[SML] XLR mic cables: Pin 1 to case/ground... yes? no?

[e-mail frank.wood95]

In my world, there are two different ground connections. One is the
safety ground, which comes with the mains supply from the sub-station
where it meets the neutral. It is connected to all exposed metalwork.

The other is the technical ground, which is usually derived from a
substantial ground plate, sometimes buried under the car park. This is
used for the cable screens of audio equipment when balanced wiring is
used. If the wiring is unbalanced, it may form a part of the signal
path, although this is not desirable.

It is a firm rule that NEVER the two shall meet. When doing big
installations, it was often the custom to connect a battery and buzzer
brtween the two, with the incoming technical ground disconnected. If
the buzzer went off, the standing instruction to the wiremen was to
stop, and to disconnect their work in reverse order until it stopped.
Then call an engineer to locate the fault.

I remember one TV studio installation where there was a heavy hum
signal on the technical ground, over the whole site. Serious experts
were called in, who found 7A of ground fault current. By working
nights, disconnecting whole areas one at a time, this was traced to
the switching and routing centre. Detailed investigation revealed that
the switch panels, supplied by an outside contractor, had not used the
specified switches, and they all had to be re-made. After that, the
problem went away.

You can see that this eliminates all the problems at the design and
build stages. There was also a policy that cable screens picked up the
technical ground from the input of the gear they were connected to.
Strictly applied, this eliminates hum loops.

On some varieties of XLR connector, the shell securing screw actually
drove into pin 1. This is bad news, as the shell inevitably picks up
the safety ground from the connector panel, so linking the technical
and safety grounds. My advice is to check all your XLR cables for this
problem, and to re-terminate those that have the shell connected to
pin 1.

These general policies were formulated by the BBC, which was learning
how to do this job back in the 1920s. They have proved sound ever
since.

On 11 March 2015 at 02:49, Jon Ares via Stagecraft
<stagecraft at theatrical.net> wrote:
> On Tue, Mar 10, 2015 at 7:31 PM, tech dept <tech.dept at inbox.com> wrote:
>> It sounds like you bought the Beh------ unit. Well, I did and it is holding up well.
>
> <sheepishly>  I did.  (The Beh----- cable tester, not the POS poopy,
> buzzy, fuzzy 4-channel mixer that started this whole mess.)  The cable
> tester seems to work fine.... and I like the signal generator function
> as well.
>
> --
> Jon Ares
> www.arescreative.com
> Follow my twits @jon_ares
>
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-- 
Frank Wood