A second channel iron is then driven into the ground
opposite the first, and the two are securely bolted
together through the pole and the remaining half of
the pole is cut clear.
[d) Iron Masts.
Iron masts are practically always used for the higher voltages and for large
power schemes, since in these cases their higher first cost is compensated for by



OVERHEAD TRANSMISSION LINES



117





Figs. 93 and 94.
118



OVEEHEAD TEANSMISSION LINES



their longer life and the greater reliability of supply which they ensure,
masts are of very varied design. The most important types are :

(1) Simple tubular masts.
(2) Multiple tubular masts.
(3) Lattice-work masts.
(4) Lattice-work structures or towers.
Such




-Bp



.rJ-.-.
B^U.4r-^..
<r I



-r--



i3>- -



U Iron
N9I2-I&



;J



Fig. 96.
Simple tubular masts (Fig. 236) and multiple tubular masts (e.g., masts made
up of three simple tubular poles arranged at the corners of a triangle) are not
very commonly used. Owing to their good appearance they are sometimes
used in villages and the smaller towns, but for long-distance transmission schemes
they are too expensive. These tubes are made in lengths up to 26 feet of wrought
iron with a breaking stress of 55,000 to 57,000 lbs. per square inch or in Siemens-
Martin steel with a breaking stress of 80,000 to 85,000 lbs. per square inch. The
Mannesmann tubes are made of the latter material. The prices vary between
17s. and 30s. per cwt.
The smaller tubular masts are made in one piece, tapering towards the top
and consisting of two or three lengths welded together. Larger masts are made up
in sections shrunk together or held together by shrink rings.
