More fallacious grounding

mbrooke

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GoldDigger

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I was thinking more of 240V circuits with a 10A breaker as opposed to 120A circuits with a 20A breaker. And UK and EU seem to have fairly tight maximum resistance requirement for earth electrodes, while the NEC has none.
Of course in consumer environments, UK and EU tend to require Residual Current detectors which will also allow a non-metallic earth fault to trip a breaker fairly easily.
 

mbrooke

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I was thinking more of 240V circuits with a 10A breaker as opposed to 120A circuits with a 20A breaker. And UK and EU seem to have fairly tight maximum resistance requirement for earth electrodes, while the NEC has none.
Of course in consumer environments, UK and EU tend to require Residual Current detectors which will also allow a non-metallic earth fault to trip a breaker fairly easily.
With an RCD of course. The tight requirements are for a TT supply, where the earth resistance must be low and supplemented with RCD (GFCI) protection.


A 1 ohm ground rod may measure such at installation, but in reality it will fluctuate up and down from that value as the moisture in soil varies.
 

GoldDigger

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or PH....or proximity to a substation.....~RJ~
In theory proximity to a substation should not have any effect on the contact resistance of a ground electrode, nor of the earth resistance to from the electrode to the substation ground system.
In practice, high earth currents near a substation because of faulty neutrals might result in ground electrode voltage being different from remote earth voltage, with interesting consequences.
 

mbrooke

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In theory proximity to a substation should not have any effect on the contact resistance of a ground electrode, nor of the earth resistance to from the electrode to the substation ground system.
In practice, high earth currents near a substation because of faulty neutrals might result in ground electrode voltage being different from remote earth voltage, with interesting consequences.
But keep in mind, a 12kv uni grounded system will start to act line an LRG, MRG, HRG and then ungrounded as you move further from the substation.
 

romex jockey

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In theory proximity to a substation should not have any effect on the contact resistance of a ground electrode, nor of the earth resistance to from the electrode to the substation ground system.
In practice, high earth currents near a substation because of faulty neutrals might result in ground electrode voltage being different from remote earth voltage, with interesting consequences.
ergo , the nec 'disconnect' Gold one

thx:thumbsup:

~RJ~
 

GoldDigger

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But keep in mind, a 12kv uni grounded system will start to act like an LRG, MRG, HRG and then ungrounded as you move further from the substation.
Actually, a very firm NO to that.
In both theory and practice, the movement of current through a near infinite three dimensional solid is such that once you correctly subtract the interface resistance of the ground electrodes at both ends, the additional resistance of the path through the earth itself, regardless of distance, is so close to zero as to be negligible. Cross country telegraph circuits proved this quite well. Higher resistance over long distances was entirely the result of the metal line impedance. Modeled as a transmission line with resistance, capacitance and inductance but with a perfect return conductor (except for electrode interface resistance.)
 

mbrooke

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Actually, a very firm NO to that.
In both theory and practice, the movement of current through a near infinite three dimensional solid is such that once you correctly subtract the interface resistance of the ground electrodes at both ends, the additional resistance of the path through the earth itself, regardless of distance, is so close to zero as to be negligible. Cross country telegraph circuits proved this quite well. Higher resistance over long distances was entirely the result of the metal line impedance. Modeled as a transmission line with resistance, capacitance and inductance but with a perfect return conductor (except for electrode interface resistance.)


Then why are uni grounded system treated as ungrounded systems miles down from the substation?
 

GoldDigger

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Then why are uni grounded system treated as ungrounded systems miles down from the substation?
1. Can you recommend a reference for that? I would like to understand it better.
2. The term uni-grounded has different consequences in the context of a single premise system and the context of a transmission line with a lumped load at the end.
3. My first impression would be that if, as you say, the system with a ground reference ONLY at the substation and not at the load end, is considered ungrounded (from the point of view of induced transients?) it is not because of earth resistance but rather because of the impedance of the line conductors themselves.
4. A system with a grounding transformer at the load end with only a local ground electrode rather than a metallic neutral would, IMHO, be considered solidly grounded. The resistance of the earth path is not a factor in whether or not the system is considered solidly grounded.
 
Earthing is like a secret society or elite club: it's hard to get in, but once you're in, you're in. I think an MGN is able to keep neutral to ground voltage low because of the extensive number of electrodes throughout the system. You kinda have to be all in or all out. If you're not all in, you probably need to just not treat the neutral as grounded.
 

mbrooke

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1. Can you recommend a reference for that? I would like to understand it better.
2. The term uni-grounded has different consequences in the context of a single premise system and the context of a transmission line with a lumped load at the end.
3. My first impression would be that if, as you say, the system with a ground reference ONLY at the substation and not at the load end, is considered ungrounded (from the point of view of induced transients?) it is not because of earth resistance but rather because of the impedance of the line conductors themselves.
4. A system with a grounding transformer at the load end with only a local ground electrode rather than a metallic neutral would, IMHO, be considered solidly grounded. The resistance of the earth path is not a factor in whether or not the system is considered solidly grounded.
I'm trying to find something good, but nothing on the physics of why that is.


If you look at lightning arrestor sizing, uni grounded has its own category like ungrounded.


I'd PM Mivey, he would know what that is.
 

mivey

Senior Member
Actually, a very firm NO to that.
In both theory and practice, the movement of current through a near infinite three dimensional solid is such that once you correctly subtract the interface resistance of the ground electrodes at both ends, the additional resistance of the path through the earth itself, regardless of distance, is so close to zero as to be negligible. Cross country telegraph circuits proved this quite well. Higher resistance over long distances was entirely the result of the metal line impedance. Modeled as a transmission line with resistance, capacitance and inductance but with a perfect return conductor (except for electrode interface resistance.)
He said unigrounded, and is correct.
 

mivey

Senior Member
1. Can you recommend a reference for that? I would like to understand it better.
2. The term uni-grounded has different consequences in the context of a single premise system and the context of a transmission line with a lumped load at the end.
3. My first impression would be that if, as you say, the system with a ground reference ONLY at the substation and not at the load end, is considered ungrounded (from the point of view of induced transients?) it is not because of earth resistance but rather because of the impedance of the line conductors themselves.
4. A system with a grounding transformer at the load end with only a local ground electrode rather than a metallic neutral would, IMHO, be considered solidly grounded. The resistance of the earth path is not a factor in whether or not the system is considered solidly grounded.
To keep it simple, picture the neutral and egc at a panel as being at the same potential then diverging as you move down the feeder. An Earth bonded wye point at a substation puts Earth at the neural point of the phases. As we move well away from that location, the neutral point begins to float away and eventually Earth is practically unbonded from the neutral point.
 
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