Measuring for 25 Ohms or Less - 250.56

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cschmid,
Now if we look at a ground rod in dry sand and a sweaty water line in the ground which one has the least resistance to the earth/ground? so if we have both the ground rod and the water line to the same point the water line is most likely the fault point. Now that would have the potential to energize the entire line during the fault. So then if that is the case than how important is 25 ohms to ground or less?
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I have no idea of what you are talking about.

My previous comment was about the terms "protective earth" and "functional earth". I have no idea what those terms mean. I have never heard or seen them used before.
Don
 
cschmid said:
.... I am assuming you are not sure of my wording of my comment. ....
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That would be exactly true - I don't have definitions for alot of the terms you use.

When you use a term I on't understand, I'm going to go the the NEC definitions first, then the IEEE dictionary, then my Funk and Wagnalls. If I still don't understand in the context used, I'm going to ask, "What's a 'functional earth' as opposed to a 'protective earth'?"

carl

edited to change 'ground' to 'earth' (two places)
 
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Pierre -
I re-read your post 31 a couple of times. Are you suggesting we don't drive the first ground that must be measured to insure <25ohms and just drive the second rod that need not be measured?

Cause if you are, that was very subtle and is pure brilliance.

carl
 
csc -
I think that part of the communications issues stems from us having somewhat differing ideas about the purpose of the residential service ground rod. Here is an example:
cschmid said:
.... one of the functions of a ground rod is to maintain the electrical equipment connected to it at the earths potential; another purpose of a ground rod is to dissipate over voltages to the earth. ...
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I don't have any evidence that a ground rod will "maintain the electrical equipment connected to it at the earths potential".

carl
 
okay lets see if I phrase this properly protective earth(PE) a connection that ensure all exposed conductive surfaces are at the same electrical potential as the earth. Functional earth is when the earth my carry a current during the operation of a electrical device. like a GFCI uses a ground connection during its operation. I hope that is a good example. Now the term earthing is, in America we call it grounding in the rest of the world they call it earthing. I hope this helps
 
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stickboy1375 said:
HUH???[/COLOR]
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I'm not sure, but I think the subject has evolved (degraded?) to "Grounding" vs "Bonding". If it has, I'll likely just listen and learn.

carl
 
A 'functional earth' (which I guess must be a non-US term) is a ground connection that carries current by design. This is very different to the normal Protective Earth, which carries current only under fault conditions. Functional earth wiring is always cream, to differentiate it from protective earth wiring, which is green or green/yellow stripe, depending on locality.

Functional Earths are generally a telecomms situation, where in a number of forms of signalling the ground is used as a conductor.

These days you would have to look pretty hard to find functional earth wiring outside a telephone exchange, whereas three decades ago, in the UK at least, it was everywhere.

Functional earths have nothing to do with a GFCI.
 
coulter said:
Pierre -
I re-read your post 31 a couple of times. Are you suggesting we don't drive the first ground that must be measured to insure <25ohms and just drive the second rod that need not be measured?

Cause if you are, that was very subtle and is pure brilliance.

carl
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You state it much better than I was trying to.:)

All I really want to understand is what is the reason for the second ground rod, if we are not required to measure/know the Ohmic value?
 
cschmid said:
okay lets see if I phrase this properly protective earth(PE) a connection that ensure all exposed conductive surfaces are at the same electrical potential as the earth. Functional earth is when the earth my carry a current during the operation of a electrical device. like a GFCI uses a ground connection during its operation. I hope that is a good example. Now the term earthing is, in America we call it grounding in the rest of the world they call it earthing. I hope this helps
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Your terms and concepts are not quite what most of us here are accostumed to. I am not sure if you were trained in the USA. If you were (you mentioned you have Soarse book) I would like to know where you developed the terms you are using.
As per 250.4(A)(5)and 250.54, the earth is not permitted to be used as an "effective ground fault current path".

GFCIs do not require the use of equipment grounding conductors (EGC)...if that is what you meant. You use the term "ground connection", is that the EGC or the Grounding electrode conductor connection (termination) to the Grounding Electrode?
 
gfci was a poor example and yes you would find functional earth in old school telephone system. no this is not a bonding vs grounding issue and yes functional is an intentional use and protective is fault usage. I apologize for the improper usage of GFCI in my explanation. No I was trained in the USA.
 
Pierre:

in your post 31 did you really say this. Are you suggesting we don't drive the first ground that must be measured to insure <25ohms and just drive the second rod that need not be measured?
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That is brilliant. While I have the capabilities to test any gnds rods we drive I'd like to try this on an inspector.

I own 5 or 6 different 2/3/4 point earth/ground resistance tester plus 2 or 3 clamp on testers, with small services we just drive two rods, less hassle.
 
now for a bunny trail for just a moment. GFCI function by measuring the current on L1 to the current on the neutral and at .005 of an imbalance it than causes the circuit breaker to trip and it is suppose to trip in .0025 sec if I am correct. What I would like to know is how does this device trip or what causes the activation of the breaker? like after it senses the .005 imbalance what happens internally in the breaker?

If neither use of the earth is permited by the NEC than why do we Ground the center point of a Y transformer? I am not talking aout grounding anywhere except at the main service now.

This started out on why we need 25 ohms to ground on a Ground rod. I just want to keep focused.

I am just trying to learn and hope anyone who reads this can learn something as well. I know I have met some smart people already.
 
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cschmid,
GFCI function by measuring the current on L1 to the current on the neutral and at .005 of an imbalance it than causes the circuit breaker to trip and it is suppose to trip in .0025 sec if I am correct.
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Actually at 5mA it is permitted to take up to 7.2 seconds to trip.
If neither use of the earth is permited by the NEC than why do we Ground the center point of a Y transformer?
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I don't see that as being functional or protective as you have defined those terms.
Don
 
An example of what the IEC term "PE" relates to would for example be the green wire in the microwave power cord, the green wire in the wall, the earth bus bar.

The GEC and supplemental bonding conductors are all included in the PE scope, as they are all designed to maintain the equipotential zone, well, equipotential, and thats part of the PE scope.
 
See I did learn something. okay if the ground rod does not fit either of thous terms, then may I ask what we would classify a ground rod as? I was always under the assumption the ground rod was to aid in clearing a fault/over voltage and stabilizing the operating voltage. bonding everything to it was to ensure that the fault cleared at the same point. I was also under the assumption that having every thing at the same potential and to the same point help clear faults faster. So If I have missed something here feel free to correct me. So I assumed that the ground rod actually fit both them terms and was trying to establish why we all need to be more diligent on the 25 ohm's to ground or less.

I thought the GFCI was suppose to trip faster so I learned again. yet can someone point me to the internal working of a GFCI?

I also apologize for the improper use of a GFCI when I really do know the proper operation just poor choice of example.

So than can someone give me some examples of a FE (functional earth) devices?
 
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cschmid said:
I ask what we would classify a ground rod as? I was always under the assumption the ground rod was to aid in clearing a fault/over voltage and stabilizing the operating voltage. bonding everything to it was to ensure that the fault cleared at the same point.
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Yep a lot of people make that mistake and assume dirt has some funtion to clearing a fault, when in reality it has no function in regards to clearing faults on low voltage applications. The NEC forbids earth to serve as any fault clearing path.

Earth an only be used as a conductor in NESC applications and has an extremely important role in high voltage transmission.

Anyway to answer your ground rod question: it is a part of the Ground Electrode System (GES). Look at it like a sub-unit of the GES as is rebar, water pipes, etc. All of these combined form the GES as a whole. They are all required to be bonded together in the event any one of them should come into accidental contact with energized conductors to operate OCPD by routing them throught the Ground Electrode Conductor, or earth for high voltages for utilities and lightning.
 
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