External source for rail track worker warning installation

摘要

The external source (1) has a sensor for detecting a train. The sensor closes an electrical contact (8), and couples an alarm voltage to a 2-wire line (9) for electrical connection with a central unit positioned at the working location. An AC voltage generator provides the alarm voltage and a test voltage of different frequency, used for testing the function of the 2-wire line and the central unit. The voltage generator may use a quartz oscillator (2,2') and 2 frequency dividers (3,4 ; 3',4').