Bigdaddy brings up a point that may have gotten lost - a star ground system is NOT designed to BYPASS any electrical code that is meant to provide SAFETY - only to SUPPLEMENT a standard install in such a way as to make sure there is only ONE common ground point for equipment, not a DIFFERENT ground point.
The code does indeed state that metal boxes will be grounded and bonded, and this is a safety concern. If a hot wire somehow either melts or gets skinned, etc, and contacts the metal parts that humans can touch, if that metal part isn't already grounded this could become a LETHAL situation. Grounding of all metal parts ensures that if the above happens, then overcurrent protection (fuses or breakers) will operate, removing the danger and alerting us to the fact that there's a PROBLEM.
For safety, this is fine - for QUIET electronics, it isn't enough. If equipment gets its ground from different points along the same grounding conductor, and ANY small amount of current is allowed to flow in that conductor, there will be a voltage drop (albiet small) - any difference in voltage will cause a current flow, and now we have noise induced into anything that's handy.
To avoid this possibility WITHOUT voiding the safety aspect, the ground that each piece of gear sees must be at exactly the same potential as the ground for every OTHER piece of gear; otherwise, when we hook grounded shielded audio cables between gear we now have a LOOP which makes current flow possible between units, hence NOISE.
The way we satisfy BOTH safety and quiet power is relatively simple, although anyone reading the code book for the first 2000 times could argue otherwise
- we first satisfy safety - all metal parts of the wiring system grounded to the main system ground. Period. For a system using METAL conduit and metal boxes, this presents a problem for technical power grounding - standard electrical outlets have their metal parts which screw into the metal box BONDED to the ground pin in the outlet - so any gear plugged into this outlet would have its ground/chassis grounded to the system ground. If each receptacle gets its ground from the common ground system, and also has its cabling grounded to other chassis, there is the potential for a LOOP, which means current can flow, which means NOISE.
To get away from this, and still satisfy safety, we need to make sure each piece of gear has a solid ground that is at the exact same potential as every OTHER piece of gear. This means NO LOOPS, and can only be done by running each piece of gear's ground from a common point, with no chance of a loop.
If using metal conduit and boxes LEGALLY, this system requires that there be no bonding of ground within the receptacle, but WITHOUT eliminating the safety of EVERYTHING being grounded. This is accomplished by using IGR's, or Isolated Ground Receptacles -
Under this method, safety ground of the wiring system itself is done in normal fashion - but the safety ground for the GEAR is ONLY connected through the ground pin of the receptacle, which is ONLY connected to the COMMON STAR GROUND - this way, each piece of gear's chassis is tied to the EXACT SAME POTENTIAL so there is no chance of current flowing in the ground.
To continue this "quiet quest" to completion, you should also NOT use metal racks unless using individual ground isolator tabs on each piece of rack mounted gear - optionally, use wood racks and rails, preferably with threaded metal inserts for durability. This keeps individual rack pieces from forming their own ground loop.
With all that out of the way, I'll try to write one paragraph that applies to each wiring method -
ALL METAL SYSTEM -
If using metal conduit and metal boxes, the receptacles MUST be of the IGR type. Mount the receptacles in the boxes normally, the conduit will be bonded to the boxes, this entire system will be bonded to the building ground, and this completes the safety ground to code. Since this is a conduit system, you'll be using separate wires and NOT romex - the STAR ground part now needs a separate (green) INSULATED wire that runs ONLY from the ground lug on each receptacle, back to the STAR point, which must be bonded to building ground. If you have 15 duplex outlets in this system, EACH outlet must have its own separate insulated green wire that ONLY connects to the outlet's ground terminal at one end, and directly to the STAR point at the other end.
Now, when you plug a piece of gear into the wall, its chassis is connected directly to the STAR point, which is at building ground potential, but does NOT have any possibility of a LOOP - therefore, every piece of rack gear is at the same potential so when you connect audio cables there will be no current flowing in the ground.
ADDED STIPULATION - code requires ALL conductors (including all grounds) to be in the same conduit for a given supply, because the mutual inductance of the wires is part of what trips safety equipment (breakers) in the event of a ground fault.
METAL BOXES, WIRED WITH ROMEX - This presents a slightly different problem - now, we have metal boxes (which must be grounded for safety, same as above) but no conduit to ground them to. For this sytem, we STILL need the IGR outlets to accomplish a STAR system - each metal box will use the ground wire included in the ROMEX for its safety ground - this wire is bare, and will be connected to the box at one end and to the ground buss in the panel at the other end; daisy-chaining these box grounds is fine, because they are NOT part of the STAR system.
To provide the STAR system, we must connect each receptacle's ISOLATED ground lug through an INSULATED ground wire back to ONLY the common STAR point.
Now comes the "glitch" - because of the code requirement that ALL wires for a circuit be run in close proximity (I'm still looking for the exact wording of this, but it IS part of the code) this means you can't run the separate STAR ground wires just anywhere - they must be close to, and parallel to, the current carrying conductors of the romex. Our bud in Colorado ran into this, and satisfied the inspector by using FMC run through the same holes in studs as the romex, with all STAR ground wires in the FMC.
Finally, if receptacle boxes are PLASTIC, and we're using ROMEX, the ONLY change is that IGR's are not necessary - because the boxes are plastic, there is no grounding requirement for the boxes.
This leaves TWO possibilities for wiring - one is to do a "home run" for each and every outlet, each receptacle gets its own romex. The ground connects to the frame of the receptacle, you make sure a second receptacle in the box doesn't touch its neighbor, and the romex ground ONLY connects to the receptacle ground at the box and to the STAR point (which is bonded to system ground) at the panel.
The second way would allow daisy-chaining of the romex conductors between plastic outlet boxes, EXCEPT that it would require a SEPARATE, INSULATED, ground wire to be run from each receptacle in each box straight to the STAR point; still no IGR's required as long as multiple receptacles in a box do NOT TOUCH each other. This isn't hard to do, just a caution that it's necessary. The downside of this is the hassle of having to run all the romex runs in the same set of holes through studs, PLUS that pesky FMC full of separate, INSULATED, star ground wires.
This is why the simplest (maybe not quite the cheapest) way for star grounding with plastic boxes is the first one.
Electrically, the graphic posted by Bryan is the one to go by - hopefully my long-winded explanation above will help figure out HOW you can accomplish this, and it's all in one place - if more gets added that I may have omitted, I can edit this post to include it - eventually, we can add it to the stickies here so it's easier to locate... Steve
PS - when I finally find the code language for the "proximity of ground conductors" thing, I'll add that too -
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