It is stipulated that the neutral wire is not allowed to be disconnected at will, so as to ensure that the terminal voltage of each phase load is symmetrical when the Y-connected asymmetric three-phase circuit works. If a fuse is installed, if one phase is short-circuited, the fuse on the middle line may blow, resulting in the disconnection of the middle line. If you accidentally pull off the switch when the three-phase load is working, it will also cause three-phase imbalance.
Switches and fuses are not allowed to be installed on the neutral line (i.e. neutral line and trunk line) of the four-wire neutral grounding power supply line. Otherwise, when the switch is turned on or the fuse is blown, it is equivalent to an open neutral line, which may generate dangerous voltage on the neutral line and threaten electrical equipment and personal safety.
Zero branch, that is, the zero line of single-phase lines supplying houses, office buildings, shops, warehouses, etc. Because it only works here, it is suggested to install switches and fuses on both the phase line and the zero line.
Extended data:
The reason for this is the following：
Compared with only installing fuses on phase lines, it can increase the chance of fuse blowing and is beneficial to short-circuit protection. If only switches and fuses are installed on the phase line, when the phase line and the neutral line are connected wrongly (which is easy to happen when repairing the line), the phase line will be unprotected.
When the phase line has a ground fault, the fuse on the neutral line will not fuse (because the short-circuit current does not pass through the fuse), and the main fuse of the transformer will not fuse (because this fuse has a large capacity), so the phase line is in a ground fault state, which poses a threat to the system.
However, there is also a problem in installing fuses on both the phase line and the zero line: when the line is short-circuited or overloaded, it is possible to blow the fuse on the zero line first. At this time, although the electrical equipment stops working, the equipment is still powered on. If the equipment is overhauled because it is thought to be out of power, an electric shock accident may occur.
In order to cut off the phase line and the neutral line at the same time, it is suggested to use a bipolar knife switch (with fuses in it) or a knife switch with two fuses to avoid the danger of cutting off the neutral line without cutting off the phase line.