Carrier 19XL - Manual (Page 100)

requested to start. After the second chiller is started and is running, the lead chiller shall monitor
conditions and evaluate whether the capacity has reduced enough for the lead chiller to sustain
the system alone. If the capacity is reduced enough for the lead chiller to sustain the CONTROL
POINT temperatures alone, then the operating lag chiller is stopped.
If the lead chiller is stopped in CCN mode for any reason other than an alarm (*) condition,
then the lag and standby chillers are stopped. If the configured lead chiller stops for and alarm
condition, then the configured lag chiller takes the lead chiller’s place as the lead chiller and the
standby chiller serves as the lag chiller.
If the configured lead chiller does not complete the start-up before the PRESTART FAULT
TIMER (user configured value) elapses, then the lag chiller shall be started and the lead chiller
will shut down. The lead chiller then monitors the start request from the acting lead chiller to
start. The PRESTART FAULT TIMER is initiated at the time of a start request. The PRESTART
FAULT TIMER’s function is to provide a timeout in the event that there is a prestart alert
condition preventing the machine from starting in a timely manner. The timer is configured
under Lead/Lag, found in the Equipment Configuration table of the Service menu.
If the lag chiller does not achieve start-up before the PRESTART FAULT TIMER elapses,
then the lag chiller shall be stopped and the standby chiller will be requested to start, if
configured and ready.