Fuse is most common protection system for electrical circuit system. It does not matter how big or small is the circuit system it must gone through a physical fuse protection system. Generally a fuse consists of a replaceable part (Fuse-link) and a fuse holder. You may already see several type of fuse during work. In this post I will discuss about the principle of fuse operation with design.
Normally the simplest fuse-link is a length of wire. It uses to mount by screw connections in a holder, which partly encloses it. When over current or short circuit current flows through the wire then the wire starts to melt and arcing commences at various positions along it. Arc voltage causes the current to fall and once it has fallen to zero then the arcs are extinguished.
If the wire cross section is larger, the larger is the current that the fuse-link will carry without operating. In the United Kingdome, fuses of this specification and type are specified for use at voltages up to 250 Volts and currents up to 100 Ampere. This type of fuses is known as semi-enclosed or re-wireable fuses. But the most common fuse-link is the cartridge type. This type of fuse consists of a barrel (Most usually of Ceramic) containing one or more elements which are connected at each other at the end to caps fitted over the ends of the barrel. If a high level current breaking capacity is required then the cartridge is filled by sand of high chemical purity and controlled grain size. Then the entire fuse-link is replaced after the fuse has operated and a fault has been disconnected.
Cartridge type fuses are used for a much wider range of voltages and currents than the semi-enclosed fuses. The fuse-links can be divided into two types like current limiting and non-current limiting types. Now sand filled cartridge fuse-link is of the current-limiting type and when it operates then it limits the peak current to a value which is substantially lower than the prospective current. A non-current limiting fuse such as a semi-enclosed fuse, does not limit the current significantly during fault. Normally melting occurs first at the notches when an over current flows and these results in a number of controlled arcs in series. The voltage across each arc contributes to the total voltage across the fuse and this total voltage results in the current falling to zero limits. Because, the number of arcs is limited. The fuse-link voltage should not be high enough to cause any damage elsewhere in the circuit system. Now the main function of the sand is to absorb intense energy from the arcs and to assist in quenching them when a high current is disconnected and the sand around the arcs is melted.
The element is usually using silver, because of its resistance to oxidation. During oxidation of the element in service would affect the current that could be carried without melting. Because the effective cross section of the element is changed. Sometime silver plated copper elements also used. Some of the elements include an m-effect blob which can be deposited on wire or notched tape. The blob is kind of solder-type alloy and it has a much lower melting point than the element. If some current flows which is large enough to melt only the m-effect blob, the solder diffuses into the silver. This use to create a higher local resistance in the element and the fuse operates at a very lower current than it would have done in the absence of the blob. Other types of fuse include the expulsion fuse which is used at high voltage and the universal modular fuse (UMF) which is used on Printed Circuit Boards (PCBs). All Fuse should offer long life without deterioration in their characteristics or performance and cartridge type fuses have the particular advantage that contain the arc products completely.
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