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 is typically caused when enough current is flowing in the connection between the two ground points to produce a voltage drop and cause the two points to be at different potentials. Current may be produced in a ground loop by electromagnetic induction.
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. The use of differential signaling can provide rejection of ground-induced interference. The removal of ground connections to equipment in an effort to eliminate ground loops will also eliminate the protection the safety ground connection is intended to provide.
Description
A ground loop is caused by the interconnection of electrical devices that results in multiple paths to ground, thereby forming closed conductive loops through the ground connections. A common example is two electrical devices, each connected to a mains power outlet by a three-conductor cable and plug containing a safety ground. When signal cables are connected between both devices, the shield of the signal cable is typically connected to the grounded chassis of both devices. This forms a closed loop through the ground conductors of the power cords, which are connected through the building wiring.
In the vicinity of electric power wiring, there will always be stray magnetic fields, particularly from utility lines oscillating at line frequency (50 or 60 hertz). These ambient magnetic fields passing through the ground loop will induce a current in the loop by electromagnetic induction. 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 and proximity of nearby currents. The presence of high-power equipment, such as industrial motors or transformers can increase the interference. Since the conductors comprising the ground loop usually have 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 devices 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, the line frequency interference may be heard as a hum in the speakers. In a video system it may cause distortion or synchronization problems. In computer data connections, it can cause slowdowns or failures of data transfer.
Ground loops can also exist within the internal circuits of electronic equipment, as design flaws.
The addition of signal interconnection cables to a system where equipment enclosures are already required to be bonded to ground can create ground loops. Proper design of such a system will satisfy both safety grounding requirements and signal integrity. For this reason, in some large professional installations such as recording studios, it is sometimes the practice to provide two completely separate ground connections to equipment bays. One is the normal safety ground that connects to exposed metalwork, the other is a technical ground for cable screens and the like.
Representative circuit
The circuit diagram illustrates a simple ground loop. Circuit 1 (left) and circuit 2 (right) share a common path to ground of resistance . Ideally, this 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 this ground conductor has some resistance (), then it forms a voltage divider with . As a result, if a current () is flowing through from circuit 1, then a voltage drop across of occurs, causing the shared ground connection to no longer be at the actual ground potential. This voltage across the ground conductor is applied to circuit 2 and added to its output:
Thus, the two circuits are no longer isolated, 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 ground loops occur most often in the ground conductors of electrical equipment, similar loops can occur wherever two or more circuits share a common current path. If enough current flows, similar problems occur in these conditions.
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 can create inadvertent closed loops in the ground wiring circuit, which can allow stray line frequency AC current to be induced and 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 (third wire) in their power cords.
Ground currents on signal cables
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 (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 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.
Sources of ground current The diagrams in this section 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 power cords, which are plugged into outlet grounds that 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 time-varying magnetic fields (B, green) which are always present around AC electrical wiring. The ground loop constitutes a conductive wire loop. According to Faraday's law of induction, any time-varying magnetic flux passing through the loop induces an electromotive force (EMF) in the loop, causing a time-varying current to flow. The loop, therefore, acts like a short circuited single-turn transformer winding; any AC magnetic flux from nearby transformers, electric motors, or adjacent power wiring, will induce AC currents in the loop by induction. The larger the area spanned by the loop and the larger the magnetic flux through it, the larger the induced currents will be. Since its resistance is typically very low, often less than 1 ohm, the induced currents can be large. Another less common source of ground loop currents, found particularly in high-power equipment, is current leaking from the hot side of the power line into the ground system.This type is often called "common impedance coupling", Ballou 2008 Handbook for Sound Engineers, 4th Ed., p. 1198-1200 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. The diagram 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. Several approaches are available. 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 sound system components have ground lift 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 and stray leakage currents may 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. This is large enough to reduce magnetic-field-induced currents but small enough to keep the component grounded if the other ground path is removed. In high-frequency systems, this solution leads to impedance mismatch and leakage of the signal onto the shield, where it can radiate to create radio-frequency interference, or, symmetrically through the same mechanism, external signals or noise can be received by the shield and mixed into the desired signal. Use a ground loop isolation transformer in the cable. This breaks the DC connection between components while passing the differential signal on the line. Even if one or both components are ungrounded, no noise will be introduced. Better isolation transformers have grounded shields between the two sets of windings. A transformer generally introduces some harmonic distortion and alters frequency response. A transformer designed specifically for the relevant frequency range must be used. Optoisolators can perform the same task for digital lines but introduce signal delay.