I've been reading about this phenomenon after reading
your question and the paper you posted . I can't see how
that much current could be travelling through ground unless
a similar percentage of Minnesota either uses SWER or is
wired incorrectly. If that's not the case then the statement
must be false.
Claims of current travelling through plumbing
is a problem with incorrect wiring for sure. If there is a
solid connection from the neutral at the service, to the
transformer in the street, to the transformer in substation
and back to the plant then it must be lower than 59%.
However there is a parallel path through ground rods connected
to transformers throughout the grid and at services. I can't
see it accounting for such a large percentage of current though.
I measured the resistance between 15 and 25 KOhms through
1 foot of damp soil with my multimeter. (Curiously it was rising
steadily like I had my meter on a capacitor and then would drop
and restart at different values with no particular pattern).
Compared to the resistance of utility wires it is obviously very
high.
I have seen (I'm an electrician) a single phase 100a service
lose the neutral in the meter socket. The customer called because
the screws connecting sections of the baseboard heater (hot water)
were glowing red. All of the power being used in the house was
travelling through any path to ground it could find.
simulate this circuit – Schematic created using CircuitLab
Figure 1 and 2. showing the danger of earthing through neutral.
In the first example the Lunatic 'Lectrician has 'grounded' the lamp case by connecting it to the neutral wire. All appears OK although the customer notices a slight tingle when she touches the lamp when it is switched on. This is because there's a slight voltage drop caused by the current through the return wire. Customer lives this time.
In the second situation one of Lunatic 'Lectrician's wires fell off. Unfortunately it was the return wire and now the metal lamp is live. (The bulb resistance isn't high enough to protect the customer.) Lunatic 'Lectrician may lose this customer.
simulate this circuit
Figure 3. Phase reversal danger with 'neutral' connected to frame of lamp.
Now not all countries use polarised plugs and unfortunately Lunatic 'Lectrician operates in one of these countries. There's a 50/50 chance of the plug going in the 'safe' way. There's a 50% chance of it going in this way.
Do you need any more convincing?
simulate this circuit
Figure 4. Properly wired lamp.
Consider what happens in a correctly wired system. If the live wire falls off the bulb and touches the metal case a large current will flow to earth. If the fuse is correctly rated it will quickly blow disconnecting the live supply. If the neutral shorts to the case the fault may not be detected and the current may split between the neutral and earth wires. An RCD / ELCB will protect against this type of fault, but that's another question.
Update after question update.
simulate this circuit
Figure 5. Break in earth wire.
No. It is never safe to use the earth wire as a neutral. Consider Figure 5: the ground wire has broken and anything else connected to it will become life once S1 is switched on. It's just too dangerous. It will cause potentially fatal confusion to the next person to modify the system. Stick to good practice, local regulations and enjoy a long life.
Best Answer
I think you are right, probably is to handle more current due to unbalance. Neutral cable's section for distribution transformer, should be the same as the phase conductor.