Thermal Inspection of Overhead Line Conductors: Splices, Clamps and Hot Spots

Why Conductors and Connections Overheat

An overhead conductor carries current through a long run of aluminium or ACSR, interrupted every few hundred meters by splices, compression connectors, clamps, and dead-ends. The conductor itself is sized to dissipate its heat — I²R losses radiate and convect away under normal loading. The problem is that a splice or clamp is a compressed interface, exactly like a busbar joint: a small resistance in series with the circuit.

When a compression splice was installed years ago with the right dies and correct compression pressure, its resistance is negligible. But corrosion, thermal cycling, vibration, and uneven compression all raise that resistance over time. The connection heats up, the aluminium anneals and softens, the resistance rises further, and the wire inside the connector begins to creep and lose contact. The end state is a glowing splice, a burned clamp, or a conductor that parts and falls to the ground.

For Latin American utilities the stakes are high: long transmission corridors through mountains and jungle, coastal salt exposure, high ambient temperatures that reduce the conductor's thermal margin, and increasing load growth on aging lines. A hot splice found by thermography is a one-day repair during a planned outage; a failed splice is an unplanned line drop that can take a week to restore.

"Our line crew replaced a 138 kV compression splice that was running 41 K above the conductor on both sides. The splice had been in service 22 years. The repair took a planned 6-hour outage — had it failed, the same section would have been down for days and we would have lost the redundant feed for half the city." — Transmission Line Engineer, Brazil

Thermal Signatures of Line Problems

Learn to recognize these patterns when you scan a line from the ground:

• Hot splice or compression connector: The most important signature. A splice running visibly hotter than the conductor on both sides indicates high resistance, corrosion, or a failed compression. The delta-T between the splice and the adjacent conductor is your key measurement.
• Hot dead-end clamp or strain clamp: Dead-ends carry the full conductor tension and current. A hot strain clamp points to a loose compression or corroded interface at the tower attachment.
• Hot jumper connection: At towers and strain points, the jumper loop between spans can develop hot bolted connections. Scan every jumper clamp and spacer.
• One phase hotter along a whole section: A complete phase running warmer than its neighbors suggests uneven current distribution, a downstream problem, or a section of undersized or damaged conductor.
• Hot armor rods or vibration dampers: Unusual heating at armor rods or dampers can indicate localized stress, broken strands, or corona activity.
• Uniformly hot conductor: If the whole line runs hot but balanced, the line may simply be heavily loaded — check the SCADA loading before flagging a defect.

Conductor emissivity is low and the background is complex (sky, sun, towers). Scan from a consistent angle, avoid direct sun reflection, and always compare the suspect connection against the conductor temperature on both sides rather than against an absolute value.

Delta-T Criteria for Line Connections

These thresholds follow standard infrared line-inspection practice (ISO 18434-based programs and utility line-patrol guides). The splice-to-conductor delta-T is more reliable than absolute temperature because both are in the same ambient, same wind, and same sun. Scan at high load whenever possible — a splice that looks normal at 40% load can be clearly hot at 80%.

"We use 10 K as our trigger for a planned repair and 20 K for urgent action. Over the last two years, ground-based thermal patrols found 14 splices above 10 K and two above 20 K on our 230 kV corridor. Every single one was confirmed as a high-resistance compression when we opened it." — Line Maintenance Supervisor, Peru

Conductor Types and Their Weak Points

Know the line construction before you patrol. On bundled lines, an entire sub-conductor running hot while its partner is cool points to a bad connection at the tower rather than a load issue. On covered conductors, thermal imaging can miss internal strand damage — combine it with acoustic corona detection to find what thermography cannot see.

Ground-Based Inspection Procedure

1. Choose the right vantage point. Position yourself where the line is broadside and the sun is behind you or overhead. Avoid scanning into the sun or with a strong reflection off the conductor.
2. Use telephoto and high resolution. Splices and clamps are small targets at distance. A camera with a telephoto lens or dual FOV and 640×480 resolution or better is essential to resolve a splice at 50–150 m.
3. Scan each connection systematically. Work tower to tower: dead-ends, jumpers, splices, clamps, spacers. Record the thermal image plus a visual reference.
4. Measure the delta-T. For each suspect connection, measure the connection temperature and the conductor temperature on both sides, and record the difference.
5. Log the line loading. Note the current from SCADA or the dispatcher. A delta-T measured at 40% load is not comparable to one at 90%.
6. Use drones for hard-to-reach spans. Where the ground vantage is poor (river crossings, steep terrain), a drone-mounted thermal camera or a closer ground position can close the gap.
7. Document for the repair crew. Save the thermal image, the exact tower number, the phase, and the delta-T so the line crew can find and verify the defect without re-scanning.

Do not rely on absolute temperature alone. A splice at 60 °C in a 40 °C ambient on a sunny afternoon is far less alarming than the same reading on an overcast morning — the delta-T against the adjacent conductor removes the weather variable.

Choosing the Right Camera

For ground-based transmission line patrols, the Hammer II is the practical workhorse: the 640×480 detector resolves a compression splice at patrolling distance, and the dual-FOV optics let you zoom from a whole-span overview to a single clamp without losing context.

Building a Line Inspection Program

Line thermography pays off when it is scheduled, loaded, and trended:

• Patrol critical corridors annually at high load (summer peak or load-growth months), with more frequent patrols for coastal, industrial, and heavily loaded sections.
• Patrol after major load changes, faults, or extreme weather. A fault current or a windstorm can stress connections that looked fine the month before.
• Trend every splice and clamp. The delta-T history of each connection is the most valuable record you have — a splice moving from 3 K to 8 K over two patrols is a warning before it crosses the 10 K threshold.
• Combine thermal with acoustic corona detection. Corona and partial discharge on line hardware produce ultrasound and visible light that thermography misses. An acoustic camera or an SC Series PD detector in the same patrol catches the electrical side of the same degradation.
• Feed results to planning. Every flagged splice becomes a scheduled repair with the thermal image attached — turning the thermal program into the utility's line-reliability engine.

In tropical and mountainous Latin American corridors, conductor connections age faster and harder than nameplate life suggests. A ground-based thermal patrol is the lowest-cost way to keep an aging line reliable until the reinforcement project is funded.