It was actually 9V but I'm not suggesting that it is typical or has any general validity. I just thought it would be a fun thing to try.
fundamental electricity question
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Carultch
Senior Member
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- Massachusetts
I know they combine in parallel. My point is, if 1 rod's contact resistance is significantly more than 50 ohms (such as 100 ohms), and 25 ohms are required, why do we get to follow the "two and done" rule? I would think we'd have to continue driving rods, until the total resistance is less than the requisite 25 Ohms. Or use another electrode.
Because 100 ohms combined in parallel with another 100 ohms, IS NOT 25 ohms.
K8MHZ
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Two rods are better because the inspector will pass the job.
FWIW, it was one of the city inspectors that told me of the study, which he was on. Two rods were in the region of 300-600 ohms. Western Michigan is basically a sand dune.
K8MHZ
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- Electrician
Where I live, the first table is usually 18-30 feet. If that table is bad, the next one is 150' or so.
We live on sand. One can go anywhere that hasn't been tilled and will hit white sand about a foot and a half down. When I install rods, I can push the first half of one in by hand. Not pound or drive, push.
K8MHZ
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- Electrician
Telegraphs used SWER systems all over the world.
If 25 ohm is not achieved by one rod, another one may be installed. But the code does not require the combined resistance be less than 25ohm because the method of grounding adopted in US ie TN-S system does nor require low earth resistance.
Actually resistivity is considered constant in the paper and resistance varies over distance, the variation becoming less and less as distance from earth rod goes on increasing.
Carultch
Senior Member
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So there is no physical reason why the requirement concedes "it is good enough" after two rods, even if the combined resistance is still well above the requirement for just one rod. It just is the way it is, because the grounding rules in the US say so.
To me, this makes about as much sense as OSHA saying one person may lift up to 50 lbs, and any load larger than that requires two people. But even if the load is 200 pounds, OSHA still only requires two people. Not necessarily an actual rule, just a hypothetical example to illustrate my point.
25ohms is a typical earth resistance value and so required by code. The physical reason for not requiring a low earth resistance is OCPD will readily operate in case of any ground fault even with high earth resistance because EGC in sound condition.
ggunn
PE (Electrical), NABCEP certified
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But as we all know, in most configurations the ground is not in any current path so the resistance to it isn't important. It's only a 0V reference.
See post #41.
does not answer my question
how far were the probes apart
a vom meter just stuck in soil?
It was done just for fun, not in depth analyses.
Open Neutral
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- Inside the Beltway
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Ufer grounds are based on the "parallel" principle. The ft^2 area of the slab in contact with the earth basically makes many parallel grounds; as well as the "always moist" aspect of concrete. (Herbert G. Ufer created his namesake to ground ammunition bunkers in the Southwest against lightning strikes.)
POCO does the same thing. Multiground Neutral has in effect thousands of parallel grounds per mi^2 of service area. Your Ufer/rod/pipe is part of the big picture.
As for SWER, the HVDC Pacific Intertie has a large ground at each end, even though it's {?now} balanced. Good place to dump faults when required.
https://en.wikipedia.org/wiki/Pacific_DC_Intertie
The grounding system at Celilo consists of 1,067 cast iron anodes buried in a two-foot trench of petroleum coke, which behaves as an electrode, arranged in a ring of 2.02 mi (3,250.87 m) circumference at Rice Flats (near Rice, Oregon), which is 6.6 mi (10.6 km) SSE of Celilo. It is connected to the converter station by two aerial 644 mm2 ACSR (aluminum conductor, steel reinforced) conductors, which end at a "dead-end" tower situated at 45.497586°N 121.064620°W.
&
The Sylmar grounding system is a line of 24 silicon-iron alloy electrodes submerged in the Pacific Ocean at Will Rogers State Beach[4] suspended in concrete enclosures about one meter above the ocean floor. The grounding array, which is 30 mi (48 km) from the converter station and is connected by a pair of 644 mm2 ACSR conductors, which are in the sections north of Kenter Canyon Terminal Tower at 34°04′04.99″N 118°29′18.5″W installed instead of the ground conductors on the pylons. It runs from Kenter Canyon Terminal Tower, via DWP Receiving Station U (Tarzana; a former switching station), Receiving Station J (Northridge) and Receiving Station Rinaldi (also a former switching station) to Sylmar Converter Station. On the section between Receiving Stations J and Rinaldi, one of the two shielding conductors on each of two parallel-running 230 kV transmission lines is used as electrode line conductor.
POCO does the same thing. Multiground Neutral has in effect thousands of parallel grounds per mi^2 of service area. Your Ufer/rod/pipe is part of the big picture.
As for SWER, the HVDC Pacific Intertie has a large ground at each end, even though it's {?now} balanced. Good place to dump faults when required.
https://en.wikipedia.org/wiki/Pacific_DC_Intertie
The grounding system at Celilo consists of 1,067 cast iron anodes buried in a two-foot trench of petroleum coke, which behaves as an electrode, arranged in a ring of 2.02 mi (3,250.87 m) circumference at Rice Flats (near Rice, Oregon), which is 6.6 mi (10.6 km) SSE of Celilo. It is connected to the converter station by two aerial 644 mm2 ACSR (aluminum conductor, steel reinforced) conductors, which end at a "dead-end" tower situated at 45.497586°N 121.064620°W.
&
The Sylmar grounding system is a line of 24 silicon-iron alloy electrodes submerged in the Pacific Ocean at Will Rogers State Beach[4] suspended in concrete enclosures about one meter above the ocean floor. The grounding array, which is 30 mi (48 km) from the converter station and is connected by a pair of 644 mm2 ACSR conductors, which are in the sections north of Kenter Canyon Terminal Tower at 34°04′04.99″N 118°29′18.5″W installed instead of the ground conductors on the pylons. It runs from Kenter Canyon Terminal Tower, via DWP Receiving Station U (Tarzana; a former switching station), Receiving Station J (Northridge) and Receiving Station Rinaldi (also a former switching station) to Sylmar Converter Station. On the section between Receiving Stations J and Rinaldi, one of the two shielding conductors on each of two parallel-running 230 kV transmission lines is used as electrode line conductor.
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- Placerville, CA, USA
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- Retired PV System Designer
Quick numbers on the net current typical and the GES resistances?
soooooo? No distance?
lol
very good primer http://assets.tequipment.net/assets/3/7/GettingDownToEarth.pdf
and the concept is simple but actual calculation complex
Basic relationship
R = p L /A
p = resistivity
L = length or distance between points of interest
S area of material i flows thru
A simplified example (numbers vary on field shape, etc)
assume 2 driven rods or plates on surface
impressed voltage across them
current flow and a voltage drop occurs
i can be a density A/sq ft
v a gradient v/ft
the shape is not unlike an American football
ovoid
the closer the rods the narrower, farther = wider
A can be described as a function of L
if L = 100 the max dia of the field is ~ L/2 for example
but we must use an avg dia, 0 at the ends L/2 middle
call avg L/4 so A = Pi (L/4 / 2)^2 = L^2/20
so R = p L / (L^2 /20) = p 20/L
so as L increases R decreases
measured a field today
not the field but the connection on a wye genset
substation 1 mva wye xfmr w/3 Ohm ground bed
1 mva wye genset on concrete pad 100' away (not on sub bed)
bolted to pad via studs welded to rebar
both feed a xfer sw which supplies a 500 hp fan
gen neut 4/0 Cu to sub bed in pvc
used the attached clamp on at the gen wye
Loop = 4/0 to sub bed back through earth to pad to gen skid to wye gen point thru internal wye bonding jumper
1.7 Ohm
ignoring metallic segments earth R path is ~1.7 Ohm or less
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junkhound
Senior Member
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- Renton, WA
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- EE, power electronics specialty
FWIW in the way of an actual number:
Have a concrete encased ground in own house, I tack welded the footing rebar together also (although contrary to IRC about welding).
Resistance to overall POCO grounding system (which is every other ground in parallel within a few miles?) is 0.61 ohms. Measured that resistance by connecting a big 10 ohm resistor between 120 Vac and the rebar and measuring the current then calculating he 0.61 ohms.
Footing is 20 inches to 30 in wide, 210 ft total length at 5 to 8 feet deep (e.g. 420 ft of rebar as ground), 3/4 of which is below the water table on hardpan.
Most of footing being at or near the ground water table probably makes for a much better ground that a rod or 2 in the dry sand in Tucson :lol:
Have a concrete encased ground in own house, I tack welded the footing rebar together also (although contrary to IRC about welding).
Resistance to overall POCO grounding system (which is every other ground in parallel within a few miles?) is 0.61 ohms. Measured that resistance by connecting a big 10 ohm resistor between 120 Vac and the rebar and measuring the current then calculating he 0.61 ohms.
Footing is 20 inches to 30 in wide, 210 ft total length at 5 to 8 feet deep (e.g. 420 ft of rebar as ground), 3/4 of which is below the water table on hardpan.
Most of footing being at or near the ground water table probably makes for a much better ground that a rod or 2 in the dry sand in Tucson :lol:
Very clever!
in a home the chance of a true earth fault is pretty small
in your case it would trip 120/0.61 ~ 200 A it would trip any ckt except the main (assiming a 200)
a 25 Ohm grid would be 5 A, no trip
but as you said your rod is in parallel with many others and possibly the sub station bed
like almost every residentialutility service
so in reality they are all likely sub 1 Ohm
if a person becomes the path doesn't matter if it 1 or 50 Ohms since the person is 1000!
120/1000 ~ 120 mA, pray the ckt has gfi lol
the higher the better since i fault will be lower
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