Circuit protection has always been main concern in industries. To add insult to the injury,
the monetary losses play a lead role in such unsought circumstances. But, the crisis due to
power failure or short circuit could be avoided if proper technique is implemented in the
industry. One of the best ways to ensure this is by proper selection and installation of
electrical circuit components like fuse. A fuse is a short length of wire designed to melt
and separate in the event of excessive flow of current. Fuses are always connected in
series with the component(s) to be protected from surplus current; so that when the fuse
blows (opens) it will open the entire circuit and stop current through the component(s). A
fuse connected in one branch of a parallel circuit, of course, would not affect current
through any of the other branches.
How are fuses rated?: Fuses are rated depending upon their ability to carry current,
voltage and interrupt capacity (i.e., its potential to break in order to stop flow of current
through circuit in case of a short circuit). In addition to all these, they are also rated
depending upon their design to operate on AC or DC circuits.
Voltage rating: The voltage rating of the fuse is the highest voltage up to which a safe
fault current interruption will occur. Consider the maximum effective operating voltage
of the circuit. In such case, the rated current of the fuse should consider factors such as:
the maximum continuous operating current of the circuit and the influence of the ambient
temperature.
Interrupt capacity: The rated breaking capacity of the fuse (interrupting rating) is the
short circuit current at which the fuse can blow (at the rated voltage) without destruction
or arcing being maintained.
Types of fuses
The various types of fuses are as follows:
·
Plug fuses
·
Type S fuses
·
Cartridges fuses
·
Mini breakers
·
Circuit breakers
Plug fuses consist of a thin metallic element, which melts when the current flowing
through the fuse exceeds the defined capacity. Plug fuses are normally threaded and they
can simply screw into the opening of the fuse panel. Although, these fuses are relatively
reliable, problems crop up when they are replaced with higher rated fuses, as in many
cases, any size plug fuse will fit into any other plug fuse opening; however, an attempt to
fit inappropriate higher rated fuse would only lead to serious current overloads.
Type S fuses are tamper-resistant fuses manufactured to prevent switching fuse sizes in
the same fuse box opening. These fuses use an adaptor base and a fuse insert. Once an
adaptor base is installed, fuses with a higher rating cannot be installed in that fuse box
opening. Due to these tamper-resistant qualities, Type S fuses are the only type allowed
by the National Electrical Code in new fuse box installations.
Cartridge fuses normally have higher current ratings and are commonly seen in higher
capacity circuits such as; water heaters, or ranges; Simultaneously, it also acts as the main
fuse for the entire electrical system. These fuses are usually found inside of a fuse block,
which is part of the fuse box.
Mini breakers are ordinary plug fuses with a reset button, which acts like small circuit
breakers. This breakers encounter problem when reset buttons are taped down or pushed
down by the fuse box cover due to which the mini breakers may not operate as designed.
Circuit breakers are enhanced version of fuses, which provide the over current
protection that fuses do, but are less likely to be altered by the building owner or tenant.
They are also restorable which means that they do not need to be replaced when they
operate; they simply need to be reset. Circuit breakers have been common in new wiring
systems since the 1960s. This Switch-like device has gained popularity because they
don’t destroy themselves in the process of breaking the circuit as fuses do. Although
over current protection placement in a circuit may determine the relative shock hazard of
that circuit under various conditions, it must be understood that such devices were never
designed to protect against any electric shock. Neither fuses nor circuit breakers were
designed to open in the event of a person getting shocked; In fact, they are made to open
only under conditions of potential conductor- overheating. Ove rcurrent device should not
be considered as Life saving device Over current devices have been designed to serve
two basic purposes, which are as follows:
1.
It is made to protect the conductors of a circuit from over temperature damage
(and the fire hazards related with overly hot conductors).
2.
It is made to protect specific pieces of equipment such as loads and generators
(some fast-acting fuses are designed to protect electronic devices particularly susceptible
to current surges). Also, it should be kept in the mind that the current levels required for
electric shock or electrocution are much lower than the normal current levels of common
power loads, Hence, a condition of over current is not indicative of shock occurring.
There are other devices designed to detect certain chock conditions (ground-fault
detectors being the most popular), but these devices strictly serve that one purpose and
are uninvolved with protection of the conductors against overheating
To sum up
·
The fuse must carry the normal load current of the circuit without nuisance
openings. However, when an overcurrent occurs the fuse must interrupt the overcurrent,
limit the energy let-through, and withstand the voltage across the fuse during arcing.
·
Fuses are rated depending upon their ability to carry current, voltage and interrupt
capacity.
·
The various types of fuses are Plug fuses, Type S fuses, Cartridges fuses, Mini
breakers, and Circuit breakers.Fuses or Circuit Breakers are not fully foolproof devices to
avoid any electrical damages