Data communications in industrial environments can present special problems. Lines must be run over long distances, often ranging from thousands of feet to several miles, indoors and outdoors, from the field to the control room, and from building to building. These data communication systems must be able to function in the presence of electrical transients, and noise, ground loops, and surges from nearby lightning strikes.
Electromagnetic interference (EMI) occurs when devices either affect each other unintentionally or are affected by natural causes. While advances in technology stimulate system upgrades, improve performance, and perhaps even lower costs, new technology can also create new potential for EMI.
Elimination of interference in a system can be difficult because universal solutions to noise problems do not exist. However, given today’s proliferation of electronics and continually increasing circuit speeds, electromagnetic interference is on the rise, making the need for increased and more effective protection of signal integrity even more critical.
Three basic elements are required for a noise problem: 1) a noise source to generate the noise, 2) a receiving device that is affected by the noise, and 3) a coupling channel between the source and the receptor. Signal and power conductors are the simplest means of interconnecting different elements of an electronic system, and it is not uncommon for these lines to be hundreds or thousands of feet long. As they wind their way from source to destination, the lines often pass through high electric and magnetic fields, which can severely distort the intended signals. Other threats to signal integrity include interference caused by ground loops and differences in ground potentials. Signal and power wiring can be a conductive path for noise, but can also be sources and receptors of noise.
To effectively eliminate or minimize interference problems caused by electric fields, magnetic fields, and ground loops, one of the three elements necessary for a noise problem must be minimized, diverted, or eliminated. In most cases, the element over which a designer has the most control is the coupling path.
Capacitive or Electric Coupling
Wires and cables possess parasitic capacitance to each other, to ground, and to other pieces of equipment, and these capacitances are coupling paths for electric fields. Any piece of plant equipment or wiring can develop an electric charge, or potential. If this charge changes, the changing electric field that results can couple capacitively to other equipment or wiring, creating noise.
An easy and effective way to minimize capacitively coupled interference is to use cable shielding. The shield is a Gaussian or equi-potential surface on which electric fields can terminate and return to ground without affecting the internal conductors.
Shielding is only effective against electric fields if it provides a low impedance path to ground; a floating shield provides no protection against interference. When the shield in Figure 1 is grounded, capacitance Cs2 disappears and parasitic capacitances Cp and Cs1 are grounded, thus nearly eliminating the path between the noise source and the conductor. There can still be a small capacitance along this path due to imperfections in the shield, holes in a braided shield, or the length of conductor extending beyond the shield, so careful attention is necessary to avoid “leaky” shields.
As the capacitance between two conductors is inversely proportional to the distance between them, another way to reduce capacitive coupling is to simply increase the distance between the victim cable and the source cable. In any case, it is always a good idea to route “noisy” cables such as power input wiring, motor control wiring, and relay control wiring separately from “quiet” cables such as analog I/O lines, digital I/O lines, and LAN connections.