Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 258 of 265
39.5.2 Demand Dependant Phases
All phases which are gaining right of way by the move from stage 1 to stage 2 which
are fixed in stage 2 or are demand dependant and demanded, including those
running gaining phase delays, will also have to appear in stage 3 for the ripple
change to be considered.
Normally a ripple change from “1 to 2” to “2 to 3” will not be allowed if a phase
appears in stage 2 but not in stages 1 and 3. However, the ripple change will be
allowed if there is no demand for the phase and it is configured as demand
dependant since the phase will simply not appear.
It is therefore possible for the controller to ripple change from “1 to 2” to “2 to 3” if a
phase appears in stage 1 and 3 but not 2 (due to a conflict with a demand
dependant phase in stage 2 for example). However, the ripple change will be
delayed until the phase has actually reached no right of way at the end of stage 1,
i.e. until it has completed any losing phase delays and amber leaving periods. This
ensures that the phase is ready to re-appear before the ripple change occurs.
Once the phase has reached no right of way, i.e. red, the ripple change may bring
the phase back to green after only a very short period, unless a gaining phase delay
for the move from 2 to 3 is configured which then would guarantee a minimum red
period.
Even without the ripple change facility, it is common to configure such a gaining
phase delay on the move from stage 2 to stage 3 if the only inter-green to the phase
in stage 3 is from a demand dependant phase in stage 2. The phase delay then
delays the appearance of the phase so that it appears with the rest of the phases in
stage 3 rather than as soon as the move to stage 3 starts. If the demand dependant
phase does actually appear in stage 2, the inter-green from that phase to the phase
in stage 3 would delay its appearance as normal.
39.5.3 Phase Delays
Phase delays specified for phases which terminate or gain right of way by the move
from stage 1 to stage 2 will be unaffected by the ripple change to stage 3 and will
continue to time off.
Phase delays configured on any phases terminated at the end of stage 2 or given
right of way by stage 3 will commence at the start of the move from stage 2 to stage
3.
Phase delays for the move from stage 1 to stage 3 will not be introduced on the
ripple change from “1 to 2” to “2 to 3” since those times would only be applicable if
they were started when stage 1 terminated. The combination of phase delays for the
move 1 to 2 and for the move 2 to 3 must take care of any combined requirements
for the move from 1 to 3.