I have to disagree with Zipse on a number of issues. The reason for a lot of neutral current flowing through the soil in rural areas is because of the practice of running running single phase and 3 wire network primaries for a kajillion miles. In the areas where the the primary is the full 4 wire wye the neutral current in both the wire and the earth adds up to a rather small value.

Since most of the cost of erecting an overhead line is in the poles and the first 2 wires adding 2 more wires does not cost that muych extra. A wooden pole itself costs about $3,000 to $5,000 plus labor. Given that modern insulators have vey low losses the extra no load loss of a full 3-phase line is negligible.

For that matter, some of the transformer primaries can be connected phase to phase so that you have a mixed multigrounded/unigrounded system. Single phase motors have better voltage drop when starting when the transformer primary is phase to phase. A sulfuric acid plant just west of Akron, Ohio has all of their transformers including some single units strictly phase to phase on 7,200Y12,470 volts. A 12,000 volt 2 bushing transformer has a tap for 12,500 volts. For that matter, connecting some transformer primaries phase to neutral and some phase to phase reduces 3rd, 5th, and 7th harmonics in the primary system. Cleveland Electric Illuminating Company does something similar on 7,620Y13,200 volts with 3-phase pad mounted transformers that have delta primaries.

Using a totally ungrounded primary system actually makes a lot of safety problems worse. Wind induces static eletricity on overhead wiring plus there is Saint Elmo's Fire when it is raining. Frequent connection of transformers phase to neutral get rids of a lot of "low level" static electricity that fries stuff. The one time that 480 volts ungrounded blew up in my face and I had to work a 16-hour shift was during a rainstorm. We lost a 300-horsepower soft start, the wires that fed it, and a 100-HP motor. The damage rate on 277Y480 is so much lower that there is no comparison.

Isolating the secondary neutral does not necessarily keep current in the earth from invading swimming pools or milking parlors. If your secondary neutral is isolated just remember that the secondary needs 4 grounding electrodes to make telephone protector blocks and CATV legal. The CATV also needs an isolator for the cable shield lest primary neutral current sneaks through. I still cannot figure out how losing the service neutral to a house that has well water does not fry the CATV service.

Also, the refusal of power companies to allow houses to have 3-phase power to operate heavier loads. A geothermal heat pump all by itself eliminates the distinction between residential and industrial. If a power company wants to use radios to force contral air conditioners onto an automatic generator during peak periods what the homeowners needs is a 3-phase generator turning a 3-phase air conditioner. There is a standard generator under the Miller brand name that provides both 10,000 watts 120/240 single phase and another 10,000 watts 277Y480 3-phase. Considering that modern electronic meters can be set up so that 1 meter totalizes the other meter there is not a really good reason why house current cannot be 120/240+277Y480 6 or 7 wire. This would actually provide better power quality in geothermal heat pump neighborhoods by putting the heat pumps on transformers for the lighting. For that matter, that way that noise abatement was done along Interstate 71 in Columbus, Ohio was to put 3-phase air conditioners into all of the hoses nearby. Those companies that do put 3-phase power into houses have shown that it does not cost that much extra, relieves some types of problems, and the zoning board thought police have not encountered problems with somebody running a machine shop in their basement or garage.