Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 154 of 265
16.3.2 Changes from a Lower to a Higher Priority Level
A change from Bus Priority mode to Emergency Vehicle mode will occur on receipt
of a priority demand or extension for an Emergency Vehicle priority unit.
A change from other lower priority modes to either Bus Priority or Emergency
Vehicle mode (as required) will occur on receipt of a priority demand or extension
for the priority unit.
Emergency vehicle priority demands and/or extensions which are received whilst an
‘inhibit period’ on a bus priority unit is running, will be serviced immediately, subject
to normal safety periods.
On a change from the bus priority level to the emergency vehicle priority level, all
stored priority demands for phases, inhibit and compensation periods, at the bus
priority level may be cancelled as the controller cycles to service the emergency
vehicle request. Normal VA demands will be stored as usual.
Should an emergency vehicle priority extension request be received for the running
phase whilst a bus priority extension period is running, the priority maximum period
for the emergency vehicle priority level will commence from the expiry of the normal
maximum running period as normal. However, should the emergency vehicle priority
extension request be received after the normal maximum period has expired and the
bus priority maximum has started, the emergency vehicle priority maximum running
period will commence from the receipt of the emergency vehicle level priority
extension.
16.3.3 Changes from Emergency Vehicle to Bus Priority Level
When all the conditions and requirements pertaining to the emergency vehicle
priority facilities have been serviced, the controller will return to normal operation, or
to bus priority mode if bus demands have been stored.
If a change from the emergency vehicle priority level occurs after the normal
maximum running period has expired, a ‘max change’ to the next demanded stage in
cyclic order will then occur, unless a bus extension is present or a bus demand
stored for another phase.
If the change from the emergency vehicle priority level occurs while a phase is at
right of way and before the normal maximum running period has expired, the receipt
of a bus priority extension request on the phase will introduce a bus priority
extension and priority maximum running period.
16.3.4 Other Changes of Level
If the required Bus or Emergency vehicle priority mode is configured higher than
UTC mode in the mode priority table, force signals from the UTC Instation will be
ignored while the priority mode is active.