Page 2 of 17 The following consequences may occur due to overcurrent a

Page 2 of 17 the following consequences may occur due

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Page 2 of 17 The following consequences may occur due to overcurrent: - (a)prolonged period of overcurrent will cause deterioration of the insulation of conductor and finally may cause a short circuit fault. (b)damage to conductor or equipment (c)fire hazard (d)injury to personnel 2.0 Protection against Overload Current A protective device shall be provided to break any overload current flowing in the circuit conductors before such a current causes a temperature rise detrimental to insulation, joints, termination, or the surroundings of the conductors. Every circuit shall be designed so that a small overload of long duration is unlikely to occur. It has to be borne in mind that protection of current using equipment, or any associated flexible cord, however connected, is not necessarily given by the overload protective devices provided for the circuit conductors. In particular, this fact needs to be taken into account in the design of non-fused socket-outlet circuits and where a flexible cord of an incorrect rating could be inadvertently connected. 2.1 Co-ordination between Conductors and Protective DevicesCharacteristics of a protective device protecting a circuit against overload shall satisfy the following conditions: (a)IbInIz(b)I21.45 Iz In= nominal current or current setting of the protective device Ib= design current of the circuit Iz = the lowest of the current-carrying capacity of any of the conductors of the circuit I2 = current causing effective operation of protective device within the conventional time The following devices will comply automatically with the condition (I21.45 Iz) above if the nominal current or current setting of the devices are selected so as not to exceed the lowest of the current-carrying capacities of any of the conductors in the circuit: (i)fuses to BS 88 part 2 or part 6 or BS 1361 (ii)miniature circuit breakers (MCB) to BS 3871 Part 1 If the device is a semi-enclosed fuse to BS3036, compliance with the requirement (I21.45 Iz) is afforded if its nominal current does not exceed 0.725 times the current carrying capacity of the lowest rated conductor in the circuit protected. The overcurrent protective devices are provided in a LV electrical distribution network in building as shown in the following electrical schematic wiring diagram.
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