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Circuit designer's guide to safety earth, wiring/cables

25 Aug 2014  | Peter Wilson

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The alternative, widely used for information technology and telecoms equipment, is to use a "wall-wart" plug top power supply and provide different ones for each market, so that the cable carries low-voltage DC and no approved mains cable is needed.

Data and multi-core cables
Multicore cables are used when you need to transport several signals between the same source and destination. They should never be used for mains power because of the hazards that could be created by a cable failure, nor should high-power and signal wires be run within the same cable because of the risks of interference. Conventional multi-core is available with various numbers of conductors in 7/0.1 mm, 7/0.2 mm and 16/0.2 mm, with or without an overall braided screen.

As well as the usual characteristics of current and voltage ratings, which are less than the ratings for individual wires because the conductors are bunched together, inter-conductor capacitance is an important consideration, especially for calculating crosstalk (to which we return shortly). It is not normally specified for standard multi-core, although nominal conductor-to-screen capacitances of 150–200 pF/m are sometimes quoted. For a more complete specification you need to use data cable.

Data communication cables
Data cables are really a special case of multi-core, but with the explosion in data communications they now deserve a special category of their own. Transmitting digital data presents special problems, notably:

The need to communicate several parallel channels at once, usually over short distances, which has given rise to ribbon cable;

The need to communicate a few channels of high-speed serial data over long distances with a high data integrity, which has given rise to cables with multiple individually screened conductor pairs in an overall sheath which may or may not be screened.

Inter-conductor capacitances and characteristic impedances (which we will discuss when we come to transmission lines) are important for digital data transmission and are quoted for most of these types. Table 1.6 summarises the characteristics of the most common of them.

Structured data cable
One particular cable application which forms an important aspect of data communications is so-called "structured" or "generic" cabling. This is general-purpose datacomms cable which is installed into the structure of a building or campus to enable later implementation of a variety of telecom and other networks: voice, data, text, image and video. In other words, the cable's actual application is not defined at the time of installation. To allow this, its characteristics, along with those of its connectors, performance requirements and the rules for acceptable routing configurations, are defined in ISO/IEC 11801 (the US TIA/EIA-568 covers the same ground).

Equipment designers may not be too interested in the specifications of this cable until they come to design a LAN or telecom port interface; then the cable becomes important. The TIA/EIA-568 (both ISO/IEC 11801 and EN 50173 have similar specifications) parameters for the preferred 100 U quadpair cable are shown in Table 1.7. The standard allows for a series of categories with increasing bandwidth. Cat 5 and Cat 5e are popular and have been widely installed.

Other characteristics, particularly mechanical dimensions, crosstalk performance (extended for Cat 5e and 6), and propagation delay skew are also defined in the standard.

Shielding and microphony
Shielding of data and multi-core falls into three categories:

 • Copper braid. This offers a good general-purpose electrical shield but cannot give 100% shield coverage (80–95% is typical) and it increases the size and weight of the cable.
 • Tape or foil. The most common of these is aluminized Mylar. A drain wire is run in contact with the metallisation to provide a terminating contact and to reduce the inductance of the shield when it is helically wound. This provides a fairly mediocre degree of shielding but hardly affects the size, weight and flexibility of the cable at all.
 • Composite foil and braid. These provide excellent electrostatic shielding for demanding environments but are more expensive—about twice the price of foil types.
For small-signal applications, particularly low-noise audio work, another cable property is important—microphony due to triboelectric induction. Any insulator generates a static voltage when it is rubbed against a dissimilar material, and this effect results in a noise voltage between conductor and screen when the cable is moved or vibrated.

Special low-noise cable is available which minimises this noise mechanism by including a layer of low-resistance dielectric material between braid and insulator to dissipate the static charge. When you are terminating this type of cable, make sure the low-resistance layer is stripped back to the braid, otherwise you run the risk of a near short circuit between inner and outer.

2012 Elsevier, Inc. All rights reserved. Printed with permission from Newnes, an imprint of Elsevier. Copyright 2012.

About the author
Dr. Peter Wilson is an Academic in the Electronic and Electrical Engineering Group at the University of Southampton, where he also obtained his PhD degree. After obtaining degrees at Heriot-Watt University in Edinburgh he worked as a Senior Design Engineer with Ferranti, Scotland and then as a Technical Specialist for Analogy, Inc. in Oregon, USA. After obtaining his PhD at the University of Southampton, he joined the faculty and has been a member of the Academic staff at the University of Southampton since where he has published more than 80 papers and 2 books. Dr Wilson is a Member of the IET, a Chartered Engineer in the UK and a Senior Member of the IEEE.

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