Selection of Proper Fuse : Fuse is a safety device or a wire of metal in a cut-out which may be fused by an excessive current. The current drawn by an appliance is restricted by the resistance in the circuit. In case of short circuit the resistance decreases and the current increases. This current produces heat in the circuit and can burn the equipment or the connecting wire. Therefore a fuse is provided in the circuit which blows off when the value of the current increases beyond its normal current rating and the faulty circuit is disconnected.
The selection of a proper fuse for a given instrument or appliance depends on the amount of current being drawn by the same. As we have discussed earlier, size of electric current is very important in relation to the effect of an electric shock on our bodies. We also need to know the size of current for each electrical appliance in the laboratory, for just as our bodies are limited to the current they can withstand, so the electrical appliances, wiring, plugs and other apparatus are limited to the current they can handle. If the flex, plug or fuse of an appliance is inappropriate to the amount of current drawn by that appliance, untold damage or injury could result. Therefore, to take a decision on the fuse to be used, one must know how to calculate the current drawn by an instrument.
Let us learn how to find out the current drawn by any particular appliance. First of all you should find out the power rating of the appliance. This is given in watts on the specification plate on most appliances. Then you would apply the formula, Where I is the current in amperes (A) which you are trying to find out, W is the power rating of the appliance in watts (W), and V is the measure of electromotive force (EMF) in volts (V), commonly known as voltage. We can illustrate this by doing the following calculation.
What current does the 100 watts light bulb draw from 240 V mains?
Applying the formula given for current calculation, i.e a 100 watts bulb will draw 0.42 A of current from 240 V mains. A fuse of 2 A may be chosen for this. What if the voltage drops down; a common phenomenon in our country. It may drop to say 200 V which means it will now draw 1001200 - 0.5 A. Therefore fuse rating is normally kept somewhat higher than that computed by the formula.
Complex equipment often contains a number of fuses of different ratings in different parts of the circuit to protect various components from damage by ensuring that a current exceeding a certain value cannot pass through them.
However, for domestic purposes or even in the laboratory /institution we do not put fuses for each fan, tube light or other appliances. Instead, the whole building's wiring in divided into a number of circuits say for each room, a block or a laboratory etc. and are individually protected by suitable fuses. The selection of fuse in such a case is done by computing the total load on the circuit. For example, a room having 2 fans of 80 W each, 6 tube lights of 40 W each will have a total load of 400 W. The use rating for this room will be calculated taking the total load in consideration. Why don't you try and solve some problems based on current calculation/selection of fuse to clearly understand these concepts.