This is a measure of the degradation of cellulose paper in the windings i.e. as the paper ages its degree of polymerisation (DP) reduces and thus so does its strength as it becomes more brittle. The DP of paper can directly related to the concentration of furan derivatives in the oil which are formed as a direct result of the breakdown of the polymeric structure of cellulose paper. New paper has a DP rating of 1250 while at 250 the paper is sufficiently brittle to fall away from the windings. It is therefore possible to use FFA to estimate the used life of the transformer as well as which will be dependent on factors such stresses on plant, overheating etc. FFA formation can also indicate low temperature overheating.