Specifies a method which is based upon the principle of holding a glass-fibre tape impregnated with the insulating liquid in a horizontal position and igniting it at one end of the impregnated length under draught-free conditions, measuring the time for the flame to travel between two lines marked on the tape, and calculating the burning time by dividing the distance between the two lines buy the burning time. Applicable to all insulating liquids, used and unused, with kinematics viscosity lower than or equal to 300 mm[2]/s at 40 degrees C; liquids of higher viscosity may also be tested but should be heated before impregnation.