# Tension from Field Data

When assessing an existing span you are able to measure dimensions such as attachment heights, and often you want to derive the stringing tension of the span, or the sag under alternative temperature conditions.

Data required for these calculations are:

1. distances – span length, attachment heights (see figure 1)
2. conductor type/properties
3. air and conductor temperatures

Mid-span sag can be calculated from item 1 data, by simple trigonometry. Since tension is inversely proportional to sag, the actual tension at the time of measurement can be calculated1 using the conductor properties.

Deriving stringing tension from actual tension requires accurate (or reasonably accurate) assessment of conductor temperature at the time the field data was taken, and also the Ruling Span (RS, also known as Mean Equivalent Span) for the strain section the span is part of.

Ruling span is not too difficult to calculate and is an important factor. Figure 2 shows calculation of actual tension in a 120m span of Hydrogen with a stringing tension of 7% CBL2 for a range of conductor temperatures (Standard temperature is 15°). If RS of 120m is used when the actual RS is 80m the tension is too high, leading to over-engineering of the pole design and under-estimation of sag. If RS of 160m is used the calculated tension is too low. This points out the need for accurate RS values in the design process.

1. AS7000:2016 R12, $$H=\frac{W_{v}L^{2}}{8D}$$, parabolic simplification