ECLYPSE User Guide 130
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 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 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 levels of the differential 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 ECY Series Controller’s S
terminal to provide a signal reference. This shield is grounded at one point only –
see Data Bus Shield Grounding Requirements on page 119.
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 Table 11-9).
Table 11-9: 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 (as shown in Figure 11-18), the voltage developed across it will be 6.4
volts. This effect is called ground lift.
Figure 11-18: Ground Lift from a Long Power Run with a 24VAC Device
Because the 24V COM terminal on ECB series controllers 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
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
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