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In this paper we present the field testing results of a low-cost all-fibre polarimetric current sensor for the monitoring of current in the European high-speed railway network. The sensor fulfils the requirements of robustness, sensitivity, accuracy and cost required for the monitoring of catenary current in changeover sections. Changeover sections are nonfed sections of the catenary that are placed between sections fed with different phases and that introduce discontinuities in the current collection done by the train. Since the train passes at high-speed between the two sections, an electric arc can be formed that may lead to significant damage of the infrastructure. To avoid this situation, it is essential to ensure the switch-off of the current collection before arriving and switch-on again when the changeover section is passed. An adequate protection system that monitors the current in the catenary before the changeover section allows to trigger the necessary protection mechanisms in the infrastructure. Efficient, robust and lightweight electrical current sensors are therefore essential for this security system. The sensor proposed here uses the Faraday magneto-optic Effect with a well-known polarimetric interrogation method. The optical configuration is extremely simplified through the use of few cost-effective, all-fibre devices with a simplified alignment. It allows high sensitivity for low current values, demonstrating a resolution below the ampere level with a dynamic range up to 500 A. The setup is electronically self-referenced to reduce the effect of small misalignments in the polarization, power variations in the optical source, temperature changes, birefringence effect and bending-induced attenuation in the lead fibre. A set of temperature tests in a climatic chamber were also performed in a range of temperature between -20 up to 80ºC, to assess the robustness of the device to operating temperature variations. The field tests include tests during commercial operations and special tests simulating fault conditions.

High sensing resolution in long-distance distributed fiber optic sensors, such as Brillouin Optical Time Domain Analysis (BOTDA), cannot be trivially achieved due to several issues including self-phase modulation, resolution-uncertainty trade-offs and fiber attenuation. These problems could be fixed by the use of differential pulse-pair techniques in combination with Raman amplification. In this work we present a Differential Pulse-width Pair (DPP) Raman-assisted BOTDA sensor, that can achieve 1 meter resolution in a 100 km range.