The ultimate assessment of reliability comes from insulation system performance in accelerated life testing, where a specified stress (electrical, thermal,
or multifactor) is applied to a group of samples and
the time to failure for each is measured.
Failure is defined as the loss of function.
Acceptance for these tests is generally comparative,
and the life of the insulation system in operation
is not directly obtained from test values because the data are statistically
valid within the range of applied stress values and a significantly
larger confidence interval appears with extrapolation to higher
or lower stresses (Figure 3).
In voltage endurance, a series of coils are subjected to a voltage of typically
2–2.5 UN, with temperature held constant. The time to failure generally
follows a Weibull distribution and is plotted as a function of
applied voltage [15], [16].