IEC 60897 (determination of lightning impulse breakdown voltage, LIBV) and IEC 61294 (determination of partial discharge inception voltage, PDIV) are designed with the purpose of evaluating insulating liquids' electrical property in a divergent field, and their current versions have existed for about two decades without revision. In view of the interest in the use of natural and synthetic esters in transformers, the suitability of IEC 60897 and IEC 61294 to evaluate new insulating liquids and the potential revisions are discussed here. Through comparative tests of mineral oil and esters, it is proposed that for IEC 60897, V-T (breakdown voltage-time to breakdown) curve should be obtained at overstressed voltages, i.e. voltages higher than LIBV, preferably under positive polarity, as a complement of LIBV. Similarly for IEC 61294, counting partial discharge (PD) numbers at overstressed voltages, i.e. voltages higher than PDIV, is proposed as a complement of PDIV. Overall, overstressing the liquids helps provide more information about electrical property of insulating liquid and thus can better assess a liquid for the purpose of insulation design. According to the above proposed procedure, both esters are found to be inferior to mineral oil in terms of lightning impulse property and PD behaviour. © 2011 The Institution of Engineering and Technology.