Time-domain measurement and spectral analysis of nonstationary low-frequency magnetic-field emissions on board of rolling stock

This paper deals with the assessment of low-frequency magnetic-field emissions (MFE) in the engine-driver cab and the passengers' compartments of rolling stock. For emissions' measurement, an experimental setup is designed, involving a three-axial magnetic-fleld sensor mounted on an antenna positioner, and optically linked to the data-handling system. Measurement is accomplished in the time domain and allows one to retrieve digitized time series of the spatial components of the magnetic-flux density (MFD) vector, at specific points in the measurement volume. Worst case emissions are related to the train operating modes by referring to an experiment that includes acceleration from an initial stop, inertial motion, and braking up to a new stop. The nonstationary behavior of the most relevant MFD sinusoidal components is evidenced, and it is accounted in the spectral analysis by resorting to short-time Fourier transform based on a fixed-length time windows of proper duration. For safety assessment, a compliance check of MFE levels with exposure limits is proposed, based on the computation of an instantaneous safety parameter, obtained by extending requirements developed in the International Commission on Non-Ionizing Radiation Protection guidelines for the simpler case of multiple-frequency steady-state fields.