Head Pressure Reference Output (See Fig. 24) --The PIC [II control outputs a 4 to 20 mA signal for theconfigurable Delta P (CONDENSER PRESSURE minusEVAPORATOR PRESSURE) leference cmwe shown in Fig. 24.The DELTA P AT 100% (chiller at maximum load conditiondefault at 50 psi), DELTA P AT 0% (chiller at minimum loadcondition default at 25 psi) trod MINIMUM OUTPUT pointsare configurable in the EQUIPMENT SERVICE-OPTIONS ta-ble. When configuring this output ensure that minimum re-quirements for oil pressure and proper condenser FLASC on-lice performance are mtdntained. The 4 to 20 mA output fromVFD TBI terminals 17 and 18 may be useful as a referencesignal to control a towel bypass valve, tower speed control,condenser pump speed control, etc. Note that it is up to the sitedesign engineering agent to integrate this analog output withany external system device(s) to produce the desired effect.Carrier does not make any claim that this output is directly us-able to control any specific piece of equipment (that is, withoutfurther control elements or signal conditioning), although itmay be.The head pressure reference output will be on whenever thecondenser pump is operating. It may also be manually operatedin CONTROLS TEST. When the head pressure differential isless than the value entered forDELTA PATO%, the output willbe maintained at 4 mA.Lead/Lag Control -- The lead/lag control system auto-matically starts and stops a lag or second chiller in a 2-chillerwater system. A third chiller can be added to the lead/lagsystem as a standby chiller to start up in case the lead or lagchiller in the system has shut down during an ahum conditionand additional cooling is required. Refer to Fig. 17 and 18 formenu, table, and screen selection information.NOTE: The lead/lag function can be configured on the LEAD-LAG screen, which is accessed from the SERVICE menu andEQUIPMENT SERVICE table. See Table 4, Example 20.Lead/lag status during chiller operation can be viewed on theLL_MAINT display screen, which is accessed from theSERVICE menu and CONTROL ALGORITHM STATUStable. See Table 4, Example 12.Lead/Lag System Requirements:• all chillers in the system must have software capable ofperforming the lead/lag function• water pumps MUST be energized from the PIC IIIcontrols• water flows should be constant• the CCN time schedules for all chillers must be identicalOperation Features:• 2chiller lead/lagDELTAPAT100% "3DE LTA PDELTA P__AT0%l NON-ZERO',EXAMPLE OF' M N MUM',REFERENCE',OUTPUTs" a"i t===7 = ..........0mA 2 mA4mA(O%) 4T0 20 mA OUTPUTFig. 24- Head Pressure Reference Output]1120 mA(100%)• addition of a third chiller for backup• manual rotation of lead chiller• load balancing if configured• staggered restart of the chillers after a power failure• chillers may be piped in parallel or in series chilled waterflowCOMMON POINT SENSOR USAGE AND INSTALLA-TION- Lead/lag operation does not require a commonchilled water point sensor However. common point sensors(Spare Temp #1 and #2) may be added to the CCM module, ifdesired.NOTE: If the common point sensor option is chosen on achilled water system, each chiller should have its own commonpoint sensor installed. Each chiller uses its own common pointsensor for control when that chiller is designated as the leadchillel: The PIC III cannot read the value of common pointsensors installed on the other chillers in the chilled watersystem.If leaving chilled water control (ECW CONTROL OPTIONis set to 0 [DSABLEI TEMP CTL screen) and a commonpoint sensor is desired (COMMON SENSOR OPTION inLEADLAG screen selected as 1) then the sensor is wired inSpare Temp #1 position on the CCM (terminals J4-25 andJ4-J26).If the entering chilled water control option (ECW CON-TROL OPTION) is enabled (configured in TEMP_CTLscreen) and a common point sensor is desired (COMMONSENSOR OPTION in LEADLAG screen selected as 1) thenthe sensor is wired in Spare Temp #2 position on the CCM(terminals J4-27 and J4-28).When installing chillers in series, either a common pointsensor should be used (preferred), or the LEAVING CHILLEDWATER sensor of the upstremn chiller must be moved into theleaving chilled water pipe of the downstream chillel: In thisapplication the COMMON SENSOR OPTION should only beenabled for the upstremn chiller if that chiller is configured asthe Lead.If ENTERING CHILLED WATER control is required onchillers piped in series, either the common point return chilledwater sensor should be used (preferred), or the LEAVINGCHILLED WATER sensor of the downstream chiller must berelocated to the LEAVING CHILLED WATER pipe of theupstream chiller. In this application the COMMON SENSOROPTION should only be enabled for the downstream chiller ifthat chiller is configured as the Lead. Note that ENTERINGCHILLED WATER control is not recommended for chillersinstalled in series due to potential control stability problems.To properly control the LEAVING CHILLED WATERTEMPERATURE when chillers me piped in parallel, the waterflow through the shutdown chiller(s) should be isolated so thatno water bypass around the operating chiller occurs. However.if water bypass around the operating chiller is unavoi&_ble, acommon point sensor in the mixed LEAVING CHILLEDWATER piping should be provided and enabled for the Leadchillel:CHILLER COMMUNICATION WIRING -- Refer to thechiller's Installation Instructions, Cmrier Comfort NetworkInterface section for infommtion on chiller communicationwiring.LEAD/LAG OPERATION- The PIC III not only has theability to operate 2 chillers in lead/lag, but it can also start adesignated standby chiller when either the lead or lag chiller isfaulted and capacity requirements are not met. The lead/lagoption only operates when the chillel_ are in CCN mode. If anyother chiller configured for lead/lag is set to the LOCAL orOFF modes, it will be unavailable for lead/lag operation.44