The reference reliability states a component type fraction failure under a defined load or operating condition. This fraction failure will not be exceeded within a specified operating time.
The reference operating condition for power capacitors to which the reliability and failure rate are referred, are the maximum ratings (rated voltage and rated temperature, without de-ratings applied and no humidity considered), unless differently specified at types specifications.
The Failure Rate is expressed in FIT (failure in time), as follows:
1 FIT= 1 x 10-9 / h (1 failure per 109 component hours)
The Failure Rate, when available, is referred to failure rate criteria like short or open circuit, main electrical parameters variation limits and so on, declared in each series specification.
Conversion factor CF can be used to estimate expected failure rate in different conditions. Please refer to General Technical Information document - paragraph B34.
The Lifetime Expectancy of the power capacitors series is, as well, typically referred to the rated voltage Ur and to the hot spot temperature of the capacitor case (the typical reference temperature is stated at type specifications).
The Life Expectancy may be improved derating the operating voltage and/ or the operating temperature.
Capacitors used in power applications are typically exposed to relevant stresses level and possible failure may result in very serious consequences. For this reason, the keeping of a wide safety margin (about 25÷30%) compared to ratings is a wise and long term profitable approach.
Life Expectancy can be expressed as a function of operating voltage and temperature. Please refer to General Technical Information document - paragraph B35.
The obtained estimations are based just on voltage and temperature parameters, NOT considering any other possible stress source, in particular the humidity level. Whichever is the obtained estimation, orders of magnitude higher than the reference value do not represent realistic data: keep a rational approach interpreting the result.
The failure rate estimation anyway prevails as a primary criteria to evaluate the reliability of the component The formulas shall not be used for estimations outside the specification rating limits.
To obtain long life and low failures incidence always consider large enough safety margins on ratings compared to application operating conditions, when choosing capacitors. The above information are referred to the typical behaviour of the single component, net the possible layout of several units connected in series/parallel, which may increase the global risk upon critical operating conditions.