Several authors have published work related to detection of fault in high voltage transformers using Sweep Frequency Response Analysis (SFRA), Wavelet and Coherence. The work published in this paper describes the result of analysis of neutral current during reduced and full impulse voltage test of 400 KV transformers with detailed analysis using Coherence Function (CF). It is aimed at determining the variation of CF due to change in neutral current for transformers which have either passed or failed during certification test. In order to establish criteria, theoretical simulation was carried out initially to determine the nature of CF for different types of noise superimposed on impulse wave. The analysis was done for high voltage transformer by recording neutral current during calibration and full impulse voltage test of high voltage winding, using data obtained during testing. Of the three types of transformers, two have passed the test and one has encountered major failure. For the purpose of analysis the two outputs i.e. neutral currents at reduced and full voltage with appropriate scaling were used. The assumption is made that for similar inputs, outputs must remain similar as long as the equivalent electric circuit remains unchanged. In the event of any change by way of fault, electromagnetic interference and jitter, the CF is likely to be less than unity.