DC track circuit

[Shomel122]

Hi,

Im new to signalling and new to the railway. Just wondering how are DC track circuit powered. What are the fault indication if current, resistance or voltage were higher than normal?

[Peter]

Hi,

Im new to signalling and new to the railway. Just wondering how are DC track circuit powered. What are the fault indication if current, resistance or voltage were higher than normal?

Basic DC tracks are fed from a simple transformer rectifier. For typical values, see the range of TC calculation questions in the past IRSE papers.

Some older DC tracks will be fed from a single 1.2v cell. More modern ones have a supply voltage more in the range 10-20v.

Simple track circuits do not have any monitoring as such - the voltage levels are checked on routine maintenance and assessed by the technician. There is no condition monitoring as such. If the supply fails, or drops below the required level, the track relay would drop and the track would show occupied on the signaller's display.

Peter

[PJW]

The only exception I know to Peter's statement is the ACC Ansaldo (Italian) signalling system installed for Manchester South, where the technician form the control centre can certainly get a voltage reading of the volts across relay coil and so can get some indication that a track is likely to be about to fail due say to a combination of wet weather and poor ballast giving low ballast resistance etc.

In the UK at least I believe "this is the exception that proves the rule", although potentially I suppose other monitoring systems could be implemented as a local initiative nowadays, given the more ready applicability of reemote sensing technology.

Reuben may be able to add something.

Hi,

Im new to signalling and new to the railway. Just wondering how are DC track circuit powered. What are the fault indication if current, resistance or voltage were higher than normal?

Simple track circuits do not have any monitoring as such - the voltage levels are checked on routine maintenance and assessed by the technician. There is no condition monitoring as such. If the supply fails, or drops below the required level, the track relay would drop and the track would show occupied on the signaller's display.

Peter

[reuben]

It's worth adding that almost universally today, a "DC" track circuit is powered by a transformer-rectifier set from a 110V AC supply. As there is a separate unit for each track circuit, this creates a "virtual battery" for each track circuit, electrically isolated from all others. The "DC" voltage will therefore actually be full-wave-rectified AC and so very lumpy compared to true DC, but the relay only cares about the mean DC level.

It would be perfectly possible to create a microprocessor-controlled track circuit with the ability to self-adjust for changing conditions or to report faults through a serial link, but this option isn't used in the UK.

I do seem to recall (i reserve the right to be wrong here!) that the new digital TI21 track circuit (EBI track 200) has some sort of self-diagnostic capability. I've also encountered the AF902 track circuit (former US&S product, now Ansaldo STS), which is microprocessor controlled and can interface to the interlocking through a serial link, so this also has that potential. It's not used in the UK.

reuben

[PJW]

Yes I beliebe the Ebitrac does have dagnostic facilities but unaware if these are actually used remotely.

Perhaps the tracks that I know are remrtely monitored at Manchester South ACC are actually AF902 track circuits............

It's worth adding that almost universally today, a "DC" track circuit is powered by a transformer-rectifier set from a 110V AC supply. As there is a separate unit for each track circuit, this creates a "virtual battery" for each track circuit, electrically isolated from all others. The "DC" voltage will therefore actually be full-wave-rectified AC and so very lumpy compared to true DC, but the relay only cares about the mean DC level.

It would be perfectly possible to create a microprocessor-controlled track circuit with the ability to self-adjust for changing conditions or to report faults through a serial link, but this option isn't used in the UK.

I do seem to recall (i reserve the right to be wrong here!) that the new digital TI21 track circuit (EBI track 200) has some sort of self-diagnostic capability. I've also encountered the AF902 track circuit (former US&S product, now Ansaldo STS), which is microprocessor controlled and can interface to the interlocking through a serial link, so this also has that potential. It's not used in the UK.

reuben