How is failure of signals guarded against?
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
