Fuse is a current interrupting device which breaks or opens the circuit by fusing the element when the current in the circuit exceeds a certain value. Fuses are mainly categorised into two types, i.e., High voltage fuses and Low voltage fuses. Low voltage fuses can be further divided into two classes, namely semi-enclosed or rewirable type and totally enclosed cartridge type.
Rewirable fuses are most commonly used in housing wiring and small current circuits. It is also known as a kit – kat fuse. It has a porcelain base which is carrying the fixed contact through which the live wires are connected.The fuse carrier is an independent part and can be taken out or stuck in the base without hazard.
The fuse wire may be of lead, tinned, copper, aluminium or alloy of tin-lead. The actual fusing current will about twice the rated current. When two or more fuse wires are used, the wire should be kept away, and the de-rating factor of the fuse element is 0.7 to 0.8. The fuse wire of any rating should not be more than 80 A. When the fault occurs, the fuse element fallout and the circuit are interrupted.
The fuse carrier takes off, the burst out fuse element is replaced by new element and the supply is repaired by re-inserting the fuse carrier in the base. Though such fuses have the advantages of easy replacement without any risk of coming into contact with a live part, and negligible replacement cost suffers from the following advantages.
Unreliable Operation – The operation of the rewirable fuse is unreliable because there is a possibility of renewal by the fuse wire of the wrong size. Also, the fuse wire deteriorates over a period due to oxidation through the continuous heating out of the element.
Small time Lag – Due to the small time lag, such fuses can blow with the transient currents which are encountered during the starting of motors and switching out the operation of transformers, capacitors, fluorescent lights, etc. Unless fuses of sufficiently high rating are used.
Low rupturing capacity – Rewirable fuses have limited breaking or rupturing capacity. For example the rewirable fuse of 16 A normal current having the breaking current of 2 KA and those up to 200 A normal current having a breaking current of 4 kA.
Slow Speed of Operation – No special mean is used to be abolished the arc that blows after the fuse melts. Thus arcing time is more in such fuses.
The risk of Flame and Fire – Because of simplicity and cheapness rewirable type fuses are extensively used as protective devices against overload and short circuit on low and medium voltage circuit.
Totally Enclosed or Cartridge Type Fuses
The fuse element is closed in a closed container and is provided metal contacts on both the sides. The fuses are further classified as D-types and link type. Link type cartridge fuses are again of two type viz., knife blade or bolted.
D-Types Cartridge Fuse
It is a non-interchangeable fuse comprising a fuse base, adapter ring, cartridge and a fuse cap. The cartridge is moved in the fuse cap, and the cap is tightened on the fuse base. After the complete screwing, their tip touches the conductor, and their circuit is completed through the fuse link.
The standard ratings are 6, 16, 32, and 63 amperes. The breaking or rupturing capacity is of the order of 4 kA for 2 and 4 amperes fuses and 16 kA for 6 or 63 A fuse. D-type cartridge fuses have none of the drawbacks of the rewirable fuses. Their operation is reliable. Coordination and discrimination to a reasonable extent are achieved with them.
Link Type Cartridge or High Rupturing Capacity Fuses
High rupturing capacity cartridge fuse, commonly called HRC cartridge fuses. The outer body is usually made of steatite; a ceramic material has good mechanical strength. The fuse element is connected to two end caps of brass or copper in the body.
The metal end caps are screwed to the ceramic body using special forced screws to withstand the pressure developed under short circuit condition. End contacts are welded to the metal end caps. An indicating device is provided to distinguish the blown a fuse. The space between the fuse element and the cartridge body is filled with power, pure quartz which acts as an arc extinguishing agent.
Here quartz powder sand is used as a filler because it can absorb heat at a very high rate and it does not involve an appreciable amount of gas. After absorbing heat, the quartz sand is converted into glass beads which obstruct the path of restriking voltage.
The fuse element is of the silver or copper with a special shape. Normally, the fuse element has two or more sections joined by mean of a tin joint. The purpose of the tin joint is to limit the temperature of the fuse under small overload conditions. The melting point of silver is 980ºC while that of pure tin is 240ºC. As the circuit is overloaded the melting of tin prevents the silver element from attaining high temperature.
The usual fusing factor for link fuses is 1.45. The fuses for a special application may have a low fusing factor as 1.2. There are two types of such fuses such as knife blade type and bolted type.