Too Well-Grounded?

One characteristic of residential electrical distribution systems is the 'neutral' often has some small? potential relative to true ground. For circuits subject to high starting current (and high short circuit loads) , it is good practice to minimize distance to good high current capable earth ground to avoid potential for 'ground loop' induced corrosion.

I don't think anything can be too well grounded, but it can be incorrectly grounded. The one thing you must remember especially in a house, is that every ground must go back to a single point of reference. Codes in the USA require that reference to be a the first means of disconnect.

If you can keep that idea straight in your design you can be reasonably sure you are grounded correctly.

You can also get a isolation transformer based power conditioner to solve this problem. Oneac makes a power conditioner in their Condition One series that has a isolation transformer and filtering circuit and then the Neutral is rebonded to ground. This results in extemly low output noise of 10v Normal mode (L-N) and .5v Common mode (N-G) in normal operation. This kind of product is also a dedicated isolated portable ground. They also offer as an option the One Ground choke circuit which eliminates high frequency ground loop noise. To answer your question No you never want to float a ground for 2 reasons. 1) Safety - ground is used to prevent a shock hazard, a floating ground is a safety risk. 2) Ground Skews or loops - if you have a network laidout of multiple circuits and the grounds are not at the same potential then anytime there is a voltage potential between to grounds there will then be current flow. This will typically result in a high frequency noise problems that could slow a system down, degrade or even destroy system components.

Pardon me for being late, I just walked into this thread.

As a native Chicagoan, I can provide some insight.

Chicago Electrical codes require all permanent wiring to be in metal pipe, all electrical boxes must be metal, and the breaker panel (or fuse box, if your house is REALLY old and desperately needs updating) has a stub of conduit that is bonded to the main cold water line in the house. The cold water line is metal pipe all the way to the water main under the street.

Because of that system, all the conduit in the building is bonded to ground, and normally no 'ground wires' are needed, since the outlets are boned to the boxes, which are bonded to the conduit, etc, etc, to the water main which provides a very solid earth grounding.

There are exceptions to the 'no ground wire needed' bit, mostly for 'specialty equipment.' An audiophile, for example, may install outlets with 'isolated grounds,' meaning the ground pin does NOT bond to the electrical box, in order to run a dedicated ground wire directly to the breaker panel. This would be done to isolate the audio equipment from ground loops in the conduit ground mesh (and it *IS* a mesh, if you try to pipe a house is a 'proper' tree setup to avoid conduit loops, you'll end up using a lot more (expensive) copper wire to complete the house wiring).

Another exception would be medical equipment, again, using an 'isolated ground' running straight back to the breaker panel, to protect the sensitive equipment.

Since I don't know what sort of AC system we are talking about, window unit, or whole-house, I'm going to make an educated guess here, and say the contractor or inspector was looking at the wattage of the AC unit, and deciding that it needed an 'isolated ground' to protect the rest of the house in case it had a fault and shorted to ground. Higher amperage equipment may be too much for the 'thinwall aluminum pipe to safely deal with if there's a short that has to run through 'conduit ground' before tripping the breaker. That's if the contractor or inspector said what the OP says they said. I'm more likely to believe that the inspector sait that the AC unit needed it's own dedicated Branch Circuit, so it wouldn't interfere with other equipment in the house, or trip breakers if it was, for example, sharing a circuit with the refrigerator, and they both decide to start their compressor cycles close together.

Yes, the Chicago Electrical Codes are a bit of overkill, but remember, the Electrical Codes are part of the Fire Codes, and Chicago had a little fire a while back, you may have heard of it. We don't want a repeat of that night and day, it was a BIIIIIIG mess to clean up.