earthing calculation

kingpb

Senior Member
I'll go out on a limb here and say, that in general, there is not a substation in this country that has been, or will be built where a local inspector has any involvement, or where the NEC will be utilized except for maybe the receptacles in the building.

These are engineered facilities designed far and above the minimum safety requirements of NEC.
 
But the underlying principle of NEC i.e being free from danger to life is not met in this case, IMO.
The NEC is a BARE MINIMUM standard and not meant as a source for design criteria.

What the OP is attempting is to design a system with a much higher standard set forth by the NEC. IMHO, the OP is focusing too much on 'the math' and not enough on the design.

When I worked with the telco, they were adamant about low resistance grounding. Their system looked nothing like anything in the NEC. It combined Ufers, rods, grids, loops and #2 solid bare tinned copper conductors. All rods were cad welded. Some of the rods were under the ufer pads, welded to the re-rods.

Grounding rods, no matter how many, will be of relatively high resistance and variability. A system that must maintain a low resistance over time should not be made solely of ground rods, no matter how many, or how long they are. You may measure 4 ohms right after installation, but in a year it may be 100 ohms.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The NEC allowable maximum ground resistance of 25 ohms does not apply to outdoor substations and other outdoor switching apparatus belonging to a consumer. The ground resistance in such cases should be maintained in the range of 4 ohms or less.
why? what purpose is served?
 

Sahib

Senior Member
Location
India
The NEC is a BARE MINIMUM standard and not meant as a source for design criteria.
NEC is not a source for design criteria but a BARE MINIMUM standard to ensure safety to life. But if followed by 25 ohms maximum resitance rule satisfied by a single ground rod in a large outdoor substation, it could lead to electrocution on HV ground fault.
Grounding rods, no matter how many, will be of relatively high resistance and variability. A system that must maintain a low resistance over time should not be made solely of ground rods, no matter how many, or how long they are. You may measure 4 ohms right after installation, but in a year it may be 100 ohms.
TAKE maintenance into account
 
NEC is not a source for design criteria but a BARE MINIMUM standard to ensure safety to life. But if followed by 25 ohms maximum resitance rule satisfied by a single ground rod in a large outdoor substation, it could lead to electrocution on HV ground fault.
Any substation owned by the POCO is not covered by the NEC. Only those owned by private companies are, and they will be rather small.

Regardless, every substation I have ever seen does not rely on ground rods for earthing. Again, the 25 ohms is a bare minimum for a single rod. At a residence in my area, two rods only get around 200 - 500 ohms. A single rod, in a test, measured 1300 ohms. We have sandy soil here. So, most residences only have 200 - 500 ohms resistance. If more were needed to reduce a hazard it would be stated as such.

The grounding at a sub station is more focused on lighting protection than line to ground faults. As such, they are overkill for the utility voltage and current capacity just by design. I have seen electrode conductors in the 750 MCM range on sub stations. Those sure aren't going to a ground rod.


TAKE maintenance into account
1) What kind of maintenance is there for ground rods?

2) How is maintenance going to eliminate the changes due to weather? Any ground rod will have a varying resistance due to the moisture content of the soil, which usually goes from dry to wet during the course of the year.

If an electrolytic ground is used, there is maintenance that can be done to insure stability.
 

Smart $

Esteemed Member
Location
Ohio
...it could lead to electrocution on HV ground fault. ...
Think about it. When around any source of electricity, is it greater or less resistance between touch points and the electricity source that keeps us from getting shocked or electrocuted?
 

kingpb

Senior Member
You are free to submit proposals to change it.
Wouldn't do any good, I'm not a manufacturer with a product to sell.:cry: Unfortunately that's what the NEC appears to have become, a way for manufacturers to get their products sole sourced by pushing some new requirement in the NEC.:rant:

I wll agree there is no basis for the 25 ohm.
 

Sahib

Senior Member
Location
India
Regardless, every substation I have ever seen does not rely on ground rods for earthing.
In addition to ground rods, a substation employs flat strip conductors connected to ground rods for earthing.
Again, the 25 ohms is a bare minimum for a single rod. At a residence in my area, two rods only get around 200 - 500 ohms. A single rod, in a test, measured 1300 ohms. We have sandy soil here. So, most residences only have 200 - 500 ohms resistance. If more were needed to reduce a hazard it would be stated as such.
A residence is a different breed from an outdoor substation, which is unlikely to have HV ground faults.

1) What kind of maintenance is there for ground rods?

2) How is maintenance going to eliminate the changes due to weather? Any ground rod will have a varying resistance due to the moisture content of the soil, which usually goes from dry to wet during the course of the year.
If an electrolytic ground is used, there is maintenance that can be done to insure stability.
In some substations, watering funnels provided to pour water periodically during dry weather. Or suitable provision is to be made during design stage itself.
 

kwired

Electron manager
Location
NE Nebraska
Wouldn't do any good, I'm not a manufacturer with a product to sell.:cry: Unfortunately that's what the NEC appears to have become, a way for manufacturers to get their products sole sourced by pushing some new requirement in the NEC.:rant:
Sadly this is what is happening to it even though it does go against its own principles.

If you throw enough money at something you eventually get what you want, is the approach taken by the manufacturers, just like lobbying at the State house or in Washington for something in your interest.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
As has been stated in countless (well I did not count them anyway) threads here and times in Mike Holt's materials, an earth ground (even as low as 4 ohms) is not an effective fault clearing path at low voltages. But for MV or higher it can be. That is point 1 for low impedance earth connections at a substation.
Lightning protection is important when dealing with long runs of open transmission wires. Point 2.
And given the possibility of current leaks and even high voltages from via electrostatic or electromagnetic induction, the best possible equipotential grid is valuable. The earth resistance is not the most important part of that, but it helps. Point 3.


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In addition to ground rods, a substation employs flat strip conductors connected to ground rods for earthing.
Flat conductors are for lightning protection. They would fuse open in the presence of a long duration fault, and are not meant to clear one. Even so, the only place I have seen flat conductors is at radio tower installations. All the subs depend on the concrete slab and the conductors bond the steel to the grid in the concrete and are large, round and stranded.


A residence is a different breed from an outdoor substation, which is unlikely to have HV ground faults.
Agreed.


In some substations, watering funnels provided to pour water periodically during dry weather. Or suitable provision is to be made during design stage itself.
For a ground rod? How do you get water to the bottom of a 40 foot long ground rod? Or even an 8 foot long one. Also, it's the conductivity of the entire sphere of influence that matters, not just the surface of the ground rod.
 

kingpb

Senior Member
In some substations, watering funnels provided to pour water periodically during dry weather. Or suitable provision is to be made during design stage itself.
I assume what you are referring too is to keep the ground wet around the rod during dry weather in order to maintain a suitable ground resistivity? Can you confirm. Even a picture would be nice.

Never seen that here in the US, is that something done where you are located?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Drive it down into the water table :lol: Pretty easy in Florida:D
Or just leave it lying on the surface in some prime real estate areas. :)
On a more serious note, a perforated pipe driven down alongside the ground rod, coupled with a "trap primer" type plumbing valve might be expected to continue working without maintenance for 10-20 years.
 

Sahib

Senior Member
Location
India
The ground resistance in such cases should be maintained in the range of 4 ohms or less.
why? what purpose is served?
If the substation ground resistance is not kept below certain low values such as 4 ohms or less, the ground fault current would be less than the required minimum to operate the earth fault relay.
Then the earth fault relay would not operate and the earth fault would remain uncleared posing danger to the operating personnel.
 
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