Why should the switch box be grounded?    

The metal shell of switch box must be grounded reliably. When the phase line in the switch box is insulated with the outer casing, the human body will not be dangerous when touching the switch box. But when the insulation between the phase line and the outer casing is not good, even with short circuit, the risk of electric shock will occur.     

When the arm touches a working switchbox and a grounded handrail at the same time, the knife switch L3 phase contacts in the switchbox are burned out and loosened, the wire insulation connected to L3 phase is damaged, and collided with the switchbox enclosure, so the switchbox enclosure is electrified. The electric shock current flows in from the finger, flows out of the arm, and flows into the earth through the grounding handrail. So they are struck by electricity. 

If the shell of the switch box is not grounded, it is not dangerous under normal conditions, but it can’t be guaranteed to be normal all the time. Because of the loose connection of the switch, it will cause the shell charged, and there may be other conditions to make the shell charged. So if the voltage in the switch box exceeds the safety voltage, the metal shell of the switch box must be grounded reliably.