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The principle of the method is 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 calculting the burning time by dividing the distance between the two lines buy the burning time. The method is applicable to all insulating liquids, used and unused, with kinematic viscosity lower than or equal to 300 mm²/s at 40°C; liquids of higher viscosity may also betested but should be heated before impregnation.