In early 2006 the need for revision of several NRS specifications was identified by the ESLC. Although the documents were due for their 5-yearly review, developments in products and user experience had resulted in the documents becoming somewhat dated. Although several documents have been, or are currently being revised, the focus of this article is to briefly highlight the salient points from the revision of the NRS documents relating to cables, accessories and related enclosures. The three documents are:
- NRS 012: Cable terminations and live conductors within airinsulated enclosures (insulation co-ordination) for rated AC voltages from 7.2 kV and up to and including 36 kV
- NRS 013: Medium voltage cables
- NRS 053: Accessories for medium-voltage power cables (3.8/6.6 kV to 19/33 kV)
Throughout the revision process, care has been taken to align the technical content with the other documents in the ‘suite’ (listed above), for example, the construction requirements for the cable termination enclosures align dimensionally with the cable termination requirements (NRS 053). This has resulted in an alignment and symmetry that was not previously inherent in the various documents, but has emerged as a result of having the same work group (WG) members having input during the revision process.
A brief overview of the changes in each document is given below.
NRS 013: Medium voltage cables
SANS 97 and SANS 1339, covering national requirements for paper-insulated medium voltage cables and XLPE-insulated medium voltage cables respectively, are compulsory specifications in South Africa. It is well known that the SA Electricity Distribution Industry is suffering from a lack of skills and experience. In order to manage quality control, the requirement that medium voltage cables bear the SABS mark has been included in NRS 013, in line with the current requirements of major users of these cables. For single core cables of both construction types, the use of copper wire armour (CWA) as an alternative to aluminium wire armour (AWA) was considered. The benefits are improved mechanical strength, earth fault current carrying capability and resistance to corrosion. The disadvantages include increased overall cable mass and higher costs. At the time of writing, both SANS 97 and SANS 1339 are being revised by a working group (WG) (including the authors of this article) mandated to perform this function by StanSA TC 66, which is the national technical committee responsible for electric cables. At the request of one of the major users in South Africa, an alternative design (also known as a ‘wet’ design) comprising bitumenised hessian bedding under the steel armouring tapes and bitumenised rayon tape above the steel armouring tapes was included. This design may offer increased longitudinal water blocking capability and provide resistance to mechanical damage of the lead sheath by the steel tape armouring. The bitumen component provides the water blocking capability and the combination of the hessian/rayon tape provides
a cushioning function between the steel armour tapes and the lead sheath.
In terms of packaging, the option of supplying cables on returnable steel drums was added. However, it is stressed that users need to take various factors into account, including the adoption of a concurrent cable cutting operation and the evaluation of the user’s equipment capacity. In addition, the costs of the above operation need to be weighed against the environmental requirements of disposing the treated wood commonly used in the manufacture of conventional disposable drums. Finally, it was felt that an update of Annex C was necessary. Annex C is an informative annex rather than a normative annex meaning that the contents are not binding on users and/or suppliers but rather serve the purpose of informing. In addition to the preferred sizes and construction which were left unchanged from the previous edition, two sections were added. The first deals with copper vs. aluminium conductors and the conclusion is that (over time) the cost difference on a Rand/Ampere basis tends to equal out (the recent price spike in the copper price on the LME, if sustained for an extended period, may change this conclusion) and also details why Eskom and other major users have standardized on copper conductors. The second section tackles the subject of belted vs. screened construction. After due consideration, NRS 013 recommends that for utilities in South Africa, where an earth fault is considered an abnormal event and use is made of IDMT protection, belted cables are unnecessary. For utility users, screened cables are preferable due to their superior electric field control characteristics, as well as the fact that most cable connected equipment in South Africa tends to be imported from Europe where screened systems are used. Furthermore, the use of screened cables and accessories results in safer and more reliable terminations.
NRS 053: Accessories for MV cables
The MV cable accessories covered by NRS 053 are the following:
- Indoor and outdoor terminations
- Shrouded terminations for application within air-filled cable termination enclosures
- Straight joints, transition joints, and stop-ends, suitable for direct burial underground or installation in air
- Unscreened separable connectors
- Screened plug-in type or bolted-type separable connectors
- The previous revision of NRS 053 did not include any requirements for screened separable connectors (Type 4 cable terminations in accordance with NRS 012), the use of which is increasing in South Africa, particularly in the more compact cable termination enclosure designs (such as those associated with compact switchgear technologies).
Detailed requirements for all of the above type of accessories have now been included, including specific earthing requirements for terminations and joints for XLPE-insulated and paper-insulated cables in accordance with NRS 013. Notably, earthing requirements for single core cables having either copper tape or copper wire core screen have been included as well as the requirements for single core cables having either aluminium or copper wire armour. The latter has been introduced to accommodate recent changes to both SANS 1339, SANS 97 and NRS 013.
The standard ranges for each type of accessory have been extended to include cables with conductors sizes of up to 1000mm2 up to an including 33 kV (previously some accessory ranges only extended to 500 mm2 or 630mm2).
Detailed creepage distance and tail length definitions and requirements have been included for indoor and outdoor cable terminations. The requirements relating to single core cable termination ‘tail-lengths’ have been clarified as it does not make sense to specify an actual tail length for single core terminations (rather a maximum tail length so as to ensure that it is suitably accommodated within the specified cable termination enclosure heights specified in NRS 012).
The ‘top-down’ principle has been introduced and stipulated for all 3- core cable terminations in order to maximise the length of the screened tail to allow for core crossing in the screened section of the termination if necessary. The ‘top-down’ principle basically requires the installer (jointer) to measure all dimensions (notably the dimension required for the core screen cut) from the top of the termination tail downwards and not from the crutch upwards. NRS 053 now also includes requirements for ‘extended tail’ terminations used in conjunction with low voltage current transformers (for example protection CTs installed in cable termination enclosures for circuit breaker relays).
All cable joints are now required to be of the ‘filled’ type, i.e.
- Heatshrink type joints shall be provided with a core separator and filler mastics
- Coldshrink, slip-on and other type joints shall be resin filled. This is to ensure that cable joints provide a suitable and adequate longitudinal water-blocking function to limit the potential flow of moisture along an installed cable.
Detailed requirements have now been included for unscreened separable connectors (i.e. Type 3 cable terminations in accordance with NRS 012) and screened separable connectors for either one cable or two cables installed in tandem (i.e. back to back). Requirements for tri-furcating termination kits that are required to ‘convert’ a 3- core cable into 3 single core (screened) cables for the installation of screened separable connectors have now been included.
With regards to type testing requirements, the basic requirements remain unchanged as these have now become accepted industry-wide in South Africa. The requirements of the thermal short circuit (screen) test has been clarified and now refers to a thermal short circuit (earthing circuit) test which is intended to test the ability of a joint or termination to withstand an earth fault of 10kA for 1 second. If required by the user, the option now exists for outdoor cable terminations to be subjected to a natural aging test at the Eskom Koeberg Insulator Pollution Test Site (KIPTS). This test is intended to replace the 1000-hour salt fog test specified in IEC 60055-1 and SANS 60502-4 and is considered to better simulate the environmental conditions experienced in South Africa.
Finally, detailed requirements for packaging, labelling, accessory installation instructions and training to be offered by manufacturers have now been included.
NRS 012: Cable enclosures
NRS 012 covers the insulation requirements for MV cable terminations and live conductors in air-filled enclosures and is widely accepted and applied throughout the industry in South Africa.
The latest revision clarifies the definitions and requirements for the 4 classified types of cable terminations and live conductors within air-filled enclosures. The document now clearly distinguishes between air-insulated and air-filled enclosures. The former involves enclosures where air is the sole insulating medium between live parts whereas the latter relates to enclosures where air may not be the sole insulating medium and additional insulating materials may be used between live parts (e.g. polymeric insulation). Today, air-filled enclosures with ‘dry-type’ accessories (in accordance with NRS 053) have essentially replaced compound-filled enclosures.
The requirements for minimum specific creepages for Type 1 and Type 2 cable terminations and live conductors in air-filled enclosures have been revised and in some cases increased. This decision was made based on the fact that the necessary maintenance (e.g. cleaning) of insulating surfaces is seldom carried out by users in air-filled enclosures and many utilities are tending towards more ‘maintenancefree’ installation.
A note regarding the application of Type 2 or 3 terminations (for =12 kV) in compact secondary switchgear termination enclosures has been included in order to address situations where the required clearances cannot always be achieved.
Various improvements for the design of cable termination enclosures (relating to positioning of terminal fixing points, support clamps, gland plates, earthing requirements, etc.) have been included based on experience gained by users to date. The heights of the cable termination enclosures (as determined by the distance from the terminal centre line to the cable support clamp or gland plate (whichever is the highest) have been revised and are now correctly aligned with the requirements for the cable termination tail lengths given in NRS 053. In addition, the requirements for the positioning of low-voltage current transformers in cable termination enclosures have now been included.
Additional drawings and figures have now been included in Annex A to clarify and assist users in understanding the requirements and concepts in the specification. Typical dimensions that can be used for separable connectors (Type 3 and 4 cable terminations) and examples of the 4 types of cable terminations have been added. In addition, requirements for range-taking ‘maintenance free’ cable support clamps and range-taking rubber grommets for sealing cable entries through gland plates have been included. A drawing has also been included showing a 3-core cable termination with the recommended core crossings done in the screened section of the cable termination in order to eliminate partial discharge problems.
This article has reflected the salient changes in the revisions of NRS 012, NRS 013 and NRS 053. It is the authors’ view that the comprehensive technical changes and harmonisation achieved amongst all three specifications will lead to ultimately, value-adding and effective specifications for the industry.
M Ryan, Technology Engineering Specialist, City Power Johannesburg,
R Kelly, Chief Engineer, Eskom Distribution Technology
NRS 092: CONSTRUCTION GUIDELINES
Because of the unique nature of the construction industry, it has become necessary to develop industry guidelines that are construction-specific. The NRS 092 working group was convened to undertake this task. The mandate of the working group is to consider the construction aspects which are included in the design, planning and construction of NRS 033 and 034 respectively. The construction industry is, in my view, in a fragmented state and therefore requires appropriate standards to provide the industry with guidance in the way in which its activities can be executed. These impact particularly on safety and health, the environment and quality (SHEQ) and results in unnecessary and additional exposure to risk. The objective of developing these guidelines is to standardize crucial practices in construction that are unique to the ESI and that will ultimately minimise the impact on SHEQ.
Why should we separate these components?
The Electricity Suppliers Liaison Committee had recommended that the construction aspects be handled separately from planning and design. Although these components will be dealt with independently, the construction-related planning and design issues will nonetheless be handled within the guidelines. A typical example will be survey activities which can both be dealt with in the planning phase as well as in construction phase. However, what is important is the small (and almost insignificant) transition between planning and design. It should however be noted that the three disciplines are not completely separate as one is dependent upon the other.
It is important to take into account the planning and design aspects when executing construction work. In the same breath it is also important to consider the impact of the planning and designs during the construction activity. It is imperative that this is appropriately handled to prevent gaps concerning the specifications and full understanding regarding the work to be carried out. Another critical issue is to consider various technologies, methods and practices in the construction industry with the intention to develop meaningful and effective guidelines that can be implemented in the ESI with ease. One critical aspect brought into the guidelines, in addition to the technical components, is the business process approach concept to ensure best practices within the industry.
Ishmael Ramora Ledingwane, Eskom: R&S, Industry Association Resource Centre