 |
| Fuse |
A fuse is the simplest device for interrupting a circuit
experiencing an overload or a short circuit. A typical fuse,
like the one shown below, consists of an element electrically
connected to end blades or ferrules. The element provides a
current path through the fuse. The element is enclosed in a tube
and surrounded by a filler material.
The National Electrical Code® defines overcurrent as any
current in excess of the rated current of equipment or the
ampacity of a conductor. It may result from overload, short
circuit, or ground fault (Article 100-definitions). Circuit protection would be unnecessary if overloads and short
circuits could be eliminated. Unfortunately, overloads and short
circuits do occur. To protect a circuit against these currents,
a protective device must determine when a fault condition
develops and automatically disconnect the electrical equipment
from the voltage source.
Nontime-delay Fuses
Nontime-delay fuses provide excellent short circuit protection.
Short-term overloads, such as motor starting current, may
cause nuisance openings of nontime-delay fuses. They are best
used in circuits not subject to large transient surge currents.
Nontime-delay fuses usually hold 500% of their rating for
approximately one-fourth second, after which the current
carrying element melts. This means that these fuses should
not be used in motor circuits which often have inrush (starting)
currents greater than 500%.
Time Delay Fuses
Time-delay fuses provide overload and short circuit protection.
Time-delay fuses usually allow five times the rated current for
up to ten seconds. This is normally sufficient time to allow a
motor to start without nuisance opening of the fuse unless an
overload persists.
Fuse Ratings
Fuses have a specific ampere rating, which is the continuous
current carrying capability of a fuse. The ampere rating of a fuse,
in general, should not exceed the current carrying capacity of
the circuit. For example, if a conductor is rated for 10 amperes,
the largest fuse that would be selected is 10 amperes.
There are some specific circumstances when the ampere rating
is permitted to be greater than the current carrying capacity of
the circuit. For example, motor and welder circuits can exceed
conductor ampacity to allow for inrush currents and duty cycles
within limits established by the NEC.
The voltage rating of a fuse must be at least equal to the circuit
voltage. The voltage rating of a fuse can be higher than the
circuit voltage, but never lower. A 600 volt fuse, for example,
can be used in a 480 volt circuit. A 250 volt fuse could not be
used in a 480 volt circuit.
Fuses are also rated according to the level of fault current that they can interrupt. This is referred to as ampere interrupting
capacity (AIC). When applying a fuse, one must be selected
which can sustain the largest potential short circuit current
which can occur in the selected application. The fuse could
rupture, causing extensive damage, if the fault current exceeds
the fuse interrupting rating.
UL Fuse Classification
Fuses are grouped into current limiting and non-current
limiting classes based on their operating and construction
characteristics. Fuses that incorporate features or dimensions
for the rejection of another fuse of the same ampere rating
but with a lower interruption rating are considered current-limiting fuses. Underwriters Laboratories (UL) establishes and
standardizes basic performance and physical specifications
to develop its safety test procedures. These standards have
resulted in distinct classes of low voltage fuses rated at 600
volts or less. The following chart lists various UL fuse classes.
 |
UL Fuse Classification
|
Reference:
.webp)
No comments: