Some thoughts.
Conversions:
Many early systems were 2400 Volts delta with two bushing transformers.
After WW2, the use of electrical energy increased rapidly and many existing circuits were reaching their capacity.
With the addition of a substation transformer and a multiple grounded neutral, many of these systems were successfully upgraded to 4160 Volt wye circuits.
The town that I lived in did such a conversion.
If for some reason the existing transformers were unsuitable for phase to neutral use, then the additional cost of replacing the distribution transformers was incurred.
For new construction there are savings in insulators and in transformers which may be fitted with only one insulator and may be insulated for line to neutral voltage rather than line to line voltage.
It depends.
Code and safety:
Linked by Mbrooke said:
The Hazardous Multigrounded Neutral Distribution System
And Dangerous Stray Currents
Copyright Material IEEE
Paper No. PCIC-03-03
Donald W. Zipse, P.E.
Life Fellow, IEEE
Many of Mr. Zipse's ponts are valid, but he is pushing an agenda.
He cites anecdotes with no supporting facts.
What current was Edison pushing through the ground?
While I cannot recall the details, I have read of a number of investigations of livestock being injured by electricity but as I recall, the cause was generally wiring issues on the low voltage side and not influenced by primary wiring methods.
In specific regards to livestock, a common fault is not electrocution but a very low stray voltage (Not current) on the surface of stack waterers.
The tingle that the animals feel discourages them from drinking. The animals tend to suffer dehydration, not electrocution.
The cause is invariably local wiring issues.
Inappropriate calculations.
He tends to deal with resistance only and fails to consider reactance and impedance.
He gives a table of resistances but with no indication of the range of resistances typical of ground conditions.
At times he compares apples to oranges with a horse of a different colour.
Lightning is a real issue, but it may be argued that when lightning strikes a secondary conductor the effect on homes may be lessened by the effect of a multiple grounded neutral.
I will counter some of Mr. Zipse's inappropriate calculations with an inappropriate calculations of my own.
Consider a 10,000 Volt circuit.
In the event of contact between the hot line and ground at the end of the circuit, the hot line and the ground return will form a voltage divider.
With equal impedances between the hot line and the ground return, the voltage drop on the ground return will be 5000 Volts.
In the real world, the voltage drop on the ground return will probably be less, so this is a conservative estimate.
Mr. Zipse focuses on current and suggests that voltage is unimportant, it is the current that matters.
But, it is the voltage that drives the current and the voltage must be considered.
Let us now consider a fault about one mile from the substation, round it down to 5000 feet for convenience.
We have 5000 Volts dropping over 5000 feet.
That's 1 Volt per foot.
That is probably why we are not overwhelmed by cases of electrocutions of persons and animals due to multiple grounded neutrals.
Mr.Zipse uses single phase circuits for examples. This is somewhat misleading.
In a three phase circuit the diversity and cancellation of neutral currents from different phases results in the neutral current at the transformer being much less than the numeric sum of the individual neutral currents.
I suggest that both multiple grounded neutrals and insulated neutrals have strengths and weaknesses.
The choice is a compromise technically.
And by the way, I live in a ranching and farming area of the prairies.
Some of the farmers and ranchers are intelligent and well educated.
They also have sons and daughters who have become electrical engineers or electricians.
The farmers and ranchers also have a very strong political influence.
If multiple grounded neutrals were a problem they would have reacted long ago.
As for unity of action:
Who are the farmers (and by extension, the government) going to believe?
The wicked power companies or their sons and daughters who they paid to educate and can still quote the cost to the cent.
No dead or dehydrated stock to be concerned with.
But, actually we don't have the typical multiple grounded neutrals here.
What would Mr Zipse's reaction to know that we use the ground return system that failed for Mr. Edison?
Yes. No neutral carried back.
A typical installation is a pole mounted transformer with a grounding electrode at the pole.
An overhead neutral line is carried back one span and connected to a second grounding electrode.
That's it.
100% ground return.
No electrocutions.
No dead animals.
Like I said, it is a compromise.
Bill
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"Why not the best?"
Jimmy Carter
