Environmental concern, safety and sustainability of oil supply are the driving forces behind research into alternative dielectric fluids to mineral oil. Natural and synthetic esters, as the alternative fluids, need to be assessed under thermal and electrical fault conditions, and their performance and likely impact on the established diagnostic techniques need to be identified. In this respect, dissolved gas analysis (DGA) and online DGA monitoring play a vital role for mineral oil filled transformers, and they are the techniques examined in this paper. Tests were designed to produce electric faults in ester fluids in a sealed vessel. Both the headspace gas and dissolved gas in oil results were used to calculate the generated fault gas amount. Although DGA readings were lower than mineral oil, Envirotempreg FR3reg generated significant amount of free gases, allowing easy detection of a fault by use of a Buchholz alarm or similar. Midelreg 7131 was the most stable fluid in terms of gas generation during these tests. With improved fluid circulation, the response time of HYDRANreg 201R was identified to be the same as with esters as with mineral oil.