Figure 4-46.Riser pole for underground distribution circuit.
UNDERGROUND POWER CABLES
Underground cables have various types of
insulation and sheaths. Because higher voltages
generate more heat, the amount of voltage carried
determines the composition of the insulation.
Cables rated at 15 kilovolts and below usually
have rubber or varnished cambric insulation and a
PVC or rubber sheath. Those rated at 600 volts to 425
kilovolts have oil-impregnated paper insulation and a
Cables rated at 5 kilovolts and above have metallic
tape shields between the insulation and sheath for
mechanical protection. Exceptions to this requirement
are for single-conductor (1/0) cable with a PVC sheath
and three-conductor (3/0) belted type of cable.
Much of the new cable being installed is cross-
linked-polyethylene (XLP) or ethylenepropylene
rubber (EPR). These are called solid type of
insulations. The size and number of conductors in the
cable depend on the job requirements.
The most common types of underground
communications cables in use today are steel-armored
with plastic insulation (STELPATH), plastic-
insulated with aluminum armor (PIC), and the new
shielded fiber-optic cables.
When installing a new run of duct, you pull in
pulling wire, usually a lo-gauge iron wire. With this
wire, you pull in a wire rope to which you attach the
cable for pulling in.
Sometimes, when the duct has been in the ground a
long time, the original iron pulling wire may be rusted
so that it is not strong enough to pull the wire rope
through. Also for a 400- to 500-foot run, it would be
difficult to push a fish tape through the duct. The job
can be simplified by using an air compressor to blow a
chalk line cord through the duct. To do this, take a
small cloth and tie the chalk line end to the four
comers, so the cloth functions like a small parachute.