One of the fundamental teachings of electrical engineering is that we reference all circuits to what we call a common ground and that we call that 0 V. This means that there is zero potential energy difference along the common ground.
Well wouldn't you know that I spent the last two days of my life considering that ground may not be 0 V after all. In a great number of cases the ground is not 0 V. These cases usually involve some amount of distance. This distance may be greater than 25 m. Over a distance of 100 or 200 m there may naturally occur in the soil and between soil layers a potential difference of a few volts. With heavy machinery or lightening about the potential difference across a few hundred meters can be significant.
The downside of potential differences between different points is that if one is trying to communicate over this distance and the ground rises the communication will fail.
It turns out that the solution to this problem has been to divide large buildings up into different grounding zones. These zones are kept electrically separate without any conductors between them. Any communication between different grounding zones must be done with differential signalling.
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