The present invention refers to a method and apparatus for testing the insulation of a direct current track circuit .

In the railway industry language, a direct current track circuit is a section of a railway track of variable length (ranging from a few metres to several kilometres) electrically insulated from the adjacent track sections and having a low voltage applied on the set of two tracks. Application of voltage onto the electrified tracks occurs at an end of the track section, while a voltage detector is applied parallel at the end opposite to such power supply point. When there are no vehicles on that track section, the detector extracts a non-zero voltage given that the electrified tracks are not short-circuited by the presence of the train axes, while should any railway vehicle reach that track section it short-circuits the tracks with each other through its axes, and causes current circulation between the axes and thus lower voltage, ideally zero-voltage, which, is calculated by the detector. Thus, such technology allows continuously generating information regarding the presence in a track section for vehicles (standstill or mobile) , and used as it is for automating the control of a railway line. The information received from the direct current track circuit is reliable if electrically insulated from the earth, or from an adjacent direct current track circuit, in that should such insulation deteriorate, the sensor could detect a voltage signal, due to dispersion towards the earth or due to the influence of another direct current track circuit connected, and transmit false circuit unoccupied information. The object of such solution is to allow measuring the insulation resistance of the joints (made up of an electrically insulating material) present between two adjacent direct current track circuits and uniquely identify the joint (or joints) revealing dispersions, in such a manner to be able to intervene through suitable actions and repairs. The measurement environment is extremely noisy - just imagine the traction noise of electric locomotives - and the measurement method must be practically immune to noise signals even of high levels. Document US20060155433 proposes a method and a system for detecting a train on a direct current track circuit wherein an unambiguous code is added to the signal transmitted to the circuit. In particular, added to the voltage signal at the carrier frequency sent on the direct current track circuit is a preset number of information bits which - if identified in the reception step - signal the absence of the train in that direct current track circuit. It is obvious that in a noisy environment like the one described above even this system could fail to signal the correct information in that the added word could erroneously not be received. The present invention overcomes the abovementioned drawbacks by providing a system in which in each of the adjacent direct current track circuits part of the railway line, the alternating low voltage signal is modulated in amplitude and/or in phase with an unambiguous digital code word. Each circuit shall have its own unique digital word. Such encoding is currently used for encoding signals in mobile phones and it is substantially a technique for access, by several sources of information, to the same transmission channel, by using the same frequency band, which consists in associating - to the information transmitted by each of these sources - a code capable of identifying it in an unambiguous manner with respect to the rest and capable of allowing a receiver, to selectively extract information associated to each of them. In such manner, it is possible to monitor whether the insulation level of a predetermined direct current track circuit is satisfactory. Only its unambiguous digital word shall be detected in such case, otherwise if there is deterioration in the insulation, traces of other digital words associated to adjacent circuits shall be detected. By evaluating and calculating the degree of presence of such digital words of other circuits it is possible to estimate the degree of insulation of the circuit in question. An aspect of the present invention regards an apparatus for testing the insulation of a direct current track circuit according to the characteristics of claim 1. A further aspect of the present invention regards a method for testing the insulation of a direct current track circuit according to the characteristics of claim 4.

Characteristics and advantages of the method and apparatus according to the present invention shall be more clear and apparent from the following exemplifying and non-limiting description of an embodiment provided with reference to the attached figure which represents a block diagram of the apparatus according to the present invention.

With reference to the abovementioned figure the apparatus according to the invention is applied on a direct current track circuit 1 which essentially comprises a pair of tracks R made of parallel metal sections B usually T-shaped, mounted transversely with respect to a bearing structure made up of crosspieces made of wood, steel or cement. Each direct current track circuit is separated from the adjacent one through electrically insulating joints G. In a railway line there is a plurality of direct current track circuits adjacent and insulated from each other. The apparatus according to the present invention for each direct current track circuit comprises a transmission block 2 capable of transmitting to the circuit an alternating voltage signal to which an unambiguous code word is associated. Such code word consists in providing a preset number of positive and negative half-waves of such alternating current according to a predetermined sequence. In particular, a so-called spread word S, for example made up of 16 chips, wherein each chip is equivalent to a sinusoidal cycle at the considered frequency, wherein the chip with value "1" provides for the sinusoidal cycle at the considered frequency with zero-phase, while the chip with value "0" provides for the sinusoidal cycle with phase 180°.

Advantageously, an unambiguous digital word of a specific direct current track circuit is orthogonal to the unambiguous code words of the adjacent direct current track circuits.

According to the present invention, the term "orthogonal" is used to indicate that the scalar product between two orthogonal words is zero. Such voltage signal on which the code word is overlapped or associated may be the traditional 50 Hz or 83.3 Hz voltage signal which is used for detecting the passage of the train on that direct current track circuit or it may be a specific voltage signal generated for such purpose. Exploiting such property, the apparatus of the present invention further comprises a block 3 for extracting or detecting such voltage signal and a block 4 for processing such signal . Under normal conditions, such signal must contain the unambiguous word regarding its direct current track circuit or it may contain more than one unambiguous word (in case of deterioration of the insulation of the joints G. By carrying out the scalar product between such signal and the various unambiguous words (the unambiguous word of the circuit in question and those of the adjacent circuits) , the presence of a correct unambiguous word may be verified and - in addition - other power supply signals may be detected, verifying the presence of other unambiguous words hence signalling, identifying and assessing the deterioration of the electric insulation between adjacent direct current track circuits . The extracted voltage signal is sent to the processing block which comprises a microprocessor and at least one analogue/digital converter for such extracted signal . Comparison between the extracted signal and the previously memorised unambiguous codes is carried out in such processing block 4, verifying the presence of the unambiguous word/s received emitting a signal for identifying the words themselves.

Barzanδ & Zanardo Milano S. p. A.