conductor is applied to circuit 2 and added to its output: Thus the two circuits are no longer isolated from each other and circuit 1 can introduce interference into the output of circuit 2. If circuit 2 is an audio system and circuit 1 has large AC currents flowing in it, the interference may be heard as a 50 or 60 Hz hum in the speakers. Also, both circuits have voltage on their grounded parts that may be exposed to contact, possibly presenting a shock hazard. This is true even if circuit 2 is turned off. Although ground loops occur most often in the ground conductors of electrical equipment, similar loops can occur wherever two or more circuits share a common current path, which can cause a similar problematic voltage drop along the conductor if enough current flows. Common ground loops A common type of ground loop is due to faulty interconnections between electronic components, such as laboratory or recording studio equipment, or home component audio, video, and computer systems. This creates inadvertent closed loops in the ground wiring circuit, which can allow stray 50/60 Hz AC current to be induced and flow through the ground conductors of signal cables. The voltage drops in the ground system caused by these currents are added to the signal path, introducing noise and hum into the output. The loops can include the building's utility wiring ground system when more than one component is grounded through the protective earth (third wire) in their power cords. Ground currents on signal cables The symptoms of a ground loop, ground noise and hum in electrical equipment, are caused by current flowing in the ground or conductor of a cable. Fig. 1 shows a signal cable S linking two electronic components, including the typical line driver and receiver amplifiers (triangles). The cable has a ground or shield conductor which is connected to the chassis ground of each component. The driver amplifier in component 1 (left) applies signal V1 between the signal and ground conductors of the cable. At the destination end (right), the signal and ground conductors are connected to a differential amplifier. This produces the signal input to component 2 by subtracting the shield voltage from the signal voltage to eliminate common-mode noise picked up by the cable If a current I from a separate source is flowing through the ground conductor, the resistance R of the