If an IG is bonded at one end via a NEC compliant method, 60hz can not be capacitively coupled into the IG conductor,
Quite correct. However the coupling mechanism isn't capacitance, it's mutual inductance. Please pardon the length, but this excerpt from an article I wrote for an audio industry newsletter should explain the issue well.
When current flows through a conductor a magnetic field is built up around it. For alternating current this field collapses and reverses each half cycle of the power frequency, thus it is a moving magnetic field. A conductor placed within a moving magnetic field will have a voltage induced upon it. This is the principle of the transformer.
When two conductors carrying current of the same magnitude and phase but flowing in opposite directions are in close proximity, their fields repel each other, which distorts both fields. While this interaction is complex, there are two important points we need to know. First at a distance from the two conductors, the field is considerably reduced through cancellation. Second, a third conductor located equal distance from both conductors, will not have a net voltage induced onto it, since the two fields are trying to induce equal and opposite voltages onto it.
This is the case for a typical power circuit where the hot and neutral form a series current loop. Since the current is the same everywhere in a series circuit, the current in the hot and neutral wires should always be the same magnitude but flowing in the opposite direction.
In a typical power circuit a third wire is added to serve as a safety ground. This wire may also serve as the signal ground reference for equipment it is powering. This wire normally should not be carrying any current and should not be generating a magnetic field of its own. However as a conductor it can have a voltage induced onto it by magnetic fields it may encounter along the way.
The NEC and other electrical codes require the ground wire to be run the same route as the power wires for the circuit it is serving. This means it will be in close proximity to the current-carrying, magnetic-field-producing power conductors. If, by a matter of its physical location, it is closer to one current carrying conductor than the other, than it will have a voltage induced upon it. The voltage induced onto the ground wire is added to the ground reference at the source panel to become the ground voltage at the load end of the circuit. It creates the ground voltage difference we are trying to avoid.
There are two ways to keep the voltage induced onto the ground wire to a minimum.
1. Maintain exactly equal spacing between the ground wire and each power conductor. This can be done with varying effectiveness by twisting the ground together with the hot and neutral or by cable construction with the ground wire held precisely equal distance from the hot and neutral.
2. Alternate the exposure to each side of the current loop and thus each polarity of the magnetic field, so there is no net addition. This can be done by twisting the hot and neutral together into a pair and running the ground wire separate from this twisted pair.
Read through the links I posted above for some good info.
While I haven't read those threads today, I'm quite familiar with the stand on IG taken by Mike Holt and others on this forum. At the considerable risk of launching a firebomb which may be better addressed elsewhere, I'll just say this. They do make some good points about IG and how, particularly when poorly implemented, it can result in poorer performance in the very area it is meant to fix. However, I see some basic misunderstandings of the real purpose of IG, and total lack of insight into the means and methods of keeping ground voltage differences low.
It was not my intent to start a thread on these issues, but anyway....
if your are installing an IG conductor that is isolated at the supply end from the service grounded conductor, then not only are you violating the NEC but also setting yourself up for other problems.
An IG has to be bonded somewhere to the service grounded conductor, or it can not be used. just running it to a ground rod is not compliant and dangerous.
I thank you for your diligence in correcting the many ignorant thoughts that come from so many in my industry. I'm also working to educate people toward avoiding the foolish and dangerous practices you mention above. I trust by now you've seen your assumptions are ungrounded and we need not discuss them further
