8.5.1 Causes of failure

There are many factors that will cause conductors to fail in service. The preponderance of one or other process depends on the way the conductor is used and the environment in which it is placed. The main causes of premature failure are:

• fatigue

• creep

• corrosion.

8.5.2 Fatigue

Fatigue is caused by the continual movement (flexing) of conductors at a fixed point where they are held. It is commonly due to the vibration that is caused by wind blowing across the conductor (Aeolian vibration). Each conductor has its tension limit above which fatigue becomes a problem, and this limits the span lengths unless antivibration methods are used. Typically, aluminium conductors can be strung to 20 per cent of their UTS and copper conductors to 33 per cent of their UTS. Cigr reports [ ^{[1] [2] [3]}] give full details. Vibration can cause conductor failure at connectors or clamps due to the dynamic bending stresses (the clamping pressure restricting strand movement) and any initiating surface damage. The vibration is caused mainly by wind speeds in the range 2 to 20 mph with most damage caused in light winds. Vibration is also dependent on the line rigidity, i.e. it increases with line tension and so is also temperature dependent as line tension increases as the temperature drops. Aeolian vibration occurs in the frequency range from 3 to 100 Hz.

A less frequent wind-induced problem is galloping, where the conductor oscillates at low frequency (<3 Hz)...