Inside building there are situations where you can see 60-115 V AC between different equipment grounding points. When going outside the building the voltage differences can between different grounding points can become larger. In electrical engineering, Earth Potential Rise (EPR) also called Ground Potential Rise (GPR) occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance from the source. Ground potential rise is a concern in the design of electrical substations because the high potential may be a hazard to people or equipment. The ground potential rise near high voltage substation can be hundreds of volts, even thousands of volts during the ground fault for a short time.
GPR will occur at several locations simultaneously. Fault current will divide among all circuit paths back to the source (metallic and earth return, for example) and create GPRs in the process. Metallic return paths include overhead ground wires, multi-grounded neutrals, bonds, station ground grids, and other conducting materials. Hazardous voltages can appear suddenly as a result of power faults or lightning strikes. Conductive objects (Copper telephone cable, metal, damp saline soils, etc.) can become energized or carry a harmful potential that, if not properly protected, can cause serious injury.
Ground potential rise is also an issue with cell phone base stations. Lightning strokes can hit the base station antenna. And those base stations are also sometimes co-located nearby high voltage electrical lines.
The use of an all-dielectric fiber optic cable in place of a copper telephone cable eliminates the possibility for high voltage to travel from one end of the system to the other. It is of utmost importance to always determine the proper protection scheme when dealing with telecommunication wire line circuits between any location where high voltage and high current equipment is co-located with these circuits. It is always best to design for the worst case situation, since more likely than not, this can happen. Protection against both High voltage and ground potential rise is the best precaution against equipment failures and human injury.
Always when operating with grounding issues remember that there is no absolute ground. There is always a certain amount of resistance to electrical current between all grounding points. This resistance can change with humidity, temperature, connected equipment and many other variables. And there is always a change that some fault current can get to that resistance.