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

[Steve Payne]

http://www.rane.com/note151.html

On Wed, Mar 11, 2015 at 3:24 PM, e-mail frank.wood95 via Stagecraft <
stagecraft at theatrical.net> wrote:

> 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
>
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