Distech Controls ECB-VAV, User Manual

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42 Network Guide
Figure 25: Ground Lift from a Long Power Run with a 24VAC Device
If the 24V COM terminal is the signal reference point for the data bus, ground lift offsets the
data bus voltage reference that is used to interpret valid data levels sent on the data bus. If the
ground lift is more than 7 volts peak, there is a risk of data corruption and offline events due to
the device being incapable of correctly reading data signals from the data bus. Thus it is
important to keep the power supply (transformer) as close to the controller as possible.
Techniques to Reduce Ground Lift
Reduce the impact of ground lift as follows:
• Use a heavier gauge wire.
• Use a 3-wire power connection when available.
• Add more wire runs. Connect these wire runs to the power supply in a star pattern.
• For controllers that accept DC power (that is, models without triac outputs): Specify a
24VDC power supply. The continuous and even voltage of a DC power supply makes
more efficient use of the power handling capabilities of a power run. A 24VDC power sup-
ply eliminates the 2.5 multiplication factor associated with the peak AC current being 2.5
times the average RMS AC current. See below.
About External Loads
When calculating a controller's power consumption to size the 24VAC transformer, you must
also add the external loads the controller is going to supply, including the power consumption
of any connected subnet module (for example, for Allure Series sensors). Refer to the respec-
tive module's datasheet for related power consumption information.
A controller can support a maximum of two Allure Series sensor models equipped with a CO2
sensor. See
Subnetwork Module Compatibility and Supported Quantity Charts on page 71 for
how many Allure EC-Smart-Vue sensors are supported by a given controller model. The
remaining connected Allure Series sensor models must be without a CO
2
sensor.
For the ECB-VAV model, see the room device calculator spreadsheet available for download
from our website to determine the power requirements and supported quantities of connected
subnet modules: VAV-Smart Room Control Device Calculator.xlsm
The equivalent resistance of a run of wire. For
example, a 1000 ft of 18 AWG copper wire
has a resistance of 6.4 Ohms. With 1 Ampere
of current, the voltage developed along the
length of the electrical power run for one
conductor will be 6.4 volts.
24V COM
24V AC/DC
BACnet Device
R
eqv
Electrical Power Run Length
I = 1 A
AC Power
Source
24 / 120 / 208 /
240 / 277 / 347 /
480 VAC, 1Ø, 2-
Wire plus Ground
Power Supply
24 VAC
Fuse: 4A Max.
Fast Acting
Electrical
System
Ground – At
Transformer
Only