Distech Controls ECB-VAV, User Manual

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60 Network Guide
Grounding a Network
Grounding Shielded Twisted Pair Cable
When using Shielded Twisted Pair, terminate the twisted pair and ground the cable shield, as
shown in the following figure.
Figure 35: Grounding Shielded Twisted Pair Cable
The twisted pair is terminated according to the guidelines listed in the previous sections. The
cable shield should be grounded using a capacitor to tie the shield to earth ground and a large
value resistor should be used to bleed off any static charge on the shield.
Tie the shield to earth ground through a capacitor, instead of with a direct connection. This
prevents DC and 50/60 Hz ground paths from forming through the shield. Typical values for
Cc and Rb are as follows:
• Cc = 0.1 μF, 10%, Metalized Polyester, ≥100V
• Rb = 470Ω, 1/4W, ±5%
The cable shield should be grounded at least once per segment, and preferably at each node.
Grounding the shield at every node (using the shield grounding circuit shown in
Figure 35 ) will
assist in suppressing 50/60Hz standing waves.
Building Entrance Protection
Use shielded twisted pair wire for networks, or portions of networks, that are run outside of
buildings. The shield should be connected to earth at each building entry point via a data-line
lightning/surge arrester, to conduct lightning strike energy or power surges directly to ground
and prevent their entry into the building via the control systems network. Data-line lightning/
surge arresters should also be used at each building entrance and should be connected to the
network data lines. Refer to the L
ON
W
ORKS
FTT-10A Free Topology Transceiver User's Guide
available from Echelon, for more information about building entrance protection.
It is recommended to use shielded cable only when the network cable is outside of
the building since it will be subjected to electromagnetic interference (EMI).