How is failure of signals guarded against?

July 14, 2019, 3:09 PM

Signal installations are designed as far as possible for fail-safe operation, which means that any failure should leave the system in a state where dangerous train movements are not allowed. For instance, in case of a failure detected at a panel interlocking installation, all signals controlled by it are designed to revert to On. Similarly, a failure detected in the block control circuit at the Starter signal causes all signals to the rear guarding the approach to the block section switch to On, and notification is sent automatically to the control centre or signal cabin.

The signals themselves have two-filament bulbs, or two-bulb assemblies for each lamp, to provide redundancy in case of a filament burning out. Where incandescent bulbs are used, the filaments are kept warm even when the lamp is off, through the passage of a small current which prevents thermal shocks on switching on the lamp and thereby reduces the chances of failure. The signals are also frequently examined and bulbs replaced in a pre-emptive manner.

There is also a trend towards using LED panels instead of incandescent bulbs for the greater safety they afford (since several LEDs on a panel can fail without compromising the safety of the signal) as well as for the power savings involved. Normally, a current relay also detects the current flowing in the signal lamp in its different states, and this allows detection of a failed lamp. (Even in the days of kerosene lamps for signals, a bimetallic thermal contact strip was used to detect the heat of the lamp and notify the signalman if the lamp was extinguished.)

Back lights for electric signals today (and small slits in the rear of kerosene-lamp signals in days gone by) allow the signalman or stationmaster to see the states of the signals at a station. Where visibility limits the use of back lights, the signal aspect is repeated in the signal cabin or (at small stations) in the station master’s office.

In the latest instances of signalling control by means of interlinked stations (e.g., Chennai – Washermanpet), failure-detection circuits are provided for each track circuit and signal circuit with notification to the signal control centres in case of problems.

Signal installations are usually powered by independent power supplies (DC) that are driven by battery installations that are charged from the regional grid (state electricity board’s supply). All the failsafe equpment and the signals themselves also have emergency fail-over to backup battery sets that keep the signals and points working in case of power failure. Most stations also have diesel generator sets to continue charging the batteries in case of power failure.

Source – IFRCA.org

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