The sensor allows the instantaneous detection of the first traces of fluid at drain outlet of a rotary union which is about to collapse, allowing the instantaneous detection of the rotary union breakage and preventing in this way greater damages, It is supplied already programmed as per operating features set at order stage. Every fluid passage causes the lighting of a LED and an external signal of fault. The sensor is delivered in a protective case so that can be installed in zones which are critical for the presence of spurts or oil fogs. The checking reference value is set at supply stage and it is fixed.