why is it not allowed for some equipment to be connected to zero while the other equipment is grounded through protection?

When protective grounding is used for electrical equipment that collides with the shell, due to the much greater resistance of the ground than the resistance of the neutral line, a short-circuit power supply is formed through the grounding electrode of the shell and the ground, which is often insufficient to cause automatic switch and safety action. The grounding power supply, in turn, increases the neutral point potential of the power supply, causing all electrical equipment shells or cabinets connected to the neutral line to have ground voltage, which will cause more chances of electric shock.