Selection of Right- Flex : It is also important that the flex we choose is appropriate to the appliance used. Like fuses, the cables and flexes are also rated according to their current carrying capacity. The selection of a right flex, therefore, necessitates the knowledge of the I= FV/V formula.
Each flex and cable is rated according to the maximum current it will safely carry. The conducting ability or the current carrying capacity of the metallic conductor depends on its cross sectional area (measured in mm2) and therefore the diameter (in mm). However, the cables and flexes are rated in terms of maximum allowable current (in amperes) through them. This rating is given in the supplier's catalogue or is stamped on the spool or cable container. Common, general purpose flex or cable is usually covered with p.v.c. for insulation and is available in a variety of ratings, e.g. 5A, 1 OA, 15A etc. It is dangerous to use a cable where there is a chance that it might carry current in excess of its rated value. Heat would be generated in the cable and, at a certain level, the current could cause the wire to bum and melt the insulation. This is a common cause of fires. This does not mean that we use flexes/cables of very high rating. One must use the cable appropriate for a given application. The correct flex for a piece of electrical equipment is the one which allows the safe passage of the current drawn by the appliance. It would be uneconomic, for example, to use an expensive heavy duty heating circuit cable rated at 30A to connect a 60W reading lamp to the mains. At the opposite extreme, it would be unsafe to use a 3A lighting circuit flex as the laboratory power supply cable. You have read about the dangers of using the wrong cable. So this is another contributory factor to electrical safety in the lab. Here are two questions to check your knowledge on this aspect. You have to tick (J) mark the correct answer.