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brian john

brian john

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I just read an article in Pure Power Magazine for the 3rd time, trying to figure out if I am just ignorant, and missing something, or if it is a waste of paper and ink. I understand the importance of getting articles published and how this may help your career and standing in your choosen field, but for some reason this article just just seemed wrong.

http://www.nxtbook.com/nxtbooks/reed/purepower_2008summer/ (starting on page 19)

While there is some usable information in this article it seems mostly a rehash of other data that is readily available. And while grounding is important (like duh), I think the article is focusing more on the earth ground connections and makes some common mistakes.

The NEC establishes a presumably acceptable level of resistance to ground as 25 ohms or less.
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If you skip the article at least read the conclusion.

I just think a subject as important as grounding could have been handled with a bit more insight? Effort?

In my expierence articles like this are common regarding grounding focusing on EARTHING, rather than the many mistakes made by electricians with critical power systems.
 
From the article:
With the growth of high-tech facilities with sensitive electronics, one will see the 5-ohm resistance value as the new target for grounding system.
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And that will accomplish what, exactly?

How do the sensitive electronics that are present on boats and airplanes work without this all important earth reference? ;)
 
080627-2002 EST

I like the article from the standpoint of a summary of some resistivity values.

However, many people, especially in the CNC field, will make broad statements like a good ground is required and that will solve your problem. They never define a good ground or why that will solve the problem.

I like to present the following illustration:
A quote from my web site
What does a given amount of ground resistance mean? Consider a supplemental ground rod at a machine with a ground resistance of 10 ohms back to the nuetral point of your main breaker box. This also means a ground rod array at the breaker box. The ground rod array at the service is the primary ground rod and the one absolutely required by the NEC. In this experiment said supplemental ground rod is connected only to earth ground. Apply 120 v to said supplemental ground rod from a hot line (not neutral or ground) in that breaker box with #12 wire which is fused by a 20 amp breaker. We will use a 50 ft piece of #12 copper wire which has a resistance of 0.08 ohms to connect from the breaker to said supplemental ground rod. The current that flows is I = E/R, or I = 120/10.08 = 11.9 amperes. This will produce a voltage on said supplemental ground rod of E = I * R = 11.9 * 10 = 119 volts relative to neutral and safety ground at the breaker box. This won't ever blow the 20 amp breaker. The failure to blow the breaker and the high voltage produced are the reason that the NEC does not allow this as a safety ground means. Do not run this experiment, but rather use an ac voltage in the range of 20 to 25 volts.
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Now consider a 10 ohm ground connection and a 10,000 A lightning current. This will raise the input end of the rod 100,000 V relative to some point down in the earth, and not too far into the earth. This means everything in the house or building connected to the EGC will have approximately this same potential. Additional paths to ground in the building will conduct some of that current thru the building to earth. Pick any other current you want to get a relative idea of what may happen.

Sensitive electronic equipment, whatever that means, should be in a Faraday shield with no external electrical paths to other points.

A high current in an EGC could thru its magnetic field affect sensitive equipment or from the potential drop along the conductor. I constantly illustrate the problem of fault currents in the EGC and how these can damage directly connected, no isolation, RS232 circuits.

Electronic circuits should be designed with isolation coupling rather than trying to get micro-ohm ground paths.

Lightning currents should be kept out of buildings by low impedance outside ground paths, high impedance paths into the building, and high frequency shunt capacitors from non-grounded wires entering the building to the ground electrode.

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It helps to remember specific purposes
  • Ground/Grounded/Grounding Conductor/GEC/Grounding Electrode are only there to ?limit voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines?, 250.4(A)(1). These are only intended to carry lightning and over-voltages from upstream failures, not circuit current for functionality.
  • Equipment Grounding/EGC/Bonding are there to enable ?Effective ground-fault current path.? 250.4(A)(3) through (5). These are only intended to carry fault current from shorts & failures, not circuit current for functionality. These are intended to allow operation of overcurrent protection.
  • Grounded Conductor is ?a system or circuit conductor that is intentionally grounded.? This conductor is intended to carry current (neutral). If this is a service it has a shared function with the EGC
Sure would be nice to only refer to everything past the GEC as bonding (not grounding!)
 
?Owners will benefit by having a good grounding system because their computers will work better, health care equipment will work better, and telecommunication systems will work better.? (Ronnie L. Ostendorf)

I disagree with Mr. Ostendorf?s conclusion! How will a good ground make these items work better?

A low resistance grounding system will protect these items but it does not contribute to their functionality. Lightning is not looking for metal or conductive objects it?s not looking for sensitive electronic equipment, IT IS LOOKING FOR EARTH! A high resistance grounding system will cause a lightning strike to flash through these items and a low resistive grounding system will help the lightning find earth ASAP.

Shorts are not looking for earth they are looking for their source. Earth is not the return path for the unbalanced load (or neutral current) the grounded conductor is, the only time these are in parallel is at the service entrance everything past the service disconnect isn?t.

Mr. Ostendorf starts out with a good topic of low impedance grounding but he muddles it by ending as if it contributes to circuit function.

Drive two rods/install a CCE ? ufer ? test if desired and leave! Any more than this should be designed for a lightning protection system.

Correctly bond throughout and circuit correctly, that?s most important to functionality and safety.
 
Correctly bond throughout and circuit correctly, that?s most important to functionality and safety.
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This was my point just not as succinctly put. BUT all the other information was a cut and paste article, heck I could have written that (put it together) but I would have written a different ending.
 
brian john said:
This was my point just not as succinctly put. BUT all the other information was a cut and paste article, heck I could have written that (put it together) but I would have written a different ending.
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I think you made a good assessment of the article.
 
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