Power Supply Requirements For 24Vac-Powered Controllers; Bacnet Ms/Tp Is A Three-Wire Data Bus; Avoid Ground Lift - Acuity Controls nLight ECLYPSE User Manual

BACnet MS/TP Communication Data Bus Fundamentals

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 voltage levels of the trans-
mitted 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 provided 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 lev-
els of the differential data signals incorrectly, and this may result in data transmission errors.
The PS120 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

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
Range
14 0.0641
16 0.0508
18 0.0403
Table 10: 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 voltage will develop over
the length of wire. For example, a 1000 ft. of 18 AWG copper wire has a resistance 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.
AC Power
Source
24 / 120 / 208 /
240 / 277 / 347 /
480 VAC, 1Ø, 2-
Wire plus Ground
Figure 103: Ground Lift from a Long Power Run with a 24VAC Device
Because the 24V COM terminal on BACnet MS/TP controllers is the signal reference point for the data bus, ground lift off-
sets 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.
nLight ECLYPSE
Data Bus Shield Grounding
Area
(mm) (kcmil)
1.628 4.11
1.291 2.58
1.024 1.62
Power Supply
Fuse: 4A Max.
Fast Acting
24 VAC
Electrical
System
Ground – At
Transformer
Only
Requirements.
Copper wire resistance
(mm ) (Ω/km)
2
2.08 8.286
1.31 13.17
0.823 20.95
Electrical Power Run Length
R
eqv
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.
(Ω/1000 ft.)
2.525
4.016
6.385
BACnet Device
24V AC/DC
24V COM
I = 1 A
106