The NEC recognizes that sensitive plug-in equipment such as cash registers, personal computers, printers, etc. can be adversely affected by currents flowing in common EGCs, such as conduit, green or bare ground conductor, building steel, etc. To minimize such problems, the NEC permits an insulated grounding conductor to be run from the insulated grounding terminal of the receptacle back to the neutral-ground bond at the power source (main panel or secondary of a separately derived system). This conductor must be run in the Same wire way, conduit, or raceway, with the conductor serving that receptacle load.
This separate conductor is usually green with a yellow stripe. The conductor should not be connected to any grounding buses or common points between the receptacle load and the neutral-ground bond at the power source. This system can eliminate much of the common mode (neutral-ground) noise on building power systems that can impact the reliable performance of plug-connected sensitive electronic equipment. To avoid any induced electrical noise into the insulated conductor ground-return path, only the supplied receptacle conductors should be in the metallic raceway and no other conductors permitted.
The isolated ground system reduced the noise, which had come from the power ground system via multiple grounding connections. In fact, it was called, in some parts of the world, a quiet ground. Since additional impedance had been introduced into the circuit, some noise was still picked up unless all interconnected electronic equipment was grounded to the same electrode group. Voltages can exist in the earth between electrodes even a meter apart; these voltages could be introduced into the different parts of an electronic equipment system connected to separate grounding electrodes.
Reference: IEEE 142