Why structure spotting matters

Structure spotting determines the height, type and exact location of every structure along a transmission line route. It ensures the line meets mechanical, electrical and clearance criteria while remaining cost-effective. Good spotting:

• Guarantees required conductor-to-ground and conductor-to-object clearances.
• Equalizes span lengths to reduce peak tensions on towers and hardware.
• Minimizes the number of special or extra-tall structures.
• Improves maintainability and operational safety over the line’s lifecycle.

Inputs: what you need before spotting

Accurate inputs are non-negotiable. The following data items are required to perform reliable structure spotting:

• Plan & profile drawings: survey centerline, elevations, feature locations and crossing items.
• Sag templates: cold, normal and hot sag curves for the selected conductors and ruling span.
• Clearance tables: minimum conductor clearances per TES-P-122.09 (roads, railways, pipelines, etc.).
• Span limits and mechanical constraints: max/min spans, wind/weight span limits, insulator swing criteria.
• Structure templates and height scales: to quickly map attachment heights to tower types.

Tools of the trade: manual vs. software spotting

Historically, engineers used a transparent celluloid sag template. Today, software like PLS-CADD performs optimization and reduces manual iterations, but understanding the template remains essential for field verification and special cases.

Manual spotting (sag template)

The sag template shows multiple curves (cold, normal, hot, ground wire) and a ground-clearance offset. Manual spotting:

• Positions the template on the profile so the clearance curve is tangent to the terrain.
• Slides the template to locate the next structure that preserves clearance while approximating the ruling span.
• Is useful for short lines, teaching, or when software results need manual confirmation.

Software spotting (PLS-CADD)

Per TES-P-122.07 Rev.02, structure spotting shall be performed using the latest PLS-CADD or equivalent. Software:

• Integrates accurate terrain, conductor sag-tension properties and design limits.
• Optimizes tower heights and positions for economy (span utilization target ≥ 90%).
• Generates structure lists, sag-tension reports and drawings for construction.

Core design principles to follow

Use these principles during spotting to achieve a robust and economical layout:

1. Uniform spans: aim for spans close to the ruling span. A span utilization factor (average span / ruling span) ≥ 0.90 is ideal.
2. Standard structures: maximize repeated use of a “basic structure” height to lower material and foundation costs.
3. Weight : wind span ratio: keep the weight span to wind span ratio ≥ 0.7 to prevent excessive insulator swing and uplift risks.
4. Limit special heights: avoid crossing structures above recommended heights (e.g. >80 m for 380 kV) unless justified.

How to perform the spotting procedure

The spotting sequence normally begins at an established take-off point (substation gantry or P.I.) and proceeds along the line profile:

• Set the starting structure height at the take-off.
• Place the sag template (or run the PLS-CADD routine) so the ground-clearance curve is tangent to the profile.
• Shift the template to find the next structure location that keeps required clearances while maintaining span uniformity.
• At angles, crossings or rough terrain, test alternative layouts to find the most economical and safe arrangement.

Special cases and checks

Crossings

Crossings are high-risk points. The higher-voltage line must cross over lower voltages, and the horizontal offset from the nearest conductor of the crossed line to the crossing structure centerline should be at least the height of that crossing structure. TES-P-122.07 discourages very tall crossing towers (limits cited for 380 kV and 230 kV) and recommends using gantry arrangements where practical.

Insulator sideswing

Lateral insulator swing reduces conductor-to-structure clearance. The horizontal wind force on an insulator equals half the wind pressure on conductors of adjacent spans; vertical holding force equals conductor weight plus half the insulator string weight. Maintain a factor of safety ≥ 2 on insulators/hardware under maximum wind loading and check the weight/wind span ratio carefully. Remedies for excessive swing include:

• Reposition structures (balance spans).
• Change heights of adjacent towers.
• Use stiffer/different structure types or dead-end towers where necessary.
• Add insulator weights only if other fixes are not feasible.

Grading and uplift

Grading produces a smooth conductor profile and balances structure loading. Uplift (negative weight span) occurs when the cold-temperature position of the conductor pulls upward on an intermediate structure. Avoid uplift by repositioning towers or raising the intermediate tower; if unavoidable, dead-end the conductor or use hold-down guys where appropriate.

Dead-ending and in-line anchors

To avoid cascading failures on long stretches, place in-line dead-end (anchor) structures at intervals not exceeding:

• Wood structures: 5 km
• Steel structures: 12 km

These anchors must be designed to resist full maximum tension for adjacent spans. Also, spans significantly longer than 1.7× ruling span or shorter than 0.5× ruling span should be dead-ended and sagged to a special ruling span.

Field reconnaissance & foundation considerations

Before finalizing the spotting, perform a full field check to identify unsuitable foundations, marshy/sandy zones, or erosion-prone areas. Avoid locating angle towers in waterlogged or weak soils. In shifting sand dune areas, add extra clearance and consider foundation protection.

Drawing check, review and structure list

After spotting, conduct a thorough drawing review to ensure compliance with criteria, accurate elevations, and confirmed clearances. The final Structure List (schedule) should include:

• Station levels and P.I. coordinates
• Structure numbers, types and deflection angles
• Span data (ahead span, wind span, weight span, ruling span)
• Footing and foundation type, soil classification
• Conductor/OPGW specs, grounding resistance, lighting/marker requirements

Practical tips for delivery and review

• Keep PLS-CADD native files and create a clear structure list for construction teams.
• Document any deviation from standard ruling spans and provide justification with alternate templates.
• At crossings and major roads, coordinate with road/rail authorities early to minimize later changes.
• Run automated checks for mid-span clearance, conductor galloping risk, and insulator swing with worst-case load conditions.

Conclusion

Structure spotting is where theoretical design meets the realities of terrain, safety, and construction logistics. Following the TES-P-122.07 Rev.02 principles — combined with modern software like PLS-CADD and disciplined field checks — produces transmission line routes that are safe, economical, and maintainable for decades. Precision at the spotting stage prevents costly fixes later in procurement, foundation works, and operation.