The present invention relates to a method for determining a fibre fuse effect in optical networks, in order to identify a fibre fuse effect with certainty and provide warning signals that trigger relief actions. The method is essentially characterised by the use of at least one diffraction grating designed with optical fibres as temperature sensors externally in contact with points of the optical fibre being monitored. The method comprises the following successive steps: checking the half-height width of the pulse as measured in the diffraction grating and comparing it with an expected interval having the values provided by the expression T2 = L/(2v), where T2 is the half-height width, L the diffraction grating length and v the speed of propagation of the thermal wave, or when L/(2v) < 20f, by the expression T2<20/f, where f is the sampling rate of the interrogation system; comparing the time interval between the pulses in the case of two diffraction gratings designed with the value provided by the expression ?t=v/d, where v is the speed of propagation of the thermal wave and d the spacing between the two gratings when the presence of a thermal wave is unambiguously detected; emitting warning signals to activate relief/confinement processes, in particular to warn of an automatic power shutdown of the emitters of the optical gratings. The invention further relates to a monitor for carrying out said method.