using current limiting fuses
are for applications where the fault duty exceeds the
interrupting capacity of expulsion fuses and to minimize
the probability of disruptive failure of equipment due to
high current faults.
ELF DROPOUT CURRENT-LIMITING
FUSE
The ELF current-limiting dropout fuse is a full range
current-limiting fuse designed for mounting in an industry
standard interchangeable cutout. The full range current
limiting rating ensures reliable operation of all overloads
and fault currents. The element construction consists of
two separate sections (low current section and high current
section) which are self-contained in one housing. The low
Current limiting fuses limit the amount of current that is
available from the system. They limit fault current by
introducing a very high impedance into the circuit, thus
forcing the current down to zero amperes before the normal
60 cycle zero occurs. By reducing the level of fault current
let-through to the fault and by reducing the amount of time
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Cooper Power Systems
current section provides consistent, reliable clearing of all
currents high enough to melt the element. The high current
section uses a ribbon design which controls peak arc
voltage levels and limits both current and energy (I2t) letthrough levels during high current fault clearing operation.
(Refer to Figure 2)
CUTOUT
HANGER
CUTOUT
INSULATOR
CUTOUT
LOWER
CONTACT
DROPOUT
ACTUATOR
(OPERATED)
LIFTING EYE
ELF FUSE
(OPERATED)
PULL RING
Figure 3. ELF Fuse in Interchangeable Cutout After
Dropping Open Due to Operation of Dropout
Actuator
Once operated, the line personnel simply use a hookstick to
remove the suspended ELF fuse. The ELF fuse is
lightweight, approximately 2.4 lbs for 95kV BIL cutout
mountings through 40A ratings, similar to that of a standard
fuseholder. A new fuse is easily installed using procedures
similar to replacing an expulsion fuseholder.
APPLICATIONS
Figure 2. Cutaway of ELF Fuse
The ELF fuse provides advantages for various applications.
The ELF dropout fuse operates at a dramatically lower
noise level than expulsion fuses. In addition, the expulsive
shower that exists with an expulsion fuse operation is
eliminated. This offers increased safety to line personnel
during circuit energization operations. In addition, the
reliable drop open design makes fault location easy. (Refer
to Figure 3.)
Line Tap Laterals
Line tap laterals are typically fused with an expulsion
cutout mounted fuse. If the cutout is an interchangeable
design, it provides a perfect opportunity for an ELF fuse.
The ELF fuse replaces the fuseholder and expulsion fuse.
Installation of the ELF fuse would increase protection of
the lateral tap to a 31kA interruption capacity at 8.3kV.
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September 1993
Underground Taps
Back-up Current-Limiting Fuse Installations
Underground cable typically fails through a breakdown of
the insulation resulting in an immediate puncture or, as a
minimum, a reduction in cable life. Utilities protect their
underground systems at the riser pole location.
Underground taps are typically fused with an expulsion
cutout mounted fuse. As with the line tap laterals, if the
cutout is an interchangeable design, an ELF fuse can be
mounted in the cutout. The dramatic reduction of energy
into the underground cable when the ELF fuse is used will
aid in reducing cable failures and extend cable life.
To increase the interrupting capability of an expulsion
fuseholder/cutout installation, the industry has commonly
used a back-up current-limiting fuse. A back-up type
current-limiting fuse is defined by ANSI/IEEE C37.401981 as "a fuse capable of interrupting all currents from
the maximum rated interrupting current down to the rated
minimum interrupting current." A back-up current limiting
fuse requires that the fuse link in the fuseholder clear low
current faults while it clears only high current faults.
At some fault current levels, the back-up fuse may be
damaged. It is recommended that it is resistance tested to
verify damage has not occurred, before putting it back into
service. This requires time to unbolt the back-up fuse, and
replace it with a new fuse prior to returning it to the shop
for testing.
Sensitive Environments
Sensitive environments include public areas that would
have sensitivity to loud noise and/or dry grassy areas that
could pose a fire hazard as a result of expulsive byproducts.
Expulsion fuses in fuseholders vent either out the bottom
of single-venting fuseholders or out both the top and
bottom of double-venting fuseholders. When clearing
occurs, exhaust gases, molten metal and fuse link fragments
will be expelled from the vent end of the fuseholder. (This
venting/clearing operation is very loud.) These are obvious
safety concerns for both personnel and environment in the
immediate area.
The ELF fuse in this application replaces the fuse link,
fuseholder and bolted in back-up current limiting fuse.
The ELF fuse is easily hookstick installed into the
interchangeable cutout and, thus, eliminates the need for
bolting a separate fuse in place and/or testing.
Pole-type Transformer Protection
The ELF fuse eliminates this concern. It's operation for
clearing both low current and high current faults is totally
self-contained within the fuse housing. Internally, an
auxiliary wire extends the length of the fuse. The upper
end of the auxiliary wire is connected to the upper cap
assembly. A trigger wire is connected to the lower end of
the auxiliary wire and extends through the lower cap
assembly, to the outside. The auxiliary wire and trigger
wire are insulated from the lower cap assembly.
Pole-type transformers are protected by current-limiting
fuses to minimize eventful failure due to internal high
current faults. An I2t value of 150,000 A2s is commonly
used in the industry as a limiting value for protection of
pole-type transformers.2 Various options are available to
protect pole-type transformers. An ELF fuse makes an
excellent alternative to these options. It reduces the I2t letthrough levels to safe levels (for instance, 50,000 A2s is the
maximum let-through I2t for a 40A, 8.3kV ELF).
The lower cap assembly includes a spark gap located
between the lower cap assembly and the trigger wire,
where the trigger wire passes out of the fuse body. Upon
experiencing a fault current, arcing across the spark gap
melts the trigger wire and releases the dropout actuator.
This allows the hinge to rotate to an extended position
which causes the ELF fuse to drop out of engagement with
the upper contact of the cutout mounting. Activating the
dropout actuator causes the ELF fuse to fall open in the
cutout, providing a visible indication of fuse operation.
Compared to the violent expulsion fuse operation, the
ELF fuse operates silently, without any expulsive
byproducts.
Safety Tool for Re-Fusing
The recommended installation of the ELF dropout fuse
into an interchangeable cutout is:
• With a hookstick, hook the lifting eye of the ELF
fuse.
• Raise the fuse towards the lower contact of the
interchangeable cutout. Lower the fuse support
hinge into the lower contact.
• Move the hook of the hookstick into the pull ring of
the ELF fuse.
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Cooper Power Systems
• To properly close, the operator should rotate the
ELF fuse to an intermediate position as shown in
Figure 4.
Station Service Transformer Protection
Station service transformers are located in a substation
where fault current levels may exceed the interrupting
capabilities of the expulsion fuse cutout. Consequently,
this is an ideal application for the ELF fuse with its high
current limiting interrupting capability. In addition, without
the expulsive violence, safety is vastly improved.
• Quickly and firmly (with minimal side thrust) drive
the ELF fuse into the upper contact of the
interchangeable cutout.
Enclosure Mounting
Because it does not expel gases and molten material when
it operates, the ELF fuse could be used in enclosures.
Sufficient clearance, however, would have to be provided
to ensure room to drop out when the fuse operates.
INTERCHANGEABLE
CUTOUT
INSULATOR
PULL RING
CONCLUSION
CUTOUT
UPPER
CONTACT
The ELF current-limiting dropout fuse meets the industry's
needs for the application of current-limiting fuses. The
ELF Fuse:
CUTOUT
HANGER
• fits common, existing mounts
ELF FUSE
• operates silently
CUTOUT
LOWER
CONTACT
HOOKSTICK
• operates without expelling byproducts
DROPOUT
ACTUATOR
• is lightweight and easy to install
• limits peak let-thru currents and, thus, energy letthrough to failed equipment
Figure 4. Intermediate Position for the Installation
of ELF Fuse into Interchangeable Cutout
• drops open to indicate operation
ELF™ is a trademark of Cooper Power Systems
If a fault is present when installing an ELF fuse, the
dropout actuator will operate. When the hookstick is
removed, the ELF fuse will drop open. The operator will
then be unable to permanently close the operated ELF fuse
into the upper contact of the cutout.3 The operation is selfcontained as described above, without expulsion
byproducts, thus, providing increased safety to line
personnel. If the fault has been cleared, the ELF fuse will
remain intact in the upper contact. Thus, faults can be
located by using the ELF fuse instead of tools such as the
NX Fault Guard Tool, which requires a special high cost
fuse with special end fittings.
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September 1993
REFERENCES
1
H. Webb, K. Leix, C. Wahlgren, "Current-Limiting
Fuses Address Safety and Power Quality
Concerns" presented at 1992 Rural Electric Power
Conference, New Orleans, Louisiana, May 3-5,
1992; C4–1-C4-5.
2
M. Bishop, K. Leix, F. Muench, "Considerations in
the Use of Current-Limiting Fuses on Pole-Type
transformers" presented at 1991 IEEE Power
Engineering Society, T & D Conference, Dallas,
TX, September 22-27, 1991; pages 516-522.
3
Cooper Power Systems Service Information
Publication No. S240-66-1, "ELF Current-Limiting
Dropout Fuse Installation Instructions".
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