Analog Input Cable Specifications; Digital Inputs - McQuay MicroTech II Installation Manual

When variable speed pump control is used, the chilled water loop differential pressure transducer allows the CSM to
change pump speed as required to maintain the desired pressure across the chilled water loop using a PI (proportional-
integral) control loop.
When sequenced pump control is used, the chilled water loop differential pressure transducer allows the CSM to maintain a
constant pressure difference between the supply and return lines by staging pumps on and off.
To provide the most efficient operation of cooling load pumps, the location of the differential pressure sensor is important.
When the load consists of multiple cooling coils equipped with two-way control valves, and the piping is in a direct return
configuration, the differential pressure sensor should be located at the far end of the system (across a cooling coil far away
from the pumps).
Two loop differential pressure sensors may be required when you have a large system with reverse return piping (install
one at each end of the loop). Computer centers and other critical loads may also require a dedicated differential pressure
sensor located close to them to ensure that these loads receive adequate flow regardless of the flow requirements of the rest
of the system. If a second differential pressure sensor is required, it can be connected to UI 9.
3. Spare Sensor (UI 9)
UI 9 provides input for a variety of uses. If a second chilled water loop differential pressure transducer is required, it
should be wired to UI 9. The CSM compares the differential pressure signals from both UI 8 and UI 9 and determines the
control pressure to be the one with the largest deviation from setpoint.
4. External Chilled Water Reset Signal (UI 10)
The external water temperature reset option resets each chiller's leaving evaporator water temperature according to a 2–10
VDC, or 4–20 mA signal. If the external reset signal is less than or equal to 4 mA (2 VDC), the System Setpoint will equal
the Minimum System Setpoint (adjustable on the CSM's User Interface). If the external reset signal equals 20 mA (10
VDC), the System Setpoint will equal the Maximum System Setpoint (adjustable on the CSM's User Interface). For more
on reset, refer to Reset in the Chilled Water Temperature Control section of OM 780-MicroTech II Chiller System
Manager Operation Manual.
5. External Demand Limiting Signal (UI 11)
An external demand limit signal (2–10 VDC or 4–20 mA) can be used to provide demand limiting for all chillers included
in a CSM network. Demand limiting will prevent chillers from operating above a specified capacity (% RLA for
centrifugal, stages for scroll and screw). As the demand-limiting signal varies between 2-10 VDC (or 4–20 mA) the % RLA
or maximum number of stages available in each chiller varies. For more on demand limiting, refer to Demand Limiting
section of OM 780-MicroTech II Chiller System Manager Operation Manual.
Note: A demand limiting value can also be provided by a BAS.
6. Relative Humidity Signal (UI 12)
A relative humidity sensor connected to the CSM can display relative humidity, dew point, enthalpy and wet-bulb values on
the CSM's User Interface. When a relative humidity sensor is used, an outside air temperature sensor integral to the sensor
should be attached to UI 5 and used as the source of the OAT to provide a more accurate calculation of dew point, enthalpy
and wet-bulb by the CSM.
The cooling tower constant approach reset option uses the outside air relative humidity and outside air temperature signals
to calculate an outside air wet-bulb value. This optional feature can then be used to reset the cooling tower Stage 1
Setpoint to the wet-bulb temperature plus a differential. For more on cooling tower constant approach reset, refer to the
Cooling Tower Control section of OM 780-MicroTech II Chiller System Manager Operation Manual.

Analog Input Cable Specifications

The cable for analog inputs must meet the following cable specifications for minimum requirements: twisted, shielded with
drain wire, 300 V, 60°C, 20 AWG, polyethylene insulated, with a PVC outer jacket. Depending on the application, either
two conductors (Belden 8762 or equivalent) or three conductors (Belden 8772 or equivalent) are required. Note that some
local codes or applications may require the use of plenum rated cable. Do not install the cable in the same conduit with
power wiring.

Digital Inputs

CSM onboard universal Inputs 13 to 16 are set to receive digital inputs from external dry contacts.
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