I have been doing some research on this question. The answer I get most of the time is that this is for safety reasons. That there can be no current in the neutral that didn't come from the live. If anyone has a clear answer, please tell me.
- Cost: One fuse in the live is adequate to cut current to the circuit.
- Isolation: If the neutral fuse blows first the circuit would stay live. It's generally best to disconnect the circuit from mains.
- Polarity: Many countries don't use polarised plug and sockets on single-phase plugs. This means that the fuse in the appliance may indeed be in the neutral. So, to answer your question, "Why is it not advisable to fuse the neutral?" - we do it all the time in many countries.
Figure 1. Unpolarised American (120 V) and European (230 V) mains plugs.
Note that with the unpolarised plugs the fuse can only be guaranteed to protect against over-current in the device itself. e.g., a motor short-circuit will cause the fuse to blow. With the fuse in the neutral wire a short to earth would not cause high current to flow through the fuse.
Having a device appear to be electrically dead while its components are electrically live can be dangerous; if the neutral were fused, an overcurrent fault could easily create that dangerous condition unless the fusing assembly ensured that an overcurrent condition would disconnect both hot and neutral simultaneously. While it's possible to construct fuse assemblies in such fashion, such assemblies are generally much more expensive than those which only disconnect the wire through which excessive current is flowing.
- To ensure complete isolation with the removal of fuse.
- To make the total current always flows through the fuse.