Answer Posted / avs
The main function of fuse is to prevent continuous flow of faulty current through the system. Whenever there is an over current flowing through the fuse element for a certain period of time, the fuse element melts and disconnects the circuit to prevent continuous flow of faulty current. Hence it is desirable to flow entire faulty current through the fuse element during any faulty condition that means it is desirable to insert a fuse unit at that point of the circuit through which entire current will flow even in the faulty condition. Now let us assume the situation where fuse element is inserted into the neutral path of the circuit. In that case in earth fault condition of the circuit, even the neutral fuse is blown out, the phase of the circuit will still get neutral or earth path through the fault of the system and hence there will be still huge current drawn from the supply by the system even after neutral fuse is blown out. Hence the faulty portion of the system is not at all isolated from the mother system although the neutral fuse is melted and disconnected.
Neutral is nothing but the return path of the current through the load. If the fuse unit inserted on phase is disconnected there will be no further current drawn from the source by the circuit, hence there will be no return current through the neutral path also. Hence after disconnecting the fuse unit of live phase, there will be no further need of disconnecting neutral path from the source. Hence there is no need of inserting fuse unit in the neutral path.
In three phase system, the neutral only carries the summation of the currents in three live phases and maintains the star point of the system at earthed potential. If a fuse element is inserted into the neutral path of a three phase system and accidentally it is disconnected then the star point will no longer be at earth potential instead it would be floating which is not desirable for three phases with neutral earthed system.
From safety point of view, fuse unit should not be inserted in neutral even in the single phase system because the single phase load is off either phase or neutral fuse is blown out. If only neutral fuse is blown out, there will be still a live voltage on the faulty equipment and hence by mistake if anyone touches the life conducting part of the equipment by thinking that the equipment is totally dead, he will get saviour electrical shock.
These are the reasons for which fuse unit is not provided in neutral.
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