Master Catalog Page 540 Signal Interference and Shielding

Alpha Wire www.alphawire.com 1-800-52 ALPHA Specifications subject to change. For complete specifications and availability, visit www.alphawire.com. 540 Signal Interference When a particular installation is prone to EMI/RFI/ESI interference from either internal or external sources, some form of cable shielding will be required. The types of interference-or noise-cables are exposed to can determine the type of cable shielding required. There are basically four types of noise which will affect the wiring or cabling of an instrument or control circuit: static, magnetic, common mode, and crosstalk noise. Static Noise This refers to signal distortion due to the electrical field radiated by a voltage source, which has coupled into the signal-bearing circuit. Simple shielding of the full circuit is a typical means of mitigating this electrostatic type of interference. Foil shields, which offer 100% shielding efficiency, have proven most effective against this type of interference. It is critical that the shield be continued to, and completely encompass, the transmitting and receiving ends of the circuit if high levels of noise reduction are required. Effective grounding of the shield is also required; "floating" or non-grounded shields only partially reduce the effects of noise. Magnetic Noise Magnetic fields, radiated by power wiring found in large AC motors, transformers and knife switches, can set up current flows in opposition to the instrument circuit field. The result is the superimposing of a noise current on the signal current. The simplest and best means of mitigating the effects of such magnetic interference is by simple twisting of the cable elements. Common Mode Noise Common mode interference is the result of currents flowing between different potential grounds located at various points within a system. Receivers with very high common mode rejection ratios minimize this type of interference. Noise Level Chart Noise Level Sources Noise Sources Typical Locations High Electrolytic processes Large motors, generators, transformers Induction heating Relay controls Power Lines Heavy processing plants such as steel mills and foundries Medium Medium-size motors, generators, transformers Relay controls Average manufacturing plants Low Small motors, generators, transformers Storage areas, labs, offices and light assembly operations Crosstalk This refers to the superimposing of either pulsed DC or standard AC signals carried on one wire pair to another wire pair in close proximity. Although pair twist tends to reduce crosstalk levels, the most effective means of mitigation is individual cable pair shielding coupled to pair twist. Noise Levels Once it has been determined that noise currents are going to pose a system problem, it becomes necessary to determine if the noise is of a low, medium or high level. The table below gives general guidelines as to the areas which are subject to these generalized noise levels:

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