To this point, the overhead ground wires or shield wires have been located so as to minimize the number of lightning strokes that terminate on the phase conductors. The remaining and the vast majority of strokes and flashes now terminate on the overhead ground wires. A stroke that so terminates forces currents to flow down the tower and out on the ground wires. Thus voltages are built up across the line insulation. If these voltages equal or exceed the line CFO, flashover occurs. This event is called a backflash. The origin of the word backflash is interesting. In the laboratory, an impulse is normally applied to the conductor and flashover occurs from the conductor to ground. For the backflash, the highest voltage is on the tower rather than on the conductor and flashover appears to occur from the tower or ground to the conductor. The flashover is backwards from that in the laboratory, thus the term backflash.