[BC] Studio Grounding Info

[Mike McCarthy]

Martin,

There is no single source publication other than the NAB handbook on 
the subject.  There is an edition of the AES Journal which many 
consider to be the best treatment of creating/handling audio common 
for studio applications.  I want to say Dec. 1995.  I will say it was 
done after 1988...which is when I became a member.

One key item however.  Instead of using the term "ground", use the 
term "audio common" for defining your purposes.  While both 
potentials will be equal or within mV at most locations, their 
purposes are VERY different and completely incompatible.  Never mix, 
connect, or bond ground and audio common together except at the 
electrical distribution panel or transformer deriving neutral bond 
point for the complete plant.  That's a tough thing to accomplish 
most of the time.  BUT, that's the only way I've learned to create a 
truly quiet system using an unbalanced AC power system.

Additionally, "ground" carries specific inferences and requires 
specific treatment under the NEC and local codes.  By specifying 
"audio common" as the purpose of the conductor, one distinguishes 
clearly to the various trades and jurisdiction that it is not to be 
considered the same as ground and should be treated accordingly.

There are two aspects to creating audio common in the studios.  One 
is the size and manner in which the conductor is installed and 
brought into the rooms which will use the system.  The second is the 
connection of cable shields and creating the audio common node for that room.

The former is directly influenced by the location of the 
facility.  If the studio/compound is near an AM TX, strap is the 
preferred method.  Installing strap is a VERY tricky prospect. One 
must insure a connection between any aspect of the AC ground system 
(conduits) and the strap used for audio common is avoided.  Which is 
next to impossible to accomplish if steel studs are to be used. The 
only two simple ways around that are to imbed the copper in the slab 
pre-pour and/or to simply lay rubber over the strap on the floor 
before the studs are installed.  Also, the NEC and most local codes 
contain no specific treatment for the installation of copper strap in 
a building that I'm aware.  So practices are at best dependant on the 
preferences of the installer and the local inspector.


For non-strap installations, use the heaviest conductor your project 
can justify installing.  If code will allow, pull that conductor in a 
PVC or non-conductive conduit to minimize/eliminate inductive 
coupling between the audio common and the AC ground of the conduit 
and those surrounding.  Bring all conductors back to a common bonding 
point if possible.  This is not practical with strap however.

RF ground and audio common can also be two different animals and need 
to be treated accordingly in those environments. I have created audio 
commons with mesh in studios which employ their own ground rods 
(electrodes) under slabs.  The key is to clearly define the purpose 
of the system and it's application.  Once the local inspector got his 
hands around our purpose and methods, he concluded the code was still met.

Also, there is much debate about some practices.  Some believe in 
grounding at the termination, others at the source.  I believe one 
has some specific research behind it.  Others believe tying at major 
common junction points like patchbays, consoles, and/or the shop for 
inter studio cabling.  Some believe in carrying shields across 
studios/rooms and in effect employing one continuous shield for the 
whole cable run.

Additionally, there are issues with the audio common of equipment DC 
coupling to the case ground and that case ground being connected to 
the AC ground via the third prong on a typical power cable.  "What 
does one do there?" is a difficult and often times an unanswered 
question due to costs/time to remediate in the field.

My one question to you is simply...analog ...why?  If you're going to 
build a new operation, one would think digital or IP based (Axia or 
Wheatstone) infrastructure would be the mode preferred over (simple) 
discrete analog between or in studios. A distributed switcher based 
operation like the SAS, Wheatstone, Logitek, Klotz, Harris, et al. 
which uses digital and/or fiber to transport inter-studio signals is 
a more efficient means to carry content than a huge snake requiring 
lots of connections to audio common.  Installing a patchbay around 
the switcher or parts of the switcher is a wise choice.  BUT, that's 
a highly focused application which employs very simple rules.

Not to be overly critical, but I would seriously reconsider designing 
a new plant with an analog mindset as it's backbone.  You are really 
bonding (not to mis-apply a term here) the operational future of the 
plant to a standard which is fast becoming less and less attractive 
in so many ways.

Good luck...

MM

At 12:36 PM 5/29/2008 +0200, Martin Sims* wrote
>But I'd really like to find some material that takes a rigorous approach to
>the topic. If someone can refer me to a paper, book or website with a title
>like "Practical approaches to grounding in multi-room studio complexes" that
>would be excellent. I haven't yet find anything that deals with the topic in
>a practical, yet "scientific" way.