Consider the case where a workstation in Building A is sending data to another network device in Building B. The ground potential of each building will be a function of the impedance of its ground system and the current flowing through the ground. The data line, in addition to carrying data, is also connecting together the ground systems of the two buildings. If the ground potentials of Building A and Building B are different, a ground current flows in the data line. This is known as a ground potential difference. The voltage level of the data signals is increased or decreased by the ground potential difference, causing data transmission errors.