My 25-Ohm Obsession

[bphgravity]

Yes, I have a copy of the 1899 code and the 1896 minutes, but want the real thing if you know what I mean...

 

[brian john]

Bryan:

Someone emailed me a copy of that also. But I would give my left toe for the real thing.

 

[bphgravity]

I would give my right leg if they through in a crutch.....

 

[iwire]

Give up a leg or even a toe for an outa date code book?

You guys need serious help.

 

[brian john]

I've posted in another thread I just recieved a book "Electrical Dictionary" a 1929 dictional of electrical terms under the word

"Ground": See Earth

under "Earth" A Synonym of "ground" meaing the grounded side of an electrical circuit or machine.

 

[George Stolz]

Here's the online version of the 1899. The PDF is hidden on the website here somewhere, I'll dig it up.

Okay, found it. And I just stumbled across the 1896 minutes Brian G. mentioned.

1896 Minutes PDF
1899 NEC PDF

 

[brian john]

Bob:

I have already lost two finger tips and end of my left thumb. Thanks to the miracle of modern medicine I got all three back and I am betting on the same thing with the toe.

 

[bphgravity]

In the 1937 NEC, Section 2579 states:

Buried or driven electrodes shall, if practicable, have a resistance to ground not to exceed 25 ohms. If the resistance is not as low as 25 ohms, two or more electrodes connected in parallel shall be used.

Following is commentary stating:

Continuous underground water or gas pipe systems in general have a resistance to ground of less then 3 ohms. Metal frames of buildings and local metallic underground piping systems, metal well casings, and the like, have, in general, a resistance substantially below 25 ohms.

It is recommended that in locations where it is necessary to use buried or driven electrodes for grounding interior wiring systems, additional grounds, such as a connection to a system ground conductor, be placed on the distribution circuit. See 2533.

It is also recommended that single electrode grounds when installed, and periodically afterwards, be tested for resistance, particularly if there are no other grounds on the secondary distribution system.

10 years later in 1947, the section was moved to 2584. The main wording remained unchanged; however the reference to Section 2533 was dropped from the commentary. Also, the last paragraph from the 1937 edition was changed to state:

It is also recommended that single electrode ground when installed, and periodically afterwards, be tested for resistance.

10 years later again, the 1959 edition Section 250-84 was changed to state:

Made electrodes shall

instead of

buried or driven electrodes shall

This remained in place up until the 1971 edition, including the commentary.

A major change took place between the 1975 and 1978 editions.

In the 1978 edition, section 250-84 was changed to:

A single electrode consisting of a rod, pipe, or plate which does not have a resistance to ground of 25 ohms or less be augmented by one additional electrode of a type specified in section 250-83 Made Electrodes.

Also the commentary was completely removed.

1. Does anyone have a copy or knowledge of what substantiation was used to change this section and remove the commentary?

2. If they (CMP) knew as far back as 1937 and earlier that other electrodes had substantially lower earth resistance, then why did 25 ohms get selected as the magic number?

I can only assume one of two possibilities. One, an IEEE study was performed somewhere around 1918 that indicated 25 ohms was about as low of resistance a single buried or driven electrodes could be reasonably obtained.

Or two, 25 ohms was somehow calculated as all that is necessary for proper functioning of the grounding systems.

 

[bphgravity]

I haven't been able to get a hold of a 1918 edition of the NEC...yet... but I just received yesterday the next best thing.

In the 1919 Standard Wiring for Electrical Light and Power, Based on the 1918 NEC, there is a small but fruitful section on grounding that quotes portions of the 1918 code as follows:

When protective grounding is required, the suggestions given on the following pages 56 to 62 are in accordance with the recommendations of the Committee on Grounding of the National Fire Protection Association which were adopted October 17, 1917....

...The combined resistance of the ground wires, and connections of any grounded circuit, equipment, or lightning arrestor should not exceed 3 ohms for water pipe connections, nor 25 ohms for artificial grounds where these are used. Where because of dry or other high resistance soils it is impracticable to obtain ground resistance as low as 25 ohms, two such grounds 6 feet apart, if practicable, should be installed, and no requirement will be made as to resistance.

One step closer, yet still so far...

Looks like I need to find a copy of the minutes from the 1917 Committee Reports. I wonder if NFPA has kept copies of all those past meetings...

 

[George Stolz]

Five bucks says they were lost in a fire.

 

[LawnGuyLandSparky]

Yes, a fire caused by the 43rd circuit!