Ground loop (electricity)

ground loop (electricity)



In an electrical system, ground loop refers to a current, generally unwanted, in a conductor connecting two points that are supposed to be at the same potential, that is, ground, but are actually at different potentials. Ground loops can be detrimental to the intended operation of the electrical system.

Description

A ground loop in a medium connecting circuits designed to be at the same potential but at different potentials can be hazardous, or produce problems to the electrical system. For example, the electrical potential at different points on the surface of the Earth can vary by thousands of volts, primarily from the influence of charged clouds. Such an occurrence can be hazardous, for example, to personnel working on long metal conductors.

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 flowing 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 to flow through the cables, eg: 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 to the electrical distribution system at one point. The next is to have shielded cables for the low currents, with the shield connected only at the source end. 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, eg: computer. These chokes are most often called ferrite core devices.

In video, ground loop 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 case 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. Alternatively, one can use a surge protector that includes coax protection. If the cable is routed through the same surge protector as the 3-prong device, both will be regrounded to the surge protector.

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 actually grounded at 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.

Ground and ground loops are also important in designing circuits. In many circuits, large currents may flow 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 together. This external ground should be connected to the ground plane of the PCB at only one point -- this avoids large current flowing through the ground plane of the PCB. 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 flowing 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.
Power devices should be placed closest to the power supply, while low-power devices can be placed further from it.
Signals, wherever possible, should be differential. Use differential signaling.