Trane TR200 Operating Instructions Manual page 13

How to Install
Bus Cabling
EMC Precautions
The following EMC precautions are recommended to achieve interference-free operation of the BACnet network. Additional
EMC information is available in the TR200 Design Guide, MG.12.IX.YY . Please also consult the BACnet MS/TP manual for further
installation guidelines.
Note
Ensure compliance with relevant national and local regulations, for example in protective earth connection.
Single Ground Shielding
For installing the bus cable on 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 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 P bus cable, the use of fiber 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.
Multiple Ground Shielding
If the distance between the individual drives is limited (e.g. inside a cabinet or in one control room) Trane 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 TR200 Series supplies various clamps and brackets to enable a proper ground connection of the
BACnet cable screen.
Trane 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.
12 TR200 Embedded BACnet Operating Instructions