A withstand voltage test is a test of how high voltage can be applied between two conductors that are not electrically in contact with each other. 

A withstand voltage test is required only when two conductors are close together, one of which has electricity and the other is exposed to come into contact with the human body, or when they are grounded. 

In other words, a withstand voltage test is required if electricity is inserted between two wires or one conductor, and the two conductors touch each other or the insulation between them is poor, causing a short circuit or electric shock.