AUSGRID REVISES NETWORK STANDARD NS220 - OVERHEAD DESIGN MANUAL
We knew it was coming, but Ausgrid’s revised Network Standard NS220 has finally made the light of day. So, what does this mean for ASP3 overhead line designers in the Ausgrid franchise area? It is time to browse to Ausgrid’s Network Standards page (click here) and download a fresh copy of NS220. Then with a mug of your favourite brew**, settle in for a hardy engineering review.
**we suggest a caffeine-based brew for the technical review, and then when you are finished, any brew of your choice for celebrating your hard work!
Ausgrid Network Standard Amendment NSA2022 states that:
“The document has undergone a full rewrite of its content
and does not contain any grey shading”
No grey shading means that changes will not be obvious, so line designers should be setting aside some time to familiarise themselves with the new NS220. It is also a timely opportunity for ASP3’s to compare the document’s revised parameters with their own line design software parameters.
The first thing to notice is a welcome reduction in page numbers. This comes from the removal of worked examples, stringing tables, conductor load tables, and some oddly empty tables and sections. With the requirement for designers to use an approved Overhead Line Design software package (refer to Clause 2 of the revised NS220), the removal of the stringing tables and conductor load tables makes sense. However, the worked examples were a good reference for junior designers and those interested in calculations using first principles. There may be a chance that Ausgrid will release a Handbook for NS220 with some worked examples (HB 331 in style), but no official word yet. We will have to wait for the next COVID-safe ASP3 roadshow for any word on this. If you liked the worked examples, or the stringing tables and conductor load tables as reference tools, it might be worth holding on to your old copy of NS220.
The biggest change in the revised document is Ausgrid’s adoption of ‘Design Security Levels’; refer to Table 1 in NS220. This concept is nothing new for those familiar with AS/NZS 7000:2016, but it should be noted that Ausgrid have adapted the Design Security Levels to suit their operating voltage levels. What results is a change to wind pressures that designers have used for some time. Refer to Table 3 for the new ‘Design Wind Pressures’ and cue an update to your line design software parameters. For those interested in where the wind pressures are derived from, scroll to Table 4. Kudos to Ausgrid for including these parameters as it makes for easy comparisons with AS/NZS 1170.2 where fine tuning of the design wind pressures is required.
While your line design software parameters window is open, a scroll through Clauses 3.6 and 3.7 will allow you to check and update parameters as required.
Design submissions may be an anxious time for some, but Ausgrid have provided a list of design deliverables to help ASP3’s through this phase. Look to Clause 4.4 if you do not already have a checklist for this critical part of the design phase. And if you do have a checklist, it would be worth your while to run it past Clause 4.4 just to be sure.
Clauses 5 and 7 provide a good update for conductor and pole information, respectively. The tables will be helpful for building or updating line design software libraries for conductors and poles. If you are in Clause 7 and looking for timber pole information, you may be disappointed to find a reference to Ausgrid’s NS128 document instead. All other pole material types are contained in Clause 7, though.
Clause 8 refreshes the information on stays and stayed structures. Table 23 presents stay wire sizing and ultimate loads in one handy location, so update your software parameters while you are perusing this section. Of note to stayed structures is an update to the Failure Containment load case in Clause 3.3.3. Ausgrid now require ASP3 designers to model stayed structures to ensure:
“that the pole and foundation shall be capable of withstanding
the Failure Containment Load with the stay wire removed”
For structures with multiple stays, the check will need to be performed with only one stay removed. This new requirement provides Ausgrid with some assurance that the structure will not collapse between the time a stay wire fails and the time when the failure is corrected through regular maintenance practices. This new Failure Containment check may require designers to specify a higher strength pole, or to replace an existing pole, to satisfy the unstayed pole strength requirements.
The start of Clause 9 presents a concept of which some designers may have never seen or been made aware. Eyebolts and eyenuts are used in standard constructions for attaching termination insulators, but they have their limits when it comes to deviation angles. Figures 26 and 27 (Ausgrid Drawings #520331 and #520324, respectively) detail how quickly the strength reduces, particularly for the eyenut, when a deviation angle is applied to these elements. The eyebolt provides a higher strength under deviation (see Figure 27), which is why steel crossarms provide two (2) eyebolt locations at the crossarm ends. When the load exceeds the limit of the eyebolt/eyenut combination (usually due to the deviation angle), then two (2) eyebolts are installed (one (1) for each termination direction). If the eyebolt load is exceeded for the required deviation angle, then it is time for an athwartship installation to account for the deviation angle (for example, refer to Ausgrid’s 4‑12 construction on drawing #513931). This tends to be an issue for sub-transmission constructions with heavy conductors and long spans, but it is a consideration for all designers.
Clause 9.5 introduces ‘Aerial warning markers’ to NS220. The coverage is not as comprehensive as in Endeavour Energy’s standards but provides enough detail for the designer to conduct their own assessment based on Australian Standards AS3891.1 and AS3891.2. There are many valleys and long-spans in the Ausgrid franchise area which may be frequented by aircraft, particularly during fire-fighting activities. There are also numerous heliports and Australia’s busiest airport (Sydney Airport), so this clause warrants designers’ attention. It would be good to see Ausgrid add some additional detail to this clause to bring it closer to Endeavour Energy’s documents around Aerial markers.
The Clearance tables of Clause 10 have been updated, which correct some anomalies in the previous document’s values. They also provide some new and useful information. In particular, the unattached conductor separation requirements (Clause 10.3.4) are explained much more clearly with the addition of Table 34. And conductor separation information (Clause 10.3.5) has been translated from AS/NZS 7000, which is helpful for all designers.
The last major change is in Clause 11 ‘Software’. Up until now, Ausgrid has been fairly relaxed about which software ASP3’s use for their design. This is still the case for distribution design with spans less than 250m, but for distribution designs with spans over 250m, and for any sub-transmission designs, software that employs Finite Element Analysis is a must. Clause 11 has a fairly extensive list of what is required from your software, so it would be worth ASP3’s (and software companies) having a review to ensure compliance.
If you come across any issues in your review that you would like to discuss, or need assistance with, know that PCE is ready to work with you. Our ASP3 team understands NS220 and its requirements, and we use this knowledge to save our clients time and money on design delivery. We also understand our software (which includes PLS-CADD) and how to use it to help you with even your most complex problems.
When you finish your review, don’t forget your celebratory brew…you will have earned it!