and second to prevent any follow-up power current from flowing to ground Lightning arresters must be installed  on  the  primary  side  of  all  substations, distribution  centers,  distribution  transformers,  and capacitor banks. CONDUCTOR SUPPORTS An important element in any overhead electrical distribution system is a structure that is designed to support  the  weight  of  the  conductors  and  all  equipment mounted on the structure. The structure is also designed to  provide  required  clearances  from  the  ground  to  the conductors  and  between  conductors.  Common  types  of structures  used  for  this  purpose  are  wood  poles, reinforced  concrete  poles,  metal  poles,  and  metal towers.  The  following  text  discusses  poles. Types of Poles Poles used in the Navy can be wood, reinforced concrete,  or  metal  (steel  or  aluminum).  However, concrete and metal poles should be used only when they are more economical or when special considerations warrant  their  use. WOODEN POLES.— Wood poles are available in various types, depending upon species of trees available in the area. For example, yellow pine is commonly used in   the   eastern   United   States.   The   length   and circumference of poles also vary. Poles are available in 5-foot  incremental  lengths  and  with  top  circumferences varying in 2-inch increments. Therefore, we have poles that measure 30, 35, 40 feet, and so on, in length and 17, 19, 21 inches, and so on in top circumference. The classification (or  class) to which a wood pole, of  given  length  and  top  circumference,  belongs  is determined  from  the  circumference  of  the  pole measured at a point 6 feet up from the butt. The class determines the strength of the pole, which is the ability of a pole to resist loads applied 2 feet from the top of the pole. Pole classes are numbered from 1 to 10, with 1 being the strongest. A Class 2 pole, for example, will withstand a force of 3,700 pounds and a Class 4 pole will  withstand  2,400)  pounds  of  force. Wood  poles  are  used  mostly  in  distribution  systems and light-duty transmission lines. The class of pole used depends on what the pole is used for. In other words, is the  pole  to  be  used  as  a  line  pole,  corner  pole,  or transformer pole? The length of pole used is determined, in part, by the clearances required for the voltage of the circuits on the poles, the number of circuits, and the location of the pole in relation to streets, railroads, buildings, and so forth. Clearances are also required to provide safe working conditions for linemen working on the lines. All clearances have minimum requirements that are set by the American National Standards Institute (ANSI) and the National Fire Protection Association (NFPA).  These  requirements  are  specified  in  the National  Electrical  Safety  Code  (NESC),  ANSI  C2-87, and the most recent edition of the  National   Electrical Code^®(NEC^®). Engineers  also  consider  local  conditions  when determining the length of poles. For example, poles located in densely popular high-traffic areas need to be higher than those located in sparsely populated rural areas. In the Navy, the MINIMUM height of a wooden transformer pole is 35 feet and of all other wood poles, 30  feet.  Other  guidance  regarding  the  heights  and classes of poles is found in Power Distribution Systems, MIL-HDBK-1004/2. CONCRETE   POLES.—   Concrete   poles   are preferred  where  the  life  of  wood  poles  is  shortened  by local conditions. Concrete poles may be solid or hollow. Solid concrete poles are made in a trough form with steel reinforcing  rods  running  lengthwise.  The  hollow  type  of pole is made by placing the concrete and reinforcing rods into a cylinder of the desired length and taper and then revolving the cylinder in a lathelike machine. The hollow type is lighter than the solid type and, in addition, provides a means for making connections through the pole to underground cables or services. This technique allows  wires  to  be  concealed  from  view  and  protected from  the  weather. The exterior form of concrete poles can be changed to  meet  almost  any  need.  Gains  (cut  notches)  for crossarms and holes for bolts are cast in the pole. Either metal  pole  steps  are  solidly  cast  into  the  pole  or prethreaded holes for the steps are installed. Although concrete poles last longer and are stronger than wood poles, they are also expensive to make and install. However, the rising cost of wood poles and their treatment and maintenance plus better landscaping have brought on an increased use of concrete poles. METAL POLES.— Metal poles used in the Navy are either steel or aluminum. Steel poles are not used in ordinary  power-line  distribution  circuits  except  for unusual circumstances, such as where there is a high stress or heavy load placed on the pole. Aluminum poles are used for lightweight distribution, such as street- lights. Guying of Poles As  poles  must  be  strengthened  sufficiently  to  carry heavy conductors and pole-mounted equipment, the 2-6