Circuit Breakers Rated on a Symmetrical Current Basis. Switching devices, as discussed below, have a separate  capacitive  switching  rating  for  the  reasons mentioned above; and the switching rating of the device must be at least 135 percent of the capacitor bank rating to which the switching device is connected. This 135 percent rating is a minimum specified by the National   Electrical   Code(NEC)^®   and   includes allowance for operation at overvoltage, allowance for capacitance manufacturing tolerance, and allowance for harmonic components above the fundamental frequency. CONSTRUCTION. —A  capacitor  unit  consists of two aluminum foil strips, or plates, with a thin high- grade  insulating  paper  or  a  synthetic  film  placed between them. The strips, or plates, are compactly wound and connected in groups, each of which is connected to a terminal. There is no contact between the two metal surfaces. When these two surfaces are connected to a source of power, energy is stored in the capacitor. The capacitor remains charged at, or above: full-line voltage when disconnected from the source of power until a discharge path is provided between the terminals. Capacitors have a built-in discharge resistor designed to drain off or reduce this residual charge. NEC^® requires capacitors rated 600 volts or more to be discharged to a residual voltage of 50 volts or less in 5 minutes.  Since  the  built-in  resistor  has  the disadvantage  that  it  cannot  be  visually  inspected  for  an open circuit, it should not be relied upon for positive drain off of the residual charge. The wound plates and discharge resistor of a capacitor are enclosed in a welded sheet steel or stainless steel container, which is hermetically  sealed  to  protect  the  capacitor  from deterioration  due  to  entrance  of  foreign  material  or moisture. The contents are vacuum dried and are usually impregnated with a dielectric fluid. As of 1 October  1977,  dielectric  fluids  containing polychlorinated  biphenyls  (PCBS)  can  no  longer  be installed. The connecting leads from the capacitor are brought up through the bushings to a joint at the top directly  under  the  brazed  terminal.  The  bushings supplied on capacitors are usually made of porcelain. As of 1 October 1988, existing PCB capacitors in unrestricted areas must be removed. TYPES  OF  INSTALLATIONS.—The greatest electrical   benefits   are   derived   from   capacitors connected  directly  at  the  loads.  This  connection permits maximum loss reduction and released line capacity.   However,   economics   and   physical limitations  are  usually  the  governing  factors. Capacitors may be divided into two classes: primary capacitors   and   secondary   capacitors.   Primary capacitors are those rated 2,400 volts and above. Secondary  capacitors  are  those  used  on  the  low- voltage side of distribution transformers or at motor terminals and are normally rated 600 volts and below. The three most common types of power capacitor installation  are  as  follows:  pole  mounted,  metal enclosed, and open rack. Pole Mounted.—Pole-mounted  capacitors  are packaged  as  a  complete  unit  containing  all  necessary items  for  a  switched  distribution  capacitor  bank installation. The banks consist of an aluminum- or steel-mounting frame that supports the capacitor units, interconnecting   wiring,   and   capacitor   switches. Overcurrent, protection is usually provided by group fuses. Metal   Enclosed.—Metal-enclosed    capacitor banks   consist   of   a   factory-assembled   group   of individual capacitor units mounted in a protective housing complete with bus connections, controls, and protective   and   switching   equipment   within   the enclosure. Personnel safety and compactness are the major  benefits.  Each  capacitor  unit  normally  is protected by an individual current-limiting fuse. Open Rack.—An  open-rack  capacitor  installation (fig. 4-29) is a field-assembled group of capacitor units mounted in an open-rack structure without enclosing plates or screens. Open-rack installations normally are made up of several stack type of capacitors connected in  parallel  to  provide  desired  kvar  capacity.  All  the units in a given stacking unit are normally connected in parallel with the steel frame forming one terminal and the insulated bus forming the other. For open-rack installations  the  capacitor  units  are  protected  by individual fuses, group fuses or relays, and a circuit breaker. CAPACITOR   MAINTENANCE.—All switched capacitor banks should be inspected and checked for proper operation once each year before the time  period  when  they  are  automatically  switched  on and off to meet system requirements. A suggested reading  source  for  capacitor  maintenance  is  The Lineman’s and Cableman’s Handbook. Capacitor-bank oil switches should be maintained on a schedule related with the type of on/off controls installed at each bank. The maximum number of open and close operations between maintenance of the switches   normally   should   not   exceed   2,500. 4-19