Circuit Breakers Rated on a Symmetrical Current
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
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
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
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
Linemans and Cablemans 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.