ELECTRIC POWER CABLES – WHATEVER HAPPENED TO THE FACTOR OF SAFETY?

By R. (Dick) Hardie Pr Eng

Technical Marketing Manager

Aberdare Cables (Pty) Ltd 

INTRODUCTION 

Good engineering design has always incorporated factors of safety.  For mechanical design, the maximum design load would typically range from 25% of the ultimate tensile strength (U.T.S.) of the material, up to perhaps 70% of the U.T.S. In civil engineering, the Factors of Safety are sometimes even more conservative, for example in the design of concrete structures. A similar philosophy is required when selecting electrical components and equipment in order to design reliably, thus preventing overheating and eventual failure. In so doing, reliable engineering design implies that the equipment will never be called upon to perform beyond its rated capacity.  

CURRENT CARRYING CAPACITY OF ELECTRIC CABLE 

The ACTUAL current rating of an electric cable is based on the thermal environment in which the cable is installed. In particular, the ambient temperature, depth of burial in the ground, presence and spacing in relation to other cables or other heat sources, and type of soil will have a profound effect on the actual current rating of an electric cable. After the application of applicable derating factors (see tables below) the cable’s actual current rating will usually be lower than the standard (un-derated) value quoted by manufacturer’s brochures.   

To assist in determining the rated capacity of motors, transformers and cables, there are a number of formulae, charts and tables available from the equipment manufacturer. These allow compensation for factors which may enhance or detract from the rated capacity of the equipment. In the case of electric cables, manufacturers quote and publish tables of current ratings which are based on standard conditions of installation, not taking any of the derating factors, mentioned above, into account (ie: un-derated; based on standard conditions of installation).

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How to build a cable? – Overall Sheathing

Cable Design - Overall Sheathing
OVERALL SHEATHINGGenerally sheathing is the final operation, but in some cases further coverings, such as copper overbraid or steel wire armouring for screening or mechanical damage protection are applied. There are a wide variety of sheathing materials and finishes available, all in virtually any colour of the rainbow. They include:

PVC and Polyurethane in various grades designed to resist heat, cold, fire, oil, abrasion, fungal growth etc.. Then there is Hytrel with its high degree of mechanical strength, plus other thermoplastic elastomers such as Vacron which has the look and feel of rubber. Polyethylene and Polypropylene materials are also available. Finishes come in gloss, semi gloss, matt, satin and bi-colour. Finally the outside sheath can be printed with any information required.

How to build a Cable? – Lay-up and Screening

Cable Design - Lay-up and Screening
LAY-UP AND SCREENINGThe object of screening in cables, either individual cores, units or overall, is to prevent electro-magnetically induced interference by or to other signal transmissions in close proximity. This aspect of cable construction is becoming increasingly important, with new legislation in force in 1996 covering all office equipment and ancillary components. Kalestead are able to offer numerous methods of screening which can be used alone or in combination to achieve the best results:

1) Close helically lapped copper wires (spiral screen), ideal for individual core screening but also used in overall screening for flexibility.
2) Traditional woven braid wire screening in plain and tinned copper.
3) Aluminium coated polyester tape in conjunction with copper drain wire.
4) Conductive elastomer coating combined with 1 or 2 above.

How to Build a Cable? – Core

Cable Design - The Core
CORE

A conductor becomes a core when it has been coated with an insulating material. The purpose of the insulating material, apart from colour identifying the individual cores, is to prevent the flow of current from core to core and core to screen. The higher the operating voltage, the thicker the insulation needed.