Testing of breakdown voltage of transformer oil - Test data dispersion
IEC 156 transformer oil tester
The scatter of individual breakdown voltages has been found to be very dependent on the value of the result. The graphical representation of figure 3 indicates the values of standard deviation/mean ratio which have been found in a large body of test data in several laboratories using transformer oil.

The full line in the graph shows the distribution of the median value of SO/mean as a function of the value of the mean. The dotted lines indicate the expected 95% range of values of SO/mean as a function of the value of the mean.