In Part 3 (July/August issue) we determined the height of
our structures and the strength of our structures and
foundations to comply with the National Electrical Safety
Code® (NESC®). The last step in the design of a high-voltage
overhead line up to 50 kV to comply with the NESC is to design
the guying. Stringing conductors between poles puts tension on
the poles. Though poles and foundations can be designed with
enough strength to hold the tension, guying is much more
economical.
Dead-End Guying
To understand the principles of anchor guying, let’s
look at a simple example. Consider a single 250 foot span of
477 AAC "Cosmos" conductor strung between two poles
at a stringing tension of 1410 pounds at 60 degrees F. The
conductor is dead-ended on each pole at a height of 35 feet.
To prevent the poles from breaking at or near the ground
line and to prevent the poles from kicking out (tipping over
due to foundation failure), we need to install an anchor guy
on each pole
The "longitudinal load" or force on the pole the
anchor guy must hold is the final tension of the conductor
under the conditions specified by Table 250-1 for the
applicable loading district. Referring back to the heavy
loading district Sag/Tension table for 477 AAC presented in
Part 2 (May/June issue), the final tension of the conductor at
0 degrees F. with 1/2 inch of ice and 4 lb./sq. ft. wind
pressure is 3601 pounds. Let’s assume we will install our
anchor in the ground about 20 feet from the pole. This
distance is called the guy lead.
The tension in the guy wire and the anchor is the
longitudinal load on the pole due to the conductor
"H" divided by the cosine of the angle
"A".
The angle "A" is the arctangent of the guy
attachment height divided by the guy lead.
A = tan-1(35'/20')
A = 66.9 degrees
T = 3601 lb / cos 66.9
T = 3601 / .496
T = 7260 pounds
For grade B and C construction, the rated strength of the
guy wire and the anchor must exceed the guy tension
"T" multiplied by the appropriate safety factors
specified in Rules 261 C and B, respectively. For a discussion
of Safety Factors, see my article "Strong Enough to be
Safe" in the January/February issue. The rated strength
of the anchor must take into consideration the type of soil in
which it is installed.
The vertical force "V" on the pole due to the
anchor is the tension in the guy wire "T" multiplied
by the sine of the angle "A".
V = T x sin(A)
V = 7260 x sin 66.9
V = 7260 x .867
V = 6300 pounds
For grades B and C construction, the vertical strength of
the pole and its’ foundation must exceed the vertical force
on the pole "V" multiplied by the appropriate safety
factor specified in Rules 261A and B, respectively, or the
pole may buckle or sink. The shorter the guy lead, the higher
the tension in the guy wire and anchor and the higher the
vertical force on the pole. Using a shortlead increases the
probability of the anchor slipping (pulling out) or the pole
sinking, either of which will cause the pole to lean and the
conductors’ sag to increase. The increased sag may cause a
clearance violation.
Multi-Span Lines
All the spans of a multi-span line are usually strung at
the same time and at the same tension. If all spans are not
the same length, this stringing technique complicates the
design. If each span is designed separately, the tension of
the conductors in each span is a function of the span length,
i.e. the longer the span, the higher the tension. If we string
the different length spans at the same tension, what stringing
tension do we use and what final tension doe we use in the
guying calculation? The tensions we are looking for are the
tensions of the "Ruling Span". The Ruling Span is a
weighted average span. The calculation method gives longer
spans more effect on the average. The Ruling Span is the
square root of the sum of each span cubed divided by the sum
of the spans.
Where S1 through SN are the individual span lengths and S13
= S1 x S1 x S1
The conductor tension we use for the guying calculation
must come from a sag/tension calculation based on the Ruling
Span length.
Let’s look at a common example of a three-phase vertical
dead-end structure shown below:
In this example, the top three conductors are 477 AAC phase
conductors. The bottom conductor is a 1/0 ACSR neutral
conductor. The longitudinal load of the phase conductors is
divided between the top two guys. The bottom guy holds the
longitudinal load of the neutral. The tension in the top guy
and its anchor is the longitudinal load of each phase
conductor multiplied by three, divided by two and divided by
the cosine of the angle "A".
T = H x 3
2 x cos(A)
A = tan-1(40/35)
A = 54.2 Degrees
T = 3601 x 3
2 x .658
T = 8209 Pounds
The rated strength of the top guy wire and its anchor must
exceed the tension "T" multiplied times the
appropriate safety factor specified in Rules 261C and B,
respectively. The tension in the middle guy wire is calculated
similarly except that angle B is different. The tension in the
lower guy wire is the longitudinal load of the neutral
conductor divided by the cosine of the angle C. The tension in
the anchor closest to the pole is the sum of the tensions in
the bottom two guy wires.
If you have general questions about the NESC, please call
me at 302-454-4910 or e-mail me at dave.young@conectiv.com.
National Electrical Safety Code® and NESC® are the
registered trademarks of the Institute of Electrical and
Electronics Engineers.
David C. Young is a Senior Engineer with
Conectiv of Wilmington, Delaware, where he has been working
with and teaching the NESC for over 27 years. He is a member
of the NESC Interpretations Subcommittee and an alternate on
the NESC Overhead Lines Clearances Subcommittee. He is also a
member of the ANSI Z535 committee
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