Purpose of ground rod

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250.4(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
Informational Note No. 1: An important consideration for limiting the imposed voltage is the routing of bonding and grounding electrode conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided.
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Straight from Article 250.
 
don_resqcapt19 said:
How does it provide a back door for damage by lightning?
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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.
 
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I have never seen Mike say that about the required grounding electrodes. They are single point grounding if installed correctly as they are only permitted to connect the the electrical system at a single point.
 
As Don stated it’s only at one point connected to the electrical systems, could that mean to the service neutral and the rest are just extensions from that via being attached EGC.
 
petersonra said:
Just how does that little ground rod provide any measure of safety?
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IMHO. It provides a path of current flow that is around people. Over the past 70 years, construction practices have changed immensely. Seventy years ago, electricity was provided to residential units and no one gave a rip about grounding. Currently, and in my neck of the woods, concrete slabs on earth are the foundation of choice. these concrete slabs can act as a conductor. In an attempt to keep people safe, the ground rod provides an underground circuit back to the source. Anyone who installs electron pathways should know that ground (or Earth, or dirt) is really not a good conductor since several factors, both physical and environmental effect its impedance. Since driving a ground has been practiced for a long time, it has to be doing some good. :) On my personal overall safety scale, a ground rod is at a 2 on a scale of 1 to 10. which can simply be demonstrated mathematically with 2 ground rods, some earth, and a transformer.
It can also provide a shortened path back to the grounded conductor. That is a different conversation. :)
 
electrofelon said:
But current takes ALL paths so I don't see how that is possible. Perhaps you can provide a very specific setup and example we could discuss to illustrate your point?
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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. :)
 
petersonra said:
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.
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and the entire electrical grid is multiple separate structures with an electrode at nearly if not all structures, to include customer side of service electrodes. One big network of electrodes all connected to a common conductor as a general rule.
 
electrofelon said:
But current takes ALL paths so I don't see how that is possible. Perhaps you can provide a very specific setup and example we could discuss to illustrate your point?
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How about a simple battery charger with positive connected to one piece of rebar, and the negative connected to a separate piece or rebar. Both rebar are driven into the earth with 5 ft distance between. The primary path for current flow would be under ground.
 
xformer said:
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. :)
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25 ohms is only good for dissipating high volt transients. Connect 277 volts to a 25 ohm resistance and you only get 11 amps of current flowing, not going to trip a 15 amp overcurrent device, will create voltage gradients and dangerous touch potential in the vicinity of the electrode.

Grounding electrodes have almost no value in protecting against low voltage fault conditions (as in under 600 or even 1000 volts systems).

If you bond the slab you earlier mentioned, you make an equipotential grid out of it, but the dangerous gradients will still exist at the edges of the slab or possibly to items not connected to the bonding network but within reach of a user.
 
kwired said:
25 ohms is only good for dissipating high volt transients. Connect 277 volts to a 25 ohm resistance and you only get 11 amps of current flowing, not going to trip a 15 amp overcurrent device, will create voltage gradients and dangerous touch potential in the vicinity of the electrode.

Grounding electrodes have almost no value in protecting against low voltage fault conditions (as in under 600 or even 1000 volts systems).

If you bond the slab you earlier mentioned, you make an equipotential grid out of it, but the dangerous gradients will still exist at the edges of the slab or possibly to items not connected to the bonding network but within reach of a user.
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I wholehearted agree Kwired. I would prefer something less than .5 ohm.
 
kwired said:
25 ohms is only good for dissipating high volt transients. Connect 277 volts to a 25 ohm resistance and you only get 11 amps of current flowing, not going to trip a 15 amp overcurrent device, will create voltage gradients and dangerous touch potential in the vicinity of the electrode.

Grounding electrodes have almost no value in protecting against low voltage fault conditions (as in under 600 or even 1000 volts systems).

If you bond the slab you earlier mentioned, you make an equipotential grid out of it, but the dangerous gradients will still exist at the edges of the slab or possibly to items not connected to the bonding network but within reach of a user.
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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.
 
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