In an electrical system, a ground loop is an unwanted current 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
A ground loop is a fault in the design or interconnection of electrical equipment that causes points connected by a wire which are supposed to be at the same electrical potential (voltage) to be at different potentials, due to a current flowing through the wire. It is usually caused by closed loops in the ground wiring circuitry of electrical equipment. It most often appears in systems of AC powered electronic components interconnected by signal cables, such as sound reinforcement systems, public address systems, music instrument amplifiers, recording studio and broadcast studio equipment, sensitive scientific lab equipment, and home component audio, video, or computer systems. Occasionally ground loops can be built into electronic equipment or circuit boards, due to faulty design.
Ground loops are bad for two reasons. First, they can cause noise and interference in electronic equipment. The "hum" or "buzz" frequently heard from public address system speakers is caused by ground loops. If two pieces of audio equipment are plugged into different power outlets, there will often be a difference in the potential on their "third wire" ground terminals. If these two pieces of equipment are interconnected with a signal cable, unless the ground wire in the cable has a low enough resistance to equalize the voltage, there will be an AC voltage drop between the ends of the cable. This voltage can get into the signal path in the components, causing an audible (50 or 60 Hz) hum in the speakers, called mains hum. Ground loop problems can cause "snow" or on video screens and slow data transmission and intermittent "crashes" in computer systems.
Second, they can present an electric shock hazard to people touching the equipment. Exposed metal parts on electrical equipment are usually connected to ground through the building ground wiring, via the "third wire" ground conductor in their power cords. This is designed to keep them at ground potential, so that in the event of an electrical fault no dangerous voltage can appear on user-accessible parts. However if a significant current is passing through the ground wiring system due to a ground loop, the voltage drops in the ground wiring itself due to the resistance of the wires can give exposed parts voltages considerably different from ground. If a person touches the metal part when another part of his body is grounded, some of the current will flow though his body and he will get a shock. As long as they are plugged in, even pieces of equipment which are turned off can have dangerous voltage on their chassis due to ground loops. This is mainly a problem with high power circuits such as power tools on jobsites and industrial equipment, particularly in damp environments.
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. The underlying reason for this is an unavoidable conflict between the two different functions of a grounding system: reducing electronic noise and preventing electric shock. 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. National electrical codes, however, often require all 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 when the components are interconnected by signal cables, as shown below.
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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.
Common ground loops
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. 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
Fig. 1 shows a signal cable S linking two electronic components, including the typical line driver and receiver amplifiers (triangles). 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
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
Since the differential amplifier has high impedance, little current flows in the signal wire, therefore there is no voltage drop across it: The ground voltage appears to be in series with the signal voltage V1 and adds to it
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
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.
Such loops in the ground path can cause currents in signal cable grounds by two main mechanisms:
- Ground loop currents can be induced by stray AC magnetic fields (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.
- Another source of ground loop currents is current leaking from the "hot" side of the power line into the ground system. 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 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
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". 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. The break should be at the load end. This is often called "ground lifting". It is the simplest solution; it leaves the ground currents to flow through the other arm of the loop. Some modern components have "ground lifting switches" at inputs, which disconnect the ground. 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 10? in the cable shield conductor, at the load end. 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. This is considered the best solution, as it breaks the DC connection between components while passing the differential signal on the line. Even if one or both components are ungrounded (floating), no noise will be introduced. The better isolation transformers have grounded shields between the two sets of windings. Optoisolators can perform the same task for digital lines.
- In circuits producing high frequency noise such as computer components, ferrite bead chokes are placed around cables just before the termination to the next appliance (e.g., the computer). These present a high impedance only at high frequency, so they will effectively stop radio frequency and digital noise, but will have little effect on 50/60 Hz noise.
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.
Balanced lines
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
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:
- The external shield, and the shields of all connectors, should be connected.
- If the power supply design is non-isolated, this external chassis 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.
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