Insulators & Conductors


A conductor is classified as a material that allows flow of electricity with
minimal resistance.  Metals primarily make up this category. Conductors used 
in todays transmission lines have several unique characteristics.  The two
metals that are used in most power transportation applications are copper
and aluminum.  They are the conductor of choice because of their non-
magnetic characteristics and of course, their low resistivity.

Conductors for transmission lines are designed differently than those for 
low-voltage circuits or distribution lines.  Electrostatic fields that form
around the surface of a high-voltage conductor can cause a phenomenon called
corona.  Corona refers to the point when the electrical breakdown strength of
air is exceeded.  This results in crackling noise and glowing.  A great deal of 
energy loss can also be attributed to corona.

To minimize corona, designers have come up with four basic designs that deal
with such factors as conductor spacing, altitude of line above sea level, as
well as conductor diameter.  The first is the hollow copper conductor with
the I-beam core.  A circle of individual copper wires are supported by an
"I" beam that spirals through it for support.  Second there is the HH
conductor.  It is made up of curved segments resembling the letter "H" that
are connected in tongue and groove fashion.  Third there is the acsr 
(aluminum conductor steel reinforced).  This is comprised of a core steel
strands used for support that is encompassed by aluminum strands.  Last is 
the copperweld conductor.  It consists of a solid steel core over which a
thin copper cylinder is welded.  All of these designs have favorable 
characteristics when it comes to high-voltage power distribution.

The atomic structure of insulators is such that relatively few free 
electrons are available; therefore, for all practical purposes, insulators
are unable to carry any current.

Letter designations are used to identify types of conductor cables with
regard to the type of insulation and their conditions of use.  The following
lists include the common types used for fixed building wiring systems 
operating on low voltage (up to 1000 volts).  

The following letters identify the insulation.

1.  According to the type of insulation material:
    A,   asbestos
    MI,  mineral insulation 
    R,   rubber
    SA,  silicon asbestos (rubber)
    T,   thermoplastic
    V,   varnished cambric
    X,   cross-linked synthetic polymer

2.  According to conditions of use:
    H,   heat resistant up to 75 degrees C
    HH,  heat resistant up to 90 degrees C 
    UF,  suitable for underground, direct burial
    W,   moisture resistant, suitable for use in wet locations

In addition to the insulation around the conductor, some types of cables have
an outer jacket or sheath, either enclosing a single conductor or a group of
individually insulated conductors.  These outer coverings provide mechanical
and/or corrosion protection.  The following letters identify some of these 
types of cables:

  -AC,  armored cable (flexible metallic interlocked armor sheath)
  -L,   lead sheath
  -MC,  metal-clad cable (metallic sheath of interlocking tape or a smooth
           or corrugated tube)
  -NM,  nonmetallic sheath cable (moisture resistant, flame retardant)
  -N,   cables with nylon jacket

Here are a few examples:
THWN - Flame retardant, moisture and heat resistant thermoplastic, with
       nylon jacket.  Rated for a maximum operating temperature of 75 degrees
       C and suitable for use in wet locations.

SHHW - Flame retardant, moisture and heat resistant cross-linked synthetic
       polymer with no jacket or covering.  Rated for a maximum operating
       temperature of 90 degrees C for dry and damp conditions, and 75 degrees
       C for wet locations.


	National Electrical CODE 1993.  Quincy:  National Fire Protection
Association, 1993

	Skrotzki, B.G.A., Electric Transmission and Distribution.  New York:
McGraw-Hill Book Company Inc., 1954

Related Links

  1. Electric Power Research Institue
  2. Electrotek
  3. University of North Dakota's Energy & Environmental Research Center
September 20, 1995