Where Did The NEC 25-Ohm Ground Requirement Come From?

[dereckbc]

I think I know the answer. First let me say, if my info is correct, then what applied 100 or so years ago does not apply today as you should ascertain.

For a long time I have searched for the answer; ?Where did the NEC 25-ohm requirement come from?? I always suspected it had something to do with using earth as a conductor, but was not sure. Well after talking to a couple of very old ?FARTS? who can?t remember what your name is, but can tell you what they did in WWI, all had about the same answer, that I think is true. I think read somewhere Mike Holt came to the same conclusion, so maybe he might see this and concur or re-direct.

Back in the 1800?s when Edison built the first distribution system, he used earth as the return conductor which we now call the grounded circuit conductor (neutral for you laymen) 25-ohm happened to be about the same impedance of the phase conductor of a #8 AWG ran 7 miles. The 25-ohm requirement seems to be the highest acceptable impedance that would guarantee Edison?s system to work.

Edison did note that electrical workers did receive a shock from employees working along the distribution path (duh). What I do not know is when exactly distribution systems started employing a grounded circuit in the distribution system that we currently use today. What I do know is the NEC requirement did not follow technology.

This seems to make perfect since too me. In the late 1800?s and early 1900?s when the 25-ohm requirement was added to the NEC (1918 cycle I believe), house hold loads were very low compared to today?s home loads. With a distribution voltage of say several kilo-volts, all one would need is a simple transformer and a ground rod or two on the secondary to complete a sufficient circuit for a household voltage up to several kilo-watts.

To further add credibility is the modern SWER systems employed in Australia and Alaska today. They use the same exact concept of using earth as the return conductor with a 5-ohm requirement

Now of course in today?s distribution system, earth has no or little part, so the 25-ohm requirement is irrelevant and plays no effect in the operation of a household system.

In fact if were to go to the other extreme, and for example we could obtain say something less than 1-ohm, then we would have big problems with voltage gradients or step voltages. If you could even possible obtain such radical values using current distribution technology. Most of the load current from your transformer would flow through earth rather than the grounded circuit conductor. That would cause extreme EMF problems in addition to voltage gradients. Well stop there.

THOUGHTS?

 

[roger]

Hello Dereck, this is all the history I know of the requirement.

Roger

 

[stickboy1375]

This should not even be code, drive two ground rods and call it done, I will tell you this, a lightning bolt is about 40,000 amperes, so if you try and drive that thru a 25 ohm ground rod, you'll end up a lightning bolt at 1,000,000 volts which would surly create some surface arcing...

 

[bphgravity]

Though Edison can be traced to the original installations of electrical distribution and even further back to telegraphy, I do not believe the 25-ohm mystery can be answered with him or his operations. Even after Edison succumbed to the use of alternating current, he was more interested in creating small power plants to serve a relatively small area with very little open wiring and distribution; whereas Tesla/Westinghouse types were working on large central or regional power plants to distribute power throughout a state and even interstate as in the case with the Niagara Hydro-Electrical Facility.

I believe the key figure to solving this great question is with Cromwell Fleetwood Varley. Varley was heavily responsible for the development and operation of telegraphs in England and throughout Europe. He made a trip here to the states around 1868 and found the telegraph distribution to be in real bad shape as compared to his homeland. At the time, companies such as Gold and Stock and Western Union were operating on make-shift systems using relatively new and experimental equipment. (post Civil War)

Varley held patents for various lightning protection equipment, the Kelvin-Varley voltage divider and many telegraph improvements. And Varley is the key individual in the successful laying and operation of the Transatlantic Telegraph System.

The fact that a “Varley” was recognized at the time to be approximately 25-ohms cannot be a coincidence. The very problem Varley was solving was the earth resistance of both telegraph pole lightning protection and long runs of telegraph cable using the earth as the return conductor. Though I can’t attach Varley to the NFPA or as a member of any committees on the development and publication of the NEC, it appears this value was used in his honor based on his research and troubleshooting methods.

Mr. Holt has both the resources and contacts to get to the bottom of this investigation and I look very forward to what he discovers. I hope it is sooner or later. This issue can perhaps be compared to the holy grail of the King Arthur legend; however I have the premonition that the true story behind this requirement is not going to be what we think it is. My guess is that a couple of gentlemen were sitting around one afternoon in a smoke filled room say in 1920, discussion the requirements of grounding in the NEC and one casually mentioned that a minimum grounding resistance should be established for pipe and plate electrodes and 25-ohms was pulled out of thin air. ( I don’t think ground rods started being used until the 30’s???)