The Ruling Span is defined
as the assumed uniform span that most likely represents actual spans that are
in any particular section of the line. In the absence of finite element
analysis tools or software (ex. PLS CADD), the ruling span is used to calculate
sag and clearances on the plan profile drawing, and it is necessary in
structure spotting.
Ruling Span Tip: When stringing
the line, the general rule is that the spans in the line should not be more
than twice the ruling span, or less than half of the ruling span.
Ruling Span is one of the most used yet misunderstood and
misused terms in the design, staking, and construction of overhead lines. “Ruling
span” is loosely used with several different meanings.
Theoretical
Ruling Span: It is the equation derived from the conductor
length equation and by making certain assumptions, approximations, and formula
substitutions. This formula must be used if the actual spans are already known.
The
theoretical ruling span equation is not exact because of the assumptions made.
Since
its
accuracy is sufficient for most line designs, it is the equation used most
often to calculate the ruling span for new overhead distribution lines.
Estimated
Ruling Span: If the actual spans are not yet determined but knowledge
gained from a reconnaissance and previous surveys of the proposed line are
known, it is possible to estimate a ruling span. A traditional “rule of thumb”
equation that may be helpful in the estimation of a ruling span is:
Se
=
Average Span + 2/3 (Maximum Span – Average Span)
Use
this rule for estimating the ruling span with caution. Use only this formula if the actual spans are
not yet known.
What
would happen if my ruling span is different from the actual design?
• If the design
sag is greater than the theoretical sag, then the actual sag
of the installed conductors will be less than the predicted sag. This condition
will lead to increased conductor tensions, which may exceed the permitted loads
of support structures and guying assemblies.
• If the design
sag is less than the theoretical sag, then the actual sag of the installed conductors
will be greater than the predicted sag. This condition may result in inadequate
ground clearances.
I
hope this helps.
additional reading and resources:
http://www.usda.gov/rus/electric/pubs/1724e152.pdf
http://www.sag10.com/documents/AppX.pdf
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