Shielded twisted pair cable may have been installed in an area of high EMI/RFI, and proper grounding of the shield wire is necessary to make the shield effective.
A segment of LON trunk with shielded, twisted-pair cable.
Proper grounding helps to avoid DC and 50/60 Hz ground paths from being formed through the shield.
Follow the Guidelines as presented in the Echelon FT 3120/ FT 3150 Smart Transceiver Data Book, page 114, Figure 5.7 "Terminating and grounding Shielded Twisted Pair Cable."
Grounding Shielded Twisted Pair Cable
When using shielded twisted pair, terminate the twisted pair and ground the cable shield, as shown in Figure 5.7.
Figure 5.7 Terminating and 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 to bleed off any static charge on the shield.
Tie the shield to earth ground through a capacitor, instead of a direct connection, in order to avoid DC and 50/60 Hz ground paths from being formed through the shield. Typical values for Rb and Cc are as follows:
Cc = 0.1 µF, 10%, Metalized Polyester, ≥100V
Rb = 470kΩ, 1/4W, ±5%
The cable shield should be grounded at least once per segment, and preferably at each device.