Sorry Jim - you've lost me. I'm very happy for you that you got to engineer
Tony Bennett's audio live, but can you try again to tell me if you agree
with me or not ? What I described is what is happening all over the known
world, as we follow US practice over here as all our refineries were US
companies to start with (Worsley Alumina now owned by BHP and Alcoa Alumina)
- with Kaiser Engineers in the driving seat !
-----Original Message-----
From: Amps <amps-bounces@contesting.com> On Behalf Of Jim Brown
Sent: Monday, April 24, 2023 3:59 PM
To: amps@contesting.com
Subject: Re: [Amps] World's worst coax connectors
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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