On 4/23/2023 11:02 PM, vk6jx@bigpond.net.au wrote:
Every shielded single pair or multi pair instrumentation cable (read
low-level signals being conveyed through the noisiest HV/LV/variable speed
AC drives environment imaginable) all over the world, has the shield
grounded only at the receiving end
Cable shields are not EARTHED, they are shields -- the late Neil Muncy
correctly taught us that the earth is not a sump into which noise is
poured. :) And not all large scale performances are of rock music --
when I've recorded Tony Bennett live, and mixed audience sound for him
outdoors a year or two later, it was a big band with strings, toys, and
a harp. 32-48 channels carrying mics several hundred feet, with the sum
of noise picked up on those cables (non-coherent noise increasing by 3
dB for each doubling of the number of inputs, 6 dB if the noise is
coherent), is very different from rock and roll, and even more different
from instrumentation.
A study of Whitlock's 1994 AES Paper (reprinted in Jun 1995 Journal of
The AES) is groundbreaking. JAES is in most engineering libraries and
the Jun '95 issue got a special re-printing, because it included
ground-breaking papers by both Whitlock and Muncy, as well as two other
EMC papers from the same session. Many years ago, it could be ordered
from the AES website for $10.
Whitlock's paper analyzed the balanced interface as a Wheatstone bridge,
which led him (and us) to the recommended practice. For balanced to be
achieved broadband, cable capacitance comes into the equation, and it
was learned that the differing dyes used to color-code conductors caused
their dielectric constant, and thus their capacitance to the shield, to
vary, thus upsetting the balance with increasing frequency. That issue
is solved by always terminating the shield at the sending end.
Remember -- the fundamental reason for breaking the shield at all is
legacy equipment with a Pin One Problem.
- which is usually within the E/I
Equipment Room adjacent to the area control room. This is where these low
level signals are connected into the Honeywell or whatever SCADA system
which monitors and controls the plant. There is a "clean" instrumentation
earth system installed under each control room which is entirely independent
from the power earth installation to which all the motor cable earth wires
are bonded. The drain wires of the shields of all of these instrumentation
cables are grounded only to the clean earth system. The shields at other
ends of all of these cables - out in the field connected to thousands of all
types of transmitters - is not connected to anything.
In North America, all ground electrodes (earths) in a facility must, by
Code, be bonded together. Isolated ground mains wiring is used for
sensitive equipment, including sound systems; ground (protective earth)
is bonded to earth and neutral at the point of entry to the premises,
but isolated from all other contact with grounded objects. When power
transformers are added for voltage transformation or noise isolation,
neutral must be bonded to building ground at its secondary, and if that
transformer fed isolated ground wiring, the grounding point would be at
that transformer secondary.
73, Jim K9YC
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