A method of analyzing dissolved gas in an oil-filled transformer, comprising: obtaining (1302, 1402) measurements of dissolved gas levels of a first number of gases in the oil-filled transformer; determining (1304, 1404) relative levels of the first number of gases; plotting (1306, 1406) a first number of points (204A-204E) representing each of the relative levels of the first number of gases on a respective one of a first number of radial axes, each of the first number of radial axes being equally angularly spaced around an origin in a two dimensional coordinate system, each of the first number of radial axes representing a relative level of one of the first number of gases, the first number of points forming a polygon within the two dimensional coordinate system; determining (1308, 1408) a centroid of the polygon; determining (1310, 1410) a fault region in which the centroid of the polygon is located, out of a plurality of fault regions defined in the two dimensional coordinate system, wherein the plurality of fault regions are defined in a composite fault region map that is a composite of a first fault region map that defines regions that classify basic electrical faults within the oil-filled transformer, wherein the definition of regions in the first fault region map is not based on carbonization of cellulose within the oil-filled transformer, and a second fault region map that defines regions that classify faults at least partially based on carbonization of cellulose within the oil-filled transformer; classifying a fault experienced by the oil-filled transformer based on the determined fault region within the composite fault region map, characterised in that the first number is five, the polygon comprises a pentagonal shape, the first fault region map comprises a first Duval pentagon, and the second fault region map comprises a second Duval pentagon. obtaining (1302, 1402) measurements of dissolved gas levels of a first number of gases in the oil-filled transformer; determining (1304, 1404) relative levels of the first number of gases; plotting (1306, 1406) a first number of points (204A-204E) representing each of the relative levels of the first number of gases on a respective one of a first number of radial axes, each of the first number of radial axes being equally angularly spaced around an origin in a two dimensional coordinate system, each of the first number of radial axes representing a relative level of one of the first number of gases, the first number of points forming a polygon within the two dimensional coordinate system; determining (1308, 1408) a centroid of the polygon; determining (1310, 1410) a fault region in which the centroid of the polygon is located, out of a plurality of fault regions defined in the two dimensional coordinate system, wherein the plurality of fault regions are defined in a composite fault region map that is a composite of a first fault region map that defines regions that classify basic electrical faults within the oil-filled transformer, wherein the definition of regions in the first fault region map is not based on carbonization of cellulose within the oil-filled transformer, and a second fault region map that defines regions that classify faults at least partially based on carbonization of cellulose within the oil-filled transformer; classifying a fault experienced by the oil-filled transformer based on the determined fault region within the composite fault region map, characterised in that the first number is five, the polygon comprises a pentagonal shape, the first fault region map comprises a first Duval pentagon, and the second fault region map comprises a second Duval pentagon.