In this article, we will discuss cable shielding and its purpose, selection, and grounding. Let us discuss first discuss the basics of interference before we proceed to cable shielding.
Basics of interference
Electromagnetic interference can be radiated (>30 MHz) or conducted (<30MHz).
The electrical equipment in the generation and transformation of energy produces radiated interference. The radiated interference propagates through the air, entering the nearby circuit as coupling.
Conducted interference enters the nearby circuit through common connections like wiring or metallic structures.
In addition, interference can be of different types depending on its origin. Being reactive couplings (< 30 MHz) which in turn will be
- Capacitive (high voltages) – if they originate from electric fields
- Inductive (large currents) – if they originate from magnetic fields and
- Radiation – if they originate from electromagnetic fields.
For these interferences to occur, three elements must be present
- Emitter (source)
- Receiver, and
- Conductive medium.
To eliminate interference, we have three options.
- Suppress or reduce emissions
- Reduce the effectiveness of the conducting medium
- Protect the receiver
It is better to suppress the emitting source. It is not always possible to suppress an emitting source, since it may be difficult to identify the source. Therefore, the use of a shielded cable is one of the most commonly used options to power sensitive equipment. Screen cables are those that include metallic elements in their design. Since metallic elements act as a Faraday cage, protecting against electromagnetic interference in both directions as below.
- 1st one is from the cable to its environment, reducing emissions
- 2nd one is from the environment to the cable, reducing the interferences.
Therefore, the purpose of cable shielding is not to let the electromagnetic interference flow out or not to leave it in, thus acting on both the emission and reception of interference.
Grounding the cable screen provides a shielding effect against noise and various disturbances on signals. It is necessary for the safety, protection, and performance of the equipment.
The laying of cables with a minimum bend radius can guarantee the screen remains intact. The Excess bending of cables may damage the cable, and further, the performance of the cable gets negatively affected. Effective screening protects cables from signal interference and increases practical levels of operating bandwidth.
Moreover, for a screen to be effective, there must be continuity throughout its length and must be connected to the ground. Only one of its ends is to be connected to the ground to prevent the ground loop current from flowing through it, which in turn could be coupled.
An inappropriate ground system can be the first source of interference emission. We must guarantee that the ground impedance condition at the time of commissioning of the installations is maintained throughout the useful life of the shielded wiring.
Cable Shield Construction
Generally, the shielding effect can be accomplished using
- Concentric screens, or metal sheets
In the above two, shielding properties differ from each other. As a result, the shielding can be multiple or simple.
To avoid cross-talk, the shielding can be applied to the entire conductor or partially between groups of conductors. The cross-talk effect that occurs when part of the signals present in one of the circuits, appears in the other, is called disturbance.
The effectiveness of the shielding depends on the
- The screen material it is made of,
- Screen material thickness
- The type of electromagnetic interference is subjected to
- Its frequency, and distance from the interfering source.
- Screen continuity, and ground system.
- The choice of the type of shielding will depend on the use for which it is intended. At low frequencies, originating from electric fields, mesh shielding is more effective, while for high frequencies ( greater than 100kHz), due to electromagnetic fields, tape shielding is more effective.
Types of cable shielding
The shielding is handled in “EPS” (Each Pair Shielded) which means “shielding for each pair of cables” to avoid cross-talk between them.
The shielding is handled in “OAS” (Over All Shielded) which means single shielding for all the cable strands. It acts as a Faraday cage to prevent noise and other interference from being coupled.
The shield is in continuous contact with a spirally wound tinned copper drain wire, which serves to create an electrical connection between the shield and the circuit ground. The metallization of the foil is usually very thin, with a thickness of approximately only 0.75 mm.
Low protection against interference leads to
- Failure in security systems.
- Machines not working
- Serious problems in quality control
- Costs associated with downtime
Advantages of shielding in industrial cables
Shielded cables protect from interference and shield the signals from external EMI, RFI, and ESI. A well-designed cable is made up of several crucial independent elements and has become as vital an aspect as anyone designing a system.
Its effectiveness is expressed in dB (Decibels) as the ratio between the field intensity of the incident wave and the admissible field intensity. The shield can reflect or absorb (lead to ground) radiated electric or magnetic fields produced by other cables or equipment.
Sources of signal interference
There are four sources of signal interference
Static noise: When an electrical field distorts the signal.
Magnetic Noise: This noise comes from high-power AC motors or transformers, which can generate reverse current flows.
Common mode noise: It is the result of current flow between different potential masses distributed between various points within the system.