Safety Strip Calculation Report for Transmission Lines: the technical study that defines the easement
The safety strip calculation report is the technical study that sizes the minimum safety width reserved along an electrical transmission line in Chile. It is prepared span by span—a span being the horizontal distance between two consecutive structures—and is one of the documents required by the Superintendency of Electricity and Fuels (SEC) when processing the final electrical concession. Its rigor determines whether the line will operate with the safety clearances required by RPTD No. 07 and, consequently, sets the minimum safety strip width that will later give rise to the legal easement over the affected properties.
In transmission line design, the safety strip is not assumed as a single value: it is calculated span by span, integrating the voltage level, the conductor properties, the maximum sag under extreme thermal conditions, and the topography of the line route. Only after that calculation, and as a legal consequence of what the report demonstrates, are the corresponding easements established.
This article describes in detail what the report contains, how it is built span by span, which regulations govern it, and why the quality of this study conditions the entire land management stage of the project, from the perspective of specialized design engineering.
| In this article: • What the safety strip calculation report is and its role • The span as the unit of calculation (wind span, weight span, ruling span) • Technical variables that enter the report • Minimum clearances under RPTD No. 07 • Methodology and software: PLS-CADD and span-by-span modeling • From the study to the easement: the cause–consequence chain • Common errors that trigger SEC observations • Frequently asked questions |
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1. What the safety strip calculation report is
The safety strip calculation report is the technical document that justifies—with calculations, drawings, and catenary modeling—the minimum width of the corridor that must be respected on both sides of the axis of an electrical line to guarantee operational safety. It is not a legal procedure or an agreement with property owners: it is an engineering study prepared by a specialized firm and delivered to the project owner as an input for the concession file.
Its role in the project chain is decisive:
- It defines the technical width of the corridor based on the criteria of the SEC’s RPTD No. 07.
- It demonstrates that the minimum clearances to buildings, vegetation, roads, and terrain are met in each span under extreme thermal scenarios.
- It provides the special drawings and longitudinal profiles with modeled catenaries that will be part of the file submitted to the SEC.
- It serves as the technical basis for the owner and their legal advisors to establish the easements over the affected properties.
Technical study, not a legal act
It is important to distinguish the safety strip from the easement strip. The report is engineering: calculation, modeling, and regulatory verification. The easement is law: an encumbrance that is established by operation of law upon issuance of the final concession decree, in accordance with DFL No. 4/20018 (General Law of Electrical Services). The engineering firm does not establish easements; it produces the technical study without which the easement cannot be sized or recorded.
This separation of roles organizes the project: the design engineer calculates the strip with technical precision; the owner’s legal team manages the concession file and the registration of the easements; the real estate registrars formalize the legal acts where applicable.
2. The span as the unit of calculation
The safety strip is not calculated with a constant value along the route. It is calculated span by span. A span is the horizontal distance between two consecutive structures (towers or poles). In each one, the topographic conditions, the length of exposed conductor, the maximum possible sag—and therefore the effective strip width that must be reserved—change.
A real transmission line can have few or many spans, depending on the location of the structure and the length of the line. In extensive projects—for example, trunk lines that cross different climatic zones and elevations—each safety strip will be associated with the length of each span.
Wind span and weight span
The report distinguishes two magnitudes associated with each structure, both critical:
- Wind span: the horizontal length over which a structure receives the lateral load of the conductor from wind action.
- Weight span: the length over which a structure bears the vertical weight of the conductor, including the elevation difference with the adjacent structures.
On irregular terrain—slopes, ravines, elevation changes—both values can differ markedly from the physical span measured in plan. This requires treating each span as an individual calculation.
Ruling span and span groupings
For consecutive sections between two anchor structures, engineering defines a ruling span (also called the ideal span or equivalent span). It is the value that allows the conductor to be tensioned uniformly along the section while respecting the permissible mechanical tensions. The safety strip calculation report uses the ruling span to set the mechanical behavior of the section, and then verifies span by span whether the minimum clearances are met under the critical thermal scenarios.
3. Technical variables that enter the report
Each span is modeled with a defined set of variables. A rigorous safety strip calculation report integrates at least the following:
- Nominal voltage level of the line (kV) and, where applicable, maximum operating voltage.
- Conductor type: material (ACSR, AAAC, ACAR, or others), diameter, unit weight, and modulus of elasticity.
- Stringing mechanical tension and permissible tensions under each load hypothesis.
- Maximum conductor sag under the maximum operating temperature (typically 50–80 °C, depending on the conductor and the zone).
- Detailed topography of the axis and longitudinal profile with actual ground elevations.
- Height and geometry of each structure, with its insulator attachment points.
- Wind loads and, in mountain zones, sleeve ice in accordance with the applicable zoning.
- Mandatory minimum clearances to buildings, vegetation, roads, railways, and other electrical lines, according to RPTD No. 07.
On this basis, the safety strip is calculated under the following design conditions: maximum wind; ambient temperature; phase conductor sag at maximum power transfer; and design deflection angle under the maximum wind condition of the phase conductor due to maximum wind pressure.
4. Minimum clearances under RPTD No. 07
The Superintendency of Electricity and Fuels establishes, in the Technical Regulatory Standard RPTD No. 07, the minimum safety clearances that every overhead electrical line must meet with respect to its surroundings. These clearances are the main regulatory input of the report. The following table summarizes the usual order of magnitude of the strip width per side of the axis, according to voltage level:
| Voltage level | Range (kV) | Reference width (per side) | Applicable standard |
|---|---|---|---|
| Medium voltage | 10–36 kV | 5 m – 10 m | RPTD No. 07 / SEC |
| High voltage | 44–110 kV | 15 m – 20 m | RPTD No. 07 / SEC |
| High voltage | 220 kV | ~25 m | RPTD No. 07 / SEC |
| Extra high voltage | 500 kV | ~32 m | RPTD No. 07 / SEC |
| Direct current | ±600 kV (HVDC) | Per Strip Study | Law 20,936 art. 93 bis |
Source: prepared by the authors based on RPTD No. 07 (SEC), DFL No. 4/20018, and Law 20,936.
The values in the table are referential: the actual width for a specific project is obtained from the safety strip calculation report of that particular route. For direct-current lines—such as the Kimal–Lo Aguirre project, in high voltage—Law 20,936 introduced the Strip Study (EdF), a public mechanism, prior to the tender, that defines authorized territorial corridors before the project is awarded.
Vertical and horizontal clearances
RPTD No. 07 distinguishes between vertical clearances (between the conductor and the ground, buildings, or objects below the line) and horizontal clearances (between the conductor and lateral objects, other lines, or structures). The report must demonstrate simultaneous compliance with both in each span, under all load hypotheses recognized by the standard.
5. Methodology and software: PLS-CADD and span-by-span modeling
Standard engineering practice for preparing the safety strip calculation report uses specialized software, with PLS-CADD (Power Line Systems – Computer-Aided Design and Drafting) being the most widely used in transmission projects internationally and in Chile.
The typical workflow includes the following stages:
- Loading the digital terrain model (DTM) from topographic surveys, aerial photogrammetry, or LiDAR.
- Positioning the structures along the route, with definition of types, heights, and orientation.
- Defining the conductor type and the regulatory load hypotheses.
- Modeling the catenary in each span and automated calculation of sags for the critical thermal scenarios.
- Graphical and numerical verification of compliance with the minimum clearances at all points of the route.
- Generation of georeferenced special drawings, longitudinal profiles, route plan, list of structures, safety strip, and load tables.
Once the model is completed, the report presents the results in the form of drawings, tables, and an explanatory report documenting how each criterion of RPTD No. 07 was met. This set is what is delivered to the owner to be incorporated into the concession file that will be submitted to the SEC.
BIM and integration with other disciplines
In modern projects, the line model does not exist in isolation. It is integrated with BIM workflows and cross-checked with parallel disciplines: geotechnics, roads, hydrology, environmental management, and the design of entry and exit substations. The safety strip report is consolidated as a chapter within a broader information model, which reduces inconsistencies between drawings and improves traceability when facing observations from the oversight body.
6. From the study to the easement: the cause–consequence chain
The electrical easement does not precede the report: it follows it. The logical chain, in order, is as follows:
- Design engineering prepares the safety strip calculation report and the special drawings of the route.
- The owner incorporates these records into the application file for the final electrical concession before the SEC.
- The SEC reviews the technical records (general works drawings, special easement drawings, descriptive report for the electrical concession) and issues a report.
- The Ministry of Energy grants the concession by decree.
- By operation of law, upon publication of the decree, the easements over the properties identified in the file come into being.
- The owner manages—voluntarily or by the route provided for in the LGSE—the formalization and compensation with the affected property owners.
This sequence explains the key phrase: the easement is a consequence of the calculation. If the report is insufficient or erroneous, the easements end up poorly sized, and the entire subsequent process carries the defect. That is why the rigor of the report is decisive for the timelines and the legal viability of the project.
Legal easement and voluntary easement
The 2013 reform of the LGSE expressly introduced the voluntary easement. In practice, many owners prefer this route: it allows them to negotiate directly with each property owner and reduce timelines. But the technical basis remains the same safety strip calculation report: the width agreed with the owner is not an arbitrary number, it is the one that engineering demonstrated to be required to comply with RPTD No. 07.
7. The role of the design engineer in the concession file
The specialized engineering firm specifically produces a defined set of deliverables for the concession:
- Georeferenced special easement drawings and general works drawings, with North–East–Elevation coordinates of each structure.
- Explanatory report of the project and technical justification of the chosen route.
- Safety strip calculation report in accordance with RPTD No. 07.
- Longitudinal profiles with catenaries modeled under different operating conditions.
- Geometries of the structures and of the insulator strings.
- KMZ files or aerial cartography when the drawings require it.
These products do not resolve the negotiation with property owners or the registration of the easements: they are the technical basis on which those actions—carried out by the owner and their legal advisors—are performed. The quality of the report conditions the SEC’s review time, the possibility of technical objections, and, ultimately, the line’s date of entry into operation.
8. Common errors that trigger SEC observations
The SEC’s oversight of the file focuses on verifying consistency between drawings, report, and calculations. The most recurring problems the body observes are:
- Underestimated conductor temperature in the sag calculation, which produces insufficient strips under real operating conditions.
- Inconsistencies between the longitudinal profiles and the plan drawings (elevations that do not match or catenaries not updated after a route change).
- Absence of explicit verification at critical spans: road crossings, parallel lines, long spans over irregular terrain.
- Omission of sleeve-ice hypotheses in zones where the zoning requires it.
- Load tables not consistent with the geometry of the catalogued structures.
- Georeferenced coordinates with different reference systems between drawings and report.
- Inconsistencies in values such as length, span length, strip widths, coordinates, and areas between the drawings and the submitted documents.
Each observation translates into requests for supplementary information that can extend the processing. A report that is well structured from the start, with strict internal review and traceability of the hypotheses, substantially reduces the risk of delays in the concession.
What is the safety strip calculation report?
It is the technical study that justifies, with span-by-span calculation, the minimum width of the corridor to be respected on both sides of the axis of an electrical line, where there can be no other installations and/or trees and vegetation above a certain height. It models the conductor’s catenary under critical thermal scenarios and demonstrates that the minimum clearances of RPTD No. 07 are met at all points of the route. It is one of the documents required by the SEC within the final electrical concession file.
Why is the strip calculated span by span?
Because each span—the horizontal distance between two consecutive structures—has its own topography, length, loads, and mechanical behavior of the conductor. A line can cross ravines, plains, and slopes within a few kilometers, and the conductor’s sag responds differently in each case. Assuming a single strip value along the route would be technically and legally incorrect.
Does the report define the legal easement or only the technical width?
The report defines the necessary technical width corresponding to the line’s safety strip. The legal easement is the legal consequence: it is established by operation of law upon issuance of the final concession decree, and it is recorded over the properties identified in the file’s drawings. The report is engineering; the easement is law. Without a rigorous report there is no properly sized easement. The easement strip is acquired by the owner of the transmission line, who must directly manage its acquisition and/or use with the landowners.
What software is used to prepare the report?
The international and national standard is PLS-CADD, complemented by BIM tools, GIS systems, and topographic modeling software. The choice of software does not exempt the responsible engineer from validating the results: each model must be reviewed against the hypotheses of RPTD No. 07 and against the actual field survey.
Who prepares the report and who establishes the easement?
The report is prepared by an engineering firm specialized in transmission lines, which delivers the study to the owner. Establishing the easement is the responsibility of the project owner and their legal advisors, in coordination with the SEC, the Ministry of Energy, and the real estate registrars. They are complementary but distinct roles.
What happens if the report contains modeling errors?
The SEC may issue observations that delay the concession by weeks or months. In operation, a deficient calculation may translate into insufficient clearances detected during oversight, route modification requirements, or, in the worst-case scenario, safety events involving the line’s surroundings. The technical quality of the study is not a cost: it is protection against far more costly contingencies.
Is the report updated during the line’s service life?
Yes. Any relevant change in the surroundings—new buildings under the line, reconductoring, uprating, modification of structures—requires updating the model and re-verifying compliance with RPTD No. 07. The report is a living document in projects with a service life of several decades. The calculation report is complemented by its corresponding safety strip drawing.
In summary
The safety strip calculation report is the technical heart of any transmission line project. It defines, span by span, the corridor needed for the line to operate with the safety clearances required by RPTD No. 07. The legal easement that will later be established over the affected properties is the legal consequence of that calculation: it does not precede it, it follows it.
The quality of this report conditions the concession timelines, the viability of land management, and operational safety throughout the entire service life of the asset. To delve deeper into the relationship between voltage levels and sizing, we recommend reviewing our article on electrical voltage levels in Chile, and our general guide on electrical transmission line design. If your organization is developing a transmission line project and requires support in preparing the technical report for the concession, contact us.
References
Chilean regulations
- Ministry of Economy, Development and Reconstruction. (2006). DFL No. 4/20018: Sets the consolidated, coordinated, and systematized text of DFL No. 1, on Mining, of 1982. General Law of Electrical Services (LGSE). Government of Chile.
- Ministry of Energy. (2016). Law No. 20,936: Establishes a new electrical transmission framework and creates an independent coordinating body for the national electrical system. Government of Chile.
- Superintendency of Electricity and Fuels (SEC). (in force). Technical Regulatory Standard RPTD No. 07: Safety strips and clearances. SEC. https://www.sec.cl
- National Energy Commission (CNE). (2024). Proposed Annual Transmission Expansion Plan.https://www.cne.cl
Institutional sources
- Superintendency of Electricity and Fuels (SEC). (n.d.). Final electrical concession. Chileatiende factsheet. https://www.chileatiende.gob.cl/fichas/2680
- Ministry of Energy. (n.d.). Electricity Engineering Division. https://energia.gob.cl
- National Electrical Coordinator (CEN). (2024). 2024 Transmission Expansion Plan — Preliminary Technical Report. CNE / CEN.
Reference technical standards
- Power Line Systems. (in force). PLS-CADD: User Manual. Software for the design and modeling of overhead transmission and distribution lines.
- IEEE Standards Association. IEEE 738: Standard for Calculating the Current–Temperature of Bare Overhead Conductors.
Related articles — ESINEL Ingenieros
- ESINEL Ingenieros. (2025). Electrical voltage levels in Chile: design, regulations, and decision-making.https://esinel.cl/niveles-de-tension-electrica-en-chile-diseno-normativa-y-decision/
- ESINEL Ingenieros. (2025). Electrical Transmission Line Design in Chile. https://esinel.cl/diseno-de-lineas-de-transmision-electrica-en-chile/