Unwanted disturbances in the data lines between two electronic devices communicating with each other due to unbalanced reference voltages in the ground loop. Data signals can be altered, and electronic circuits can be damaged. Causes are poor building grounding at one of the electric panels, power supplies that filter unwanted noise into the ground line (ground noise injection) and voltage surges due to ground faults and lightning strikes.
RS-232 lines, long printer cables and USB cables are subject to inter-system ground noise, while optical fibers and Ethernet are not. There is no electrical current in optical fibers, and Ethernet adapters are electrically isolated from earth ground. See ground loop
Ground Loop Interference
Various causes can create an imbalance in the ground loop. When communications equipment uses earth ground as a voltage reference, the voltage differences can cause signal loss and hardware malfunction.