Purpose of ground rod

[kwired]

I wholehearted agree Kwired. I would prefer something less than .5 ohm.

But grounding isn't really about clearing faults at low voltage levels. Bonding is.

If we really wanted to eliminate stray voltages on the grid we would stop using grounded conductors as current carrying conductors.

 

[kwired]

IMHO that is also true. They do have some.... not much. Hence the need to reteach and retrain people not to wholly trust the green conductor.

That green conductor is for fault clearing. it is bonded to the system grounded conductor. There can be temporary voltage on it during a fault, how long will depend on system characteristics and OCPD trip curves, but in general a pretty short time until opening compared to how long it may take for something to open (if it even does) if you connect an ungrounded conductor to an electrode in the earth of some sort.

 

[FionaZuppa]

Isn't 25ohm back to supply via earth better than infinite ohms? NEC is a book of best options, not always ideal options.

 

[kwired]

Isn't 25ohm back to supply via earth better than infinite ohms? NEC is a book of best options, not always ideal options.

For fault clearing on 600 volt and less systems it is pointless in most cases.

To help dissipate a high voltage transient imposed on a low volt system - yes.

 

[tom baker]

The purpose of ground rods is to promote discussion.

 

[Dsg319]

The purpose of ground rods is to promote discussion.

The 5/8” ones also work well for putting 500’ #12s on to pull.

 

[LarryFine]

When lightning hits electrical systems, everything becomes temporarily energized relative to the earth. That forces current through all available pathways to earth, proportionate to their conductances.

I see the electrode system at the premises as an attempt to minimize lightning-induced currents in pathways through the premises by providing what is hopefully a lower-impedance pathway around them.

I also think that line-to-neutral voltages are irrelevant during these events. While those voltages may even remain, the entire system is temporarily elevated way above normal relative to surrounding earth.

 

[don_resqcapt19]

That's not true at all. Not only does it permit multiple points of connection but in the case of a separate structure it mandates them.

It has not permitted a second connection from the earth to the grounded conductor at a second building since the 2008 code. Again, my understanding of single point grounding is a single point of connection between the earth and the grounded conductor of the system.

 

[synchro]

Benjamin Franklin was an early proponent of ground rods for lightning protection:

From his 1753 Poor Richard's Almanac:

“How to secure houses, etc. from Lightning
It has pleased God in his goodness to mankind, at length to discover to them the means of
securing their habitations and other buildings from mischief by thunder and lightning.
The method is this: Provide a small iron rod (it may be made of the rod-iron used by the nailers)
but of such a length, that one end being three or four feet in the moist ground, the other may
be six or eight feet above the highest part of the building."

 

[tom baker]

I would like to redirect on ground rods
100 years ago the required means of grounding was an underground metallic water line, as it was available at nearly every premise. And the water lines connected to a metal water main.
And remember originally electrical services were not grounding, there was a debate on this that was not resolved until after 1910.
One issue was early transformers would have the windings short, and cause an overvoltage on the secondary side, and grounding helped reduce that overvoltage
At the time grounding was the issue and bonding was not emphasized like it is currently, grounding was done for 250.4 A 1.
In the 1950's metal water lines were being replaced with plastic, and it was in the early 70's that the language changed to a grounding electrode system - I am not sure when ground rods were first required.
So we have gone from an low resistance water pipe to a high resistance ground rod. What we don't have is evidence that ground rods at a residence actually do work for lightning. We know they won't clear a fault

 

[synchro]

I would like to redirect on ground rods
100 years ago the required means of grounding was an underground metallic water line, as it was available at nearly every premise. And the water lines connected to a metal water main.
...
In the 1950's metal water lines were being replaced with plastic, and it was in the early 70's that the language changed to a grounding electrode system - I am not sure when ground rods were first required.
So we have gone from an low resistance water pipe to a high resistance ground rod. What we don't have is evidence that ground rods at a residence actually do work for lightning. We know they won't clear a fault

Along those lines, pages 853-854 of the Nov. 1920 Electrical Review at the link below have some interesting comments:

"Discussion of New Code Rules and of Grounding to Water Systems Before Convention of National Association of Electrical Inspectors.
...
If anxiety to assure good grounds had started functioning along more rational lines, ... we should have been attacking the real, weak spots of early grounding methods--high resistance in the actual connection to earth and poor or unreliable joints in the ground connections. What is really essential is a connection to moist earth exposing to the earth an extended contact surface. Nothing approaches the underground water-piping system for these qualities.
...
"Recently the American Water Works Association indorsed the use of ground connections between alternating-current secondaries and water piping. This does away with the last obstacle to good grounding which the high resistance of artificial grounds of small extent has imposed in many localities."

https://books.google.com/books?id=kFI_AQAAMAAJ&pg=PA854&lpg=PA854&dq="national electrical code" "rule 15A" transformer&source=bl&ots=dVkLu55qj_&sig=ACfU3U1Z3XYt_rwlWBxe0yJcLc5jy5VvlQ&hl=en&sa=X&ved=2ahUKEwjRk_HOwZH2AhUmI0QIHY_YDMIQ6AF6BAgSEAM#v=onepage&q="national electrical code" "rule 15A" transformer&f=false


In the webpage linked below about the 1920 NEC it says:

"Driven rods (ground rods) were not yet recognized by the code. “Artificial grounds” in the 1920 edition included driven pipes or buried plates (Rule 15A.). The 1918 edition added driven pipes and the 1923 edition added driven rods. The “25-ohm” rule first appeared in the 1918 edition. Article 250 now contains the rules for grounding and bonding."

https://www.linkedin.com/pulse/1920-national-electrical-code-bryan-p-holland-mcp-

 

[GoldDigger]

That is correct. It does take ALL paths and thats where parallel impedance some into play. But, providing a connection of lower impedance other than through myself is always preferred, even if it is as high as 25 ohms.

Providing an alternate path of lower impedance is only useful if that path reduces the voltage that you are exposed to. In the case of a solid fault which still does not trip the OCPD because of high GES impedance you are exposed to the full line voltage. And that line voltage source will, for all practical purposes, drive the same current through you whether there is a ground rod in parallel with you or not.
Your argument has some merit when looking at stray voltages caused by capacitive coupling or high impedance insulation failure. But in those cases an EGC would protect you far better than a ground rod.

 

[jaggedben]

When lightning hits electrical systems, everything becomes temporarily energized relative to the earth.

Or vice versa. When lightning hits the earth, the earth becomes temporarily energized relative to electrical systems. Did you watch the video?

That forces current through all available pathways to earth, proportionate to their conductances.

I see the electrode system at the premises as an attempt to minimize lightning-induced currents in pathways through the premises by providing what is hopefully a lower-impedance pathway around them.

I see grounding electrodes as more likely to reliably perform this function in the case of high voltage systems faulting onto lower voltage systems than in the case of lightning.

 

[romex jockey]

what is their purpose?

to pass inspection

~RJ~

 

[romex jockey]

Applies mainly to auxiliary electrodes but also arguably to multiple GECs as allowed by 250.64(F)(2). Electrodes at detached buildings, too.

Mike Holt would recommend a single main GEC, i.e. 'single point grounding', but the NEC permits lots of alternatives. I think it cannot be stressed enough that while NEC grounding electrodes may mitigate lightning strikes in many situations, they are not a lightning protection system, and the NEC permits installations that may make lightning damage worse.

theory lite video Jag

~RJ~

 

[FionaZuppa]

Providing an alternate path of lower impedance is only useful if that path reduces the voltage that you are exposed to. In the case of a solid fault which still does not trip the OCPD because of high GES impedance you are exposed to the full line voltage. And that line voltage source will, for all practical purposes, drive the same current through you whether there is a ground rod in parallel with you or not.
Your argument has some merit when looking at stray voltages caused by capacitive coupling or high impedance insulation failure. But in those cases an EGC would protect you far better than a ground rod.

Or the rod will guide a dump of a billion joules into the wet soil below, vs letting all that energy fan out on the surface looking for something to bite?

What if the joules are dumped onto the surface 10ft away from rod? The energy will find a path, and a rod into wet soil below is better than not finding it?

No ground rod (loop back) is less than 25ohm?

GEC's are almost always low ohms to EGC.

 

[Dsg319]

Or the rod will guide a dump of a billion joules into the wet soil below, vs letting all that energy fan out on the surface looking for something to bite?

What if the joules are dumped onto the surface 10ft away from rod? The energy will find a path, and a rod into wet soil below is better than not finding it?

No ground rod (loop back) is less than 25ohm?

GEC's are almost always low ohms to EGC.

I don’t follow what is in red. Can you explain?

 

[FionaZuppa]

I don’t follow what is in red. Can you explain?

You have bldg GEC that bonds to say 5 rods around the bldg. You install equip that has EGC tied to everything metal around it (frames, cases, conduit, jboxes, etc etc). You ohm between GEC point to a piece of conduit or motor frame (EGC), what you get?

 

[electrofelon]

Or the rod will guide a dump of a billion joules into the wet soil below, vs letting all that energy fan out on the surface looking for something to bite?

What if the joules are dumped onto the surface 10ft away from rod? The energy will find a path, and a rod into wet soil below is better than not finding it?

Again, 'lectricity takes ALL paths, so I don't see that argument holding any water.

 

[xformer]

Why do we even use a GE or a GEC? Wy not include the GEC with the Service conductors and just bond?