3.2 Bus Cabling
3.2.1 EMC Precautions
The following EMC precautions are recommended to achieve
interference-free operation of the BACnet network.
Additional EMC information is available in the
VLT
®
HVAC
Drive Design Guide, MG.11.Bx.yy
. Please also consult the
BACnet master manual for further installation guidelines.
Ensure compliance with relevant national and local
regulations, for example in protective earth connection.
3.2.2 Single Ground Shielding
For installing the bus cable on MS/TP, two different
strategies can be followed, Single ground of shield and
multiple ground of shield. Each strategy has both
advantages and disadvantages. The following chapter
explains the different between the two strategies. The single
ground shield is specified in the ANSI/ASRAHE 135-2004
standard. The solution benefits by having only one ground
connection of the shield, by doing so the possibility for
ground loop of equalizing current is heavily reduced. In
these systems the shield of the MS/TP cables has to be
isolated from ground at all stations, except one. At each
station the shield from the two cables has to be connected
with each other, and isolated from ground. The best solution
for this has been proven to be the use of shrink tubes. The
single ground shielding is a good approach where the
system uses long bus cables. If two buildings have to be
connected over the same MS/TP bus cable, the use of fibre
optic has to be considered. This will prevent that a lightning
stroke will be carried from one building to another, and
problem with difference in earth potential can be neglected.
3.2.3 Multiple Ground Shielding
If the distance between the individual drives is limited (e.g.
inside a cabinet or in one control room) Danfoss
recommends connecting the screen to ground at both ends
of the bus cable. This ensures the maximum protection from
EMC noise. Connecting the screen at each end will require
that each BACnet device has the same earth potential or an
equalizing current will flow in the screen of the cable and
cause disturbance and poor performance of the system. Low
impedance to ground connection of the screen can be
achieved by connecting the surface of the screen to ground,
by means of a cable clamp or a conductive cable gland. The
FC 100 Series supplies various clamps and brackets to enable
a proper ground connection of the BACnet cable screen.
Danfoss recommends to connect the screen to ground at
both ends of the bus cable. This ensures the maximum
protection from EMC noise. Connecting the screen at each
end will require that each BACnet device has the same earth
potential or else an equalizing current will flow in the screen
of the cable and cause disturbance and poor performance of
the system. Where this is not possible, the screen can be
isolated from the chassis of the drive by use of shrink-tubing.
It must be pointed out that the routing of the BACnet cable
must be established with a maximum distance to other
cables such as mains, motor cable, etc.
How to Install
VLT
®
HVAC Drive Native BACnet Operating Instructions
10
MG.14.C1.02 - VLT
®
is a registered Danfoss trademark
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