6.7.11 Preventing Hazardous Displays
Hazardous displays arise from failures in the mechanisms that switch power to the signal lanterns. These hazards can be minimised by interlocked switching and/or conflict monitoring.
The general principle is that if a signal group is showing green when it should not, then conflicting signal groups are forced to red, or are switched to flashing yellow.
Interlocked switching is used with relay switching of lamp circuits. Switching a green to one signal group will open the circuit to conflicting green signal groups and close the circuit to the red of these signal groups.
One method of interlocking signal groups is affected by connecting relay contacts in series to create a ‘chain’ from the lamp active supply to the green feeds of the signal groups. When a signal group is switched to green, the green feed to signal groups lower down the chain is open circuited (green interlocking). The groups which are higher in the chain must ensure the red is displayed for groups which are lower in the chain (red interlock). Signal groups which are designed to have an off display should be in the lowest positions in the chain. They do not require a red interlock as this would override the off display.
Conflict monitoring is mandatory where solid state lamp switching is used. The circuits to the signal group colours should be monitored as closely as possible to the controller output terminals. The state of each circuit is compared with a table of conflicting signal groups specified in the personality so that unsafe displays are avoided.
A detailed study of the techniques used is beyond the scope of this Guide. Figure 6.13 indicates the acceptable, undesirable and unsafe lantern displays for conflicting movements shown on the respective axes. The adaptive engineering required is equipment dependent.