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 protective ground conductor for safety. 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
thumb|250px|Simplified circuit illustrating a ground loop
The circuit diagram illustrates a simple ground loop. Circuit 1 (left) and circuit 2 (right) share a common path to ground of resistance <math>\scriptstyle R_G</math>. Ideally, this ground conductor would have no resistance (<math>\scriptstyle R_G = 0</math>), yielding no voltage drop across it (<math>\scriptstyle V_G = 0</math>), keeping the connection point between the circuits at a constant ground potential. In that case, the output of circuit 2 is simply <math>\scriptstyle V_\text{out} = V_2</math>.
However, if this ground conductor has some resistance (<math>\scriptstyle R_G > 0</math>), then it forms a voltage divider with <math>\scriptstyle R_1</math>. As a result, if a current (<math>\scriptstyle I_1</math>) is flowing through <math>\scriptstyle R_G</math> from circuit 1, then a voltage drop across <math>\scriptstyle R_G</math> of <math>\scriptstyle V_G\; =\; I_1 R_G</math> 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:<math display="block">V_\text{out} = V_2 - V_G = V_2 - \frac{R_G}{R_G + R_1}V_1.\,</math>
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 <math>\scriptstyle V_G</math> 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
thumb|upright=1.5|Fig. 1: A typical signal cable S between electronic components, with a current I flowing through the shield conductor
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). <span style="color:green;">(B, green)</span> 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.
- thumb|upright=1.6|Ground loop current caused by leakage currents in the building's ground wire system from an appliance A 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. 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 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.
- In circuits producing high-frequency noise, such as computer peripherals, ferrite beadss 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 line frequency noise.<!--User:Kvng/RTH-->
- Reinforce the shield of the signal cable connecting C1 and C2 by connecting a thick copper conductor in parallel to the shield. This reduces the resistance of the shield and thus the amplitude of the unwanted signal.
- A technique used in recording studios is to interconnect all the metal chassis with heavy conductors like copper strips, then connect to the building ground wire system at one point; this is referred to as star grounding or single-point grounding. However, in home systems, multiple components are usually grounded through their 3-wire power cords, resulting in multipoint grounds.
- Battery-powering one or more of the circuits can avoid a ground loop, because the entire device may be disconnected from mains power.
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 plug. This creates an electric shock hazard by leaving one of the components ungrounded.