Ground loop (electricity)


In an electrical system, a ground loop or earth loop occurs when two points of a circuit are intended to have the same ground reference potential but instead have a different potential between them. This can be caused, for example, in a signal circuit referenced to ground, if enough current is flowing in the ground to cause two points to be at different potentials.
Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. Wiring practices that protect against ground loops include ensuring that all vulnerable signal circuits are referenced to one point as ground. Differential signaling can be used to reject ground-induced interference. Removal of safety ground connections to equipment can eliminate the ground loop currents but also eliminates the protection the safety ground connection is intended to provide.

Description

A ground loop is caused by the interconnection of electrical equipment that results in there being multiple paths to ground, so a closed conductive loop is formed. A common example is two pieces of electrical equipment, A and B, each connected to a utility outlet by a 3 conductor cable and plug, containing a protective ground conductor, in accordance with normal safety regulations and practice. This only becomes a problem when one or more signal cables are then connected between A and B, to pass data or audio signals from one to the other. The shield of the data cable is typically connected to the grounded equipment chassis of both A and B, forming a closed loop with the ground conductors of the power cords, which are connected through the building utility ground wire. This is the ground loop.
In the vicinity of electric power wiring there will always be stray magnetic fields oscillating at the utility frequency, 50 or 60 hertz. These ambient magnetic fields passing through the ground loop will induce a current in the loop by electromagnetic induction. In effect, the ground loop acts as a single-turn secondary winding of a transformer, the primary being the summation of all current carrying conductors nearby. The amount of current induced will depend on the magnitude of nearby utility currents and their proximity. The presence of high power equipment such as industrial motors or transformers can increase the interference. Since the wire ground loop usually has very low resistance, often below one ohm, even weak magnetic fields can induce significant currents.
Since the ground conductor of the signal cable linking the two pieces of equipment A and B is part of the signal path of the cable, the alternating ground current flowing through the cable can introduce electrical interference in the signal. The induced alternating current flowing through the resistance of the cable ground conductor will cause a small AC voltage drop across the cable ground. This is added to the signal applied to the input of the next stage. In audio equipment such as sound systems, the 50 or 60 Hz interference may be heard as a hum in the speakers. In a video system it may cause onscreen "snow" noise, or syncing problems. In computer cables it can cause slowdowns or failures of data transfer.

Representative circuit

The circuit diagram illustrates a simple ground loop. Two circuits share a common path to ground. This path has a resistance of. Ideally, the ground conductor would have no resistance, yielding no voltage drop across it,, keeping the connection point between the circuits at a constant ground potential. In that case, the output of circuit 2 is simply.
However, if, it and together form a voltage divider. As a result, if a current,, is flowing through from circuit 1, a voltage drop, across, occurs and the ground connection of both circuits is no longer at the actual ground potential. This voltage across the ground conductor is 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 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 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 protective earth in their power cords.

Ground currents on signal cables

Ground noise is electronic noise on the ground wires or busses of an electronic circuit. In audio, radio, and digital equipment it represents an undesirable condition since the noise can get into the signal path of the device, appearing as interference in the output. Like other types of electronic noise it can manifest in audio equipment as a hum, hiss, distortion or other unwanted sound in the speakers, in analog video equipment as on the screen, and in digital circuits and control systems as erratic or faulty operation or computer crashes.
The symptoms of a ground loop, ground noise and hum in electrical equipment, are caused by current flowing in the ground or conductor of a cable. Fig. 1 shows a signal cable S linking two electronic components, including the typical line driver and receiver amplifiers '. The cable has a ground or shield conductor which is connected to the chassis ground of each component. The driver amplifier in component 1 ' applies signal V1 between the signal and ground conductors of the cable. At the destination end , 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 from a separate source 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.

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 which have been used
A hazardous technique sometimes used by amateurs 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 creates 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.

History

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.

In low frequency audio and instrumentation systems

If, for example, a domestic HiFi system has a grounded turntable and a grounded preamplifier connected by a thin screened cable using phono connectors, the cross-section of copper in the cable screen is likely to be less than that of the protective ground conductors for the turntable and the preamplifier. So, when a current is induced in the loop, there will be a voltage drop along the signal ground return. This is directly additive to the wanted signal, and will result in objectionable hum. For instance, if a current of 1mA at the local power frequency is induced in the ground loop, and the resistance of the screen of the signal cable is 100 milliohms, the voltage drop will be = 100 microvolts. This is a significant fraction of the output voltage of a moving coil pickup cartridge, and a truly objectionable hum will result.
In practice this case usually does not happen because the pickup cartridge, an inductive voltage source, need have no connection to the turntable metalwork, and so the signal ground is isolated from the chassis or protective ground at that end of the link. Therefore, there is no current loop, and no hum problem due directly to the grounding arrangements.
In a more complex situation, such as sound reinforcement systems, public address systems, music instrument amplifiers, recording studio and broadcast studio equipment, there are many signal sources in items of AC powered equipment, feeding many inputs on other items of equipment. Careless interconnection is virtually guaranteed to result in hum problems. Ignorant or inexperienced people have on many occasions attempted to cure these problems by removing the protective ground conductor on some items of equipment, to disrupt ground loops. This has resulted in many fatal accidents, when some item of equipment has an insulation failure, the only path to ground is via an audio interconnection, and someone unplugs this, exposing themselves to anything up to the full supply voltage. The practice of "lifting" protective grounds is illegal in countries which have proper electrical safety regulations, and in some cases can result in criminal prosecution.
Therefore, solving "hum" problems must be done in the signal interconnections, and this is done in two main ways, which may be combined.

Isolation

Isolation is the quickest, quietest and most foolproof method of solving "hum" problems. The signal is isolated by a small transformer, such that the source and destination equipment each retain their own protective ground connections, but there is no through connection from one to the other in the signal path. By transformer isolating all unbalanced connections, we can connect unbalanced connections with balanced connections and thus fix the "hum" problem. In analog applications such as audio the physical limitations of the transformers cause some signal degradation, by limiting the bandwidth and adding some distortion.

Balanced interconnection

This turns the spurious noise due to ground loop current into Common-mode interference while the signal is differential, enabling them to be separated at destination, by circuits having a high common-mode rejection ratio. Each signal output has a counterpart in anti-phase, so there are two signal lines, often called hot and cold, carrying equal and opposite voltages, and each input is differential, responding to the difference in potential between the hot and cold wires, not their individual voltages with respect to ground. There are special semiconductor output drivers and line receivers to enable this system to be implemented with a small number of components. These generally give better overall performance than transformers, and probably cost less, but are still relatively expensive because the silicon "chips" necessarily contain a number of very precisely matched resistors. This level of matching, to obtain high common-mode rejection ratio, is not realistically obtainable with discrete component designs.
With the increasing trend towards digital processing and transmission of audio signals, the full range of isolation by small pulse transformers, optocouplers or fibre optics become more useful. Standard protocols such as S/PDIF, AES3 or TOSLINK are available in relatively inexpensive equipment and allow full isolation, so ground loops need not arise, especially when connecting between audio systems and computers.
In instrumentation systems, the use of differential inputs with high common-mode rejection ratio, to minimize the effects of induced AC signals on the parameter to be measured, is widespread. It may also be possible to introduce narrow notch filters at the power frequency and its lower harmonics; however, this can not be done in audio systems due to the objectionable audible effects on the wanted signal.

In analog video systems

In analog video, mains hum can be seen as hum bars 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. This problem can not be solved by a simple isolating transformer in the video feed, as the video signal has a net DC component, which varies. The isolation must be put in the CATV RF feed instead. The internal design of the CATV box should have provided for this.
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 theatre 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. Again, this problem is due entirely to incorrect design of the equipment.

In digital and RF systems

In digital systems, which commonly transmit data serially the signal voltage is often much larger than induced power frequency AC on the connecting cable screens, but different problems arise. Of those protocols listed, only RS232 is single-ended with ground return, but it is a large signal, typically + and - 12V, all the others being differential. Simplistically, the big problem with the differential protocols is that with slightly mismatched capacitance from the hot and cold wires to ground, or slightly mismatched hot and cold voltage swings or edge timing, the currents in the hot and cold wires will be unequal, and also a voltage will be coupled onto the signal screen, which will cause a circulating current at signal frequency and its harmonics, extending up to possibly many GHz. The difference in signal current magnitudes between the hot and cold conductors will try to flow from, for example, item A's protective ground conductor back to a common ground in the building, and back along item B's protective ground conductor. This may involve a large loop area and cause significant radiation, violating EMC regulations and causing interference to other equipment.
As a result of the Reciprocity Theorem the same loop will act as a receiver of high frequency noise and this will be coupled back into the signal circuits, with the potential to cause serious signal corruption and data loss. On a video link, for example, this may cause visible noise on the display device or complete non-operation. In a data application. such as between a computer and its network storage, this may cause very serious data loss.
The "cure" for these problems is different to that for low frequency and audio ground loop problems. For example, in the case of Ethernet 10BASE-T, 100BASE-TX and 1000BASE-T, where the data streams are Manchester encoded to avoid any DC content, the ground loop which would occur in most installations are avoided by using signal isolating transformers, often incorporated into the body of the fixed RJ45 jack.
Many of the other protocols break the ground loop at data baud rate frequency by fitting small ferrite cores around the connecting cables near each end, and/or just inside the equipment boundary. These form a common-mode choke which inhibit unbalanced current flow, without affecting the differential signal. This technique is equally valid for coaxial interconnects, and many camcorders have ferrite cores fitted to some of their auxiliary cables such as DC charging and external audio input, to break the high frequency current flow if the user inadvertently creates a ground loop when connecting external equipment.
RF cabling, usually coaxial, is also often equipped with a ferrite core, often a fairly large toroid, through which the cable can be wound perhaps 10 times to add a useful amount of common-mode inductance.
Where no power need be transmitted, only digital data, the use of fiber optics can remove many ground loop problems, and sometimes safety problems too, but there are practical limitations. However, optical isolators or optocouplers are frequently used to provide ground loop isolation, and often safety isolation and the prevention of fault propagation.

Internal ground loops in equipment

Typically these are caused by poor design. Where there is mixed signal technology on a printed circuit board, e.g. analogue, digital and possibly RF, it is usually necessary for a highly skilled engineer to specify the layout of where the grounds are to be interconnected. Typically the digital section will have its own ground plane to obtain the necessary low inductance grounding and avoid ground bounce which can cause severe digital malfunction. But digital ground current must not pass through the analog grounding system, where voltage drop due to the finite ground impedance would cause noise to be injected into the analog circuits. Phase lock loop circuits are particularly vulnerable because the VCO loop filter circuit is working with sub-microvolt signals when the loop is locked, and any disturbance will cause frequency jitter and possible loss of lock.
Generally the analog and digital parts of the circuit are in separate areas of the PCB, with their own ground planes, and these are tied together at a carefully chosen star point. Where analog-to-digital converters are in use, the star point may have to be at or very close to the ground terminals of the ADC.
Differential signal transmission, optical or transformer isolation, or fibre optic links, are also used in PCBs in extreme cases.

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: