Circuit Protection-- Fuses
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Circuit Protection-- Fuses
 
Article Introduction
Unexpected power cut has always been unwelcome guest, creating nuisance and money loss. But knowing factors responsible for it and preventing it in the right time could prove out to be a big benefactor

Article Description
Circuit protection has always been main concern in industries. To add insult to the injury, the monetary losses play a lead role in such unthough 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 overcurrent 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.
Note: -
Although overcurrent 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.
Overcurrent device should not be considered as Life saving device
Overcurrent 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 overtemperature 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 overcurrent 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.
Posted : 12/6/2005

 
 
Circuit Protection-- Fuses