Distech Controls ECLYPSE APEX User Manual

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

Distech Controls ECLYPSE APEX Specifications

General

Brand	Distech Controls
Model	ECLYPSE APEX
Category	Control Systems
Language	English