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 light-load conditions. Sometimes the capacitor banks are installed to enable incremental
switching, depending on the system reactive requirements and the amount of system voltage
required. To remove a switched capacitor bank from service, the control box should be opened
and the automatic control lever or control switch should be placed in the off position. The circuit
breaker or the switching device should then be tripped. To ensure the circuit breaker or
switching device remains open, the fuses should be removed from the control circuit. Before it
can be assumed that the capacitor bank has been deenergized, the position of the switching
device should be inspected. On a circuit breaker, the position indicator should be checked. For
oil switches, the position of the operating handle can be checked with a switch stick.
3.7.7 Types of Switching Devices. Switching capacitors imposes severe duty on switching
devices because of the differences in phase relationship between the current and voltage on a
capacitor circuit. When a capacitor bank is energized, high transient overvoltages and
overvoltages may easily be three times the rated line voltage, and transient inrush currents may
approach the short-circuit current duty values. These factors are especially important when one
or more capacitor banks is already energized and another one at the same location is switched on
to the bus. The methods for determining the values of inrush current, transient overvoltage and
resonant frequency of the circuit are discussed in more detail in ANSI C37.99, IEEE Guide for
Protection of Shunt Capacitor Banks, and ANSI C37.012, Application Guide for Capacitance
Current Switching of AC High-Voltage 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 rating is a minimum
specified by the National Electric Code, and includes allowances for operation at overvoltage,
allowance for capacitance manufacturing tolerance, and allowance for harmonic components
above the fundamental frequency. Some common types of switching devices used on capacitor
banks are discussed in the following paragraphs.
3.7.7.1 Indoor Circuit Breakers. Metal-enclosed capacitor equipment on circuits 13.8 kV,
or below, often use indoor air or oil circuit breakers. These breakers are housed in a separate
compartment to protect them from the weather and to increase operating personnel safety.
3.7.7.2 Outdoor Circuit Breakers. On large open-rack outdoor installations, outdoor oil
circuit breakers are usually used. In some cases, the interrupting chambers of these breakers is
modified to ensure proper switching of the capacitive load by controlling prestrikes when
energizing or restrikes when interrupting.
3.7.7.3 Oil Switches. Single-phase or three-phase oil switches are often used on
pole-mounted or metal-enclosed capacitor banks. These switches are generally solenoid or
motor-operated and do not have fault interrupting capability.
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