Ground loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed equipment are a major cause of noise and interference in audio and video systems. They can also create an electric shock hazard, since ostensibly "grounded" parts of the equipment, which are often accessible to users, are not at ground potential.
How it works 
The circuit diagram, right, illustrates a simple ground loop. Two circuits share a common wire connecting them to ground, which has a resistance of . Ideally, the ground conductor would have no resistance (), so the voltage drop across it, , should be zero, keeping the point at which the circuits connect at a constant ground potential, isolating them from each other. In that case, the output of circuit 2 is simply . However, if , it and will together form a voltage divider. As a result, if a current, , is flowing through from circuit 1, a voltage drop , across will occur and the ground connection of both circuits will no longer be at the actual ground potential. This voltage across the ground conductor will be applied to circuit 2 and added to the 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 will 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 they occur most often in the ground conductors of electrical equipment, ground loops can occur wherever two or more circuits share a common conductor or current path, if enough current is flowing to cause a significant voltage drop along the conductor.
A ground loop in a system which connects circuits designed to be at the same potential but which are actually at different potentials can be hazardous, or cause problems with the electrical system, because the electrical potential and soil resistance at different points on the surface of the earth can vary.
In a floating ground system, that is, one not connected to earth, the voltages will probably be unstable, and if some of the conductors that constitute the return circuit to the source have a relatively high resistance, or have high currents through them that produce a significant voltage (I·R) drop, they can be hazardous.
Low current wiring is particularly susceptible to ground loops. If two pieces of audio equipment are plugged into different power outlets, there will often be a difference in their respective ground potentials. If a signal is passed from one to the other via an audio connection with the ground wire intact, this potential difference causes a spurious current through the cables, creating an audible buzz at the AC mains base frequency (50 or 60 Hz) and the harmonics thereof (120 Hz, 240 Hz, and so on), called mains hum. Sometimes, performers remove the grounding pin from the cord connecting an appliance to the power outlet; however, this creates an electrocution risk. The first solution is to ensure that all metal chassis are interconnected, then connected to the electrical distribution system at one point (often referred to as a "single-point ground"). The next solution is to have shielded cables for the low currents, with the shield connected only at one end (this, however, increases the possibility of radio frequency interference (RF) since the shield may act as an antenna). Another solution is to use isolation transformers, opto-isolators, or baluns to avoid a direct electrical connection between the different grounds. However, bandwidth of such is of consideration. The better isolation transformers have grounded shields between the two sets of windings. In circuits having high frequencies, such as computer monitors, chokes are placed at the end of the cables just before the termination to the next appliance (e.g., the computer). These chokes are most often called ferrite core devices.
In video, ground loops can be seen as hum bars (bands of slightly different brightness) scrolling vertically up the screen. These are frequently seen with video projectors where the display device has its case grounded via a 3-prong plug, and the other components have a floating ground connected to the CATV coax. In this situation the video cable is grounded at the projector end to the home electrical system, and at the other end to the cable TV's ground, inducing a current through the cable which distorts the picture. As with audio ground loops, this problem can be solved by placing an isolation transformer on the cable-TV coax.
Ground loop issues with television coaxial cable can also affect any connected audio devices such as a receiver. Even if all of the audio and video equipment in, for example, a home theater system is plugged into the same power outlet, and thus all share the same ground, the coaxial cable entering the TV is sometimes grounded to a different point than that of the house's electrical ground by the cable company. The potential of this ground is likely to differ slightly from the potential of the house's ground, so a ground loop occurs, causing undesirable mains hum in the system's speakers.
A cheap and often easily available way to resolve this problem is a to use a pair of back-to-back 300-75 ohm transformers (which may also be known as baluns), as commonly used with TV antennas. The 300 ohm twin lead of the transformers is simply wired together using any available means, resulting in a short jumper with coaxial connectors at each end that can be easily inserted in-line. (The transformers work by breaking the ground path from the TV set to the cable company ground, since the two sides are now AC coupled only)
Ground and ground loops are also important in designing circuits. In many circuits, large currents may exist through the ground plane, leading to voltage differences of the ground reference in different parts of the circuit, leading to hum and other problems. Several techniques should be used to avoid ground loops, and otherwise, guarantee good grounding:
- The external shield, and the shields of all connectors, should be connected. If the power supply design is non-isolated, this external ground should be connected to the ground plane of the PCB at only one point; this avoids large current through the ground plane of the PCB. If the design is an isolated power supply, this external ground should be connected to the ground plane of the PCB via a high voltage capacitor, such as 2200pF@2KV. If the connectors are mounted on the PCB, the outer perimeter of the PCB should contain a strip of copper connecting to the shields of the connectors. There should be a break in copper between this strip, and the main ground plane of the circuit. The two should be connected at only one point. This way, if there is a large current between connector shields, it will not pass through the ground plane of the circuit.
- A star topology should be used for ground distribution, avoiding loops.
- High-power devices should be placed closest to the power supply, while low-power devices can be placed farther from it.
- Signals, wherever possible, should be differential.
- Isolated power supplies require careful checking for parasitic, component, or internal PCB power plane capacitance that can allow AC present on input power or connectors to pass into the ground plane, or to any other internal signal. The AC might find a path back to its source via an I/O signal. While it can never be eliminated, it should be minimized as much as possible. The acceptable amount is implied by the design.
See also 
- Sound System Interconnection — from Rane Corporation
- Grounding and Shielding Audio Devices — from Rane Corporation
- Signal purity
- Information technology in combination with medical devices Risks and solutions for electrical safety