In 1975, CIGRE working group 15.01 assessed over 100 sets of DGA results
from faulty transformers using interpretation schemes that were in use in Europe.
As a result of this assessment, the IEC issued Code 599 [68] in 1978. This code was
based on the Rogers’ ratios, however, the ethane/methane ratio was omitted as it only
covered a limited temperature range of decomposition but did not assist in further
identification of the fault. The diagnosis table is code based with a rating from 0 to 2
being given depending on the value of the ratio (Figure 4.13).