Two Different Grounds
Different grounds can have different potentials. You must be aware of different grounds on a network because electricity will flow whenever there is a difference in voltage.
Typically, each building has a different ground and you should assume this when designing a network.
Sometimes different floors of the same building have a different ground because they are on different transformers and each transformer is grounded.
If you use copper wire as backbone on a network between different floors or buildings, electricity may flow between the two points.
In this scenario, a person connecting network media could be shocked.