STRUCTURES Conditions* at which clearances are to be maintained
• Line angle Force due
to line angle Force due to line angle Force due to line angle
• Wind force 0 6 psf minimum 10 year mean wind,
min.recommended value
• Temperature 60ºF 60ºF or lower Temp. at which wind
value is expected • Conductor tension Final tension Final tension Final tension a = No wind clearance b = Moderate wind clearance c = High wind clearance
*See text for full explanation of conditions.
a O1 O2 b wind O3 c wind a Ø1 b Ø2 wind c wind Ø3
TABLE 7-1
RUS RECOMMENDED MINIMUM CLEARANCES IN INCHES AT CONDUCTOR TO SURFACE OF STRUCTURE OR GUY WIRES Nominal voltage, Phase to Phase,
kV
34.5 46 69 115 138 161 230
Standard Number of 5-3/4”x10” Insulators on Tangent Structures
3 3 4 7 8 10 12
Max. Operating Voltage, Phase to Phase, kV
34.5 46 72.5 120.8 144.9 169.1 241.5
Max. Operating Voltage, Phase to Ground, kV
19.9 26.6 41.8 69.7 83.7 97.6 139.4
Clearance in inches
No Wind Clearance (Not NESC)
Min. clearance to structure or guy at no
wind in inches Notes A, B 19 19 25 42 48 60 71
Moderate Wind Clearance (NESC Table 235-6)
Min. clear. to structure at 6 psf of
wind in inches. Notes C, D 9 11 16 26 30 35 50
Min. clear. to jointly used structures and a 6 psf of wind in inches. Notes C, D
11 13 18 28 32 37 52 Min. clearance to anchor guys at 6 psf
in inches Notes C, D 13 16 22 34 40 46 64
High Wind Clearance (Not NESC)
Min. clearance to structure or guy at high wind in inches
3 3 5 10 12 14 20
Notes:
(A) If insulators in excess of the standard number for tangent structures are used, the no wind clearance value shown should be increased by 6 in. for each additional bell. If the excess insulators are needed for contamination purposes, this additional clearance is not necessary.
(B) For post insulators, the no wind clearance to structure or guy is the length of the post insulator. (C) A higher wind may be assumed if deemed necessary.
(D) The following values should be added as appropriate where the altitude exceeds 3300 feet
Additional inches of clearance per 1000 feet of altitude above 3300 feet
Voltage, kV 34.5 46 69 115 138 161 230
Clearance to structure 0 0 .14 .43 .57 .72 1.15
7.2.4 Example of Clearance Calculations: The following examples demonstrate the
derivation of the minimum clearance to anchor guys at 6 psf.
To determine the minimum clearance of a 115 kV line to an anchor guy (Table 7-1) at 6 psf, the clearance is based on NESC Table 235-6 and NESC Rule 235E.
NESC Clear. in any direction. = NESC Basic Clearance(Table 235-6) + .25(kVL-L – 50)
= 16 inches + .25(120.8-50) inches = 16 inches + 17.7 inches
NESC Clear. in any direction. = 33.7 inches (RUS recommended clearance is 34 inches)
7.3 Backswing: Insulator swing considerations are illustrated in Figure 7-1. For angle
structures where the insulator string is attached to the crossarm, the most severe condition is usually where the force of the wind and the force of the line angle are acting in the same direction. However, for small angle structures, it is possible that the limiting swing condition may be when the wind force is in a direction opposite of that due to the force of the line angle. This situation is called backswing, as it is a swing in a direction opposite of that in which the insulator is pulled by the line angle force. Figure 7-2 illustrates backswing.
When calculating backswing, it is necessary to assume those conditions that would tend to make the swing worse, which usually is low conductor tension or small line angles. It is recommended that the temperature conditions for large angle structures in Figure 7-1 be used, as they result in lower conductor tensions.
FIGURE 7-2: FORWARD AND BACKWARD SWING ANGLES
7.4 Structure Insulator Swing Values: Table 7-2 provides the allowable insulator swing angle
values for some of the most often used standard RUS tangent structures. These values represent the maximum angle from the vertical that an insulator string of the indicated number of standard bells may swing in toward the structure without violating the clearance category recommendation indicated at the top of each column. For tangent structures, the most restrictive angle for the particular clearance category for the entire structure is given. Thus, for an asymmetrical tangent structure (TS-1 for instance) where the allowable swing angle depends upon whether the
insulators are assumed to be displaced to the right or left, the use of the most restrictive value means that the orientation of the structures with respect to the line angle need not be considered. For certain angle structures the insulator string has to be swung away from the structure in order to maintain the necessary clearance. These situations usually occur for large angle structures where the insulator string is attached directly to the pole or to a bracket on the pole and where the force due to the change in direction of the conductors is relied upon to hold the conductors away from the structure.
forward swing back swing
direction of line angle
normal position of insulators (no wind, no ice)
TABLE 7-2
INSULATOR SWING ANGLE VALUES IN DEGREES (For insulator string with ball hooks)
Structure and
Voltage Number of Insulators Swing Angle No Wind Moderate Wind Swing Angle Swing Angle High Wind 69 kV TS-1, TS-1X 4 21.3 41.4 74.9 TSZ-1, TSZ-2 4 41.7 61.2 82.6 TH-1,TH-1G 4 35.6 61.2 85.6 115 kV – TH-1A 7 28.3 58.7 80.8 161 kV – TH-10 10 16.4 53.2 77.7 230 kV – TH-230 12 16.5 47.5 74.8
7.5 Line Design and Structure Clearances: Insulator swing has a key effect on acceptable horizontal to vertical span ratios. Under a given set of wind and temperature conditions, an insulator string on a structure will swing at an angle toward the structure a given number of degrees. The angle of this swing is related to a ratio of horizontal to vertical forces on the insulator string. A relationship between the horizontal span, the vertical span, and if applicable, the line angle can then be developed for the structure, conductor, and weather. Horizontal and vertical spans are explained in Figure 7-4.
The acceptable limits of horizontal to vertical span ratios are plotted on a chart called an insulator swing chart. Such a chart can be easily used for checking or plotting out plan and profile sheets. Figures 7-3 and 7-5 show simplified insulator swing charts for the moderate wind condition only. There is one significant difference between the chart for tangent structures, and the chart for angle (running corner) structures. In Figure 7-3 for a typical tangent structure, the greater the vertical span for a fixed horizontal span the less swing occurs. The reverse is true for chart of Figure 7-5 for a typical angle structure. This occurs because the swing chart in Figure 7- 5 is for a large angle structure where the force of the line angle is used to pull the insulator string away from the structure. As such, the less vertical force there is from the weight span, the greater the horizontal span can be.
FIGURE 7-3: TYPICAL INSULATOR SWING CHART FOR A TH-230 TANGENT STRUCTURE (Moderate Wind Swing Condition Only, 9 psf assumed instead of minimum NESC 6 psf, No Line Angle Assumed)
600 700 VE R T IC AL S PA N 800 800 900 1000 700 600 500 400 HORIZONTAL SPAN 32° INI TIAL C URVE IN SU LA T IO R SW IN G A LL OW AB LE IN SU LA T IO R SW IN G EX C ES SIV E
INSULATOR SWING CHART