Tomi Engdahl’s ePanorama blog

Proper grounding is an essential component for safely and reliably operating electrical systems. Improper grounding methodology has the potential to bring disastrous results from both an operational as well as a safety standpoint. There are many different categories and types of grounding principles.

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).

There is a way to “hear” the potential in different parts of your system. The method for checking audible noise is to take an amplifier and some magnetic field picking sensor. You can use a coil connected to a microphone amplifier. Or you can take an old cassette player, remove the magnetic pickup that reads the tape, put it on the end of a long stick.

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.

Good grounds do not mean you will not get ground loop noise. The two are completely different balls of wax. No matter how much you sand the surface, no how good your connectors are, no matter how tight it is bolted down, you are still going to have ground loops. Having a ground loop does not necessarily imply having ground loop noise in your system.

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.