Appendix D - Mechanical And Loading Data For Conductors

Method of Determining the Construction Requirements for a typical Crossing of “Class H” Supply Lines Over an Important Railroad and Signal Lines to Comply with the Requirements of These Rules for Light Loading Conditions.

The purpose of this appendix is to provide an example of a typical crossing of supply lines over railroads and signal lines to determine the various clearances and construction requirements necessary to comply with the rules. The actual construction of the crossing is made to conform to current good practice and indicated where and in what manner the minimum requirements of the rules are exceeded or maintained. The crossing diagram and dimensions are shown on the following page after this appendix. The primary data chosen for the crossing follow:

DATA OF CROSSING.

Number of Circuits, Voltage and Phase.

Two 3-phase 60,000-volt circuits. One private telephone circuit.

Configuration.

Power conductors of each circuit in vertical plane on opposite ends of the crossarms. Private telephone circuit in horizontal plane.

Conductors.

Power circuits-6 No. 00 A.W.G. copper, medium hard drawn, stranded, bare.
*Private telephone circuit-2 No. 8 A.W.G. copper, medium hard drawn, solid, bare.

*It is assumed that the private signal circuit on the power line pole is run, operated and maintained consistently, as a signal circuit as defined in Rule 17 (d), and for this reason change in conductor size at the crossing is required.

Insulators.

Pin Type porcelain, to meet the requirements of Rules 64 and 126 (d) 3, and 4.

Ties and Conductor Fastenings.

To meet the requirements of Rule 62(a).

Pins.

Iron pipe (extra strong) 1 ˝’’ (nominal) X 20 ˝’’ for power conductors – pins of each circuit metallically bonded together to conform to the requirements of Rule 126 (d) 2. 1 ˝’’ X 9’’ locust pins for conductors of private telephone circuit.

Crossarms.

Douglas fir 5 ľ’’ X 5 ľ’’ X 12’-0’’, 1.9’’ pin holes –11/16’’ hole for pole bolt. (Telephone circuit crossarm 3 Ľ’’ X 4 ˝’’.)

Crossarm Braces.

To conform to Rule 62(a).

Poles.

Western red – cedar National Electric Light Association specifications.

Pole Framing.

As per dimensional diagram.

Location of Poles.

As per dimensional diagram.

Length of Crossing Span.

200 feet

Length of Adjacent Spans.

150 feet

CONSTRUCTION REQUIREMTNS.

1. Conductor Sags (Normal Conditions, 60°F., no wind).

The required sags of power and telephone conductors are determined from the minimum sags curves of Appendix C.

For No. 00 A.W.G. copper, medium hard drawn, stranded, bar – Span 200 feet, Curve Sheet, curve 2, gives the sag as 12.0 inches.

For No. 8 A.W.G. copper, medium hard drawn, solid, bar – Span 200 feet, Curve Sheet, curve 2, gives the sag as 13.2 inches.

In the spans adjacent to the crossing span the supports are at different levels, and the “apparent sags” are required. These are equal to the normal sags for spans of the same length between supports at the same level. Thus from, the curve 2 of Appendix C, the apparent sags are 7.2 inches for the No. 8 A.W.G. wire and 6.9 inches for the No. 00 A.W.G. wire.

2. Conductor Sags (Light Loading 25° F., 8 pounds wind)

Since no ice loading is considered, the vertical sags under the light loading condition will be less than normal sags at 60°F. and no wind. Consequently the clearances provided under light loading will be greater than normal conditions, and it is therefore only necessary to determine the elevations of supports to provide the required clearances under normal conditions, 60°F. and no wind. The tensions in the conductors loaded are found from Tables 13 and 14, Appendix B, to be as follows:

Copper Conductors	150-foot span	200-foot span
No 00 A.W.G. medium hard drawn, stranded bare	2,610 lbs.	2,610 lbs
No 8 A.W.G. medium hard drawn, solid bare	324 lbs.	324 lbs.

3. Conductor Separations at Supports.

Table 1, case 6, column F, gives the minimum clearance of the conductors from the center of the pole for supply conductors of this class as 18 inches. For circuits of the character under consideration, where vertical configuration is employed, current good practice employs a spacing of 5’6’’ from the center of the pole, which is greatly in excess of the minimum required.

For a private telephone circuit, Table 1, case 6, column B, provides a minimum clearance for conductors of 15 inches from the center of the pole. Eighteen (18) inches is employed in this case.

4. Crossarm Spacing.

Table 2, case 11, column H, gives the minimum vertical separation between wires on different crossarms on the same poles for 60,000 volt circuits as 4 feet. Note (r) modifies this to allow a conductor spacing equivalent to the table values for pin spacing of wires of the circuit voltage which is (by Table 2, case 7, Column H) 36 inches. In the case under discussion the vertical separation between crossarms has been taken as 5’6’’, conforming to current good practice.

The minimum separation between the lowest crossarm supporting power conductors and the crossarm supporting a private telephone circuit is given by Table 2, case 11, column C, as 6 feet. Current good practice usually employs an 8-foot separation and it is so taken in this case.

5. Clearances of Conductors Over Crossarms.

Table 1, case 9, column F, gives the minimum clearance of conductors in the 60,000-volt class above the supporting crossarms as 9 inches. Since in this case a 20 ˝’’ pin is employed, the clearance of the conductor above the crossarm is 15 inches, which is in excess of the minimum requirements.

Table 1, case 9, column E, gives the minimum clearance of the private telephone conductors above the supporting crossarm as 3 inches. This is adhered to in the case discussed.

6. Vertical Clearance of Conductors Over Highway, Pole lines and Railroad Tracks.

The poles supporting the crossing span are 55 feet in length set 7 feet in the ground. From the dimensions of the pole framing diagram, the distance from the conductors of the private telephone circuit to the ground is 28’6’’. The ground level is given as the same elevation the as top of the rail.

The sag of the signal conductors in the crossing span is given above as 13.2 inches. The plan gives the distances from supporting pole C to the various structures crossed over as follows:

Telephone pole line – 37’6’’

Highway (center) – 60’0’’

Telegraph pole line on R.R r/w – 97’6’’

Center railroad tracks – 138’9’’

Railroad signal pole line – 180’0’’

Total length of crossing span, 200 feet.

Considering as an example the clearance of the private telephone circuit conductors above the crossed over telephone pole lead.

This point of crossing is from the nearest crossing pole C 37’6’’ or 18.75 percent of the crossing span length. The sag at 18.75 percent of the crossing span length is (from curve 12, Appendix C) 61 percent of the center sag. The center sag is 13.2’’, making the sag at the point of crossing 13.2’’ X 0.6’ =8.0’’. Therefore the clearance of private telephone conductors over the top of rail at the point of crossing with the telephone pole line is 28’6’’ - 8.0’’ = 27’10’’. As the top wire of the telephone pole line at this point is given as 24’, the clearance at this point from the private telephone circuit conductors will be 3’10’’, which is in excess of the minimum required clearance of 2’ given by Table 2, case 2, column c.

In a like manner the clearances of the private telephone circuit conductors at the other points of crossing are as follows:

Points of Crossing	Clearance	Minimum by rules
Highway (center)	27’7’’	18’0’’
Telegraph pole line	3’5’’	2’0’’
Railroad tracks (center)	27’7’’	25’0’’
Railroad signal pole line	6’1.3’’	2’0’’

The clearances of the lower power conductors at the various points of crossing can likewise be computed and are as follows:

Points of Crossing	Clearance	Minimum by rules
Telephone pole line	13’0’’	8’0’’
Highway (center)	36’9’’	30’0’’
Telegraph pole line	12’7’’	8’0’’
Railroad tracks (center)	36’9’’	34’0’’
Railroad signal pole line	15’3’’	8’0’’

7. Insulators.

In addition to the electrical requirements set forth in Rules 64 and 126 (d) 3, and 4, the insulators supporting the supply conductors shall have a safety factor (mechanical) of over 3 the stresses they will be required to withstand for the most severe condition of loading applying. (Rule 52 (c).)

8. Pins, Ties and Conductor Fastenings.

Pins, ties and conductor fastenings used in connection with pin type insulators shall have sufficient strength to withstand a tension in the conductor of 700 pounds per pin or fastening as a minimum (Rule 62 (a)). When greater strength is necessary they shall be designed to withstand with a safety factor of 2 the tension in the conductor under the specified loading condition. In the case under discussion wrought iron pipe pins are to be employed of the dimensions and construction indicated below:

Bending moment (at crossarm) = M = 2,610 X 15=39,150 inch-pounds

Moment of inertia

where and

Section modulus

Fiber stress

where c=D/2

Since the ultimate fiber stress for wrought iron is 45,000 pounds per square inc, a single pin is of this type is insufficient. Four such pins are required to provide a factor of safety of 2 and, as the unbalanced stress may be distruted over one or more poles adjacent to the crossing, double pins are required at poles B, C, D and E.

Locust pins 1 ˝’’ X 9’’ are employed for supporting the private telephone conductors. Since locust pins of these dimensions provide strength up to 1000 pounds tension in the conductor, with the conductor 3 ˝ inches above the crossarm (Rule 62 (a)) and the maximum tension in the conductor is only 324 pounds, the required factor of safety (2) is amply met.

9. Crossarms.

Crossarms supporting the 60,000 volt wires are to be of Douglas fir, dimensions 5ľ’’ X 5 ľ’’ X 12’, bored as illustrated below:

Dead load on crossarms - The vertical load on the double crossarms (double arms are required on poles B, C, D and E by virtue of the double pin requirement) under maximum conditions of loading will be for each conductor equal to the weight per foot of wire (since no ice loading is considered) times one-half the sum of the length of the crossing and adjacent spans or

The weight per arm for conductors is 36 pounds.

The point of maximum bending moment will be at the trough bolt attaching the arm to the pole, at which point the cross-section of the arm is reduced by the amount of the bolt hole.

The following shows the method of figuring the fiber stress at this point for one crossarm:

As the ultimate fiber stress is 5,400 pounds per square inch, the stress due to the dead load with safety factor 4 is unimportant.

The dead load on the telephone crossarm computed in a similar manner is found to be 16.2 pounds per square inch.

In the above calculations no account has been taken of weight of pins and insulators and no allowance made for additional strength afforded by crossarm braces.

10. Unbalanced Wire Load on Crossarms.

It is assumed that double crossarms (5ľ’’ X 5ľ’’ X 12’) will be employed space 8 inches apart, the separation being maintained by spreader blocks and bolts. Crossarm construction of this type is assumed to have 40 percent* of the strength of two crossarms of the same dimensions connected as a rigid truss.

*When double crossarms are connected together at the ends with spacing blocks, bolts or plates, their combined strength in the direction of the pull of the conductors is in excess of double the strength of a single crossarm and is less than the strength of two crossarms of similar strength dimensions and spacing, connected as a rigid truss.

(a) Double wood or steel crossarms, attached to poles, connected together at the ends by bolts with spacing nuts and washers or by bolts with pipe spacers of thirty (30) percent of the strength of two crossarms, of similar dimensions and spacing, connected as a rigid truss.

(b) Double wood or steel crossarms, , attached to poles, connected together at the ends by spacing blocks or plates, bolted or riveted to the crossarms will be considered as having a strength not in excess of forty (40) percent of the strength of the strength of two crossarms, of similar dimensions and spacing, connected as a rigid truss.

(c) Steel crossarms designed to take shear between the main members will be allowed a factor of strength in accordance with their construction.

m = 66 inches; o = 6 inches;

Pull = 2,610 pounds on each pin

As it is necessary to consider the total load applied to one side only, the total bending moment is

2,610 X 66 =172,200 inch-pounds

Section of Double Crossarms.

P=5.75’’ q (diameter of bolt hole) = R=5.75’’ S = 8’’

For a factor of safety of 4 required by the rules the allowable fiber stress is 1,350 pounds.

Thus double crossarms of this type upon the crossing poles C and D only would not be sufficient to meet the strength requirements. However, since, as previously discussed, double pins of the type employed herein are required at poles B, C, D and E, double crossarms are also required at the same poles and the strength requirements are therefore amply met.

Single crossarms, Douglas fir 3 Ľ’’ X 4 Ľ’’ X 42’’, are assumed for supporting the private telephone circuit at the crossing and the adjacent spans.

Since the allowable fiber stress for factor of safety 4 is 1,350 pounds the stress sue to the unbalanced load is well within the limit.

11. Poles

The crossing poles are Western red cedar and their dimensions, considering the clearances required, are as follows:

Length 55 feet

Height above ground 48 feet

Circumference at top 28 inches

Diameter at top 8.91 inches

Circumference 6 feet from butt 49 inches

Distance from ground line to conductors supported is given as follows:

Top supply conductors 48’7’’

Middle supply conductors 43’1’’

Lower supply conductors 37’7’’

Private telephone conductors 28’6’’

Ground level at base of pole is considered to be at the same elevation as top rail.

Dimensions of adjacent poles B and E are

Length 50 feet

12. Transverse Load on Poles C and D

Due to 8-pound wind on projected areas of pole and conductors, maximum stress assumed at ground line.

where
P=Pressure in pounds per square foot on projected area of pole
H=Height of Pole above ground
D₁=Diameter of pole at ground in inches.
D₂=Diameter of pole at top in inches.

P_h= Horizontal load per lineal foot due to an 8 pound wind pressure on projected area of wire

L = Height of conductors above ground in feet

n = Number of wires

S₁ and S₂= Length of crossing and adjacent spans, respectively

M_p= Moment due to wind pressure on pole

M_c0= Moment due to pressure on top supply conductors

M_c1 = Moment due to pressure on middle supply conductors

M_c2= Moment due to pressure on lower supply conductors

M_c3= Moment due to pressure on telephone conductors

Total Moment =M_p + M_c0 + M_c1 + M_c2+ M_c3

Ph = 0.279 pounds per linear foot for No. 00

A.W.G. copper, stranded, bare

Ph = 0.085 pounds per linear foot for No. 8

A.W.G. copper, solid, bare

The allowable fiber stress for Western red cedar poles to provide a factor of safety of 4 is 1,275 pounds, hence the crossing poles do not require to be side guyed.

If poles of the minimum dimensions given in Rule 126(b) 3 were employed as is permissible, then

Length 55 feet

Height above ground 48 feet

Circumference at top 25.1 inches

Diameter at top 8 inches

Circumference 6 feet from butt 41 inches

Diameter 6 feet from butt 13.05 inches

The minimum sized poles thus permitted will also meet the requirements for transverse load without the use of side guys.

13. Side Guying

If side guying were required for poles C and D, and it was assumed physically impossible to side guy these poles, then poles B and E should be side guyed and the entire line between them must be Grade A, except as to transverse strength of poles, which shall not be dimensions smaller than set forth in Rule 126 (b) 3. The method of figuring side guys for poles B and E is as follows:

Side guys are figured to take the entire transverse load (between the guyed poles B and E), the pole acting simply as a strut.

The transverse force acting on the poles will be due to wind pressure on the pole and the transverse wind pressure on the conductors supported. The length of conductor used in figuring this transverse force will be equal to one-half the distance between the guyed poles B and E, plus one-half the length of the span adjacent to these poles, or to

The specified factor of safety for guys is 3, hence a guy having an ultimate strength of not less than 7,503 pounds should be employed which can be obtained by using one Siemens-Martin 7/16- inch guy its equivalent having an ultimate strength of 9,000 pounds.

14. Unbalanced Load on Crossing Poles “C” and “D”.

(Rule 126 (b) 2) “To withstand at all times without failure, the unbalanced stress due to the combined pull toward the crossing of all the conductors supported, the pull in each conductor being taken as the tension due to the specified loading…”

The allowable fiber stress under this unbalanced load is 5,100 pounds per square inch, hence poles C and D must be head guyed.

Under an unbalanced longitudinal stress as above considered, the pole, or, in this case the guy can be figured to withstand this unbalanced stress “without failure”; that is the guys can be figured, therefore, guys with ultimate strength of 20,080 pounds. There can be used, therefore, three 3/8 inch Siemens-Martin steel strands or equivalent having an ultimate strength of 11,000 pounds each. The unbalanced stress may be assumed to be distributed over one or more poles adjacent to the crossing poles. If so assumed one head guy may be installed on each of the crossing poles C and D and also on each of the adjoining poles B and E, each of these guys to be ˝-inch Siemens-Martin galvanized steel strand or equivalent having an ultimate strength of 11,000 pounds.