Always with appliances V= 230V FUSES A fuse is simply a wire that melts when the current exceeds a particular value To decide what fuse to use in a domestic appliance we need to find out the POWER of the appliance, and use P = IV To work out the current. Always with appliances V= 230V The appropriate fuse to use is the one with current closest to, but slightly ABOVE the rated current, calculated from above. Choose from 3A or 13 A

FUSES When several appliances are used together, the total power is just added together, to find the total current supplied.

Fuse is an overcurrent protection device which consists of a metal wire or strip which melts down in a condition of overcurrent and isolates the circuit.

How do they work A fuse consists essentially of a metal strip or wire that melts at a lower temperature than the wire and other components in the rest of the circuit. Resistance of the fuse element is constant. If current in the circuit exceeds more than the rated current, then the heat generated also exceeds and which causes the meltdown of fuse wire. Fuse is connected in series in a circuit, due to the rupturing it halts the current flow and thus protecting the devices from overcurrent damage. Healthy Fuse Blown Fuse

Types of fuses Time delay fuses: They allow temporary and harmless increases in the electrical current to pass without opening the fuse. Instead, the fuse will only open on sustained overloads and short circuits. Time delay fuses are used in circuits that experience a temporary current surge when the power is first turned on, such as in motors, transformers and incandescent lamps. Fuses with a very long time delay will have two different elements that must melt before the circuit will be opened.

2) Fast acting fuses: Fast-acting fuses are used in circuits that do not experience an increase in the current when the power is turned on, because the fast-acting fuses are not designed to withstand temporary overload currents. Their elements have less mass than the elements in time delay fuses, which causes them to heat up and melt more quickly. 3) Very fast acting fuses: Very fast-acting fuses limit the magnitude and the duration of the current flow during excessive current flow. They open within milliseconds of a current overload. They are used to protect the electrical equipments which are very sensitive to overload currents such as semiconductor circuits.

Other classification of fuses Blade fuse: These kind of fuses are generally used in automobile industry and are rated for a DC voltage not more than 24V. Plug fuse: It is a fuse with a thread that screws into a socket. It looks somewhat like mushroom. To insert a new plug fuse, simply hold the head of the fuse and screw the base into the fuse outlet, thus “plugging” the hole. They are rated generally for an A.C. voltage less than 125V and current rating of less than 30A.

Cartridge fuses: They are useful in all types of buildings, and they can handle a larger electrical current than do plug fuses, so they are used for different circuits. Depending on the type of cartridge, these fuses work on circuits rated for 250 or 500 volts, and they have a capacity of approximately 600A.

Fuse Construction A typical fuse is made up of following elements:-- Fuse element Fuse Body Terminal blades or end caps Fuse element: Selection of fuse element is heavily affected by the rating of the fuse and the time delay required. It is generally made of alloys with certain proportions of elements depending upon the rating and melting point. Elements generally used are Copper, Silver, Aluminum, Lead and silver alloys. Cross section area determines the current handling capability of the fuse. Thickness and the material used determines the melting point of the element.

Fuse Body: Body of the fuse should be such that it can withstand the mechanical forces and the heat produced during the operation of the fuse. Moreover, it should be capable of providing proper electrical insulation between the two points after the fuse is blown. It can be made of transparent glass, ceramics or fiber glass. Terminal blades or End caps: End caps hold the element between the two ends of the fuse. Terminals are provided, in some high current fuses for the ease of installation. End caps and terminals are made of copper to provide low resistance. In plug fuses element and screw wends are made of copper.

Fuse Construction ( continued) Arc quenching and cooling: It is important to protect the fuse from arc during its operation. It is achieved by two ways: By creating a vacuum in the fuse body. It protects oxidation of the fuse element. Filling the fuse with quartz sand which act as cooling agent and also removes air from the fuse thus eliminating oxidation. It also helps in arc quenching by making a high resistance glass that is formed under high heat when the fuse element melts.