14.1.1 The bath-tub curve

The bath-tub curve (see chapter 11) represents the simple lifecycle of almost any mechanical device. After early faults are corrected and the failures weeded out, there normally exists a period of stable performance.

Hence the curve which represents the average performance of any overhead line component starts off with a high fault level before dropping down to a steady operational low fault-level performance. This, simplistically, should carry on till the elements of old age creep in and things start to go wrong, fault levels increase and a time comes when a decision has to be made to put an end to its life before it becomes a total economic liability.

This is a simplistic view as the units may have maintenance either regularly or when inspection demands. Each sub-maintenance period will have its own sub-bathtub curve. At each inspection the decision has to be made as to the probable economic performance or lifetime expectancy of the unit to determine whether maintenance is necessary. Maintenance can extend life but it is not always economic. At some point a decision has to be made to run the unit to its natural end-of-lifecycle and then replace rather than maintain.

14.1.2 Environment

The particular lifecycle of any type of unit is dependent on the environment in which it operates and the workload it has to undergo. It is necessary, in order to obtain optimum economic life, to use units that are fit for purpose. The overhead line (OHL) must be built...