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Ground loop (electricity)
Encyclopedia
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.
The simplified circuit diagram on the right illustrates in principle how a ground loop works. Two circuits share a common wire connecting them to ground. Ideally the ground conductor should 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 this case the output of circuit 2 is simply 
. However, if the ground conductor has significant resistance, 
, and current
of magnitude
is flowing through it 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.
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 harmonic
s 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 transformer
s, opto-isolator
s or balun
s to avoid a direct electrical connection between the different grounds. However, bandwidth of such is of consideration. The better isolation transformer
s 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 projector
s 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. Alternatively, one can use a surge protector that includes coax protection. If the cable is routed through the same surge protector as the three-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 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 way to resolve this problem is a 75-ohm coax combiner/splitter and a matching Transformer. The 75-ohm coax combiner/splitter converts the impedance from 75 ohms to 300 ohms and the matching transformer converts the impedance from 300 ohms to 75 ohms. Both parts connected together will act as a "poor man's" isolation transformer.
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:
System
System is a set of interacting or interdependent components forming an integrated whole....
, 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
Potential
*In linguistics, the potential mood*The mathematical study of potentials is known as potential theory; it is the study of harmonic functions on manifolds...
, often ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
, but are actually at different potentials. Ground loops created by improperly designed or improperly installed equipment are a major cause of noise and interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
in audio and video systems. They can also create an electric shock
Electric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
hazard, since ostensibly "grounded" parts of the equipment, which are often accessible to users, are not at ground potential.
How it works
), 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 this case the output of circuit 2 is simply . However, if the ground conductor has significant resistance, , and currentElectric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
of magnitude
is flowing through it 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 shockElectric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
hazard. This is true even if circuit 2 is turned off.
Description
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
Floating ground
A floating ground is a circuit's ground that is not grounded, rather, it is just a common node in the circuit.This can occur in 3 possible ways* as the result of intentional design, and entirely harmless...
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
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
·R
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
) 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
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
) and the harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
s thereof (120 Hz, 240 Hz, and so on), called mains hum
Mains hum
Mains hum, electric hum, or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity, which is usually 50 Hz or 60 Hz, depending on the local power line frequency...
. 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 transformer
Isolation transformer
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety...
s, opto-isolator
Opto-isolator
In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is "an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output"...
s or balun
Balun
A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
s to avoid a direct electrical connection between the different grounds. However, bandwidth of such is of consideration. The better isolation transformer
Isolation transformer
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety...
s have grounded shields between the two sets of windings. In circuits having high frequencies, such as computer monitors, chokes
Choke (electronics)
A choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which...
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 projector
Video projector
A video projector is an image projector that receives a video signal and projects the corresponding image on a projection screen using a lens system. All video projectors use a very bright light to project the image, and most modern ones can correct any curves, blurriness, and other...
s 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
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
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
Isolation transformer
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety...
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 three-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 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 way to resolve this problem is a 75-ohm coax combiner/splitter and a matching Transformer. The 75-ohm coax combiner/splitter converts the impedance from 75 ohms to 300 ohms and the matching transformer converts the impedance from 300 ohms to 75 ohms. Both parts connected together will act as a "poor man's" isolation transformer.
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 together. If the power supply design is non-isolated, this external ground should be connected to the ground plane of the PCBPrinted circuit boardA printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
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 differentialDifferential signalingDifferential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...
. - 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
- Ground currentTelluric currentA telluric current , or Earth current, is an electric current which moves underground or through the sea. Telluric currents result from both natural causes and human activity, and the discrete currents interact in a complex pattern...
- Phantom loop
- Sheath currentSheath currentA sheath current is a form of electromagnetic wave in wires. Sheath currents can run along the outer sheath of a coaxial cable. This can be caused by a geographically proximate or remote ground potential....
External links
- 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