The 10 kHz circuit belongs to a group of tone rail circuits, which do not require isolated sections and expensive equipment to follow them. It is used for control of short sections at the level crossing (it detects the presence of a train in narrow zone of LC, at 15 m average). It operates based on the principle of transmitting a high frequency current of 10 kHz by the transmitter, passing the current through the rails and receiving it on the receiving side. The receiver receives the current and sends it back to the LC housing on the track relay DrL after the voltage correction. When there is no train on the LC, the voltage is 10 VDC and energizes the relay coil. When the train is in the LC zone its shafts shortly connect two rails, causing a voltage drop in the receiving part of the 10 kHz circuit and the DrL relay dropout. The corresponding relay contact prevents the switching off the LC after the train passing over the disconnection contacts. In addition to this function, the 10 kHz circuit has more functions, such as, preventing the activation of the LC when the train, after a long standing (for any reason), moves causing unwanted activation of the LC.