10.1. Survey
The line route survey should be undertaken in accordance with the Standard for Line Survey.
10.2. Layout Clearance Buffer
An electricity entity must ensure the distance from the conductors of its overhead electric lines is in accordance with the Electrical Safety Act and Electrical Safety Regulation. There are tolerances in the design and the construction of an overhead line such that “as-built” clearances will differ from the design clearances. These tolerances are closely tied to the design and construction practices.
Traditionally a deterministic clearance buffer has been applied to maintain compliance with the code. The method detailed below offers a probabilistic method that incorporates the various design and construction practices into the design clearance.
A ground clearance buffer shall be provided to allow for the following sources of error:
Error Source Method New Construction
(Note A)
Rating Study (Notes A & B)
Survey Level Errors
GPS 0±100 0±100
Theodolite 0±50 0±50
Lidar 0±150 0±150
Rangefinder 0±200 0±200
Pegging Error or Pole Planting Error
1 m in 300m, C = 1500m 0±50 or 0.7% sag N/A 1 m in 150m, C = 1500m 0±25 or 1.3% sag N/A Conductor Modelling
No creep allowance – AAC,
AAAC (not recommended) -25±10oC? N/A
No creep allowance – ACSR (not recommended)
-30±10oC
Typical Span Only
Creep prediction 0±5°C N/A (creep virtually
complete) RS assumption, max span/ min
span less than 2 ? ?
RS assumption, max span/ min
span less than 4 ? ?
Parabola instead of catenary L =
300m, C = 1500m -10±0
temperature N/A 0±2°C
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Error Source Method New Construction
(Note A)
Rating Study (Notes A & B)
Conductor Temperature - Using Ambient Temperature Instead Of Measuring Conductor Temperature
Measured at mid-day, no cloud, lightly loaded feeder, slight breeze
N/A 10±10°C Measured at mid-day, no cloud,
moderately loaded feeder, slight breeze
N/A 15±15°C Measured at mid-day, cloud
cover, lightly loaded feeder, slight breeze
N/A 5±5°C Sag Error In Unsagged Spans
Sheave friction – large sheaves, rugged terrain, less than 10 successive unsagged spans
± N/A Sheave friction – small sheaves,
flat terrain, up to 10 successive unsagged spans
± N/A Tensioning Errors
Dynamometer (used correctly) 0±250 N/A Line-of-sight (eyeballed, poor
target, no correction for temperature changes during sagging operation)
0±150 and
0±10°C N/A
Line-of-sight (telescope, accurate target, corrected for temperature changes)
0±50 and 0±5°C
Theodolite (offset method) 0±50 N/A
Theodolite (tangent method) 0±100 N/A
Return wave method (1s in 3
returns, L = 300m, C = 1500m) ? N/A
Error Source Method New Construction (Note A)
Rating Study (Notes A & B)
Foundation Depth – Direct Buried Poles
Well supervised ±150 N/A
Poorly supervised ±400 N/A
Benched sites ±300 N/A
Foundation Depth
Flange based pole 50±100 (50 grout) N/A
Towers 0±100 N/A
Notes
A. Tabulated values of offset and typical tolerance (mean and standard deviation) in millimetres unless indicated otherwise. The offset is positive when the additional ground clearance is provided. For example if the tower height is measured from the K point and the K point is typically 300mm above the centre peg then the offset is 300mm.
B. The “as-built” line is surveyed and there are no construction errors to account for. However if modifications to the line are required to uprate the line then construction errors will be
introduced depending on the nature of the modifications. For example, retensioning the conductor will introduce sagging errors. Unfortunately this means that a different buffer is required for different portions of the line, a task that layout software applications do not normally accommodate.
It is assumed that each error source is statistically independent and is normally distributed. The sum of these random variables is also normal such that:
∑
=Where N is the number of errors involved, µn is the offset (mean) of error number n and σn is the tolerance (standard deviation) of error number n.
To allow for these sources of error with a confidence of 84% the required clearance buffer is
)
1σ
= − ( µ − σ B
To increase the confidence to 98% requires
) 2
2σ
= − ( µ − σ B
Reding (2007) details this method adopted by Bonneville Power Administration, Pacific Northwest region of USA.
10.3. Using the Buffer
Lines shall be designed with a clearance buffer and a temperature buffer based on the 84%
confidence limit. The assumed values tabulated above may be altered to suit the degree of control exercised during the design and construction phases.
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For example, a new line with a thermal rating of 75°C and a temperature buffer of 5°C shall be spotted with a temperature of 80°C. If the statutory ground clearance requirement is 6.7m and the clearance buffer is 0.5m then the line shall be spotted with 7.2m of clearance.
The records shall state the following:
Maximum operating temperature = 75°C with 5°C buffer Minimum ground clearance = 6.7m with 0.5m buffer
No line shall be designed with a vertical clearance buffer of less than 0.3m and a temperature buffer less than 5°C.
The clearance buffer does not apply to
• Vertical separation for unattached crossings
• Vertical separation for attached crossings
• Mid-span phase separation
• Electrical clearances to the supporting structures The clearance buffer does apply to
• Unroofed terraces, balconies and sun decks
• Roofs
• Covered places of traffic
• Structures not normally accessible to persons
The horizontal clearance buffer is much more difficult to evaluate because of
• Structure deflection
• Insulator swing
• Distributed nature of wind gusts
Neither the Electrical Safety Regulations nor AS/NZS7000 specifies a wind pressure to use for horizontal clearances. The “blowout” weather case shall be used to calculate horizontal
clearances. Clearances may be infringed with a lower wind pressure because the conductor moves in an arc. No additional horizontal buffer is required because the hazard to humans is considerably lessened during “blowout” wind speeds.
10.4. Clearances
Unless otherwise specified in the project scope, structures shall be located so that the required clearances below are satisfied at a maximum operating temperature of 75°C, 10 years after commissioning. That is, the designer shall allow 10 years of conductor creep. For the purpose of calculating creep the “creep” weather case shall be used.
Where the statuary guidelines (AS/NZS 7000, Electricity Act) differs from the figures below, the more onerous shall apply. The horizontal clearances shall be satisfied under the high wind weather case.
CLEARANCES FROM GROUND AND ROADS
High load corridor routes Vertically 8.0m 9.0m
Other than roads Vertically 7.0m 7.5m
Over truck stop areas / high load areas Vertically 9.0m 10.0m Extremely steep or swampy terrain that cannot be crossed
by traffic or mobile machinery Vertically 5.5m 6.0m
Road cuttings, embankments etc. Horizontally 4.6m
Over or adjacent cultivation Vertically 8.5m 12.0m
Over or adjacent to cane Vertically 8.5m 12.0m
Sugar cane bin unloading areas Vertically 12.5m
Waterways – Recreational/navigable Refer to the Distribution Design Manual Dwg 3143 Sh 1 to 10.
Vertically As agreed with appropriate
controlling body / AS6947
Waterways & other areas subject to flooding – Above flood Vertically 5.5m 6.0m
CLEARANCE FROM STRUCTURES, BUILDINGS AND BOUNDARIES
DISTANCE
LOCATION DIRECTION
66kV 132kV Vertically 5.5m 7.0m Unroofed terraces, balconies, sun decks, paved areas and
similar areas subject to pedestrian traffic only, that have a handrail or wall surrounding the area and on which a
person may stand. Easement Boundaries. Horizontally 4.6m 5.5m
Vertically 4.6m 6.1m Roofs or similar structures not used for traffic, but on which
a person may stand – includes parapets Horizontally 4.6m 6.0m
Covered balconies, open verandas, opening windows In any
direction 4.6m 6.5m
Blank walls and windows, which cannot be opened. Circuit
separation. Horizontally 3.0m 4.5m
Vertically 3.0m 4.5m Other structures not normally accessible to persons
e.g. TV aerials, clothes hoists, etc. Horizontally 3.0m 4.5m
Real property boundaries Horizontally 0.0m 0.0m
Intercircuit clearances shall be determined in accordance with the Ergon Distribution Design Manual and AS/NZS7000.
10.5. Layout Checks
The following additional requirements should also be met:
• Avoid structure and stay locations in erodible areas and flooded areas.
• Avoid Telecom(s) facilities and other infrastructure, refer to Telstra Agreement Section 11.4.
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• Avoid areas of Cultural Heritage significance.
• Constrain maximum wind limit state loads (factored) to termination structures in substations to the requirements shown on Ergon Energy Drawing EESS 10075.
• Allow for extra clearance over riparian vegetation and rare and endangered flora.
• Satisfy the specified electrical clearances on the structures.
• Avoid overloading of structures.
• Consider aesthetics and amenity where there is impact around local community.
• Gain agreement from local councils and other corporations and authorities with infrastructure in the area.
10.6. Layout for Security – Cascade Failure Prevention
The maximum length of line between termination structures shall be 5km. These termination structures shall be capable of supporting the full termination loads of all conductors and earthwires.