Fuse

FUSE

The electrical equipment are designed to carry a particular rated value of current under normal conditions. Under abnormal conditions such as short circuits, overload, or any fault; the current rises above this value, damaging the equipment and sometimes resulting in fire hazard. Fuses come into operation under fault conditions.

A fuse is short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuits. Under normal operating conditions it designed to carry the full load current. If the current increases beyond this designed value due to reasons mentioned above, the fuse melts, isolating the power supply from the load.

(a) Desirable characteristics of a Fuse Element

The material used for fuse wires must have the following characteristics

(1) Low melting point e.g., tin, lead.

(ii) High conductivity e.g., copper.

(iii) Free from deterioration due oxidation e.g., silver.

(iv) Low cost e.g., tin, copper.

(b) Materials

Material used are tin lead or silver having low melting points. Use of copper or iron is dangerous, though tinned copper may be used.

(c) Types of Fuses

Fuses are classified into following types

(i) Re‒wireable or kit‒Kat fuse and

(ii) High rupturing capacity (H.R.C) cartridge fuse

Re‒wireable or Kit‒Kat Fuse

Re‒wireable fuse is used where low values of fault current are to be interrupted. These fuses are simple in construction, cheap and available up to a current rating of 200 A. They are erratic in operation and their performance deteriorates with time. An image of re‒wireable fuse is as shown in figure (7.9)

High Rupturing Capacity (HRC) Cartridge Fuse

Figure (7.10) shown an image of HRC cartridge fuse and the essential parts of a typical HRC cartridge fuse. It consists of a heat resisting ceramic body having metal end‒caps to which a silver current‒carrying element is welded. The space within the body surrounding the elements is completely packed with a filling powder. The filling material may be chalk, plaster of Paris, quartz or marble dust and acts as an arc quenching and cooling medium. Therefore, it carries the normal current without overheating Under normal loading conditions, the fuse element is at a temperature below its melting point. When a fault occurs, the current increases and the fuse element melts before the fault current reaches its first peak. The heat produced in the process vaporizes the melted silver element.

The chemical reaction between the silver vapors and the filling powder results in the formation of a high resistance substance which helps in quenching the arc.