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Overhead power line masts
Condition assessment of steel poles for 35–750 kV overhead lines: what counts as a defect and how it is classified
An overhead line pole usually gives warning signs in advance: corrosion at the base, a loose anchor nut, a crack at a flange or a weld. Below we go through the kinds of inspection and how often they are made, what counts as a defect on a multi-sided and a lattice pole, what is measured, how defects are ranked by danger and what is done about them. The text is for the power engineer who runs the line and for the designer who prepares strengthening or replacement. The inspection intervals in the table come from the rules for consumer lines up to 150 kV; for higher voltages they are taken from the line operator’s documents.
Contents
- Defect and damage: what the difference is
- Kinds of inspection and how often they are made
- Where corrosion is more dangerous: height zones and joints
- Multi-sided pole: what is looked for and how it is checked
- Lattice pole: what is looked for
- What is measured and how it is read
- How defects are categorized
- When inspection is not enough: the survey and the verification calculation
- Repair, strengthening or replacement
- Which documents are needed and what remains after the survey
- Where assessments go wrong
- What we can do
- Standards and validity
Defect and damage: what the difference is
A defect is a departure from the design or technological documents built in at the design or fabrication stage: cavities in the metal, excessive bow of members after rolling, poorly applied coating, a mistake in the soil or wind data. Damage appears later, in transport, erection and service.
For the calculation the difference is small: a pole with any departure is checked as it actually is. For the cause it matters. A factory defect is looked for on other poles of the same batch, an erection defect is tied to the way the pole was set, a service defect to the conditions it stands in.
Part of the damage arises during construction. If the base of the pole does not match the anchors, the leg or flange is pulled to the bolts, and forces the design did not foresee appear in the wall, the flange and the bolts. Another example: the foundation is set at the wrong level, a pack of shims is put under the pole, and the anchor is no longer long enough for two nuts.
The age of a pole is not a defect in itself. But on a line that has been in service for a long time, all poles along the whole length are surveyed, not a sample, and not only because of wear: in that time the wind and ice maps have changed, as have the design strengths of steel and the corrosion protection requirements.
Kinds of inspection and how often they are made
The names in the documents of line owners may differ, but in substance there are four kinds of inspection. The intervals in the table are taken from the Rules of Technical Operation of Consumers’ Electrical Installations, which apply to consumers’ lines up to 150 kV inclusive. For the lines of power companies and for higher voltage classes the intervals and forms of inspection are taken from the line operator’s documents (see the section on standards), and the figures given here should not be carried over to them without checking.
| Kind | When and how often | What is done | Who |
|---|---|---|---|
| Periodic ground inspection | Along the whole length of the line at least once a year; the schedule is approved by the person responsible for the electrical installations | A walk along the line: pole tilt, visible corrosion and coating condition, fixing to the foundation, presence of bolts and nuts, weld integrity, plates and numbers, trees and foreign objects in the line corridor | Operating staff |
| Climbing inspection | For lines of 35 kV and above: once in 5 years if the line is 20 years old or more, or runs through a heavily polluted zone or open terrain; once in 10 years for the rest | Climbing the pole: conductors and earth wires in clamps, insulators, fittings, crossarm connections and the top of the shaft seen at close range | Operating staff, climbing the pole |
| Extraordinary inspection | After ice accretion, conductor oscillation, ice drift and river floods, forest and steppe fires and other natural events; after a trip by relay protection with an unsuccessful auto-reclosing | Inspection of poles and the line on the section the event affected | Operating staff |
| Technical survey | On the owner’s decision: a line with a long service life, mass signs of corrosion or deformation, reconstruction (a new conductor, OPGW, a branch), an accident, defects found by inspection | A defect register, measurements of thickness and geometry, inspection of foundations and earthing, a verification calculation, a report with a condition rating | A specialized organization, specialists with a qualification certificate |
Inspection intervals are from the Rules of Technical Operation of Consumers’ Electrical Installations (PTEEP), cl. 6.1.16 and 6.1.17. For 0.38–20 kV lines, climbing inspections are made as needed. Major repair of lines on steel and reinforced concrete poles is done to a schedule depending on condition, but at least once in ten years (cl. 6.1.24). The survey row describes practice, not a period from a standard.
Climbing the pole and safety of inspection
Climbing inspection and repair need a safe way up. PUE (2026 edition), cl. 2.5.136 and 2.5.137, require that servicing and repair can be carried out on the pole, that the climb is convenient and that there are places to fix devices for the work. For multi-sided bent shafts, fixed ladders without a cage are provided up to the height of the earth wire attachment. How this is solved in practice: ladders and fall arrest systems on poles and masts.
Where corrosion is more dangerous: height zones and joints
Corrosion is not spread evenly over a pole. Most damage occurs in the transition zone around ground level, a little below and a little above it: moisture stays there for long, and temperature swings and vegetation intensify electrochemical processes. Higher up, in the atmospheric zone, wear is much smaller. Below ground the rate depends on the soil (drainage, composition, water table) and falls with depth.
The second centre is the joints. Where parts fit loosely, corrosion products accumulate, and crevice corrosion is more dangerous than uniform corrosion: rust bends back the edges of gusset plates, pulls members apart and changes the calculation scheme. The volume of corrosion products in such places is several times that of the metal they came from: it bends back the gussets and is visible from outside even where the metal cannot be measured.
There is an exception to the rule of a fall with height. In an industrial zone, where the surface is coated with fallout, the corrosion rate does not fall upward, so the zone near the ground cannot be taken as representative of the whole pole there. On a free surface, a dense film of rust on the contrary protects the metal somewhat, and corrosion slows with time. How section loss enters the calculation is covered in Corrosion wear and load capacity.
Multi-sided pole: what is looked for and how it is checked
A multi-sided shaft is a tube of varying section, so defects gather around the joints: the base, flanges, splices and hatch. The joints are described in Multi-sided pole joints and wear of lighting poles in Wear and corrosion of metal poles. Here is what is added on line poles.
| Zone | What counts as a defect | How it is checked |
|---|---|---|
| Shaft base and transition zone | Rust, flaking zinc, delamination of the metal, water and dirt in the corner at the plate or flange | Inspection; wall thickness around the perimeter at several levels, the surface cleaned; for buried poles, with the soil opened |
| Flange and anchor bolts | Missing, loose or corroded nuts (and lock nuts if the design calls for them), bent bolts, a gap under the flange, chips and cracks in the concrete head, tilt | Inspection; tightening to the torque in the design; the gap measured with a feeler gauge; verticality with a theodolite or level |
| Flange splices of sections | Uneven gap, rust from under the flange, missing bolts, cracks in the weld of the flange to the wall | Inspection during a climbing inspection; bolts tightened to torque; doubtful welds by non-destructive testing |
| Telescopic splice | Worn zinc and rust streaks from under the edge, section slip, a dent in the edge of the outer section | Outside inspection, comparison with as-built dimensions; the rest is judged by calculation |
| Longitudinal and circumferential welds | Cracks, lack of fusion, undercuts, rust streaks along the weld | Inspection; in doubtful places liquid penetrant, magnetic particle or ultrasonic testing |
| Shaft along its height | Dents, buckling, ovality, bow, traces of vehicle impact or falling objects | Inspection; bow measured from a stretched string, deviation from vertical with a theodolite |
| Hatch | Corrosion of edges, a door that does not lock, poor earthing contact on the door or frame | Inspection, check of the lock and the contact |
| Crossarms, brackets, string attachments | Cracks at welds, loose bolts, crossarm deflection, deviation from horizontal, rotation relative to the line axis | Inspection; deflection compared with the permissible value (see the measurement table) |
| Earthing | A broken or corroded down-lead, poor contact on the bolt, electrodes that have come out of the ground, resistance above the norm | Inspection; measurement of earth resistance; selectively, the soil opened at the electrode |
Lattice pole: what is looked for
A lattice pole consists of many members, so the assessment goes by components: chords, braces, gusset plates and the joints of the chords with the foundation. The defect register records:
- general and local bends of chords and braces, «broken» angle legs, missing members;
- joints and gusset plates: crevice corrosion, bent-back edges, misaligned axes, poor fit; welded-on plates that close the space between the gusset and the base plate and form a cavity where water collects;
- bolted connections: missing or loose bolts and nuts;
- base joints: a leg pulled to the anchors, bent bolts without nuts, a pack of shims between the foot and the footing where the bolt is no longer long enough for two nuts, a foot that does not bear fully on the foundation;
- geometry: deviation from vertical along and across the line, displacement from the line, a crossarm that is not horizontal, rotation of the crossarm;
- foundations: settlement or heave, water standing around the footings, the top of the foundation exposed over a noticeable height (a sign of violated installation technology), cracks and chips in the concrete, exposed reinforcement;
- guys (for guyed poles): unlaying of strands at the clamps and on the crossarm where the rope bends over a small radius, corrosion of the attachments to the foundation.
What is measured and how it is read
Inspection gives a list of signs, and measurements turn them into numbers for the calculation. Below are the parameters entered in the defect register and what they are compared with. The permissible values in the table are from PTEEP (table 6) for lines of 35 kV and above; in the line operator’s instruction they may differ.
| Parameter | Measured with | How it is read |
|---|---|---|
| Wall and section thickness | Ultrasonic thickness gauge, micrometer, dial caliper; the surface cleaned of flaking coating and corrosion products | The number of readings on a member is set by the survey programme: pitting needs more than uniform corrosion. The design thickness is taken not at the mean but with the scatter of readings allowed for. The initial thickness is measured on a part without corrosion or under a washer. The ratio of the remaining section to the design section must be at least 0.9 for load-bearing members, 0.8 for non-load-bearing ones and 0.7 for gusset pieces |
| Zinc thickness | Magnetic thickness gauge | Compared with the requirements for a new coating under DSTU EN ISO 1461; the same points are repeated in later surveys ([zinc coating thickness](/en/services/hot-dip-galvanising/galvanizing-coating-thickness/)) |
| Tilt and position of the pole | Theodolite, level, measuring tape | Deviations along and across the line and displacement from the line are taken from the current document; on tension and angle poles the tolerance is tighter and is set by the design |
| Member bow | A stretched string and a rule, a level | Crossarm: up to 1:300 of its length; shaft or strut: up to 1:750 of the length but not more than 20 mm; chord angles: up to 1:750 of the member length |
| Bolt tightening | A wrench that shows the tightening torque | Torque and bolt class come from the design; a loose bolt is not simply tightened until the cause is found: tilt, settlement, vibration |
| Welds | Inspection, magnifier; then liquid penetrant, magnetic particle or ultrasonic testing | A crack or lack of fusion in a load-bearing weld is treated as critical; the method and extent of testing are fixed in the work programme |
| Foundation | Inspection, level, portable microscope or magnifier for crack width | 2–3 % of the poles in total are checked; sizes of chips and cracks and anchor bolt deviations are compared with the normative values |
| Guys | Dynamometer, caliper | Tension with the pole deviation within tolerance differs from the design by no more than 20 %; reduction of the guy rope section is no more than 10 % |
| Earthing | Earth resistance meter | Compared with the norm for lines ([earthing of line poles above 1 kV under the PUE](/en/products/overhead-power-line-masts/overhead-line-pole-earthing-above-1kv-pue-2026/)); measured in the period of greatest soil dryness |
| Steel grade and properties | Testing of specimens; hardness measurement without sampling | Needed when there are no certificates: the number of specimens or measurements for each kind of section is set by the survey programme |
Tolerances for bow, corrosion, tightening and the checking of foundations and guys are from PTEEP, table 6 (item 2); the values for pole tilt are placed in an appendix of that document which is not reproduced in the article. The number of readings and where they are taken along the height are set by the survey programme.
How defects are categorized
The line operator’s documents or a survey method may have their own condition scales. We use three levels because they match the decisions that are actually made: at once, to a plan, during routine maintenance. The level is set not by the defect alone but by the defect plus context.
What is taken into account besides the defect itself:
- the position of the pole in the line and the consequences of its failure: tension, angle and crossing poles carry a different responsibility from intermediate ones, and the fall of one can pull down its neighbours;
- the importance of the line and the direction of prevailing winds relative to its axis: whether the design load will really occur;
- the trend: corrosion rate, whether a crack is growing, whether the tilt has stopped.
| Category | Signs (examples) | Decision |
|---|---|---|
| Critical | A crack in a load-bearing member or weld that is growing; buckling or a deep dent in the shaft; a destroyed base joint; missing or stripped nuts on several anchor bolts; tilt or settlement of the foundation that is increasing; a damaged guy; section loss that the calculation does not cover | At once. The operating mode of the line and temporary measures are decided by operations together with the designer; then an extraordinary repair, strengthening or replacement |
| Significant | Corrosion with section loss within the margin; deformed individual braces or crossarms; loose or missing individual bolts in less loaded joints; cracks in the concrete head; a large area of damaged coating under which the steel is already rusting | A verification calculation with the measured sections; repair or strengthening to a plan within a set period; more frequent inspections |
| Minor | Local coating damage without loss of steel; missing numbers and plates; bird nests; surface scratches | Fixed during routine maintenance; recorded in the defect register for comparison at the next inspection |
A working classification for decisions, not a scale from a standard. The move between levels depends on the calculation and on the position of the pole in the line.
Why the same defect is rated differently
A crack on an intermediate pole in the middle of a field and the same crack on a tension pole at a river crossing get different categories and different deadlines. The same goes for corrosion: a pole working under small loads carries a given section loss, while a pole at the end of a long span with a high wind pressure does not. So the category is assigned by calculation together with inspection, not by inspection alone.
When inspection is not enough: the survey and the verification calculation
The assessment rests on a strength check. The pole is recalculated for the real loads with the measured sections: the actual span lengths, the conductor and earth wire type, the climate, the defects found and the corrosion wear. The calculation scheme takes the actual condition, with bends, poorly made joints and a fixing that differs from the design. The calculation is made under the standards in force at the time of the survey, not those under which the pole was built.
This calculation does more than expose weak points. Standard poles carry a strength margin built in for extreme conditions, and with the real spacing, shorter spans or a lighter conductor it is sometimes enough to keep a pole with a defect without strengthening. How defects enter the calculation is covered in Condition assessment of steel structures, and loads on overhead lines in Loads on overhead lines, ice loads and wind regions.
If there are no certificates for the steel and fasteners, their properties are established by testing specimens or measuring hardness. The class of bolts, if it cannot be determined, is taken as the lowest possible or checked by breaking bolts from the batch. The extent of testing is agreed by the organization carrying out the survey.
The survey is widened when defects are mass rather than isolated: then all poles of the line are checked, not only those where a defect was found. For isolated defects the damaged pole and its neighbours in the same conditions are surveyed, and after an accident the whole line is checked for the same departures.
Repair, strengthening or replacement
The decision depends on the cause of the defect, its scale and the result of the verification calculation. If the pole, with the measured wear taken into account, carries the design loads, it stays in service; if not, strengthening to a design or replacement is needed. Repair returns the design properties to the structure, strengthening raises them.
- repair: tightening or replacing bolts, restoring the coating, repairing welds to an agreed procedure, correcting local deformations. The methods are in Typical damage to steel structures and repair methods;
- strengthening: plates, ribs, additional members or replacement of a single section to a design, when the section has lost part of its capacity but the pole can be kept. See Strengthening of steel structures;
- replacement of a member, a section or the pole: buckling, repeated cracks, section loss that strengthening cannot restore, a destroyed base joint. For a multi-sided pole, replacing a section is possible if the flanges and the fixing pattern match; a new pole on an old foundation needs the same anchor pattern (anchor bolts for poles and masts).
The coating separately. A zinc coating protects the pole for its whole service life and keeps working even when part of it is destroyed (see service life of hot-dip galvanising). PUE (2026 edition), cl. 2.5.19, requires metal poles to be protected from corrosion, mainly by hot-dip galvanising to DSTU EN ISO 1461. A paint coating renewed in the field lasts noticeably less than a factory one, because it is hard to keep to the application procedure; metal that is insufficiently cleaned holds moisture under the new layer and corrosion goes faster.
After repair the pole is checked again, and the date of the next survey is set by its condition and by the line operator’s requirements.
Which documents are needed and what remains after the survey
Input materials: the line passport, design and as-built documents, the schedule of pole positions, certificates for steel and fasteners, hidden-works records, inspection and repair logs, data on failures and accidents, records of measurements of soil resistivity and earthing. The more there is, the fewer measurements are needed: with drawings the pole need not be measured up, and with a steel certificate the metal need not be tested.
- a defect register for each pole with its number, type, member and description of the defect;
- photographs with the date, name of the object and pole number; members are photographed against a measuring rule so that the dimensions of joints that are hard to reach can be restored in the office;
- records of measurements: thicknesses with the number of points, tilts, bows, tightening torques, earth resistance;
- a report: analysis of the documents, survey materials, assessment of operating conditions, refined loads, condition categories, the calculation, recommendations on the scope and timing of work;
- it is convenient to keep the data in a table by pole number, so that all poles of one category can later be selected and the repair volumes counted.
How the report and passport are prepared is in Survey procedure and passporting, and how condition is diagnosed in Diagnostics and condition assessment.
Where assessments go wrong
- judging by the look of the zinc: a thin white film is a normal patina, and brown rust does not always mean metal loss. The two are told apart by measuring the zinc and wall thickness (Wear and corrosion of metal poles);
- measuring only what can be seen from below and carrying it over to the whole pole: in the joints and the transition zone wear differs from that on a free surface;
- tightening the anchor nuts without finding out why they loosened: foundation tilt, settlement and vibration do not go away with a wrench;
- not photographing with a reference to the pole number: data without a date and number cannot be compared with the next survey;
- painting over insufficiently cleaned metal: moisture stays under the new layer and the same defect appears a year or two later;
- confusing the defect and the category: an identical crack on different poles needs a different response time.
What we can do
Technical survey of structures is our service: we agree the scope and methods for the pole or the line and fix them in the work programme. Who may sign the conclusion is explained in Who may carry out a technical survey, and what the cost depends on in Cost of technical survey. About the service as a whole: Technical survey.
If the survey showed a need for strengthening or replacement, we prepare KM and KMD drawings, fabricate poles, sections, crossarms, flanges and anchor cages (the cages without coating), hot-dip galvanise them (bath 13 000 × 1 600 × 3 200 mm, item mass up to 7 t, DSTU EN ISO 1461:2024) and supply the fasteners for the connections together with the structures. Erection is done by a licensed partner construction company; the lifting scheme comes from its works execution design. We prepare the foundation design to a separate order and for a separate fee, or we only pass on the loads.
For a preliminary assessment send a list of poles with numbers, type, year of installation and voltage, photos of the base and of the zones with signs of wear, the line passport and earlier inspection records. Leave blank what you do not know.
Standards and validity
Operation of overhead lines is regulated by «Technical Operation of Power Stations and Networks. Rules» (GKD 34.20.507-2003, order of 13.06.2003 No. 296, as amended by the order of 21.06.2019 No. 271) and SOU-N EE 20.502:2007 «Overhead power lines of 35 kV and above. Operating instruction» (order of 09.07.2007 No. 324, as amended by the order of 29.12.2009 No. 771). The assessment of lines of 35–750 kV is covered by SOU-N EE 20.571:2007 «Assessment of the technical condition of overhead power lines of 35 kV to 750 kV. Methodical guidelines» (order of 27.08.2007 No. 415). All three are named in the list of documents that an operator must follow on the website of the State Inspectorate for Supervision of Electric and Thermal Energy Consumption; the intervals and scales from these documents are not given in the article: take them from the line operator’s instruction.
The inspection intervals and tolerances in the tables are taken from the Rules of Technical Operation of Consumers’ Electrical Installations (order of the Ministry of Fuel and Energy of 25.07.2006 No. 258, registered 25.10.2006 as No. 1143/13017; in the document card on zakon.rada.gov.ua it is in force, current edition of 21.02.2017). They apply to operating electrical installations of consumers up to 150 kV inclusive. The protection zones of a line and restrictions on work in them are set by the Rules for the Protection of Electric Networks (Resolution of the Cabinet of Ministers of Ukraine of 27.12.2022 No. 1455).
Requirements for poles and earthing are in the PUE (2026 edition): cl. 2.5.19 (corrosion protection), cl. 2.5.136 and 2.5.137 (climbing the pole, devices for repair), table 2.5.29 (earth resistance, see earthing of line poles above 1 kV under the PUE). The technical survey of structures follows the current survey procedure and the Methodology for carrying out a survey and presenting its results (approved by the order of 06.08.2022 No. 144) and DSTU 9273:2024; the verification calculation follows DBN V.2.6-198:2014 «Steel structures. Design standards» and DBN V.1.2-14:2018 (both with Amendment No. 1). Hot-dip galvanizing: DSTU EN ISO 1461:2024.
The validity and current edition of each of these documents should be checked before they are used in a project. The tolerances in the tables are reproduced from a compiled text, not from the official publication; for the line of a particular operator the values are taken from its instruction.
Updated:
Frequently asked questions about the condition assessment of steel line poles
How often should steel poles of 35 kV and above lines be inspected?
Under the Rules of Technical Operation of Consumers’ Electrical Installations: a ground inspection along the whole length at least once a year, a climbing inspection once in 5 years for lines 20 years old or more, in a heavily polluted zone or in open terrain, and once in 10 years for the rest. Extraordinary inspections follow ice, conductor oscillation, natural events and a trip with an unsuccessful auto-reclosing. For the lines of power companies the intervals come from the operator’s instruction.
What counts as a critical defect of a pole?
Whatever puts the load capacity at the design load in doubt: a growing crack in a load-bearing member or weld, buckling or a deep dent in the shaft, a destroyed base joint, missing nuts on several anchor bolts, foundation tilt or settlement that is increasing. Critical defects are removed at once, and the line mode for that time is decided by operations.
Is a visual inspection enough to assess the condition of a pole?
No. Inspection cannot see corrosion under soil, concrete or coating, does not give the thickness of the metal and does not show how a defect affects strength. An assessment needs measurements and a verification calculation with the actual sections.
Must a pole be replaced because it is old?
No, age alone is not a reason. For a line that has long been in service all poles are surveyed, and then the result of a calculation with the measured sections decides: leave in service, strengthen or replace.
Who can carry out a technical survey of line poles?
A specialized organization with specialists who hold a qualification certificate matching the consequence class of the object; more in the article on the right to carry out a survey. The requirements for safe work in electrical installations and at height apply separately. A survey of structures differs from routine inspections, which are done by operating staff.
Related products: Technical inspection · Overhead power line masts
More on this topic: Typical Lattice Power Line Poles: Classification and Checks · Typical Lattice Transmission Poles 35–750 kV · Steel Multi-sided Poles for 6–10 kV Lines · Power Line Pole Types by Purpose: Suspension, Tension, Angle, Terminal · Power Line Protection Zone: Widths by Voltage Class · Distance Between Power Line Poles: What Determines the Span Length
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