OK first it is important you understand how Balanced Power Systems operate and induce noise into ground circuits. So that was not a waste of your time and I am not sure you really understand the point of how a 120 volt asymmetrical system induces noise currents into grounding circuits.
First define NOISE as any unwanted signal. I do not care what that signal is, if it is unwanted, it is a noise. For Sparkies, no offense intended guys, grounding conductors can and do serve other purposes other than safety. But Sparkies are only concerned and have been taught safety and code. The code recognizes ground circuits can and do serve other purposes, and makes allowances for modifications to grounding systems to mitigate NOISE. In the communications world ground also serves as a SIGNAL REFERENCE POINT in addition of power. Problem is the 120 volt single phase world is inheritable noisy by injecting unwanted currents into grounding conductors.
The unwanted currents, or NOISE, is injected by two primary sources:
1. L-G and N-G cable capacitance. As the circuit conductors get longer, the coupling capacitance increases, and thus injects or couple 60 Hz line power and harmonic frequencies into the ECG conductors.
2. At the utilization equipment most electronic devices like amplifiers or really anything with a semi-conductors have RFI and surge protections devices installed between L-G and N-G on the incoming line power conductors. Although these devices have fairly high impedance to power line frequencies they still inject power line frequencies into the EGC. The L-G common mode is th emost offensive of the two.
OK number 1 & 2 are additive and compound the problem. Result is you get some 60 Hz line current flowing in ground conductors. Not enough to generate a high enough voltage to be a safety concern, but more than enough to drive signal circuits crazy. All of us know one of the most common problems that annoying hum in PA speakers, stereos, ect.
With that said let me say Isolated Ground Receptacles are antiquated and really serve no purpose other than to pad a Sparkies check and give a warm fuzzy feeling to the AV nerd. In the old days of stereos, PA systems etc used unbalance signal transmission meaning they used ground as a signal path and in the COMMON MODE. Today most all communications circuit used either balanced signal transmission, Wireless RF, or completely isolated with optical or digital balanced Pulse Code Modulation.
So how did that annoying hum get into the audio stream? Well it came in at low signal levels called Line Level. Line Level would be the output of a microphone, electric guitar, mixer, pre-amps etc... All those analog line levels used ground as a signal path and reference level. Remember line level is very low power at 600 Ohms @ 0 dB or about 1 volt RMS at full output level. So here is what happens you have a guitar on stage through a long line cord to a stage mixer Powered on a AC circuit. From there to the main mixer board and EQ powered by another AC circuit. From there to a rack of Power Amps powered again by another AC circuit. See where this is going. Each of the AC circuit receptacles are referenced by the EGC. Along the length of th eEGC noise current is flowing. All the mics, pre-amps, and gizmos are connected the EGC at different points along its length. There is impedance between those points of connections. If there is a current flowing through an impedance then there is a Voltage Drop. Th emixers and mostly the Power Amps are picking up those very small signals and amplifying them and you hear it as that dang annoying HUM.
So some 50 years ago a bunch of engineers came up with the idea of a Isolated Ground Receptacle, or the forth wire in a standard 120 VAC circuit. It was magic so most thought. In reality an IGR has 3 outcomes, and what follows is liste din order of what happens.
1. Nothing
2. Makes the problem worse.
3. Desired effect.
The idea or principle is sound but corrupted by either installation or the connected equipment itself. That principle is a SINGLE GROUND POINT of power supplies, equipment racks, and signal point. In a true SPG there can be NO current flowing. If NO currents flow, there is no noise or voltage. For current to flow there has to be a complete circuit or a loop for current to enter and exit. It works if you can pull it off, but very easy to corrupt and just about all the signal ground reference equipment corrupts the SPG by the interconnect cable carrying the signal you are trying to clean up.
In reality it is an easily solved problem especially in a home environment or even commercial. Just pug all the equipment into the same receptacle. OK so you are saying that only has two ports. Well install a quad or even better by a decent Power Strip with 10 ports. That way all the EGC are physically connected to a SINGLE POINT, thus no difference of potential between them. No potential difference, no NOISE
OK Smalltime now that is out of the way and hopefully you understand what I just said lets answer your question. Don't get offended. But your understanding of 640.2 is incorrect. No ground can be isolated from the premises ground. To do so would not only be extremely dangerous, but you give you all kinds of technical problems like NOISE.
What 640.2 is telling you is the Isolated Ground is allowed to pass through all panels and junctions without bonding to them like you are required to do with standard EGC. It allows you to go all the way back to the service entrance point where the holly grail of ground is right at the Main Bonding Jumper point where N & G are bonded to each other. Th eN-G bond is where the real SINGLE GROUND POINT originates or ZERO POTENTIAL reference point. It does not have to go all the way back to the service entrance, but allowed if you wish to do so and most do prefer that point.
However someone who really knows what they are doing will not use any IGR as we have a much more effective means at our disposal. We create a new reference ground point near where we need and use it. How you ask? Simple we install a Isolation Transformer with a new N-G bond point. A Isolation transformer even a dry one gives us at least 60 dB of common mode noise rejection isolating the output from everything. Use a true isolation transformer like farromagnetic resonator and we can get 140 dB of common mode noise rejection. FWIW 60 dB = 10,000,000 : 1 or reduces 10 million volts to 1 volt, 140 dB is 1,000,000,000,000,000 : 1 rejection. Only God can comprehend that number)
Ok today none of that is needed. Modern engineering of electronics has eliminated all those headaches and problems. On stage they use Wireless mics and equipment pick ups, optical, digital, or balanced analog signal transmission and whatever power is available.
See how that word "balanced" keeps popping up?
Two last points to blow your mind but very true.
1. In any stick built house that uses NM cable and plastic junction and receptacle boxes is an Isolated Ground Circuit. Well that assumes you only have one Main Panel and no sub panels.
2. Any 2-wire plus ground in a stick built house as above is a Balanced Isolated Ground Circuit.
