General Information (Continued)
3S69 is a binary input device used on all rooftop units equipped with an exhaust fan. It is located in the exhaust fan section of the unit and is connected to the RTM (1U48). During a request for fan operation, if the differential switch is detected to be open for 40 consecutive seconds, the economizer is closed to the minimum position setpoint, the request for exhaust fan operation is turned "Off" and locked out, and a manual reset diagnostic is initiated. The fan fail- ure lockout can be reset; at the Human Interface located in the unit's control panel, by Tracer, or by cycling the control power to the RTM (1S70 Off/On).
Lead-Lag
Is a selectable mode of operation on 40 thru 130 Ton units within the Human Interface. It alternates the starting be- tween the first compressor of each refrigeration circuit. Only the compressor banks will switch, not the order of the com- pressors within a bank, providing the first compressor in each circuit had been activated during the same request for cooling.
Supply and Exhaust Fan Circuit Breakers (1CB1, 1CB2) The supply fan and exhaust fan motors are protected by cir- cuit breakers 1CB1 and 1CB2 respectively. They will trip and interrupt the power supply to the motors if the current exceeds the breaker's "must trip" value. The rooftop module (RTM) will shut all system functions "Off" when an open fan proving switch is detected.
Low Pressure Control
Is accomplished using a binary input device on CV & VAV applications. LP cutouts are located on the suction lines near the scroll compressors.
The LPC contacts are designed to close when the suction pressure exceeds 22 ± 4 psig. If the LP control is open when a compressor is requested to start, none of the com- pressors on that circuit will be allowed to operate. They are locked out and a manual reset diagnostic is initiated.
The LP cutouts are designed to open if the suction pressure approaches 7 ± 4 psig. If the LP cutout opens after a com- pressor has started, all compressors operating on that cir- cuit will be turned off immediately and will remain off for a minimum of three minutes.
If the LP cutout trips four consecutive times during the first three minutes of operation, the compressors on that circuit will be locked out and a manual reset diagnostic is initiated.
Saturated Condenser Temperature Sensors (2RT1 and 2RT2)
Are analog input devices used on CV & VAV applications mounted inside a temperature well located on a condenser tube bend. They monitor the saturated refrigerant tempera- ture inside the condenser coil and are connected to the SCM/MCM (1U49). As the saturated refrigerant temperature varies due to operating conditions, the condenser fans are cycled "On" or "Off" as required to maintain acceptable op- erating pressures.
Head Pressure Control
is accomplished using two saturated refrigerant tempera- ture sensors on CV & VAV applications. During a request for compressor operation, when the condensing tempera- ture rises above the "lower limit" of the controlband, the Compressor Module (SCM/MCM) starts sequencing con-
denser fans "On". If the operating fans can not bring the condensing temperature to within the controlband, more fans are turned on. As the saturated condensing tempera- ture approaches the lower limit of the controlband, fans are sequenced "Off". The minimum "On/Off" time for condenser fan staging is 5.2 seconds. If the system is operating at a given fan stage below 100% for 30 minutes and the satu- rated condensing temperature is above the "efficiency check point" setting, a fan stage will be added. If the satu- rated condensing temperature falls below the "efficiency check point" setting, the fan control will remain at the present operating stage. If a fan stage cycles four times within a 10 minute period, the control switches from control- ling to the "lower limit" to a temperature equal to the "lower limit" minus the "temporary low limit suppression" setting. It will utilize this new "low limit" temperature for one hour to reduce condenser fan short cycling.
High Pressure Controls
High Pressure controls are located on the discharge lines near the scroll compressors. They are designed to open when the discharge pressure approaches 405 ± 7 psig. The controls reset automatically when the discharge pressure decreases to approximately 300 ± 20 psig. However, the compressors on that circuit are locked out and a manual re- set diagnostic is initiated.
Outdoor Air Humidity Sensor (3U63)
Is an analog input device used on CV & VAV applications with 100% economizer. It monitors the outdoor humidity lev- els for economizer operation. It is mounted in the fresh air intake section and is connected to the RTM (1U48).
Return Air Humidity Sensor (3U64)
Is an analog input device used on CV & VAV applications with the comparative enthalpy option. It monitors the return air humidity level and compares it to the outdoor humidity level to establish which conditions are best suited to main- tain the cooling requirements. It is mounted in the return air section and is connected to the ECEM (1U52).
Low Ambient Control
The low ambient modulating output on the compressor module is functional on all units with or without the low am- bient option. When the compressor module has staged up to it's highest stage (stage 2 or 3 depending on unit size), the modulating output will be at 100% (10 VDC). When the control is at stage 1, the modulating output (0 to 10 VDC) will control the saturated condensing temperature to within the programmable "condensing temperature low ambient control point".
Status/Annunciator Output
Is an internal function within the RTM (1U48) module on CV
&VAV applications that provides;
a.diagnostic and mode status signals to the remote panel (LEDs) and to the Human Interface.
b.control of the binary Alarm output on the RTM.
c.control of the binary outputs on the GBAS module to inform the customer of the operational status and/or diagnostic conditions.
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