ABSTRACT
An overhead transmission line (OHTL) is a very complex, continuous, electrical=mechanical system. Its
function is to transport power safely from the circuit breaker on one end to the circuit breaker on the
other. It is physically composed of many individual components made up of different materials having a
wide variety of mechanical properties, such as:
. flexible vs. rigid
. ductile vs. brittle
. variant dispersions of strength
. wear and deterioration occurring at different rates when applied in different applications within
one micro-environment or in the same application within different micro-environments
This discussion will address the nature of the structures which are required to provide the clearances
between the current-carrying conductors, as well as their safe support above the earth. During this
discussion, reference will be made to the following definitions:
Capability: Capacity () availability Reliability level: Ability of a line (or component) to perform its expected capability
Security level: Ability of a line to restrict progressive damage after the failure of the first component
Safety level: Ability of a line to perform its function safely
Present line design practice views the support structure as an isolated element supporting half span of
conductors and overhead ground wires (OHGWs) on either side of the structure. Based on the voltage
level of the line, the conductors and OHGWs are configured to provide, at least, the minimum
clearances mandated by the National Electrical Safety Code (NESC) (IEEE, 1990), as well as other
applicable codes. This configuration is designed to control the separation of:
. energized parts from other energized parts
. energized parts from the support structure of other objects located along the r-o-w
. energized parts above ground
The NESC divides the U.S. into three large global loading zones: heavy, medium, and light and
specifies radial ice thickness=wind pressure=temperature relationships to define the minimum load
levels that must be used within each loading zone. In addition, the Code introduces the concept of an
Overload Capacity Factor (OCF) to cover uncertainties stemming from the:
. likelihood of occurrence of the specified load
. dispersion of structure strength
. grade of construction
. deterioration of strength during service life
. structure function (suspension, dead-end, angle)
. other line support components (guys, foundations, etc.)
Present line design practice normally consists of the following steps:
1. The owning utility prepares an agenda of loading events consisting of: . mandatory regulations from the NESC and other codes . climatic events believed to be representative of the line’s specific location . contingency loading events of interest; i.e., broken conductor . special requirements and expectations
Each of these loading events is multiplied by its own OCF to cover uncertainties associated with it to
produce an agenda of final ultimate design loads (see Fig. 9.1).
