CHAPTER XX._Electric Transmission of Water Power

INSULATOR PINS.

Wooden insulator pins are among the weakest elements in electric

transmission systems. As line voltages have gone up it has been

necessary to increase the distances between the outside petticoats of

insulators and their cross-arms and to lengthen the insulators

themselves in order to keep the leakage of current between the

conductors within permissible limits. To reduce the leakage, the wires

on most lines are located at the tops instead of in the old position at

the sides of their insulators.

All this has tended to a large increase of the mechanical strains that

operate to break insulator pins at the point where they enter the

cross-arm, because the strain on each line wire acts with a longer

leverage. Again, it is sometimes necessary that transmission lines make

long spans across rivers or elsewhere, and a very unusual strain may be

put on the insulator pins at these places.

As long as each electric system was confined to a single city or town a

broken insulator pin could be quickly replaced, and any material

interruption of service from such a cause was improbable. Where the

light and power supply of a city, however, depends on a long

transmission line, as is now the case in many instances, and where the

line voltage is so great that contact between a wire and a cross-arm

will result in the speedy destruction of the latter by burning, a broken

pin may easily lead to a serious interruption of the service.

Besides the increase of mechanical strains on insulator pins, there is

the danger of destruction of wooden pins by charring, burning, and other

forms of disintegration due to leakage of current over the insulators.

This danger was entirely absent in the great majority of cases so long

as lines were local and operated at only moderate voltages. These

several factors combined are bringing about marked changes in design.

On straight portions of a transmission line the insulator pins are

subject to strains of two principal kinds. One of these is due directly

to the weight of the insulators and line wire, and acts vertically to

crush the pins by forcing them down onto the cross-arm. The other is due

to the horizontal pull of the line wire, which is often much increased

by coatings of ice and by wind pressure, tending to break the pins by

bending--most frequently at the point where they enter the cross-arm. A

strain of minor importance on pins is that encountered where a short

pole has been set between two higher ones, and the line at the short

pole tends to lift each insulator from its pin, and each pin from the

cross-arm.

Where the line changes its direction, as on curves and at corners, the

side strain on pins is greatly increased, and such places give by far

the largest amount of trouble through the breaking of pins. The latter

seldom fail by crushing through the weight of the lines they support,

because the size of pin necessary to withstand the bending strain has a

large factor of safety as to crushing strength. Insulators are sometimes

lifted from wooden pins, and the threads of these pins stripped where a

short pole is used, as already noted; but failure of this kind is not

common.

Iron pins are either screwed or cemented into their insulators, but the

cemented joint is much more desirable, because where a screw joint is

made the unequal expansion of the iron and the glass or porcelain is apt

to result in breakage of the insulator. Where cement is used, both the

pins and insulators should be threaded or provided with shoulders of

some sort, so that, although the shoulders of threads do not come into

contact with each other, they will, nevertheless, help to secure a

better hold. Pure Portland cement, mixed with water to a thick liquid,

has been used with success, the insulator being placed upside down and

the pin held in a central position in the hole of the insulator while

the cement is poured in. Another cement that has been used for the same

purpose is a mixture of litharge and glycerin. Melted sulphur is also

available.

The same forces that tend to lift an insulator from its pin tend also to

pull the pin from its socket in the cross-arm or pole top. With wooden

pins the time-honored custom has been to drive a nail into the side of

the cross-arm so that it enters the shank of the pin in its socket. This

plan is good enough so far as immediate mechanical strength is

concerned, but is not desirable, because it is hard to remove a nail

when a pin is to be removed, and also because the rust of the nail rots

the wood. A better plan is to have a small hole entirely through each

cross-arm and insulator pin at right angles to the shank of that pin in

its socket, and then to drive a small wooden pin entirely through from

side to side.

Some of the important factors affecting the strains on insulator pins

vary much on different transmission lines, as may be seen from the

following table of lines on which wooden pins are used. On the older

line between Niagara Falls and Buffalo, the regular length of span is 70

feet, and each copper conductor of 350,000 circular mils is attached to

its insulator 7.5 inches above the cross-arm. On the new line the length

of span is 140 feet, and each aluminum conductor of 500,000 circular

mils is attached to its insulator 10 inches above the cross-arm.

TABLE I.--DATA OF LINES ON WOODEN PINS.

+----------------------------+-------------+-----------+-------------+

+----------------------------+-------------+-----------+-------------+

|Colgate to Oakland | [B]133,100 | ... | 13 |

|Electra to San Francisco | [A]471,034 | 130 | 15 |

|Cañon Ferry to Butte | [B]105,600 | 110 | 13-1/2 |

|Shawinigan Falls to Montreal| [A]183,750 | 100 | 16-1/4 |

|Niagara Falls to Buffalo | [B]350,000 | 70 | 7-1/2 |

|Niagara Falls to Buffalo | [A]500,000 | 140 | 10 |

|Chambly to Montreal | [B]133,100 | 90 | 8-1/2 |

|Colgate to Oakland | [A]211,600 | ... | 13 |

+----------------------------+-------------+-----------+-------------+

[A] Aluminum conductors.

[B] Copper conductors.

TABLE II.--DIMENSIONS OF WOODEN PINS IN INCHES.

+-----------------------+------+------+------+------+--------+--------+

| Location of Lines. | of | of | eter | eter | of | of |

| | | | | der. | | |

+-----------------------+------+------+------+------+--------+--------+

|Colgate to Oakland |10-3/8| 5-3/8| 2-1/8| 2-1/2| 1-3/8 | 2 |

|Francisco |12 | 4-7/8| 2-1/4| 2-3/4| 1-3/8 | 2 |

|Cañon Ferry to Butte |12-1/2| 5-1/8| 2 | 2-1/2| 1-1/8 | 3 |

|Montreal |13-1/2| 5 | 2-3/4| 3 | 1 | .. |

|Buffalo[A] | 5-1/4| 6 | 2 | 2-3/4| 7/8 | 1-1/2 |

|Buffalo[B] | 7-3/4| 6 | 2-1/4| 2-3/4| 1-1/2 | 2-1/2 |

|Chambly to Montreal[C] | 7 | 5 | 1-1/2| 1-7/8| .. | .. |

|Cañon Ferry to Butte[D]|12-3/8| 7-7/8| 2-1/8| 2-1/2| 1-1/8 | 3 |

+-----------------------+------+------+------+------+-------+---------+

[A] Pins on old line.

[B] Pins on new line.

[C] Approximate dimensions.

[D] Pole top pins.

To compensate for the greater strains introduced by doubling the length

of span and using pins of longer stem, the diameter of the shank of the

new pins was increased to two inches. One line between Colgate and

Oakland is of copper, and the other is of aluminum conductors, but the

same pins appear to be used for each. On the line between Cañon Ferry

and Butte, Mont., the pin used in pole tops has a shank 2-3/4 inches

longer and 1/8-inch greater in diameter than the pin used in cross-arms.

The weakest pin included in the table seems to be that in use on the

line between Chambly and Montreal, which is of hickory wood, about 1-1/2

inches in diameter at the shank, and carries its No. 00 copper wire

8-1/2 inches above the cross-arm.

The following dimensions for standard wooden insulator pins to be used

on all transmission lines are proposed in vol. xxi., page 235, of the

Transactions of the American Institute of Electrical Engineers. These

pins are designed to resist a uniform pull at the smaller end and at

right angles to the axis in each case. The length of each pin, in inches

between the shoulder and the threaded end, is represented by L, and the

diameter of each pin at its shank by D.

L. D.

1 0.87

2 1.10

3 1.26

4 1.39

5 1.50

6 1.59

7 1.67

8 1.75

9 1.82

10 1.88

11 1.95

13 2.06

15 2.17

17 2.25

19 2.34

21 2.42

The two strongest pins in Table II. appear to be those in use on the

line between Shawinigan Falls and Montreal and on the line from Niagara

Falls to Buffalo. The former have a diameter of 2-3/4 inches at the

shank, and the wire is carried 16-1/4 inches above the shoulder of the

pin. On the new Niagara line the shank diameter of each pin is only

2-1/4 inches, but the line wire is only 10 inches above the shoulder. It

was found by tests that a strain of 2,100 pounds at the top of the

insulator and at right angles to the axis of this Niagara pin was

necessary to break it at the shank. This strain is about six times as

great as the calculated maximum strain that will occur in service on the

line.

Some of the pins here noted are much stronger than those proposed in the

above specifications for standard pins. The pins on the old Niagara line

have a shank diameter of 2 inches, with a stem only 5-1/4 inches long,

while the proposed pin of 2 inches diameter at the shank has a stem 11

inches long. On the Colgate and Oakland line a shank diameter of 2-1/8

inches goes with a length of 10-3/8 inches in the stem, but the proposed

pin with this size of shank has a stem 13 inches long. For a shank of

2-1/4 inches diameter the proposed pin has a stem 15 inches long, but

the pins with this diameter of shank on the Electra line are only 12

inches long in the stem.

The 2-1/4-inch diameter of shank in the pins on the new Niagara line

goes with a length of only 7-3/4 inches in the stem. The new Niagara pin

is thus almost exactly twice as strong as the proposed pin, since the

strength of a pin where the shank joins the stem varies inversely as the

length of the stem, all other factors being the same.

Pins on the Shawinigan Falls line have a shank 2-3/4 inches in diameter,

with a length of 13-1/2 inches in the stem; but the largest of the

proposed pins, that with a stem 19 inches long, has a diameter of only

2-1/2 inches in the shank.

It is hardly too much to say in the interest of good engineering that

the wooden pin of about 5 inches length of stem and 1-1/2 inches

diameter of shank, as well as all longer pins of no greater strength,

should be discarded for long transmission lines of high voltage. These

pins have done good service on telegraph and telephone lines, and on

local lighting circuits of No. 6 B. & S. gauge wire or smaller, and they

may well be left for such work.

To meet the conditions of transmission work a change in both the shape

and size of pins is necessary. In the first place, the shoulder on pins

where the shank and stem meet, that relic of telegraph practice, should

be entirely discarded. This change will save considerable lumber on pins

of a given diameter at the shank, and will increase the strength of the

pin by avoiding the sharp corner at the junction of the shank and stem.

Another change of design should leave an excess of strength in the stem

of the pin, to provide for deterioration of the wood, and particularly

for charring by current breakage. This increase of diameter and strength

near the top of the pin will cost nothing in lumber, for the wood is

necessarily there unless it is turned off. The shank of each pin should

be proportionately shorter than in the older type, and the pin hole

should be bored only part way through the cross-arm. A saving in lumber

for pins and for cross-arms will thus be made, since the size of the

cross-arm may be less for a given resistance to splitting.

With these changes in general design the pin is a simple cylinder in the

shank, with a gentle taper from the shank to form the stem. An example

of this design, which might well serve as a basis for a line of standard

pins, would be a pin 2 inches in diameter and 3-1/2 inches long in the

shank, and tapering for a length of 5 inches from the shank to form the

stem, with a diameter of 1-1/2 inches at the top. The hole in a

cross-arm for this pin should be 3-1/2 inches deep, and this, in an arm

4-3/4 inches deep, would leave 1-1/4 inches of wood below the pin. From

the lower end of the pin hole, a hole 1/4-inch in diameter should run to

the bottom of the cross-arm to drain off water. A line of longer pins

designed to resist the same line pull as this short one would be strong

enough for small conductors, say up to No. 1 B. & S. gauge wire.

For larger wires, long spans and sharp angles in a line, a pin 2-1/4

inches in diameter and 4-1/2 inches long in the shank, tapering for 5

inches to a diameter of 1-3/4 inches at the top, or longer pins of equal

strength, should be used.

Where the pin holes do not extend through the cross-arm there is no need

of a shoulder on the pin to sustain the weight of the line wire. In the

cross-arm on the new Niagara Falls line each pin hole is bored to a

depth of 5 inches, leaving 1 inch of wood below the hole. On the line

from Electra to San Francisco the depth of each pin hole is again 5

inches, and the depth of the cross-arm 6 inches.

The pins for use on the Electra line were kept for several hours in a

vat of linseed oil at a temperature of 210° F. The pins for the

Shawinigan line were boiled in stearic acid. All wooden pins should be

treated chemically, but the object of this treatment should be to

prevent decay rather than to give them any particular insulating value.

The lack of strength in wooden pins and their destruction in some cases

by current leakage are leading to the use of iron and steel pins. Such a

pin, in use on the lines of the Washington Power Company, of Spokane,

Wash., is made up of a mild steel bar 17-1/2 inches long and 1-1/8

inches in diameter, cast into a shank at one end, so that the total

length is 18 inches. The cast-iron shank has a diameter of 2-1/16

inches, with a shoulder of 2-1/2 inches diameter at its upper end. To

prevent the pin from lifting out of its hole a small screw enters the

top of the cross-arm and bears on the top end of the shank. Above the

cast-iron shank the length of the steel rod is 12 inches, and starting

3/4 inch down from its top a portion of the rod 3/4 inch long is turned

to a diameter of one inch.

It is said that this pin begins to bend with a pull of 1,000 pounds at

its top, but that it will support the insulator safely even when badly

bent.

Insulators may resist puncture and prevent surface arcing from wire to

pin, but still allow a large though silent flow of energy over the pins

and cross-arms between the conductors of a transmission circuit. The

rate at which current flows from one wire of a transmission circuit to

another in this way depends on the total resistance of each path over

insulator surfaces and through air to the pins and cross-arm, and then

over these parts.

If the pins and cross-arm are entirely of iron, the total resistance of

the path through them from wire to wire is practically that of the

insulator surfaces. If the pins and cross-arm are of wood which is dry,

they may offer an appreciable part of the total resistance of the path

through them between the wires of a circuit; but if the wood be wet, its

resistance is very much reduced.

The resistance of wooden pins and cross-arm may be so small compared

with that of the air and insulator surfaces that complete the path

through them from wire to wire of a circuit, that the effect of these

wooden parts in checking the flow of current between conductors is

relatively unimportant, and yet the resistances of these pins and the

cross-arm may affect their lasting qualities.

The current that flows over the pins and cross-arms from one wire to

another of a high-tension circuit may be so small as not to injure these

wooden parts when evenly distributed over them, and yet this same

current may char or burn the wood if confined to a narrow path. Such a

leakage current will naturally cease to be evenly distributed over pins

and their cross-arms when certain portions of their surfaces are of much

lower resistance than others, because an electric current divides and

follows several possible paths in the inverse ratio of their

resistances.

These narrow paths of relatively low resistance along wooden pins and

cross-arms are heated and charred by the very current that they attract,

so that the conductivity of the path and the heat developed therein

react mutually to increase each other, and tend toward the destruction

of the wood.

Among causes that tend to make some parts of pins and cross-arms better

conductors than others, there may be mentioned cracks in the wood, where

dirt and moisture collect, dust, with a mixture of salt deposited on the

wood by the winds at certain places, and sea fogs that are often blown

only against one side of the pins and arms and deposit salt.

To make matters worse, the same cause that creates a path of relatively

good conductivity along wooden pins and cross-arms often materially

lowers the resistance offered to the leakage of current by the insulator

surfaces. Thus an increase of the rate at which energy passes from wire

to wire of a circuit, and the concentration of this energy in certain

parts of the wooden path, are sometimes brought about at the same time.

Where the line insulators employed are so designed that the resistance

of the dry wooden pins and cross-arms forms a material part of the total

resistance between the wires of a circuit, a rain or heavy fog may cause

a very large increase in the rate at which energy passes over these

wooden parts between the conductors.

As long as only moderate voltages were carried on line conductors, the

charring and burning of their pins and cross-arms was a very unusual

matter; but with the application of very high pressures on long circuits

the destruction of these wooden parts by the heat of leakage currents

has become a serious menace to transmission systems. Even with low

voltages there may be charring and burning of pins and cross-arms if the

line insulators are very poor or if the conditions as to weather and

flying dust are sufficiently severe.

In vol. xx. of the Transactions of the American Institute of Electrical

Engineers, pages 435 to 442 and 471 to 479, an account of the charring

and burning of pins on several transmission lines is given, from which

some of the following examples are taken.

In one case a line that ran near a certain chemical factory was said to

be much troubled by the burning of its pins, though the voltage employed

was only 440, and the insulators were designed for circuits of 10,000

volts. In rainy weather, when insulators, pins, and cross-arms were

washed clear of the chemical deposits, there was no pin burning. Similar

trouble has been met with on sections of the 40,000-volt Provo line, in

Utah, where dust, mixed with salt, is deposited on the insulators, pins,

and cross-arms. On page 708 a 2,000-volt line is mentioned on which fog,

dust, and rain caused much burning of pins.

When circuits are operated at voltages of 40,000 to 60,000, no very

severe climatic conditions are necessary to develop serious trouble in

the wooden pins by leakage currents, even where the transmission lines

are supported in insulators of the largest and best types yet developed.

Striking examples along this line may be seen in the transmission

systems between Colgate and Oakland, Cal., and between Electra and San

Francisco. Both of these systems were designed to transmit energy at

60,000 volts, but the actual pressure of operation seems to have been

limited to about 40,000 volts during much of their period of service.

Insulators of a single type and size are used on both of these

transmission lines, and are among the largest ever put into service on

long circuits. Each of these insulators is 11 inches in diameter, and

11-1/4 inches high from the lower edge to the top, the line wire being

carried in a central top groove. The wooden pins used on the two lines

vary a little in size, so that on the Electra line each pin stands

11-1/2 inches above its cross-arm, while on the Colgate line the

corresponding distance is 12 inches. As the insulators are of the same

size in each case, the length of the pin between the lower edge of each

insulator and the top of the cross-arm is 4 inches on the Colgate line

and 3-1/2 inches on the Electra line.

On the latter line a porcelain sleeve, entirely separate from and making

no contact with the insulator, covers each pin from the top of its

cross-arm to a point above the lower edge of the insulator. On the

Colgate line each insulator makes contact with its pin for a length of

2-1/2 inches down from the top of its thread, and on the Electra line

the contact of each insulator with its pin runs down 3-1/2 inches below

the top of the thread. This leaves 9 inches in the length of the pin

between the insulator contact and the top of each cross-arm on the

Colgate line, and a corresponding length of pin amounting to 8-1/2

inches on the Electra line. Of this 8-1/2 inches of pin surface, about 6

inches is covered by the porcelain insulating sleeve used on each pin of

the Electra line, so that only about 2-1/2 inches of the length of each

pin on that line is exposed to the leakage of current from the insulator

directly through the air. Both the sizes of pins just mentioned were

made of eucalyptus wood, boiled in linseed oil.

Each one of three pins taken from a pole, between North Tomer and

Cordelia, on the Colgate line, was badly charred and burned on its side

that faced the damp ocean winds. This charring extended all the way down

each pin from the point where the insulator made contact with it, a

little under the threads, to the top of the cross-arm nine inches below.

Two of these pins were located at the opposite ends of a cross-arm, and

the third was fixed in the top of the pole. This cross-arm was charred

or burnt, as well as the pin, but no defects could be detected in the

insulators that the pins supported.

As to these three pins, the most reasonable explanation seems to be that

enough current leaked over both the outside and inside surfaces of each

insulator and through the air to char the pin and cross-arm. In flowing

down each pin, the current was naturally concentrated on the side

exposed to the damp winds of the ocean, because the deposit of moisture

by these winds lowered the resistance on that side. When these winds

were not blowing, and before a pin became charred on one side, its

resistance was probably about the same all the way around, and the

leakage current, being distributed over the pin, was not sufficient to

char it. The damp wind would, of course, lower the surface resistance of

each insulator, and this, with the deposit of moisture on the pins and

cross-arm, many have made a very material reduction in the total

resistance from wire to wire.

The insulators used on these pins each had two petticoats, an upper one,

11 inches in diameter, and a lower one, 6-1/2 inches in diameter, the

lower edge of the smaller petticoat being 7-1/2 inches beneath the lower

outside edge of the larger petticoat. As the inner surface of the larger

petticoat was much nearer to a horizontal plane than the inner surface

of the smaller petticoat, moisture would have been more readily retained

on it, and the greater part of the surface resistance of the insulator

during wet weather must therefore have been on the inside of the smaller

petticoat. At its lower edge the smaller petticoat was distant radially

about 1-3/4 inches from the pin, and the distance between the pin and

the inside surface of the smaller petticoat gradually decreased to

actual contact at a point 5-1/2 inches above this lower edge.

The path of the current from the line wire to the pin in this case seems

to have been first over the entire insulator surface to the lower edge

of the smaller petticoat and then partly up over the inner surface of

this petticoat and partly from that surface through the air. On each of

these three pins the charring was quite as bad just below the thread as

it was further down, so that a large part of the leakage current seems

to have gone up over the interior surface of the smaller petticoat. The

charred portion of these pins extended but little, if at all, into the

threads near the tops or into the part of the pin fitting into the

cross-arm. The preservation of the part of each pin that entered the

cross-arm seems to have been due to the increase of surface and decrease

of resistance of the cross-arm in comparison with the pin. Preservation

of the threaded part of each pin seems to have been due to its

protection from moisture and its high resistance, so that little or no

current passed over it.

Another pin taken from the same line as the three just considered was

badly burned at a point about 1.75 inches below the threads, but on

sawing it completely across at two points below the charred spot the

entire section was found to be perfectly sound and free from any sign of

burning. The explanation of the condition of this pin is, perhaps, that

the resistance of the burned part, owing to its additional protection

and dryness, was high compared with that of the lower part of the pin,

and thus developed most of the heat on the passage of current. It is not

clear, however, why this pin should burn only just below the thread,

while other pins of the same kind on the same line were charred all the

way down from the thread to the cross-arm.

Another curious result noticed in some pins on this same line is the

softening of the threads so that they can be rubbed off with the

fingers.

RELATION OF PINS AND INSULATORS.

+------------------------------+-------+--------+--------+-------------+

| Location of Line. | of | of | of | Covered by |

| | | tor. | tor. | |

+------------------------------+-------+--------+--------+-------------+

|Electra to San Francisco | 60,000| 11 | 11-1/4 | 12 |

|Colgate to Oakland | 60,000| 11 | 11-1/4 | 8 |

|Cañon Ferry to Butte | 50,000| 9 | 12 | 10-1/2 |

|Shawinigan Falls to Montreal | 50,000| 10 | 13 | 10-1/4 |

|Santa Ana River to Los Angeles| 33,000| 6-3/4 | 4-7/8 | 2-1/2 |

|Provo around Utah Lake | 40,000| 7 | 5-3/4 | 4-3/4 |

|Spier Falls to Schenectady | 30,000| 8-1/2 | 6-3/4 | 5-1/4 |

|Niagara Falls to Buffalo | 22,000| 7-1/2 | 7 | 5 |

+------------------------------+-------+--------+--------+-------------+

The softened wood of the threads is not charred, but is said to have a

sour taste and to resemble digested wood pulp. While the threads of a

wooden pin are destroyed in this way the remainder of the pin may still

remain perfect and show no charring.

RELATIONS OF PINS AND INSULATORS.

+------------------------------+----------+----------+-----------+

| |Cross-arm.| Air. | Air. |

+------------------------------+----------+----------+-----------+

|Electra to San Francisco | 0 | 10-1/2 | 3-1/2 |

|Colgate to Oakland | 3-1/2 | 10 | 2-1/2 |

|Cañon Ferry to Butte | 1-1/2 | 0 | 1-1/2 |

|Shawinigan Falls to Montreal | 3-1/4 | 9-1/2 | 1 |

|Santa Ana River to Los Angeles| 3-1/2 | 2-3/4 | .. |

|Provo around Utah Lake | 3-1/2 | 2-1/2 | .. |

|Spier Falls to Schenectady | 4 | 4 | 5/8 |

|Niagara Falls to Buffalo | 3 | 4-1/2 | 2 |

+------------------------------+----------+----------+-----------+

In explanation of this disintegration of the threads of wooden pins it

was stated that a number of these pins, the tops of which were reduced

to a white powder, had been taken from the line between Niagara Falls

and Buffalo, on which the voltage is 22,000, and that this powder proved

on analysis to be a nitrate salt. This salt was thought to be the result

of the action of nitric acid on the wood, it being supposed that the

acid was formed by a static discharge acting on the oxygen and nitrogen

of the air between the threads of the insulator and pin. In support of

this view it was stated that an experimental line of galvanized-iron

wire at Niagara Falls, which was operated at 75,000 volts continuously

during nearly four months, turned black over its entire length of about

two miles. This surface disintegration was not due to the normal action

of the air, for similar wire at the same place remained bright when not

used as an electrical conductor.

These facts seemed to indicate that the brush discharge from the wires

carrying the 75,000-volt current developed nitric acid from the oxygen

and nitrogen of the air, and that this acid attacked the wire.

One of the above-mentioned pins used on the Electra line was much

charred and burned away at a point a little below the threads. The

charred path of the current could also be traced down the side of the

pin to the cross-arm, but this path was not as badly burned as the spot

near the top of the pin.

A composite pin from a 33,000-volt line, probably a part of the

transmission system between the Santa Ana River and Los Angeles, was

burned through its wooden threads to the central iron bolt, along a

narrow strip at one side. Every pin burned on this line was said to show

the effects of the current in the way just described, but no cross-arms

were burned and very few insulators punctured.

The composite pin was made up of a central iron bolt 10-5/8 inches long,

1/2-inch in diameter, and with a thin head above the wooden threads, a

sleeve of wood 2-5/8 inches long and 1 inch in diameter in its threaded

portion, and a sleeve of porcelain 3-1/8 inches long and 1-1/4 inches in

diameter at its upper and 2-11/16 inches at its lower end. The sleeves

of wood and porcelain were slipped over the central iron bolt so that

the portions of the pin above the cross-arm measured 5-7/8 inches. In

this case the path of the leakage current seems to have been over both

the exterior and interior surface of the insulator and then through the

wooden sleeve to the central bolt and the cross-arm.

The facts just outlined certainly indicate a serious menace to the

permanence and reliability of long, high-voltage transmission lines

supported by insulators on wooden pins. If such results have been

encountered on the lines above named, where some of the largest and best

designs of insulators are employed, it is only fair to assume that

similar destructive effects of leakage currents are taking place on many

other lines that operate at high voltages.

It seems at least doubtful whether any enlargement or improvement of the

insulators themselves will entirely avoid the destruction of their

wooden pins in one of the ways mentioned. It is probable, but not

certain, that further extension of distances through air and over

insulator surfaces, both exterior and interior, between line wires and

wooden pins, will prevent charring and burning of the latter by leakage

currents. Much has already been done in the way of covering most of the

pin above its cross-arm with the insulator parts, but even those

portions of the pin that are best protected in this way are not free

from burning.

Thus, on the Colgate line, eight inches of each pin is protected by the

interior surface of its insulator, but these pins were charred quite as

badly where best protected, up close to the thread, as they were down

near the cross-arm. The same is true of the Electra line, where a

porcelain sleeve runs up about the pin from the cross-arm to a point

above the inner petticoat of each insulator, so that the entire length

of the pin above the cross-arm is protected. On the Cañon Ferry line, a

glass sleeve that virtually forms a part of each insulator, though

mechanically separate from it, protects the pin from its threaded

portion to within 1.5 inches of the cross-arm.

Insulators on the line from Shawinigan Falls to Montreal are each 13

inches long and extend down over the pin to within 1.5 inches of the

cross-arm. The burned portion of each pin from the Santa Ana line was

that carrying the threads, and thus in actual contact with that part of

the insulator which was separated by the greatest surface distance from

the line wire.

Aside from the burning of pins is the destruction of their threaded

parts by some chemical agency that is developed inside of the tops of

the insulators, as shown in the cases of the Colgate and Niagara lines.

It does not appear that any improvement of insulators will necessarily

prevent chemical action.

Though it may not be practicable to so increase the surface resistance

of each insulator that the burning of wooden pins by leakage current

will be prevented, the substitution of a conducting for an insulating

pin may remedy the trouble. As the insulators, pins, and cross-arm form

a path for the leakage current from wire to wire, the wooden pins by

their resistance, especially when dry, must develop heat. In pins of

steel or iron this heat would be trifling and would do no damage. With

pins of good conducting material, like iron, the amount of leakage from

wire to wire, with a given design of insulator, would, no doubt, be

somewhat greater than the leakage with wooden pins.

It will be cheaper, however, to increase the resistance of new

insulators up to the combined resistance of present insulators and

their wooden pins than it will be to replace these pins when they are

burned.

From all the evidence at hand, it seems that insulators which reduce the

leakage of current over their surfaces to permissible limits as far as

mere loss of energy is concerned, even with iron pins, will not prevent

the charring and destruction of wooden pins.

[Illustration: FIG. 90.--Glass Insulator and Sleeve on 50,000-volt Line

Between Cañon Ferry and Butte, Mont.]

When any suitable insulator is dry and clean it offers all necessary

resistance to the leakage of current over its surface, and any

resistance in the pin that carries the insulator is of small importance.

If the resistance of an insulator needs to be reinforced by that of its

pin in any case, it is when the surface of the insulator is wet or

dirty. Unfortunately, however, the same weather conditions that deposit

dirt or moisture on an insulator make similar deposits on its pin, and

the resistance of the pin is lowered much more than that of the

insulator by such deposits. The increase of current leakage over the

surface of an insulator during rains and fogs usually does no damage to

the insulator itself, but such leakage over the wet pin soon develops a

surface layer of carbon that continues to act as a good conductor after

the moisture that temporarily lowered the resistance has gone. Reasons

like these have led some engineers to prefer iron pins with insulators

that offer all of the resistance necessary for the voltage employed on

the line.

It may be suggested that the use of iron pins will transfer the charring

and burning to the wooden cross-arms, but this does not seem to be a

necessary result. The comparative freedom of cross-arms from charring

and burning where wooden pins are used seems to be due to the larger

surface and lower resistance of the cross-arms. With iron pins having a

shank of small diameter, so that the area of contact surface between the

pin and the wooden cross-arm is relatively small, there may be some

charring of the wood at this contact surface. Should it be thought

desirable to guard against any trouble of this sort, the surface of the

iron pin in contact with the cross-arm may be made ample by the use of

large washers, or by giving each pin a greater diameter at the shank

than elsewhere.

It may be noted that the pins with a central iron bolt only half an inch

in diameter, that were used on the 33,000-volt Santa Ana line, were said

to have caused no burning of their cross-arms in those cases in which

the wooden threads about the top of the central bolt were burned

through.

Another possible trouble with iron pins is that they, by their greater

rate of expansion than glass or porcelain, will break their insulators.

Such results can readily be avoided by cementing each iron pin into its

insulator, instead of screwing the insulator onto the pin. Iron pins

will, no doubt, cost somewhat more than those of wood, but this cost

will in any event be only a small percentage of the total investment in

a transmission line. Considering the cost of the renewals of wooden

pins, there seems little doubt that on a line where the voltage and

other conditions are such as to result in frequent burning, iron pins

would be cheaper in the end.

Iron pins have already been adopted on a number of high-voltage lines.

Not only iron pins, but even iron cross-arms and iron poles are in use

on a number of transmission lines. On a long line now under construction

in Mexico, iron towers, placed as much as 400 feet apart, are used

instead of wooden poles, and both the pins and cross-arms are also of

iron. The 75-mile line from Niagara Falls to Toronto is carried entirely

on steel towers.

The Vancouver Power Company, Vancouver, British Columbia, use a pin that

consists of a steel bolt about 12 inches long fitted with a sleeve of

cast iron 4-1/2 inches long to enter the cross-arm, and a lead thread to

screw into the insulator. On the 111-mile line of the Washington Power

Company, of Spokane, which was designed to operate at 60,000 volts and

runs to the Standard and Hecla mines, a pin consisting of a steel bar

1-1/8 inches in diameter, with a cast-iron shank 2-1/16 inches in

diameter to enter the cross-arm, and with the lead threads for the

insulator, is used.

[Illustration: FIG. 92.--Iron Pins on Spier Falls Line.]

On the network of transmission lines between Spier Falls, Schenectady,

Albany, and Troy, in the State of New York, the insulators are supported

on iron pins of two types. One of these pins, used at corners and where

the strain on the wire line is exceptionally heavy, is made up of a

wrought-iron bolt 3/4-inch in diameter and 16-1/2 inches long over the

head, and of a malleable iron casting 8-3/4 inches long. This casting

has a flange of 5 by 3-3/4 inches at its lower end that rests on the top

of the cross-arm, and the bolt passes from the top of the casting down

through it and the cross-arm. Threads are cut on the lower end of the

bolt, and a nut and washer secure it in the cross-arm. The total height

of this pin above the cross-arm is 9-1/4 inches.

For straight work on this line a pin with stem entirely of malleable

iron, and a bolt that comes up through the cross-arm and enters the base

of the casting, is used. The cast top of this pin has four vertical

webs, and its rectangular base, which rests on the top of the

cross-arm, is 3-1/2 by 4 inches. The bolt that comes up through the

cross-arm and taps into the base of the casting is 3/4-inch in diameter.

The cast part of this pin has such a length that the top of its

insulator is carried 10-3/4 inches above the cross-arm. For the casting

the length is 9-1/4 inches.

Both of the types of iron pins in use on the Spier Falls lines are

secured to their insulators with Portland cement poured into the pin

hole while liquid when the insulator is upside down and the pin is held

centrally in its hole. The top of each casting is smaller in diameter

than the hole in the insulator, and is grooved so as to hold the cement.

[Illustration: FIG. 93.--Standard Pin, Toronto and Niagara Line.]

On a long line designed for 60,000 volts, and recently completed in

California, wooden pins are used with porcelain insulators, each 14

inches in diameter and 12-1/2 inches high. Each of these pins is

entirely covered with sheet zinc from the cross-arm to the threaded end,

and it is expected that this metal covering will protect the wood of the

pin from injury by the leakage current.