Answer / carl_ellis

1) A shield (screen) connected at both ends will carry
ground loop current due to the difference in ground
potentials at each end. A shield (screen) carrying current
will induce noise into the wires it is shielding.
Historically, shields (screens) have been connected at one
end only, typically at the receiver
(DCS/PLC/PAC/RTU/controller) end.

Current thinking as outlined in IEC 61000-5-2
Electromagnetic compatibility (EMC) - Part 5 Installation
and mitigation guidelines - Section 2: Earthing and cabling,
recommends grounding the shield at each
end for point-to-point and at every point for multi-drop.
Two point grounding provides protection against EMC and for
safety as well. However, this method assumes that all
points are on the same ground (i.e. use an equipotential
grounding grid).

Typically, Siemens has adopted this recommendation for
Profibus DP, an RS-485 multi-drop network. Of course, the
gotcha is "all points at the same ground potential". Not
in the real world, Mr. Siemens.

2) Although 24Vdc power supplies can be noisy, major
manufacturers produce DC power supplies with very little
ripple and excellent regulation.

DC power supplies used for HART need to meet the HART spec:
Maximum ripple (47 to 125 Hz) = 0.2V p-p
Maximum noise (500 Hz to 10 kHz) = 1.2 mV rms
Maximum series impedance (500 Hz to 10 kHz) = 10 ohm

24Vdc power supplies are not typically the source of noise

Carl Ellis
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