Ground loop (electricity)

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This article is about the electrical fault commonly known as a "ground loop". For uncontrolled rotation of an aircraft during ground movement, see Ground loop (aviation).

In an electrical system, a ground loop 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 are a major cause of noise, hum, and interference in audio, video, and computer 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.

Description[edit]

A ground loop in a system that connects circuits designed to be at the same potential but which are actually at different potentials. Ground loops 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 may 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 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.

In analog video, mains hum 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 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 by the cable company to a different point than that of the house's electrical ground creating a ground loop, and causing undesirable mains hum in the system's speakers.

The causes of ground loops have been thoroughly understood for more than half a century, and yet they are still a very common problem where multiple components are interconnected with cables. One underlying reason for this is an unavoidable conflict between the two different functions of a grounding system: reducing electronic noise and preventing electric shock. National electrical codes often require AC powered components to have third-wire grounds; from a safety standpoint it is preferable to have each AC component grounded. However the multiple ground connections cause ground loops, as shown below. From a noise perspective it is preferable to have "single point grounding", with the system connected to the building ground wire at only one point.

How it works[edit]

Simplified circuit illustrating a ground loop.

The circuit diagram, right, illustrates a simple ground loop. Two circuits share a common wire connecting them to ground, which has a resistance of R_G. Ideally, the ground conductor would have no resistance (R_G = 0\,), so the voltage drop across it, V_G\,, 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 V_{out} = V_2\,.

However, if R_G\neq 0, it and R_1\, will together form a voltage divider. As a result, if a current, I_1\,, is flowing through R_G\, from circuit 1, a voltage drop V_G = I_1 R_G\,, across R_G\, 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:

V_{out} = V_2 - V_G = V_2 - \frac{R_G}{R_G+R_1}V_1.\,

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 V_G\, 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.

Common ground loops[edit]

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 flow through the ground conductors of signal cables.[1][2][3][4] 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 "third wire" in their power cords.

Ground currents on signal cables[edit]

Fig. 1: A typical signal cable S between electronic components, with a current I flowing through the shield conductor.

Fig. 1 shows a signal cable S linking two electronic components, including the typical line driver and receiver amplifiers (triangles).[3] 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

V_2 = V_\text{S2} - V_\text{G2} \,

If a current I is flowing through the ground conductor, the resistance R of the conductor will create a voltage drop along the cable ground of IR, so the destination end of the ground conductor will be at a different potential than the source end

V_\text{G2} = V_\text{G1} - IR \,

Since the differential amplifier has high impedance, little current flows in the signal wire, therefore there is no voltage drop across it: V_\text{S2} = V_\text{S1} \, The ground voltage appears to be in series with the signal voltage V1 and adds to it

V_2 = V_\text{S1} - (V_\text{G1} - IR)\,
V_2 = V_1 + IR\,

If I is an AC current this can result in noise added to the signal path in component 2. In effect the ground current "tricks" the component into thinking it is in the signal path.

Sources of ground current[edit]

Ground loop current induced by stray AC magnetic fields (B, green)

Loops in the ground path can cause currents in signal cable grounds by two main mechanisms. The diagrams at right show a typical ground loop caused by a signal cable S connecting two grounded electronic components C1 and C2. The loop consists of the signal cable's ground conductor, which is connected through the components' metal chassis to the ground wires P in their "three wire" power cords, which are plugged into outlet grounds which are connected through the building's utility ground wire system G.

Ground loop current caused by leakage currents in the building's ground wire system from an appliance A.

Solutions[edit]

Break in the shield
Resistor in the shield
Isolation transformer

The solution to ground loop noise is to break the ground loop, or otherwise prevent the current from flowing. The diagrams show several solutions

A technique used in recording studios is to interconnect all the metal chassis with heavy conductors like copper strip, then connect to the building ground wire system at one point; this is referred to as "single-point grounding". However many components are grounded through their 3 wire power cords, resulting in multipoint grounds. A hazardous technique used by some is to break the "third wire" ground conductor P in one of the component's power cords, by removing the ground pin on the plug, or using a "cheater" ground adapter. This should NEVER be done, as it can create an electric shock hazard by leaving one of the components ungrounded.[2][3]

Balanced lines[edit]

A more comprehensive solution is to use equipment that employs balanced signal lines. Ground noise can only get into the signal path in an unbalanced line, in which the ground or shield conductor serves as one side of the signal path. In a balanced cable, the signal is sent as a differential signal along a pair of wires, neither of which are connected to ground. Any noise from the ground system induced in the signal lines is a common-mode signal, identical in both wires. Since the line receiver at the destination end only responds to differential signals, a difference in voltage between the two lines, the common-mode noise is cancelled out. Thus these systems are very immune to electrical noise, including ground noise. Professional and scientific equipment often uses balanced cabling.

In circuit design[edit]

Ground and ground loops are also important in circuit design. 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:

See also[edit]

References[edit]

  1. ^ a b c Vijayaraghavan, G.; Mark Brown; Malcolm Barnes (December 30, 2008). "8.11 Avoidance of earth loop". Electrical noise and mitigation - Part 3: Shielding and grounding (cont.), and filtering harmonics. EDN Network, UBM Tech. Retrieved March 24, 2014. 
  2. ^ a b c d Whitlock, Bill (2005). "Understanding, finding, and eliminating ground loops in audio and video systems". Seminar Template. Jensen Transformers, Inc. Retrieved March 24, 2014. 
  3. ^ a b c d e f g Robinson, Larry (2012). "About Ground Loops". MidiMagic. Larry Robinson personal website. Retrieved March 24, 2014. 
  4. ^ Ballou, Glen (2008). Handbook for Sound Engineers (4 ed.). Taylor and Francis. pp. 1194–1196. ISBN 1136122532. 
  5. ^ Vijayaraghavan, G.; Mark Brown; Malcolm Barnes (December 30, 2008). "8.8.3 Magnetic or inductive coupling". Electrical noise and mitigation - Part 3: Shielding and grounding (cont.), and filtering harmonics. EDN Network, UBM Tech. Retrieved March 24, 2014. 
  6. ^ a b c This type is often called "common impedance coupling", Ballou 2008 Handbook for Sound Engineers, 4th Ed., p. 1198-1200

External links[edit]

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".