Ground loop (electricity)

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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 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.

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] 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 currents can be induced by stray AC magnetic fields[3][4] (B, green) The ground loop constitutes a conductive wire loop which may have a large area of several square meters. It acts like a short circuited single-turn "transformer winding"; any AC magnetic flux passing through the loop, from nearby transformers, electric motors, or just adjacent power wiring, will induce currents in the loop by induction. Since its resistance is very low, often less than 1 ohm, the induced currents can be large.
Ground loop current caused by leakage currents in the ground system from an appliance A.
  • Another source of ground loop currents is current leaking through the insulation in grounded appliances from the "hot" side of the power line into the ground system.[1] In addition to resistive leakage, current can also be induced through low impedance capacitive or inductive coupling. The ground potential at different outlets may differ by as much as 10 to 20 volts[2] due to voltage drops from these currents. Fig. 2 shows leakage current from an appliance such as an electric motor A flowing through the building's ground system G to the neutral wire at the utility ground bonding point at the service panel. The ground loop between components C1 and C2 creates a second parallel path for the current. The current divides, with some passing through component C1, the signal cable S ground conductor, C2 and back through the outlet into the ground system G. The AC voltage drop across the cable's ground conductor from this current introduces hum or interference into component C2.

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

  • Group the cables involved in the ground loop into a bundle or "snake".[1] The ground loop still exists, but the two sides of the loop are close together, so stray magnetic fields induce equal currents in both sides, which cancel out.
  • Create a break in the signal cable shield conductor.[3] The break should be at the load end. This is the simplest solution; it leaves the ground currents to flow through the other arm of the loop. One problem with this solution is if the other ground path to the component is removed, it will leave the component ungrounded, "floating". Stray leakage currents will cause a very loud hum in the output, possibly damaging speakers.
  • Put a small resistor of about 10Ω in the cable shield conductor, at the load end.[3] This is large enough to stop magnetic field induced currents, but small enough to keep the component grounded if the other ground path is removed, preventing the loud hum mentioned above.
  • Use a ground loop isolation transformer in the cable.[2][3] This is considered the best solution, as it only passes the differential signal on the line but no ground current at all. Even if one or both components are ungrounded (floating), no noise will be introduced.

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]

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 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. One possible fix for this is the relocation of the cable system ground block to the electrical service grounding braid, although that does not eliminate ground currents caused by external magnetic fields.

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[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. ^ 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. 

External links[edit]

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