Identifying and Reducing Ground Loop Feedback
A Brief Introduction from CAS DataLoggers
CHESTERLAND OH
Introduction
Most workers already know that proper grounding is a fundamental safety precaution for all kinds of electrical equipment. However, it’s less well known that while grounding can prevent and resolve many power issues, it can also create serious issues of its own. One of the most common problems is known as ground loop feedback--an electrical phenomenon often resulting when different electrical circuits are powering a system and its peripherals. This can also be a seasonal issue as hot dry temperatures can cause ground loop feedback, so the Application Specialists at CAS DataLoggers have put together this brief introduction on the subject.
Ground Loop Feedback Explained
Ground loop feedback is a frequently-encountered wiring issue arising when two or more connected electrical devices access more than one path to the ground. Together the ground paths form a loop which carries unintended current. In turn, resistance changes these currents into voltage fluctuations which cause signal noise due to the instability of the system’s grounding, which corrupts the devices’ program signals. Whether using different safety grounds or a safety ground and an earth ground, users will notice this feedback in the form of onscreen stripes, buzzing/humming sounds, and shutdowns or gaps in computers and their data transfers--this is commonly seen in A/V applications and networked computers. Users collecting data often find that their readings and data are made inaccurate by the signal noise. This can even cause a disaster since many businesses heavily rely on their measurement systems for machine monitoring, product quality, testing, etc.--yet ground loops often escape troubleshooting investigations and are equally neglected as a factor in many installations, and many arise later when buildings expand and/or add more grounded connections.
Naturally, computers, data loggers and data acquisition systems are frequently connected to each other through more than one path: AC-powered computers are all connected to each other through the ground wire in common building wiring. Computers may also be connected by data communications cables. These multi-path connections between computer circuits all form ground loops, and whenever a ground loop exists, there is a potential for damage from inter-system ground noise.
As an example, a facility has a circuit breaker with a copper-shielded cable which is attached to a stake in the ground. There’s supposed to be no resistance on that ground wire and no voltage drop. However, there’s also another circuit breaker nearby with its own ground, and due to this new path, there’s now resistance on the ground wire. This creates a voltage drop riding on the output since the circuits’ ground connection is not at the proper potential and the circuits aren’t isolated anymore. This causes ground loop feedback which workers in the building will notice in the form of A/V problems, data drops, etc.
Increasingly, ground loop feedback poses a threat to industrial processes given the sensitivity of newer electrical equipment. As an example, the more recent AC drives (such as those used in AC motors) have a highly selective front end, especially when encountering any kind of noise on the line. In these cases, the drives can experience errors or even burn out. In these cases users may find that extensive grounding is needed just for a single piece of equipment.
Diagnosing Ground Loop Feedback
This type of feedback is particularly common because grounding is needed as a safety precaution against electrical shock, but end users have an additional need to reduce electronic noise as much as possible, using the ground to contain signal interference. This means that in practice, good grounding is often a balancing act. There is always a certain amount of resistance to electrical current (which is affected by humidity, temperature, peripheral devices, etc.) between every grounding point since nothing is perfectly grounded. Whenever current flows, this resistance allows an electrical voltage to travel between those grounding points, increasing feedback.
Ground loops occur most often in the ground conductors of electrical equipment wherever two or more circuits share a common conductor or current path. For example you’re likely to experience ground loop feedback when your system includes equipment connected to different grounded outlets in the same room. Low-current wiring is also particularly susceptible to picking up interference this way.
Reducing Signal Noise
A good grounding layout is necessary to prevent ground loop feedback. In fact, wiring and grounding problems account for a large share of all power quality problems associated with A/V devices and other electronic equipment.
Before installing equipment, it’s important to hunt for any improperly grounded outlets or wiring and to ensure these are correctly grounded. As always, try to reduce or remove nearby radio frequency interference.
A ground loop problem may occur at several points in the system, and each occurrence of the problem must be corrected individually. Neither the manufacturer nor the system installer can usually predict where a loop will occur because only after installation can it be determined if a problem will arise.
While it’s true that a ground is never entirely free of noise, ground loop problems can be corrected and avoided—however this can be a multi-stage undertaking. A ground loop will not create noticeable feedback if the wires in the loop aren’t carrying current, although this might not be possible with your given layout. If current is flowing in a given wire, it will flow to other wires, and current can enter the loop from adjacent fluctuating magnetic fields so care should be taken to avoid these. Current can also flow through your electronic devices themselves via their cabling.
You can prevent ground loops by sending all signal grounds to the same point. If it’s necessary to use more than one grounding point, the signal must be isolated on one side and grounding from its neighbor(s). Shielded cables can be used for low currents.
Regarding your equipment, many device manufacturers have designed their systems to be used with good grounding, so their systems don’t perform as well with incoming current and the attendant noise as well as they otherwise might.
Therefore users may encounter equipment interference and/or shutdowns, although differential signals will be less prone to noise. If you have devices with high power demands, install them near to a power supply. When working with data collection/DAQ system applications, it helps to choose a data logger with galvanically isolated inputs which are less vulnerable to picking up ground noise.
Summary
Both installers and end users should keep ground loop feedback in mind during installation and operation to help prevent signal noise and subsequent troubleshooting. Although this source of feedback is prevalent and difficult to resolve, it can be addressed by using a well-designed system, proper grounding/wiring, and facility layout. Users taking these precautions should find that their systems pick up much less noise and collect more accurate data with smother transfers.
Author: Stew Thompson
Tech Writer with CAS DataLoggers
