New AS/NZS 3008.1 coming early 2017 | NECA

New AS/NZS 3008.1 coming early 2017

The Australian Standard AS/NZS 3008.1 is the selection guide for Low Voltage cables (up to 0.6/1 kV). It is a legally mandated Standard as it is called up in the Wiring Rules (AS/NZS 3000) and is the ‘bible’ which electrical Contractors, consultants, engineers, equipment manufacturers, software developers and anyone else will base the selection of the appropriate cable size upon.

The current version was published in 2009 and in line with Standards Australia policy for periodic revision to maintain currency and the need for the inclusion of modern cable selection methods, the Standard is being revised and is nearing completion with publication due either end of 2016 or early 2017.

There are two separate documents available on the Standards Website, AS/NZS 3008.1.1 for use in Australia and AS/NZS 3008.1.2 for use in New Zealand. NECA Members have free access to the Australian version through the NECA Technical Knowledge Base (TKB).

WHAT HAS CHANGED

One of the primary drivers for the revision of the standard was to introduce modern cable sizing techniques such as those based on economic grounds rather than purely safety and technical grounds. With the growth in energy efficient buildings the cost of losses, including those generated by cables carrying electric currents, is today prominent in the design considerations of modern energy efficient buildings. While the principles of economic cable sizing methods are not new, they have yet to be included or referred to within the AS/NZS 3008.1 cable selection standard, which was first published in 1984.

Economic cable sizing was first introduced within the IEC 60287 series of standards in 1995 and is also considered in a number of international papers, standards and texts. It is not the purpose of this article to go into the details of the calculation method, other than to say that inclusion into the AS/NZS 3008.1 series of standards is by way of a worked example in Appendix A9 and relevant text references to the method in the scope of the standard and a new clause 2.6 under selection process. All of the worked examples in the Appendices of AS/NZS 3008 are classified as “Informative” meaning that they are for information only and not intended to be applied on a mandatory basis.

Other notable changes to the standard are listed below:-

1.  Low Magnetic Field Configurations: Addition of Table D2 Single-Core cable configurations that result in low radiated magnetic fields. The standard previously had the one table, D1, which was intended to cover single core cable configurations that resulted in BOTH good load current sharing and low magnetic fields. Industry feedback and further investigation on the subject highlighted the need for separate tables since the configurations that give good load current sharing do not always give the lowest level of magnetic field.

The technical investigation also revealed the complex nature of magnetic field calculations and some associated limitations, such as impact of load current sharing on the magnetic field generated by parallel groups of cables and cable group spacing, such that additional guidance notes have been included to address these limitations.

2.  Definition of “Circuit”: Inclusion of a new definition of the term “circuit” for the purpose of determining the appropriate derating factor associated with mutual heating of groups of cables in close proximity to each other.

To align with this new definition and improve the clarity of applying Tables 3(1), 3(2), 3(3) and 3(4), the column title of column 6 has removed the words “for more than one circuit”, therefore the title will simply read “Derating table”. During the review of the standard it became clear that the derating tables 22 and 23 may need to be applied for single circuits, eg single circuit installed on unperforated or perforated trays, as well as multiple circuits. Since the current ratings for unenclosed cables are only applicable to cables mounted on a solid surface (touching) or cables installed on ladder tray, suspended from a catenary wire or unsupported in a switchboard (spaced), factors are required to adjust these ratings for other common installation types, such as perforated trays.

3.  Cables Surrounded by Thermal Insulation: Additional wording/guidance has been added to the definitions of installations where cables are either completely surrounded or partially surrounded by thermal insulation. This additional guidance ensures alignment with the standards AS/NZS 3000 and AS/NZS 3999 where the distance that cables are surrounded becomes a determining factor. eg completely surrounded definition is limited to cables surrounded by more than 400 mm of thermal insulation.

Cables that are completely surrounded by thermal insulation by a distance of more than 150 mm but not more than 400 mm shall now be classified as partially surrounded only.

Cables that are prevented from being completely surrounded by thermal insulation, eg where cables are clipped to a structural member or are lying on a ceiling for more than 150mm are also classified as partially surrounded. The change here is that if cables are surrounded by thermal insulation for a distance of less than 150mm then they do not need to be derated.

4.  Non-Standard Cable Spacing – Reactance: Additional notes/guidance has been added to Tables 30 and 31 for reactance of single core cables in installations where non-standard spacing are used. Reactance values are provided for single core cables installed in touching configurations, ie either trefoil or flat. New notes have been added that give guidance on how to adjust these values for a spacing of 0.5D, 1D or 2D. A correction factor is simply added to the values from these tables.

Further guidance is provided for cables with small conductor sizes (less than 25mm2) where adjustment of reactance is not necessary when using the reactance values for determining the voltage drop factor for the cable as the resistance value of cables with small conductor sizes becomes the dominant value in the overall voltage drop factor and the error is quite small (less than 2.5%).

5.  Non-Standard Cable Spacing – Voltage Drop: Similar to (4) above, additional notes/guidance has been added to Tables 40, 41, 43, 44, 46, 47, 50 and 51 for voltage drop of single core cables in installations where non-standard spacing are used. Voltage drop values are provided for single core cables installed in touching configurations, and new notes have been added that give guidance on how to determine the voltage drop values for non-standard spacing by using the impedance of the cable (Clause 4.3) with the adjusted reactance values of (4).

6.  AC Resistance for LV ABC: New columns have been added to Table 34 for AC resistance at 80oC being aligned with the maximum operating temperature of LV ABC cables. Columns for both copper and aluminium conductors have been added, but only values for conductor sizes in common use for these cable types are included, ie. 6mm2 up to 25 mm2 for copper conductors and 16mm2 up to 150mm2 for aluminium conductors. A suitable note has been added to this table to add guidance for users of the Standard relating to the use of this table being appropriate for LV ABC.

7.  Three Phase Voltage Drop for LV ABC: New columns have been added to Tables 40 and 43 for Three Phase Voltage Drop at 80oC being aligned with the maximum operating temperature of LV ABC cables. Columns for both copper and aluminium conductors have been added, but only values for conductor sizes in common use for these cable types are included, ie. 6mm2 up to 25 mm2 for copper conductors and 16mm2 up to 150mm2 for aluminium conductors.

A suitable note has been added to these tables to add guidance for users of the Standard relating to the use of these tables being appropriate for LV ABC.

8.  Three Phase Voltage Drop at 80oC: Columns 8 and 9 of Table 45 have been deleted from the table of Three Phase Voltage Drop values for multicore cables with circular aluminium cables, since the values applicable to LV ABC have been added to Tables 40 and 43. (Noting that it is more appropriate to apply the voltage drop factors for single core cables to LV ABC due to the thicker insulation used on these cable types as required by AS/NZS 3560 compared to the insulation thickness of other 0.6/1 kV cables covered by AS/NZS 5000.1.)

A suitable note has been added to this table to point users of the Standard to the tables appropriate for LV ABC.

9.  Three Phase Voltage Drop at 80oC: Similar to (8) above, Columns 8 and 9 of Tables 50 and 51 have been deleted from the tables of Three Phase Voltage Drop values for bare or insulated aerial cables,  since the values applicable to LV ABC have been added to Tables 40 and 43. Further, since the values in Tables 50 and 51 are applicable to cables with a spacing of 0.4m, eg. bare systems, these are not appropriate for LV ABC systems as the cores are touching and would result in very conservative values.

Suitable notes have been added to these tables to point users of the standard to the Tables appropriate for LV ABC.

10. AC Resistance for Single Core 630mmCopper Flexible Cables: A new row has been added to Table 37 for AC resistance at 630mm2 single core copper flexible cables since this size is one that is commonly available in the market today.

11. Derating Factors For Circuits: During the review of the standard several anomalies were highlighted with respect to some of the derating factors detailed in Table 23 for grouping of single core cables in air on cable trays. Table 23 includes derating factors for both single circuits (due to the influence of cable trays that restrict the circulation of free air around the cables, eg. perforated and unperforated trays) and multiple circuits which are installed in close proximity to each other either on unperforated, perforated or ladder trays, in single or multiple horizontal or vertically oriented rows.

The anomalies lie with the derating factors for single circuits, eg. using the example of unperforated trays. Column 6, row 1 shows a derating of 0.95 for a single circuit while Column 6 Row 12 shows a derating of 0.98 for a single circuit. In reality these two situations represent the same circuit configuration and therefore the same derating should apply. All such instances of this type of anomaly were reviewed and adjustments made to obtain consistency.

SUMMARY

While this new revision of AS/NZS 3008.1 does not bring any major changes to the Standard none the less the changes introduced are important as they:

(1) introduce new techniques such as economic cable selection and cable configurations that minimise magnetic fields and

(2) align the standard more closely with cables and installation methods commonly used, some examples include adding the relevant cable parameters for LV ABC which have a maximum operating temperature of 80oC, and introducing techniques to allow users of the standard to compute voltage drop values for single core cables with non-standard cable spacing.

Original information from Martin Muxworthy (Voltimum) – Standards Australia Committee EL-001/10

For more information, contact Leon Dickson, NECA's Member Services and Technical Advisor on 1300 361 099 or email memberservices@neca.asn.au.

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