Ground loops and audio racks
The quipments connected to metal audio racks will get connected to each other through the metal rack. This can lead th ground loop problems if the wiring is not done carefully. If you face this type of problems do not use insulating tape or other non-conducting materials to "cure" the problem. That could end up being very dangerous. Instead think of a redesign.
How to construct rack system
Assuming that this rack is the hub of your plant then proceed as follows. It's called hub and spoke grounding.
Begin with an empty rack, standing in the middle of the floor. First, bring AC to the rack, allowing for expansion. Buy one or two five-foot pieces of wire mold with recepticles spaced every four inches. Bond the green wire from the recepticles to the rack so that the rack is at the same electrical potential as the earth ground for the AC. You can add more racks and wiremold strips as needed until you have adequate rack space. Bolt all racks together until there is NO question that they are electrically just one piece of metal.
Mount the power supply for the console in this rack. Mount all outboard equiopment in the rack. View the rack as a hub for ALL business at your plant. Start substituting the word "hub" for rack.
Spoke #1: Run a single piece of grounding from the hub to the console. Make it big. Wire the power supply to the console. Now cable other console items back to the hub. NO OTHER COMPLETE ground connection allowed between hub and any other terminal point: Open the shield, for the sake of unformity, at the terminal end for every piece of shielded cable, leaving each shielded cable grounded at the hub end. Your single piece of grouding is what keeps it all safe -- and silent.
Spoke #2: From the hub, run AC to, perhaps, a free-standing tape machine. Ground it back to the hub. As above, run signal cables to the hub, grounding the shields at the hub only.
Spoke #3: From the hub, run AC to, perhaps, a rack in another studio in your plant. This new rack becomes a hub for the world it lives in and it has ground wires emanating from it to new terminal points.
Spokes #4 through 400: As above.
NEVER NEVER NEVER: Here it is, here is what keeps it all honest and hum-free: NEVER take a shortcut and run a cable directly from one terminal point to another. ALWAYS run such cabling back up the hubs spokes to the main hub and then down the appropriate spoke to the desired terminal point, tapping into hub grounds all along the way.
How to test that you don't have ground loops
Buy a digital clamp-on current meter. There should be NO current on any ground wire. None. If you can measure current flow on a ground wire then fix it: you either have a circular path or else a faulty piece of equipment.
If you can't get clamp-on meter you can try to measure the current flowing on audio cables by removing the cable from one and and connecting an ordinary multimeter between the ground pin in the audio wire and the audio connector ground pin in you equipment it was connected to.
How to make connections to other equipments not part of the system
When connecting a system which has well done non-humming grounding and a system running from another power source, the signal grounds of the systems need to be isolated.
With balanced audio signals, lift the shields at the inputs. With unbalanced audio or video signals, use an isolation transformer or wire the signals to balanced input where the ground connection is left open.
Ather rack wiring ideas where ground loops are not a problem
If all your equipments are just in one rack then you can also use another wiring method. Instead of avoiding gound loop you can try to make the whole system to have a very stable common grounding. This stable common grounding can be achieved by making sure that all equipments have very good connection to metal rack. The mains power to all equipments mut be take from one power distribution unit installed to the rack case. If all wirign is done using short wires the system does not have grounding problems because there can't be any significant ground potential differences because all equipments are well tied to one very low resistance ground (the audio rack+the metal equipment cased tied together).
Tomi Engdahl <Tomi.Engdahl@iki.fi>