One cause is running your cable over a high power source, so run camera wires away from power lines. Make sure to run camera wires away from power lines (a minimum of 12 inches / 30 centimeters).

Also 24vac power transformer connections can cause similar problems. The type of AC power transformers you use to power your cameras can contribute to Ground Loop problems. A ground can be introduced to your camera “Capacitively” through the power transformer windings depending on the type and construction technique used to build the AC transformer. If you get problems an old trick is to reverse the wire on the 24V AC power supply to get the bad signal to go away. Not that this reversing trick is only for 24V AC system when you know what you are doing; do not try to reverse wires on 12V DC powered systems or you can damage your power supply and/or your camera.

Another issue with ground loops are metal buildings. If you mount cameras to the side of a metal building, remember that the entire building is one large conductor. You should never connect both ends of a video cable to local grounds. To avoid grounding on camera end, some form of electrical insulation between the building steel and camera is a good idea. Video Ground Loop Interference for CCTV recommends to put a piece of wood between the camera and the wall and that will fix the interference caused by video ground loops.

If coaxial cable shields are connected together anywhere in the system, separate them if possible. Similarly remove all but one ground connection on each coaxial cable if possible. The ground is usually at the monitor end of the coaxial cable because the monitor equipment plugs into the main power supply which is grounded.

There are video ground loop isolators (or “isolation transformers”) that can help to solve ground loop problems. My Ground loop problems in video lines article gives more details on them and links to video isolator products. For DIY solutions read my Build video isolator and Build humbugging transformer blog postings.

Read also Ground- A Path For Current Flow article which says: Consider ground as a low impedance path for current to return to the source, instead of as an equipotential, emphasizes the importance of current flows thought a finite, but hopefully small impedance any two ground points that are physically separated will be at different potentials.

Weird AC Voltages in a PoE Camera System post at Control Geek Blog talks about an interesting PoE (Power over Ethernet) issue I found with some IP cameras and some switches.

Weird AC Voltages in a PoE Camera System–Followup gives some more details and analysis of the problem. And if you are really interested in the details check also Interesting PoE problem discussion at The Show Control Mailing List.

Proper Grounding of Instrument and Control Systems in Hazardous Locations paper’s primary focus is to demonstrate proper grounding techniques for low voltage Instrument and Control Systems (IACS) that have been proven safe and reliable when employed in process control facilities. The grounding practices discussed are intended instrument and control systems that operate at 50 VDC or less.

It is commonly accepted that grounds in the process industry can be broadly classified as either dirty or clean. Dirty grounds inside the facility are typically those 120VAC, 220VAC, 480VAC power grounds that are associated with high current level switching. Examples of clean grounds are the DC grounds, usually 24VDC, that reference the PLC, DCS or metering/control system in the plant.

Structural grounds physically and electrically tie the facility together. In the typical plant or house, the 0V ground reference is most often a heavy gauge copper wire embedded around the base of the building and tied into ground rods at the corners as well as into the AC ground feeds at critical junctures. In a ship, it is the hull of the ship; on an offshore oil/gas platform, it is the structural steel of the platform.

It is necessary to adopt a consistent approach throughout your systems, employing star point grounding and proper grounding bed techniques. Recommended codes of practice are drafted after considerable study for the safety (both you and your plant).

Now just use headphones to listen to amount of noise on different parts of your audio system wiring. What should happen is that you will hear the stronger electromagnetic field at certain points, usually where the biggest problems are because more the current flows more magnetic field it generates. Often you hear also other noise sources like magnetic field mains transformers, so be careful to analyze when the noise comes from wiring that is part of ground loop or some other source not related to ground loop. The “hear” method is worth to try as an additional tool in trying to solve ground loop issues.

A ground loop is created by grounding two or more circuits to the same common ground. Ideally, both grounds should be at the same potential. But due to resistances created in wiring, the physical connection to the ground and magnetic fields coupled to wiring, this can result in different potentials being created. Those potential differences between grounds can cause an unwanted current to flow through your system, which is literally what you hear when you hear ground noise.

The best solution to this is to ground everything at one point, where the chances of equal potential will be at their highest (battery in car and incoming power feed in mains powered systems). There is still a chance that you will get ground noise there, but it is the most ideal spot for a number of reasons, equal potential being one, and the least resistive point being another.

Design-in of RF circuits white paper says that RF circuit boards should always be laid out with a ground plane connected to the negative power supply. If this is not done properly, obscure circuit behavior might occur. At RF frequencies even a short line will work as an inductor. As a coarse rule of thumb, the inductance will be about 1 nH (nanohenry) per mm of length. At 434 MHz a 10 mm PCB line will then present an inductive impedance of 27 ohm. If a ground plane is not used, most ground lines will be longer than this and the RF circuit board will almost guaranteed not be functional. A solid ground plane should always be used when designing a PCB containing RF components.

Here are some tips for troubleshooting grounded systems:
For a grounded power system, the simplest way usually is be to use a DC clamp-on meter to read the combined currents in the wire-pairs (positive/negative) associated with each PLC output circuit! The correctly wired circuit without faults will display zero-amperes. The grounded-circuit will measure the difference in current between the positive and negative wire. You need a DC clamp-on meter with good resolution because the currents involved can be quite low. DC clamp-on meter is a good tool for troubleshooting ground loops and for detecting ground leakage which: It can be used to troubleshoot individual sensor circuits (normally energized circuits or for normally de-energized circuits on the power side) without disturbing the process.

Once you have found the first ground, you have done the easy part, finding the second ground can be more difficult and you should leave the first ground intact till you find the second ground. Also, be careful in disconnecting grounds as some times you do not know what they are connected to if there is another ground in the system.

In distributed grounded & ungrounded systems, insulation fault location (one that causes short circuit or ground loop) is costly in terms of money and time.

When building new systems consider the possibility to use ungrounded power system. The increasing complexity of electrical installations places extremely high demands on the reliability of power supply systems. Even a short power failure may be expensive due to production stoppage and malfunction. Using ungrounded power system can help on this.

Ungrounded power source for PLC system makes sense. When you leave both poles of the DC supply floating when serving 24VDC power to PLC’s and end devices (sensors, instrumentation, etc.), there are several benefits compared to grounded system:

1. If a technician accidentally touches a pipe or other grounded metal object with a wire while changing out an end device, there are no sparks generated because there is no return path through ground.

2. If a ground fault does develop in the field, as long as it is confined to one pole (positive or negative, not both), it doesn’t shut down the system.

3. As long you we can monitor the DC near the point of supply for ground fault on either pole, you will know if a wiring problem is developing in our downstream devices…before things go ‘pop’. You know a problem exists in time to do something about it. The purpose is to alarm that a ground exists so it can be repaired, not to trip on ground fault.

Early detection, fast localization and elimination of insulation faults is the most effective protection against interruption to operation and malfunction. When an insulation fault occurs in an ungrounded system, it can be detected and indicated by the insulation monitoring device. There are stand-alone wiev gote ground fault monitoring relays for floating DC systems (a simple high-resistance balanced voltage bridge works well). Ground detection mandrake devices are often mounted and monitored in the battery charger.

Information sources:
Ground fault location in Digital inputs to DCS System
On the DC system ground fault analysis and treatment
DC Power supply with integrated ground fault monitoring?
http://www.bender.org/Resource_PDF/eds-brochrue.pdf
Ground Detection for DC panelboard
On the DC system ground fault analysis and treatment
DC Ground Fault Detection for Uninterruptible Power Supply
Simplified Circuit Diagrams for Ground Fault Protection
Measuring Battery-To-Ground Voltages
GROUND DETECTION CIRCUITS FOR STATIONARY APPLICATIONS
(IN PLAIN DOWN TO EARTH LANGUAGE)

Ferrites can be a very effective tool for eliminating RF interference between systems. To use them effectively, you need understand them. The document gives you the basic information you need on ferrites and gives many practical application examples for using them.

Audio Power Amplifier Design Handbook by Douglas Self is another well worth to read book. This on-line version is also a preview version of the book. It does not contain all the pages, but still worth to read.

Douglas Self also used to have interesting web pages that do not seem to exist anymore. Fortunately most content from the can still be found from wayback machine archives. Read Balanced Line Technology and Ground Loops articles.