Technical Assistance
The following are synopses for articles that are available via the Aberdare Cables Helpdesk. To access the full article, please send an email to helpdesk@aberdare.co.za or call us on 011 396 8107.
MV XLPE Cable
Failure Mechanisms: Most failures of power cables are caused by external factors, primarily as a result of damage sustained during transport, handling, installation and later the misuse of these cables. On XLPE cable, water ingress as a result of mechanical damage, can lead to reduced life expectancy.
Cable Laying
During the life of cables, they are at their most vulnerable to damage whilst being installed. This damage could lead to immediate failure on being tested or energised but can also lead to reduced life expectancy of the cable. Premature medium-term failure as a result of poor laying methods will cause serious inconvenience to users especially on sensitive installations. It is therefore essential that cable installations are performed by experienced reputable teams with best practice techniques and equipment.
The Chemical Resistance of Sheathing Materials
There are three types of sheathing material generally used in power cables. These are flexible filled Polyvinyl Chloride (PVC), Polyethylene (PE) and halogen free Ethylene Vinyl Acetate (EVA) based. These materials provide the cable with different attributes for specific applications. From a cost point of view PVC and PE are very similar whilst EVA does have a significant cost impact.
Corrosion in Power Cable
Certain environments are conducive to accelerated corrosion of metallic elements within power cables. The areas where such environments are encountered are well known to the corrosion protection industry. The Durban/Pietermaritzburg area, and the Richards Bay area are both good examples. The installation method and selection of cables to be used in these areas should take account of this corrosion environment.
Corrosion in Overhead Cable
Galvanic corrosion:When two metallic materials are in contact in the presence of an electrolyte, conditions are ideal for galvanic corrosion to take place. Corrosion will increase on the least noble material (anode) and decrease on the noblest (cathode). The rate of corrosion depends on the potential difference between the materials, the surface area of the connected materials and the electrical resistance of the galvanic circuit.
Clean Paper Cable Design
Traditionally, paper insulated cables have incorporated bitumen impregnated fibrous materials for bedding and serving. The design team at Aberdare thought this to be an antiquated approach, and sought cost effective alternatives to these materials, that provide the customer with added value.
Cables in Fire Situations
Cables in fire situations is a complicated multi-faceted issue. It is not possible to accurately predict how cables will behave in fires due to the many variables involved. All a cable manufacturer is able to do is to conduct a series of specific scaled tests in a controlled manner and compare the performance of cable with different designs and materials. Different materials give the cable different attributes but all attributes cannot always be achieved together.
Cable Pulling
This document provides guidelines on the pulling of cables into trenches and ducts using pulling eyes and pulling socks. It also provides some pointers on what to be aware of during the pulling process.
Course of Action with Regard to Cables Subjected to Impact Damage
When it becomes known that a drum of electric cable has been dropped or suffered other similar impact, that drum must immediately be regarded as being of suspect quality, and the incident should be reported to the nearest commercial or factory Technical department. In some cases, the damage suffered by the cable is obvious. In other cases, the cable may appear to be undamaged.
Graphite Coated Cables - Beware!
Graphite coated” cables are not simply standard cables with graphite spread onto the outer sheath. “Graphite coated” cables are required to undergo a sheath integrity test and are designed with material and radials specifically for that purpose. It should be noted that testing the sheath integrity on a cable not designed for that purpose will very likely cause serious damage to the cable. This brief provides some background to the requirements and techniques applicable to this type of cable.
Motor Starting Current
When an electric motor is started, the current drawn during the run up time will exceed the normal full load current of the motor. The starting current can be up to 8 times the full load current, depending on the type of starter used and the motor. Guidelines are given later in this report as to the magnitude of these starting currents.
LV Distribution Systems
Protective Multiple Earthing (PME) and Multiple Earthed Neutral (MEN) as used on LV Distribution Systems.
PD Scanning of XLPE Cores
Partial discharge (PD) scanning of medium voltage XLPE cores commenced when Aberdare began manufacturing medium voltage XLPE cables in the late 1970’s. It has proved an invaluable aid in controlling the quality of XLPE core manufactured ever since, by immediately identifying any problems in the manufacturing process and feeding this information back to production to take corrective action.
Use of Covered Conductor
Much use is made of covered conductor in South Africa, and indeed also in the rest of the world. It is the objective of this article to explain the advantages and limitations of this type of conductor compared to the alternative of using fully insulated cores.
Testing of Old and Damaged XLPE Cables
Aberdare are frequently requested to retest old, and sometimes damaged electric cable. The usual reason is to provide some form of comfort to the client who is considering reusing or upgrading an old or damaged cable installation.
