Cut Sheet

Volume 4—Circuit Protection CA08100005E—May 2015 www.eaton.com V4-T3-79
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3.3
Power Breakers, Contactors and Fuses
Fuses General
Expulsion vs. Current Limiting (Definitions per ANSI C47.40-1993)
An expulsion fuse is a vented
fuse in which the expulsion
effect of the gases produced
by internal arcing, either
alone or aided by other
mechanisms, results in
current interruption.
An expulsion fuse is not
current limiting and as a
result limits the duration of a
fault on the electrical system,
not the magnitude.
A current limiting fuse is a
fuse that, when its current
responsive element is melted
by a current within the fuse’s
specified current limiting
range, abruptly introduces
a high resistance to reduce
current magnitude and
duration, resulting in
subsequent current
interruption. Refer to Fuse
Types Protection Range
figure on Page V4-T3-80 for a
features comparison.
Fuse Types
There are three current
limiting fuse types: Backup,
General Purpose and Full
Range. It is important that the
user have an understanding
of these definitions to ensure
proper application of the fuse
(see Fuse Types Protection
Range figure on Page
V4-T3-80).
Backup Fuses
A fuse capable of interrupting
all currents from the
maximum rated interrupting
current down to the rated
minimum interrupting current.
Backup fuses are always
used in a series with another
interrupting device capable of
interrupting currents below
the fuse’s minimum
interrupting current.
General Purpose Fuses
A fuse capable of interrupting
all currents from the rated
interrupting current down
to the current that causes
melting of the fusible element
in no less than one hour.
General Purpose fuses are
typically used to protect
feeders and components
such as transformers.
Full Range Fuses
A fuse capable of interrupting
all currents from the rated
interrupting current down
to the minimum continuous
current that causes melting
of the fusible element,
with the fuse applied at
the maximum ambient
temperature specified by
the manufacturer.
General Fuse Component Terms
Fuse Refill Unit (of an
Expulsion Fuse)
A fuse refill unit is a
replaceable assembly
containing the calibrated
current-responsive fuse
element and certain other
items that facilitate current
interruption. On its own, the
refill unit has no interrupting
ability. A refill unit must be
mounted in a fuseholder with
a spring assembly to form a
refillable fuse unit. The refill
unit is the section of the fuse
that must be replaced after a
fuse operation.
Fuseholder (of an
Expulsion Fuse)
A fuseholder is a reusable
holder that when equipped
with a fuse refill unit forms
a fuse unit, capable of
interrupting an overload or
fault current. A fuseholder
is supplied with a spring and
shunt assembly, necessary
to complete the internal
interrupting assembly. The
spring and shunt assembly is
supplied with the fuseholder
but is also available as a
replacement part, as it may
need replacement after
several of heavy operations.
Fuse Unit
A fuse unit is a replaceable
unit or assembly that is
able, on its own, to perform
current interruption. In
the case of a refillable fuse
unit, the refill unit must
be replaced after a fuse
operation. Where a complete
fuse unit is supplied from the
factory, the complete fuse
unit must be replaced after a
fuse operation. All current-
limiting fuses are fuse units.
Exhaust Control Device
When expulsion fuses are
used in enclosures, exhaust
control devices (filters,
condensers or mufflers) are
used to control the sound
of the fuse operation, and
to de-ionize and absorb the
fuse exhaust products. These
devices are normally supplied
separately, because of
different characteristics and
ratings. They are reusable but
may need replacement after
several heavy operations.
Mounting
A mounting provides all the
necessary parts to safely
mount a fuse in its intended
piece of equipment. The
base is the metal support to
which all other pieces attach.
Insulators attach to the base
and insulate the live fuse unit
from the base and everything
beyond the base. Live parts
are the parts of the mounting
that are energized once
electricity is flowing. The live
parts provide the means to
hold the fuse unit in place,
electrical contact, and a
place to make line and load
connections.
Non-Disconnect Mounting
A non-disconnect mounting
does not provide a means for
removing the fuse unit until
the circuit is dead and the
fuse unit can be removed
manually. The fuse unit is
held in place by friction
through the use of fuse
clips or by a cross bar.