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FIG. 3-WOODEN-POLE CONSTRUCTION, DOUBLE THREE-PHASE
CIRCUIT-POLE WITH GROUND WIRE EARTHED

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FIG. 4-WOODEN-POLE CONSTRUCTION, DOUBLE THREE-PHASE CIRCUITHORN ARRESTER PROTECTION, EACH PHASE PROTECTED ON DIFFERENT POLE

or more such wires might give better protection, but the added expense is not justified. In the pole line shown in Figures 1 to 5 the ground wire specified is galvanized solid-steel wire. The reason this is preferred to stranded wire is that rusting will be slower. The wire to earth shown is stranded copper wire tinned. Tinning often prevents ground wires from being stolen for copper. The ground wire should be earthed at least every fifth pole. 10. Lightning rods above wooden poles are advantageous. A novel manner of earthing is shown in Fgure 5. A

6 STRANDS TINNED

COPPER FOR GROUNDING CABLE

STAPLED DOWN POL&

FAN OUT COPPER CABLE

AN SOLDER TO NETTING

STRIP GALY FENCING ABOUT 2EFT WIDTH

IN CYLINDER FOR GROUND NET

TINNED TO PROTECT AGAINST

BEING STOLEN

FIG. 5-WOODEN-POLE CONSTRUCTION-NOVEL MANNER OF

EARTHING

section of galvanized-iron wire fence is placed around the outside of the pole hole after the pole has been put in position. The stranded earthing wire is then spread out into its strands and each strand soldered to a different wire on the fencing. The hole is then filled and packed.

12. On each large system it is recommended that at least one aluminum cell arrester be installed in addition to regular multigap equipment. This arrester can discharge with ease enormous currents and therefore easily relieves the line in case of power surge. It is especially designed to relieve

the current surges on an insulated delta or Y system when one phase is grounded through a flaring arc. In this case surges reappear at every half cycle of the generator wave and any arrester that protects must be capable of carrying current resulting from the continuous discharges. Since the aluminum cell can be connected directly to the circuit, it gives ideal conditions for relieving the strains on the line. This arrester should best be connected across the high-tension 'buses.

Constant-Voltage Underground Cable Systems

I. Cables should be protected in the station by lightning. arresters, but no choke-coils should be used.

2. Cables should also be protected by lightning arresters wherever joining overhead systems.

3. In stations cables that do not connect with pole line wires require only static dischargers; unless, however, danger of low-frequency surges exist, in which case the aluminum arrester is recommended. Horn arresters are extremely dangerous on constant-voltage cable systems.

4. Cable sheaths should be grounded as solidly as possible. as a spark between a cable sheath and ground may set up dangerous oscillations.

Constant Voltage, Direct Current

I. On all direct-current circuits up to 1200 volts the magnetic blowout arrester should be used. For voltages higher than this, to any voltage desired, either the liquid-electrode or the aluminum cell arrester can be used. The multigap arrester, however, will not operate on direct current. Arresters should be placed on outgoing lines, in cars, and on poles of long lines.

2. With third-rail construction, danger from lightning is not serious and arresters are recommended only in stations and on long feeders.

Constant Current

For all constant-current lines, whether direct-current, mercury-arc rectifier, or alternating-current, horn lightning arresters. with resistance rods are to be used. They are to be placed in the station on each outgoing line and also, where cables are used, on the pole where the cable joins the overhead wiring.

GRADED RESISTANCE MULTIGAP ARRESTER

The general theory of the multigap arrester is as follows: When voltage is applied across a series of multigap cylinders, the voltage distribution is not uniform. The voltage distributes according to the capacity of the cylinders, both between themselves and also to ground, and the capacity of the cylinders to ground results in the concentration of voltage across the gaps nearest the line. Figure 6 shows the theoretical voltage gradient along an arrester for a certain given case. When the voltage across the end gaps reaches a certain value, they arc across, passing the strain back to the other gaps, which in turn arc over

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FIG. 6-THEORETICAL VOLTAGE GRADIENT ACROSS LIGHTNING ARRESTER

until the spark has passed entirely across. The arrester in this manner arcs over at a voltage much lower than would be required if the voltage distributed evenly. When the arrester has arced over and current is flowing, the voltage then does distribute evenly between the gaps and is for this reason too low to maintain an alternating-current arc. The arc therefore lasts only to the end of the half cycle and then goes out. The maximum voltage per gap at which the arc will extinguish at the end of the half cycle depends to a great extent upon the metal of the cylinders. Thus some metals are more efficient than others in extinguishing the arc. When the voltage of an alternating-current arc passes through zero of course no current flows. Before the

current flows in the reverse direction the voltage must again break through the dielectric; the voltage required to do this depends upon how much the dielectric has been weakened by the passage of the arc. The cooler the arc, the less is the dielectric 'weakened and the higher will be the voltage required to reverse the arc. As the temperature of the arc depends upon the boiling point of the cathode metal, in very much the same way as the temperature of steam depends upon the boiling point of the water, metals with low boiling point are used for the lightningarrester cylinders in order to keep down the arc temperature. The boiling temperatures of a few metals with a low boiling point are given below:

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The metal with low boiling point can be alloyed with other metals and still serve the purpose of keeping the arc cool; the other metals, with high boiling point, not melting and thus acting as a skeleton to keep the cylinders in shape. In the actual lightning arrester much depends on the choice of the alloy for the cylinders. Tests with a great number of different alloys and other substances were made before settling on the particular alloy used. The temperature of the arc also depends upon the current that flows; thus, if the current is limited by resistance the number of cylinders can be less than if no resistance is used.

The use of resistance in a lightning arrester needs very careful consideration. Lightning does not readily pass through resistance, especially when in series with multigaps, and, therefore, series resistance should not be used. At the same time it is very desirable in some way to limit the line current. This problem has at last been solved by use of low shunt resistance, shunting a part of the gaps, and so proportioned as to divert the arc from the gaps after the discharge has crossed to ground. Shunt resistance has been used before, but never for this purpose, and was never made low enough to divert the arc in this way.

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