Ground Rod Theory

[Speedskater]

So how does that translate to the 6' minimum rod spacing required by the NEC?

Well it doesn't translate. It's all about trade-offs and convenient numbers.
* * * * * * * * * * * *
If a high voltage line falls across a 120/240V wire, it's low resistance to Earth that matters.
With lightning, it's low RF impedance to Earth that matters.
* * * * * * * * *
I would think the once spacing is greater than longest rod length, we quickly get into diminishing returns.

 

[roger]

Why 25 ohms in the first place?

It's a holdover from way back and not really related to electrical safety.

The Orgin of the Magical 25-Ohms?

Varley unit: A unit of electrical resistance, 19th century, standardized at the Electric Telegraph Company in Great Britain, and in concept equal to the resistance of 1 mile of copper telegraph wire. The unit was named for C. F. Varley, an engineer working for the company who devised a...

forums.mikeholt.com

 

[infinity]

Another flaw I see in this rod spacing or for rods in general is that they're often very close to the foundation so the sphere of influence can be diminished by almost half since there is no earth on one side of the rods.

 

[ptonsparky]

Another flaw I see in this rod spacing or for rods in general is that they're often very close to the foundation so the sphere of influence can be diminished by almost half since there is no earth on one side of the rods.

Is it reduced or does the foundation add to it? IDK
Every situation changes so it probably doesn't matter much. Two rods and done.

 

[drcampbell]

Another flaw I see in this rod spacing or for rods in general is that they're often very close to the foundation ...

If low resistance to the Earth (and elimination of voltage gradients) is important, why are we not requiring Ufer grounds wherever & whenever there's concrete in contact with the soil?

I understand it's currently a coordination-of-trades hassle while Ufers are used only occasionally, but if they were required for every building, the concrete workers would eventually figure it out. (in a generation or two)

 

[infinity]

Is it reduced or does the foundation add to it? IDK
Every situation changes so it probably doesn't matter much. Two rods and done.

I don't know either. Just adds to the mundane theory that ground rods are actually useful.

 

[Seven-Delta-FortyOne]

If low resistance to the Earth (and elimination of voltage gradients) is important, why are we not requiring Ufer grounds wherever & whenever there's concrete in contact with the soil?

I understand it's currently a coordination-of-trades hassle while Ufers are used only occasionally, but if they were required for every building, the concrete workers would eventually figure it out. (in a generation or two)

It is required

 

[jim dungar]

Another flaw I see in this rod spacing or for rods in general is that they're often very close to the foundation so the sphere of influence can be diminished by almost half since there is no earth on one side of the rods.

Buried underground objects, like tanks and foundations, can negatively impact ground resistance testing results.

 

[infinity]

It is required

It can be but there are a few conditions that need to be met like a minimum of #4 rebar being present and that the footing is in direct contact with the earth. For places where there is no rebar or there is a barrier under the footing the CEE is typically not used. I agree that if there is no rebar installing 20' of copper in the footing is a good idea.

 

[petersonra]

Some people seem to be making the assumption that a better grounding electrode system, whatever better might mean in that context, somehow improves electrical safety. I don't think there's any real evidence to suggest that the grounding electrode system has much of anything at all to do with electrical safety. It's just a way of pinning the voltage to Earth.

 

[junkhound]

Safest system is ungrounded - with caveat that it is monitored so disconnect or warning flasher if one side gets grounded.

So, some type of ground reverence (aka ground rod) needed for reference point.

watch for it in a future code revision, the mfg need new requirements sell more 'advanced' and pricey hardware.

 

[electrofelon]

Safest system is ungrounded - with caveat that it is monitored so disconnect or warning flasher if one side gets grounded.

So, some type of ground reverence (aka ground rod) needed for reference point.

watch for it in a future code revision, the mfg need new requirements sell more 'advanced' and pricey hardware.

I don't know if that would ever happen just because we are so obsessed with grounding here in the US. It took years for the NEC to accept and realize that ungrounded PV systems are safer.

 

[ggunn]

I don't know if that would ever happen just because we are so obsessed with grounding here in the US. It took years for the NEC to accept and realize that ungrounded PV systems are safer.

Tell me about it. Grounding is Black Magic and it seems like every AHJ has a different Book of Spells.

 

[Rock86]

I'm not sure if I missed anyone saying it, but what about the fact that there is a difference if you drive ground rods 6' apart in NY soil, Nevada sand, Virginia clay, or Mississippi mud. you could test the ground rods on a dry day and have a different result after it has rained.

 

[PaulMmn]

I'm not sure if I missed anyone saying it, but what about the fact that there is a difference if you drive ground rods 6' apart in NY soil, Nevada sand, Virginia clay, or Mississippi mud. you could test the ground rods on a dry day and have a different result after it has rained.

In 1950s science fiction they spoke of 'grounding' massive antennae and other installations-- a circle of earth 50' or more around the installation, heavily soaked with salt water. Don't remember what they used as ground rods!

 

[LarryFine]

Don't remember what they used as ground rods!

Buried radial wires?

 

[don_resqcapt19]

Safest system is ungrounded - with caveat that it is monitored so disconnect or warning flasher if one side gets grounded.

So, some type of ground reverence (aka ground rod) needed for reference point.

watch for it in a future code revision, the mfg need new requirements sell more 'advanced' and pricey hardware.

As long as those using the ungrounded system understand that with a system of any size the capacitance to ground from any of the circuit conductors can result in a fatal shock if a person touches the ungrounded conductor and is connected to earth or any conductive object that is connected to earth..

 

[mtnelect]

There should be a requirement, to water your ground rod every day.

 

[GoldDigger]

In 1950s science fiction they spoke of 'grounding' massive antennae and other installations-- a circle of earth 50' or more around the installation, heavily soaked with salt water. Don't remember what they used as ground rods!

Relatively low frequency radio waves (as low as 400-500kHz), used for ship to shore radio telegraphy used transmitting stations which were typically installed at salt marshes near the coast. The receiving stations could be much smaller and were typically located directly on the coast. One of the last of these stations was KFS. When I was at Stanford in the 1970's I could hear code transmissions from KFS as interference in other radio receivers. The transmit power was high enough to be easily audible via intermodulation distortion in all but the best receivers.
I didn't really know International Telegraphy Code (not actually Morse Code,btw), but still remember "dah-di-dah-dit dah-dah-di-dah (repeating) dah-di-dah-di-di-dah-dit di-di-dit (CQ KFS)
The transmitter was located on the west shore of San Francisco Bay, since the coast itself had no marshlands. And, yes, submerged wires radiating from the base of the tower did the job of grounding.
The very large "ground plane" of the marsh served as a mirror to allow a vertical tower to act like a vertical dipole antenna for good directionality parallel to the earth's surface. For interplanetary radio or deep space radio it would have to have been used as a reflector for an upward facing antenna suspended above the ground.

 

[PaulMmn]

Buried radial wires?

Sounds likely. We're talking absorbing energy rays from space ships shooting at the ground installations!