Distech Controls ECLYPSE APEX, Руководство пользователя

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132 ECLYPSE APEX
Power Supply Requirements for 24VAC-Powered
Controllers
BACnet MS/TP is a Three-Wire Data Bus
Even though data is transmitted over a 2-wire twisted pair, all EIA-485 transceivers interpret the volt-
age levels of the transmitted differential signals with respect to a third voltage reference common to all
devices connected to the data bus (signal reference). In practice, this common signal reference is pro-
vided by the building’s electrical system grounding wires that are required by electrical safety codes
worldwide. Without this signal reference, transceivers may interpret the voltage levels of the differen-
tial data signals incorrectly, and this may result in data transmission errors.
ECY-PS100-240 Power Supply is a double-insulated device and therefore is not grounded. The reference for the
BACnet MS/TP data bus is made by connecting the shield of the BACnet MS/TP data bus to the ECLYPSE
Controller’s S terminal to provide a signal reference. This shield is grounded at one point only – see
Data Bus Shield
Grounding Requirements .
Avoid Ground Lift
24V Power wiring runs should not be too long, nor have too many devices connected to it. Wiring used
to supply power to devices has a resistance that is proportional to the length of the wiring run (See the
table below).
AWG Diameter Area Copper wire resistance
Range (mm) (kcmil) (mm
2
) (Ω/km) (Ω/1000 ft.)
14 0.0641 1.628 4.11 2.08 8.286 2.525
16 0.0508 1.291 2.58 1.31 13.17 4.016
18 0.0403 1.024 1.62 0.823 20.95 6.385
Table 17: Resistance of Common Copper Wire Sizes
If the power run from the power supply is relatively long and it supplies power to many devices, a volt-
age will develop over the length of wire. For example, a 1000 ft. of 18 AWG copper wire has a resis-
tance of 6.4 Ohms. If this wire is supplying 1 Ampere of current to connected devices (See the figure
below), the voltage developed across it will be 6.4 volts. This effect is called ground lift.
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
Figure 107: Ground Lift from a Long Power Run with a 24VAC Device
BACnet MS/TP Communication Data Bus Fundamentals