earthing calculation

Status
Not open for further replies.
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.
 
Sahib said:
But the underlying principle of NEC i.e being free from danger to life is not met in this case, IMO.
Click to expand...

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.
 
Sahib said:
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.
Click to expand...

why? what purpose is served?
 
Sahib said:
But the underlying principle of NEC i.e being free from danger to life is not met in this case, IMO.
Click to expand...

where does it say that in the NEC?

this is what it does say.

90.1 Purpose.
(A) Practical Safeguarding. The purpose of this Code is
the practical safeguarding of persons and property from
hazards arising from the use of electricity.
Click to expand...
 
K8MHZ said:
The NEC is a BARE MINIMUM standard and not meant as a source for design criteria.
Click to expand...
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.
K8MHZ said:
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.
Click to expand...
TAKE maintenance into account
 
Sahib said:
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.
Click to expand...

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.


Sahib said:
TAKE maintenance into account
Click to expand...

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.
 
Sahib said:
...it could lead to electrocution on HV ground fault. ...
Click to expand...
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?
 
kwired said:
You are free to submit proposals to change it.
Click to expand...

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.
 
K8MHZ said:
Regardless, every substation I have ever seen does not rely on ground rods for earthing.
Click to expand...
In addition to ground rods, a substation employs flat strip conductors connected to ground rods for earthing.
K8MHZ said:
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.
Click to expand...
A residence is a different breed from an outdoor substation, which is unlikely to have HV ground faults.

K8MHZ said:
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.
Click to expand...
In some substations, watering funnels provided to pour water periodically during dry weather. Or suitable provision is to be made during design stage itself.
 
kingpb said:
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:
Click to expand...

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.
 
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.


Sent from my DROIDX using Tapatalk 2
 
Sahib said:
In addition to ground rods, a substation employs flat strip conductors connected to ground rods for earthing.
Click to expand...

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.


Sahib said:
A residence is a different breed from an outdoor substation, which is unlikely to have HV ground faults.
Click to expand...

Agreed.


Sahib said:
In some substations, watering funnels provided to pour water periodically during dry weather. Or suitable provision is to be made during design stage itself.
Click to expand...

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.
 
Sahib said:
In some substations, watering funnels provided to pour water periodically during dry weather. Or suitable provision is to be made during design stage itself.
Click to expand...

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?
 
K8MHZ said:
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.
Click to expand...

Drive it down into the water table :lol: Pretty easy in Florida:D
 
kingpb said:
Drive it down into the water table :lol: Pretty easy in Florida:D
Click to expand...
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 said:
The ground resistance in such cases should be maintained in the range of 4 ohms or less.
Click to expand...

petersonra said:
why? what purpose is served?
Click to expand...

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.
 
Status
Not open for further replies.
Top