Fault diagnosis for LTCs based on DGA data alone is not an exact science and needs to be undertaken with
caution. An abnormal value of the ethylene/acetylene or TDHG/acetylene ratio usually indicates coking or
deterioration of arcing contacts. Gas concentrations exceeding their caution limits — especially if there is
more than one gas involved — often indicate abnormal tap changing activity or some kind of fault.
Comparison with case histories of similar LTCs, if available, may provide a means of arriving at a better
diagnosis.