The life of a high voltage line
Some of the maintenance tasks we do on transmission lines (Rural Australia; 11 kV - 132 kV)


Grade the access roads along the line corridor - access roads tend to get overgrown / washed out over time. Inacessible roads are very inconvenient when "running the line" to find a fault at night-time.
Cut down trees encroaching on the line.
Perform visual inspection of the line - look for damaged conductors or hardware i.e. broken insulators.
Scan for hot joints using a thermal camera. Current-carrying connections can corrode or become loose over time, resulting in overheating and eventual failure. Crimped connections are usually OK. Most of our problems are with bolted connections, particularly "parallel-groove clamps".
For wood poles- check that the wood hasn't been eaten by termites. The simplest test is a "knock test" i.e. hit the pole with a hammer and listen to how it sounds. If the pole sounds hollow, it's probably been eaten by termites.
For steel structure towers - check the structural condition of the tower. Rust is bad. Missing fasteners (nuts/bolts) are bad. Loose fasteners are also bad. All these things reduce the strength of the structure under load.
For all kinds of towers - check the earthing at the base of the tower/pole - each tower/pole should have a earth grading ring and an earth electrode. This is important for lightning protection and personnel protection (touch/step voltage). The earth electrode should be 10 ohms or less.  Replace/refurbish earthing electrode if required. 


We don't really worry too much about the conductor (wire) itself.


We have conductors that have been in service for 60 years with no issues.
Where conductors have failed, this is usually due to lightning strikes. (I have an example on my desk.)
Metal fatigue doesn't tend to be an issue - any lines that would be susceptible to wind-induced oscillations will also have dampeners or other engineering controls, to mitigate that.