Even though all data lines stay within the building, communication interfaces are still susceptible to damage. There are two reasons for this. 1. Induced voltages from a nearby lightning strike when control/communications lines run near electrical power wires, metal in the building structure, or near lightning rod ground leads. 2. Differences in AC power voltage references between two devices connected together by control/communication lines. When an event, such as a nearby lightning strike, migrates in on the AC power, individual equipment within the building can see large voltage reference differences. When these devices are connected together by low voltage control/communication lines, the control/communication lines try to equalize the difference, thus causing damage to the interface chips.