This article deals with problem of carbon brush`s length measurements. There are many applications where regular inspection is not feasible because of a number of factors including, for example, time, labor, cost and disruptions due to down time. Thus, there is a need for a system that can monitor the brush`s length to calculate it`s wear rate, while the component is in operation or without removing of the component from its operational position. We propose a novel method for characterization of carbon brush`s length. This method based on the usage of advantages of the multiplicative response of FBGs and FBG arrays: spectral parameters depend on several aspects, such as grating`s period, refractive index, it`s physical length and so on. We are the first, in our point of view, who proposed to use third parameter for sensing application and prospectively all three parameters for complex measurement: the change of FBG`s length is used to measure length of the brush and it`s wear rate, grating`s central wavelength shift for temperature (due to refractive index change) and mechanical stress (due to grating`s period variations) measurements. The results of modelling and experiments are presented.