sberry
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Everything stays together, including the IG ground.
Section 250.54 allows for what the code calls "supplementary grounding electrodes". Some electronic equipment in the installation instructions require this electrode. Why they want these I have no idea, but some electronic people think that if it isn't connected to earth, then it isn't grounded. They don't understand that the earth is loaded with stray currents from many things, but so be it. These stray currents come from many sources. One is the fact that between your grounding electrode at home the system transformer, there are currents through the earth in parallel with your service neutral. These are a fact of life and how much current depends on a lot of things. Also, you, or your neighbors could have a piece of UF going out to a yard light that has a nick in the insulation. Black or white, it don't matter. Some current is leaking out of this nicked insulation and will find it's way back to the electrical system through the earth. Then there are the installations that now grounded to a ground rod because that is the way they were put in. A good example of these are mall parking lot lights, or any large area lighting. A lot of these have no ground wire to them, they are grounded by way of a rod. If one of these lights develops a ground-fault, the current flow is down the rod, then back to the source through the earth. There are many references in the code that prohibit using the earth for an equipment grounding conductor, but these installations exist by the hundreds. If a ground rod is driven into an area that has any of these conditions, the current will be imposed on the equipment connected to this "isolated ground rod". This creates a shock hazard when touching the equipment grounded this way, and any equipment in the building that is connected to the building grounded system.
Take at least one example: A piece or equipment is grounded to a ground rod to satisfy the electronic people. They insist that the equipment be connected to an isolated ground. The average ground rod will megger well over 100 ohms. But just suppose you are lucky and manage a 10 ohm ground rod. 120 volt divided by 10 means that leas than 12 amperes will flow in case of a ground-fault. Will this clear a 15- or 20-amp overcurrent device? No way, but there is 120 volts going down the rod, and 120 volts on the metal of the equipment, just waiting for someone to come in contact with it. Where is this current going from the ground rod. Pretty much where ever it wants, and if there is a swimming pool nearby that for some reason wasn't properly bonded, or has a bad bonding connection, this could be time for an electrocution. About the overcurrent device, in order to clear a standard 20 amp circuit breaker in a reasonable time, it must have close to 80 to 100 amperes pass through it. Kind of hard to do when the rod will only pass 10 or 12 amperes.
I strayed, back to 250.54. This section permits a supplemental grounding electrode at the equipment, but the electrode must be connected to the equipment grounding conductor in the circuit to the equipment. A lot of mall lighting is installed this way. The supplemental electrode is bonded to the equipment ground of the light, and it provides added safety for lighting hits. A lot of the lightning energy will dissipate down the rod. Otherwise it would go back on the equipment ground in the circuit and since the insulation is only 600 volts, it causes a lot of damage to conductors.
As for isolation transformers, these are no different electrically than any other transformer. All transformers, other than autotransformers, are isolation transformers. By this I mean the primary is isolated from the secondary. The difference between a standard and an isolation transformer is the addition of a grounded metal shield between the primary and the secondary winding. All separately derived systems, transformers, must be grounded and bonded to comply with section 250.30. No exceptions are made for isolation transformers, nor for electronic equipment Again, some people feel that these transformers are neither bonded or grounded because they are special. If the bonding connection is not made, there is no way at all to complete the ground-fault return circuit for opening the overcurrent device. You wind up with phase voltage imposed on the entire structure, and this can be deadly.