Consider for a moment what the grounding of a secondary circuit involves. Take the simple case of a two-wire, 120- olt circuit fed by a transformer. The transformer keeps the difference of potential between the two wires of the circuit at 120 volts, but, in the absence of grounding, the potential of the circuit with respect to earth is indefinite. If there is no foreign electrical influence, the average potential of the two conductors will hover around zero or earth potential, but if one of the conductors becomes crossed with a foreign wire of high potential to earth, it will mmediately become charged to the same high potential (assuming that no breakdown of insulation occurs). This brings about a hazardous state of affairs, for if a person in contact with earth or with any grounded metal should touch the charged conductor, he would be liable to receive a dangerous shock. Such hazardous condition might be produced by an accidental cross with a foreign wire or by breakdown of insulation of the transformer in such way as to impress potential from the primary circuit upon a secondary circuit conductor.
As a safeguard against such hazard, it was proposed that secondary circuits be grounded, i.e., that one conductor, in case of a two-wire circuit, or the neutral conductor, in case of a three-wire circuit, be connected to earth, thus assuring that no conductor of such grounded secondary circuit could have a potential to earth greater than the normal circuit voltage.