API Capacitors’s definition of terms, where applicable, are within the context of IEC 61071 (Power Electronics Capacitors) and IEC 61881 (Capacitors for Power Electronics used on Rolling Stock), summarised below:

  1. Capacitance: The ability of a capacitor to store electrical charge.
  1. Dielectric: An insulating material placed between the plates of a capacitor.
  1. Electrode: The conductive plates of a capacitor.
  1. ESR (Equivalent Series Resistance): The total internal resistance of a capacitor, including the resistance of the conductive plates and the dielectric material.
  1. Farad: The standard unit of capacitance, defined as the amount of charge that can be stored per unit of voltage.
  1. Leakage current: The small amount of current that flows through a capacitor even when it is fully charged.
  1. Polarisation: The orientation of the electrodes in a polarised capacitor, which can only be connected to a DC voltage source in one direction.
  1. Ripple voltage: The small, fluctuating voltage that remains after a power supply has been filtered by a capacitor.
  1. Self-healing: The ability of some capacitors to repair small defects in their dielectric material, which can prevent catastrophic failure.
  1. Tolerance: The maximum allowable deviation from the rated capacitance value for a given capacitor.
  1. Voltage rating: The maximum voltage that a capacitor can withstand without breaking down.
  2. X/Y Capacitors: Special types of capacitors used for EMI/RFI suppression in electrical circuits. They are rated for use in high-voltage and high-frequency applications.
  3. UDC: dc voltage: The normal dc voltage that may be continually applied to the capacitor for a lifetime expectancy of 100,000 hours at 70°C ambient.
  4.  UN: Peak repetitive voltage: The maximum operating peak voltage of either polarity but of a non-reversing type wave form, for which the capacitor has been designed for continuous operation.
  5. US: Peak non-repetitive voltage: A peak voltage induced by a switching or any other disturbance of the system which is allowed for a limited number of times and for durations shorter than the basic period.

    It is specifically equivalent to the Vdrm (Ugto) of GTO devices for the GT and GC ranges of capacitors.

  6. IMAXMaximum current: The maximum rms current for continuous operation.

  7. IP: Maximum peak current: The maximum current amplitude that occurs instantaneously during continuous operation. Equivalent to Î as defined in IEEC 61071.
  8. IS: Maximum surge current: The admissible peak current induced by a switching or any other disturbance of the system which is allowed for a limited number of times.
  9. R0: (See IEC 61071)
  10. 0amb: Cooling air temperature: The temperature of the cooling air measured at the hottest position in the bank, under steady state conditions, midway between two units. If only one unit is involved, it is the temperature measured at a point approximately 0.1 m away from the capacitor container and at two-thirds of the height from its base.
  11. L: (See IEC 61071)
  12. Rs: Associated series resistance that contributes to the I²R losses of the capacitor.