The phone line is not concerned with the meaning of the signal that it carries, it is merely moving those signals from one point to another physically. This is the physical layer, the conduit in which communications pass from one point to another. Although the physical layer is working, the language or protocol is not correct, and communications cannot exist. The industrial world has developed a variety of different physical and protocol communications standards. This allows a greater distance between the transmitter and receiver.
Industrial buses cover a large area.
Often different areas of the network are supplied by different power sources. Even though all of the sources are grounded, a voltage difference can exist between the grounds of these voltage sources. Another source of excess voltage potential can be caused by intermittent sources. Power line surges and lightning are causes of this type of disturbance, but other causes, such as large electric motors starting and stopping, can temporarily affect the ground reference voltage. Additional protection can be achieved by using a fiber optic link between the network and areas known for voltage problems such as a power house or a water treatment plant.