For many years the American Electrician's handbook has described the
use of a Mershon diagram for finding voltage drop. The Diagram in the
AEH is about 2 inches square. Does anyone know where a larger diagram
can be found?
On 11/22/06 11:19 AM, in article
snipped-for-privacy@j44g2000cwa.googlegroups.com,
" snipped-for-privacy@electrician2.com" wrote:
What does a Mershon diagram do that cannot be done with Ohm's law?
Bill
-- Fermez le Bush
If I could find my AEH, I would give a detailed account. What the
heck, I am going into Fairbanks this PM; I will stop by Barnes and
Nobel and buy the new one I was looking at a couple days ago, and let
you know. They have three methods of finding voltage drop in the AEH,
and it has been there for a long time. But I don't think most people
read the AEH. I haven't read it in about 25 years.
I did search for Mershon at Google and found a grave site for a rather
famous EE. I think he was a renowned inventor back in the 20th
century. I also went up to the University of Alaska EE dept several
years ago and asked the power Prof about it and he never heard of it.
I did find this. Apparently he was a Mechanical Engineer, and he was
the founder of the college ROTC program.
RALPH DAVENPORT MERSHON, both an excellent engineer and an american
patriot, worked untiringly for the goals and ideals he deemed important
to society. In his passing he served to perpetuate his work through
his generous contribution to The Ohio State University where he
received his formal education.
Dr. Mershon was born in Zanesville, Ohio, July 14, 1868 and
passed on --------, 1952. He graduated from the Mechanical Engineering
Department of The Ohio State University in 1890. From 1891 to 1900 he
worked as an engineer at Westinghouse Electric and Manufacturing
Company. While at Westinghouse his inventive skill and dedication
combined to produce many devices for which he was granted a total of
thirteen patents. The Compensating Voltmeter and 6-phase Rotary
Converter are two of his inventions. In 1900 he left Westinghouse and
began a private consulting practice where he designed various
electrical power plants in the United States, South Africa and Japan.
He was chief engineer during the designing and construction of the
Niagara, Lockport and Ontario Power Company and was a pioneer in
measurements of electrical transmission. During his career Dr. Mershon
was granted a total of 93 patents.
RALPH D. MERSHON'S career was interrupted by World War 1 where
he served as a Major in the Engineers Officer's Reserve Corps and
then as a Lieutenant Colonel in the Engineers Corps. He was in active
service from 1917 to 1919 detailed to the Naval Consulting Board.
During the early days of the war Colonel Mershon, in conjunction with
Brigadier General Edward Orton, Ohio State Graduate, Ohio State
President, William Oxley Thompson and Colonel George L. Converse, held
a series of meetings respecting a need for a comprehensive plan
embracing military training in the country's educational
institutions. This evolved into the "Ohio Plan" for training of
reserve officers. It was presented to the Military Affairs Committee
of the House of Representatives and was incorporated into the National
Defense Act of 1916. This dream of Colonel Mershon became the Reserve
Officers Training Corps (ROTC).
DEGREES
Degree of Mechanical Engineer - The Ohio State University - June
25, 1890
Honorary Degree of Doctor of Engineering - The Ohio State University
- 1936
Honorary Degree of Doctor of Engineering - Tufts College - 1936
That's a kind of nomograph. Those were popular in the days before
programmable calculators, PCs and PDAs.
Nomographs necessarily make some simplifying assumptions to make their
use simple, or in some cases possible. If you want one for the sake of
nostalgia, I'd suggest looking in the engineering/science section of a
good used book store. If you want one for design work, I'd suggest
pursuing a more up to date solution.
Nostalgia might be the reason for wanting to ask about the Mershon
Nomograph. As we look back so little credit is given to individuals
that have done so much. It seems unfair that the only thing left for
us to remember him by is a headstone in Ohio with the initials R.D.M.
Maybe the history channel will do something on this. As for an up to
date solution, we electricians for the most part are still using ohm's
law to find voltage drop.
I appreciate all this engineering advice, but we still use ohm's law.
The basic formula for voltage drop is
VD = (2*k*L*I/CMA) * Multiplier
(or else we simply use the 75 degree resistances from Table 8 from
Chapter 9 of the NEC.)
Multiplier is 1.0 for single phase, 0.866 for three phase 3 wire and
0.5 for 3-phase 4 -wire and K=12.9 for copper and 21.2 for aluminum.
This is the formula you had better use if you want to pass the
electrician journeyman's test.
That is about all there is to it. I might add it works fine for us
electricians and apparently has for the last fifty or so years. For
Corps jobs the rule of thumb is size up for every 70 feet for No. 12
AWG on 20 ampere circuits.
As for Maxwell and voltage drop, we leave that for the engineers on the
big design jobs. There aren't enough of them to go around so we often
have to do our own field designs for the smaller shop jobs.
---------
For what you are doing, the theory has been reduced to a set of rules or
formulae. Nothing wrong with that. Paul is trying to point out some of the
background behind such rules. They work very well as you know but there are
situations where they don't work well. You may not run into these situations
but will run into situations where you know just what practical problems
arise where a more theoretical person may not (e.g. being used to
considering transmission systems where line inductance is more important
than resistance, I did lose sight of the fact that in commercial/residential
and some industrial distribution, resistance is the dominant factor -and was
corrected by an electrician- was it you- if so -thanks). I hope that
"engineers" do learn from "electricians" as well as the other way around.
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