In many installations, the ground potential between one rack and another, or between one point in a building and another, may be different. When two points are connected that do have a potential difference, this causes a ground loop. A ground loop is the flow of electricity down a ground wire from one point to another. Any RF or other interference on a rack or on an equipment chassis connected to ground will now flow down this ground wire, turning that foil or braid shield into an antenna and feeding that noise into the twisted pair. Instead of a small area of interference, such as where wires cross each other, a ground loop can use the entire length of the run to introduce noise.
If the building can be installed with a Star Ground, the ground potential will be identical throughout the building. In this case, the connection between any two points will have no potential difference. However, if this modification is not possible, the following solutions are available::
Use a Ground Telescopic system which is based on literally cutting the ground at one end of the cable. The cut should be on the load end (which, for analog audio, is the high impedance end), while maintaining the connection at the source end. However, it will lose 50% of shielding effectiveness.
Using an Audio Isolator with Ground Lift: in addition to providing a high CMRR it will lift the ground.
Using an unshielded cable: leaving all responsibility for noise rejection to the balanced pair.
Bibliography: Handbook for Sound Engineers. Fourth Edition. Glen Ballou. Focal Press.
