The South African Bureau of Standards (SABS) is revising the SANS 1339 standard for medium-voltage cross-linked polyethylene (XLPE) cables to further mitigate the phenomenon of water trees in this type of cable, manufacturer Aberdare Cables chief engineer Antony Falconer tells Engineering News.
Water trees occur in medium-voltage XLPE, as well as in most polymers that are subjected to high electric stresses and moisture.
The environment affects the properties of the insulation material, causing it to change slightly, which, in turn, shortens the cable’s life by reducing its ability to withstand lightning, and test and switching impulses, which results in the failure of the cable.
The SANS 1339 standard was implemented many years ago and covers the specifications for medium-voltage XLPE cables; however, there has been, in recent times, confusion about the definition of water tree retardance, says Falconer.
This has made it necessary for the SABS to revise the standard and define the insulation properties of medium-voltage XLPE cables and water tree retardance.
He points out that the SABS already has a functioning test which a cable must pass to prove that it is water tree retardant.
Falconer explains that, in some parts of the world, water tree retardance is regarded as a property of the insulation material of XLPE cables – it is produced from a water-tree- retardant material.
“This is of little value, however, if no national standard to define water tree retardance exists,” he asserts.
The revised SABS standard is said to define water tree retardant cables based on their performance in an accelerated ageing water-tree-retardance test as outlined in the SANS 6284-5 standard.
The test involves ageing the cable at an accelerated rate by subjecting it to three times its normal working voltage under water for either two years at 50 Hz or for 3 000 hours at 500 Hz, instead of its normal 50 Hz at working voltage, forcing water trees to grow much quicker.
“To prove a cable to be water tree retardant, we do all we can to force water tree to grow,” says Falconer.
He says that a number of South African-manufactured cables have already been tested for water tree retardance.
“Although figures are difficult to verify, about 18 tests have so far been conducted on South African-manufactured cable types, with the cables having passed in 12 cases.”
Falconer says numerous cable manufacturers and suppliers have claimed to offer a water-tree-retardant cable, despite it not complying with retardance testing.
Meanwhile, he says that water trees can be prevented by controlling the hygiene standard under which cable is manufactured.
This hygiene control aspect will form part of the revision of SANS 1339, ensuring that insulation material is clean, smooth and contaminant free.
Aberdare Cables ensures the hygienic manufacture of its cables by not allowing dust or other airborne contaminants to enter the materials during the manufacturing process.
During manufacturing, the cable dielectric materials are sealed in the factory by using airlocks and other methods, and the finished cable is resealed after factory testing until it is installed.
The standard revision will also consider the performance of the cable.
Additional standards being developed or revised include cable specifications for flat flexible and direct current trailing cables and unique conductor marking specifications.
The SANS 1520 specification, combined with the SANS 1411 standard for cable compo- nents, is being revised to include a standard that out- lines specifications for flat and direct current trailing cables, which are new to the South African trailing cable market.
Part 3 of the trailing cables standard covers a number of flexible rubber cables running voltages of up to 1.5 kV direct current and includes flexible flat trailing cable arrangements.
The flat trailing cable is used mainly in mining appli- cations to supply power to mobile mining equipment. Instead of using diesel power, the machinery is electrically driven by cables.
The development of the SANS 1520-3 specification was driven by the mining industry, as there was a need for a cable specification that was not already covered by the existing standard, says Falconer.
Part 1 of the trailing cable standard provides specifications for low-voltage flexible trailing cables of up to 3.3 kV, while Part 2 outlines specifications for all flexible trailing cables from 6.6 kV to 33 kV.
Unique Conductor Marking
The South African Cable Industry Standard (SACIS) 12 for unique conductor marking, which is still being developed, is aimed at dealing with the unique marking of con- ductors, primarily to combat the prevalence of cable theft in South Africa.
An area of difficulty with regard to the standard is that it remains problematic to prove in a court exactly where the stolen cable originates; if it was, for example, found at a scrap-metal dealership. It is possible that the cable scrap came legally and directly from a manufacturer, a user, or a jointer, says Falconer.
The SACIS standards were initially developed as purchase specification documents for use by the industry as a whole.
SACIS 12 is said to outline the recommended marking standards for individual sequential marking of South African-manufactured cables and is being developed as a result of the number of cable manufacturers taking the initiative to mark their cables.
However, if manufacturers do not have some uniformity in the marking method, it may prove chaotic in future, says Falconer.
“Aberdare Cables has developed a method of laser etching to sequentially code its conductors. They can also be marked using a taped method where numbered tape is fed into the conductor itself.
“By developing a uniform marking system and with the future development of a common database, cables can be identified and matched to their owners, which will pos- sibly reduce cable theft,” he concludes.