Grounding Basics Part 2

A brief note: At no time should you design the grounding for a facility unless you are qualified to do so and are very knowledgeable in the field of grounding. A qualified grounding engineer should always be involved.

Let’s begin today talking briefly on the projects and the many different impacts that affect the way we design and install grounding. All NEC references shall be to the 2014 code book.

Substations and cellular antenna sites: We are just going to glance over this and give key points to what is required and move on to other topics.

Substations common to large projects and because many times they are built we have to make sure grounding is installed and handled. Grounding in substations is covered in IEEE and NEC.

1. The substation grounding system shall be dimensioned and installed in such a way that during a fault in the electrical installation, no danger to life, health or material shall occur neither inside nor outside the installation. The grounding system shall be constructed to fulfill the following demands, which apply to all voltage levels: [2]

• Provide personnel safety against dangerous touch voltages including at highest earth fault current.
• Prevent damage to property and installations 
• Be dimensioned to withstand corrosion and mechanical stress during the entire lifetime of the installation 

· Be dimensioned to withstand the thermal stress from fault currents The risk connected to electric shock to human beings is primarily connected to current flowing through the heart region and of a magnitude large enough to cause auricular fibrillation. The “current-through-body” limit, see Appendix A, is transformed into voltage limits in order to be compared to calculated step and touch voltages when considering the following factors:

• The amount of current flowing through the heart region 

• The body impedance along the current path 

• The resistance between the contact spot of the body and for example a metal construction against the hand including glove, or feet including shoes or shingle towards remote earth The duration of the fault 

• One must consider that the fault frequency, magnitude of the current, fault duration and presence of human beings are probabilistic factors [3]. 

2. Conductors must be large enough to handle any anticipated faults without fusing (melting). Table 1, which is derived from IEEE 80-1986, IEEE Guide for Safety in AC Substation Grounding, lists the maximum allowable fault current (in kA) for various conductor sizes and fault durations.

3. The connections between conductors and the main grid, and between the grid and ground rods, are as important as the conductors themselves in maintaining a permanent low-resistance path to ground. You must consider the type of bond the connection creates with the conductor or ground rod and temperature limits.

4. The length, number, and placement of ground rods affect the resistivity of the path to earth ground. Each doubling of ground rod length reduces resistivity 45%, if you're working with uniform soil conditions. Usually, soil conditions are not uniform, so it's vital to obtain accurate resistivity data by measuring ground rod resistivity with appropriate instruments.

5. Soil conductivity is an important consideration in substation design. The lower the resistivity, the easier it is to get a good ground. In areas where soil conductivity is low or where dry weather can change soil conductivity, consider using a ground-enhancement material. Another option, especially in areas where deep frost occurs, is to use deep-driven rods.

6. Limiting step and touch potential to safe values in your substation is vital to employee safety. Step potential is the voltage difference between a person's feet and is caused by the dissipation gradient of a fault entering the earth. Just 30 in. away from the entry point, voltage usually will have been reduced by 50%. For example, a 1000A fault in a 5-ohm grounding system will enter the earth at 5000V. So, 30 in. away, less than the distance of a normal step, a fatal potential of 2500V will exist. This is shown in Fig. 2.

7. Touch potential represents the same basic hazard, except the potential exists between the person's hand and his or her feet. However, since the likely current path runs through the arm and heart region instead of through the lower extremities, the danger of injury or death is even greater.

8. Because it's nearly impossible to isolate a metal structure from its foundation, the use of "Ufer" grounds has significantly increased in recent years. Ufer grounds utilize the concrete foundation of a structure plus building steel as a grounding electrode. Even if the anchor bolts are not directly connected to the reinforcing bars (rebar), their close proximity and the semi-conductive nature of concrete will provide an electrical path. Two additional facts need to be considered in Ufer grounding.

History on Ufer grounding: During World War II, the U.S. Army required a grounding system for bomb storage vaults near Tucson and Flagstaff, Arizona. Conventional grounding systems did not work well in this location since the desert terrain had no water table and very little rainfall. The extremely dry soil conditions would have required hundreds of feet of copper rods to be inserted into the ground in order to create a low enough impedance ground to protect the buildings from lightning strikes.

In 1942, Herbert G. Ufer was a consultant working for the U.S. Army. Ufer was given the task of finding a lower cost and more practical alternative to traditional copper rod grounds for these dry locations. Ufer discovered that concrete had better conductivity than most types of soil. Ufer then developed a grounding scheme based on encasing the grounding conductors in concrete. This method proved to be very effective, and was implemented throughout the Arizona test site.

After the war, Ufer continued to test his grounding method, and his results were published in a paper presented at the IEEE Western Appliance Technical Conference in 1963.[1] The use of concrete enclosed grounding conductors was added to the U.S. National Electrical Code (NEC) in 1968. It was not required to be used if a water pipe or other grounding electrode was present. In 1978, the NEC required rebar to be used as a grounding electrode if present. The NEC refers to this type of ground as a "Concrete Encased Electrode" (CEE) instead of using the name Ufer ground.

Over the years, the term "Ufer Ground" has become synonymous with the use of any type of concrete enclosed grounding conductor, whether it conforms to Ufer's original grounding scheme or not. (Info and text Courtesy or Wikipedia)

9. Utilities vary in their fence-grounding specifications, with most specifying that each gate post and corner post, plus every second or third line post, be grounded. All gates should be bonded to the gate posts using flexible jumpers. All gate posts should be interconnected. In the gate swing area, an equipotential wire mesh safety mat can further reduce hazards from step and touch potentials when opening or closing the gate. NEC Article 250.194 Grounding and Bonding of Fences and Other Metal Structures.

• Special Notes: Not everyone agrees and since the NEC and IEEE do not directly call for the fence to be grounded on facilities where the conditions require the grounding and bonding not need to be done. I have argued this several times that fences and gates no matter where should be bonded to prevent the very event that requires them to be installed in substations. 

• Fences step and touch potential still exist even if it is on a much lower scale. And many times we have transformers, underground conduits and a ground grid, all capable of creating levels of electrical current capable of shocks and other potentials. So it is recommended even if it is not required.

• The IEEE® and NEC® both discuss the need for proper fence grounding and step and touch-potential limitation. Grounding and bonding of fence posts and gates is especially important at substations and cellular antenna sites because the outside of the fence is typically accessible to the public. During a fault condition, the fence could have a very high “touch” potential. In addition, the fence is often at the edge of the ground grid where the surface potentials are at the highest. A solidly bonded counterpoise 30" outside the fence can mitigate the risk this high-touch potential poses.

10. To protect the switch operator in case of a fault, place a safety mat on or under the earth's surface at all switch handles. There are four types of safety mats.

• A steel grate or plate on supporting insulators. This works only if the operator can be kept completely isolated on the grate. Therefore, insulators must be kept clean. Any vegetation in the vicinity should be cut or eliminated completely.
• A steel grate on the surface, permanently attached to the grounded structure. This arrangement has the operator standing directly on the grate.
•  Bare conductor buried (in a coil or zigzag pattern) under the handle area and bonded to the grounded structure.
• Prefabricated equipotential wire mesh safety mat buried under the handle area and bonded to the grounded structure, as in Fig 6 (on page 46). This is likely to be the least expensive choice.

11. Surge arrestors pass surge energy ("spikes") to ground. To transfer current at minimum voltage drop (which provides maximum protection), each surge arrestor ground lead should have a short direct path to earth and should be free of sharp bends.

12. The NEC in Art. 318 details the requirements for cable trays, which cannot be treated the same as conduit. All metallic tray sections must be bonded together because mechanical splice plates do not provide an adequate path for fault currents. Therefore, the bonding jumpers (either the welded type used on steel trays or the lug type) must be placed across each spliced joint. If a metallic tray comes with a continuous grounding conductor, the conductor can be bonded inside or outside the tray. When cable tray covers are used, they should be bonded to the tray with a flexible conductor. The trays themselves should be bonded to the building steel (usually at every other column) and to all conduits containing conductors common to the cable tray system.

13. When personnel work on high-voltage electric structures or equipment, any conductive bodies should be grounded. The usual grounding method is to attach a flexible insulated copper cable with a ground clamp or lug on each end, as shown in Fig. 7. These flexible jumpers require continual inspection and maintenance.

For copper conductors, the exact formula from IEEE Std. 80 can be simplified to:
I = A / K [square foot of S]

Where:

I = RMS current in amperes

A = Conductor size in circular mils

S = Current duration in seconds

K = Constant for maximum allowable temperature

Although we discuss the grounding of substations, remember there is never a substitution for a qualified grounding engineer and good engineering practices where engineering and substation design are needed and required.

Facilities Grounding will be in the next Blog. Grounding Basics Part 3

Grounding Basics

Grounding Basics

Article 250 of the NEC is not always the easiest to read and understand, then you throw in IEEE standards and wow you have a confusing mess to contend with.

There are a lot of engineers, electricians, inspectors “City, State and Company”, and customers who do not grasp the need nor understand even the basics of what grounding is and why we install it. Is it because they are ignorant of it, or is it because they have been taught the way they currently see its need? 

The answer is never the same and when you think about those who design, engineer, inspect and build the homes, buildings and facilities we live, work and visit. We would hope, the engineer, designer and the electrician not only understand grounding, but do the engineering, designing and installing to the highest degree of professionalism & safety keeping in in mind all who will use that which has been designed and built.

As the areas we live, work and play are all very different, we must treat grounding as though its importance is as very critical as the very breath we take. Without proper grounding electricity from any number of sources can cause death, burns or mild discomfort for a person and cause untold damage to property.

At what levels does Electricity hurt us?

Electric Current (1 sec Contact).

1.   0.5 – 2 mA = Threshold of feeling, tingling sensation. We've all felt that buzzing or tingling sensation without experiencing injury.

2.   2 – 16 mA = muscular contraction, experience pain.

3.   16 – 30 mA = Respiratory paralysis. “Can’t let go!” current.

4.   50 mA – 200 mA = “Can't let go!” current – Onset of Ventricular Fibrillation. Above 50 mA, an electric current can trigger cardiac arrest if it passes through the heart.

5.   200 mA and above = Burns to tissue and organs and could result in death.

Will the 120 volt common household voltage produce a dangerous shock? It depends!

If your body resistance is 100,000 ohms, then the current which would flow would be:

I = 120 volts / 100,000 Ohms = 0.0012 A = 1.2 mA

Above is just about the threshold of perception, so would produce a tingle. 

But if you have just played a couple of sets of tennis, are sweaty and barefoot, and then your resistance to ground might be as low as 1000 ohms. Then the current would be:

I = 120 volts / 1,000 Ohms = 0.12 A = 120 mA

This is a lethal shock, capable of producing ventricular fibrillation and death!

The severity of shock from a given source will depend upon its path through your body.

Electric burns to tissue and organs:

Above 100 mA, electrical marks appear on the body at the points of contact.

Above 10,000 mA (10 amperes), burns are severe and amputation is required.

Special Note: If you see someone who is or was injured as a result of Electric Shock, follow these steps:

1.   Call 911

2.   Look First. Don’t touch! Person may still be in contact with the source of shock!

3.   Turn off the source of electricity, if possible. If you cannot turn off the power, move the source away from you and the person safely, using dry nonconducting objects made of cardboard, plastic or wood.

4.   Check for signs of circulation (breathing, coughing or movement. If none are found start CPR immediately.

5.   Prevent shock. Lay the person down and, if possible, position the head slightly lower than the trunk with the legs elevated.

Caution

·         Don't touch the person with your bare hands if he or she is still in contact with the electrical current.

·         Don't get near high-voltage wires until the power is turned off. Stay at least 20 feet away — farther if wires are jumping and sparking.

·         Don't move a person with an electrical injury unless the person is in immediate danger.

·         Never touch Power Lines, sources of power or anything you do not know what it is as you could become a victim and injured as well.

Now that we have covered the basics of electric shock and its effects on the body let’s get moving forward to our subject.

To understand grounding we have to first get some information about the environment we are working and start off at the beginning of any project that is to be designed and built.

Electrical Grounding or "Grounding" originally began as a safety measure used to help prevent people from accidentally coming in contact with electrical hazards. The term "Grounding" is used in America to discuss both below-grade earthing and above-grade grounding. The process of electrically connecting to the earth itself is often called "earthing".

While electrical grounding may have originally been considered only as a safety measure, with today's advances in electronics and technology, electrical grounding has become an essential part of everyday electricity.

Types of Grounding electrodes vary and the type you use on your project will depend upon Soil resistivity tests and what is needed for your location or project. An Engineer with the test results will make the best design for the project to be built and the equipment to be protected.

Common Electrodes used are: Driven Rods, Advanced Driven Rods, Buried Copper Plate, Ufer Ground or Concrete Encased Electrodes, Water pipes and Electrolytic Electrode.

	Driven Rods	Advanced Driven Rods	Grounding Plate	Concrete Encased Electrode	Building Foundation	Water Pipe	Electrolytic Electrode
Resistance to Ground (RTG)	Poor	Average	Poor	Average	Above Average	Poor to excellent **	Excellent
Corrosion Resistance	Poor	Good	Poor	Good*	Good*	Varies	High
Increase in RTG in cold weather	Highly Affected	Slightly Affected	Highly Affected	Slightly Affected	Slightly Affected	Minimally Affected	Minimally Affected
Increase in RTG over Time	RTG Worsens	RTG unaffected ***	RTG Increases	RTG Unaffected ***	RTG Unaffected ***	RTG Unaffected ***	RTG Improves
Electrode Ampacity	Poor	Average	Average	Average *	Above Average *	Poor to Excellent **	Excellent
Installation Cost	Average	Excellent	Below Average	Below Average	Average	Average	Poor
Life Expectancy	Poor 5-10 years	Average 15-20 years	Poor 5-10 Years	Average * 15-20 Years	Above Average* 20-30 Years	Below Average * 10-15 Years	Excellent 30-50 Years

* High-Current discharges can damage foundations when water in the concrete is rapidly converted into Steam.

** When part of extensive, bars, metallic, electrically continuous water system.

*** Typically

Each project grounding design starts with a site analysis, collection of geological data, and soil resistivity of the area.

“Soil Resistivity Test”.

Soil resistivity testing is the process of measuring a volume of soil to determine the conductivity of the soil. The resulting soil resistivity is expressed in ohm-meter or ohm-centimeter.

Soil resistivity testing is the single most critical factor in electrical grounding design. This is true when discussing simple electrical design, to dedicated low-resistance grounding systems, or to the far more complex issues involved in Ground Potential Rise Studies (GPR). Good soil models are the basis of all grounding designs and they are developed from accurate soil resistivity testing.

There are many types of soil and will depend upon your location and soil resistivity varies dramatically throughout the world and is heavily influenced by electrolyte content, moisture, minerals, compactness and temperature.

Type of Surface Materials	Resistivity of Sample in Ohmmeters
	Dry	Wet	Soil Types or Type of Earth	Average Resistivity in Ohm-meter
Crusher granite w/ fines	140 x 106	1,300	Bentonite	2 to 10
Crusher granite w/ fines 1.5”	4,000	1,200	Clay	20 to 1,000
Washed granite – pea gravel	40 x 106	5,000	Wet Organic Soils	10 to 100
Washed granite 0.75”	2 x 106	10,000	Moist Organic Soils	100 to 1,000
Washed granite 1-2”	1.5 x 106 to 4.5 x 106	5,000	Dry Organic Soils	1,000 to 5,000
Washed granite 2-4”	2 x 106 to 3 x 106	10,000	Sand and Gravel	50 to 1,000
Washed Limestone	7 x 106	2,000 to 3,000	Surface Limestone	100 to 10,000
Asphalt	2 x 106 to  30 x 106	10,000 to 6 x 106	Limestone	5 to 4,000
Concrete	1 x 106 to 1 x 109	21 to 100	Shale’s	5 to 100

Ground Potential Rise Study

A Ground Potential Rise Study is the process of automated timed and/or continuous resistance-to ground measurement.

Ground Potential Rise (GPR) Definitions

Ground Potential Rise or Earth Potential Rise (as defined in IEEE Standard 367) is the product of a ground electrode impedance, referenced to remote earth, and the current that flows through that electrode impedance.

Ground Potential Rise or Earth Potential Rise (as defined by IEEE Standard 80-2000) is the maximum electrical potential that a (substation) grounding grid may attain relative to a distant grounding point assumed to be at potential of remote earth. This voltage, GPR, is equal to the maximum grid current times the grid resistance.

Ground Potential Rise or Earth Potential Rise events are a concern wherever electrical currents of large magnitude flow into the earth. This can be at a substation, high-voltage tower or pole, or a large transformer. In cases where an Earth Potential Rise event may be of special concern, grounding precautions are required to ensure personnel and equipment safety. 

Electrical potentials in the earth drop abruptly around the perimeter of a grounding system, but do not drop to zero. In fact, in a perfectly homogeneous soil, soil potentials are inversely proportional to the distance from the center off the grounding system, once one has reached a distance that is a small number of grounding system dimensions away.

Ground Potential Rise Study: The primary purpose of a Ground Potential Rise Study is to determine the level of hazard associated with a given high-voltage location for personnel and/or equipment. When the degree of hazard is identified, the appropriate precautions must be made to make the site safe. To do this, the engineer must identify what the minimum grounding system for each location will be. The engineer must also take into consideration all local and federal guidelines, including utility company and other requirements.

The grounding engineer needs three (3) pieces of information to properly conduct a Ground Potential Rise Study: Soil resistivity data from a Soil Resistivity Test, Site drawings with the proposed construction and Electrical data from the power company.

The grounding engineer will be required to develop safety systems to protect any personnel working where Ground Potential Rise hazards are known to exist. Federal Law mandates that all known hazards must be eliminated from the work place for the safety of workers. It is the engineer’s choice on which voluntary standards to apply in order to comply with the law. Federal law 29 CFR 1910.269 specifically states that Step and Touch Potentials must be eliminated on transmission and distribution lines that include any related communication equipment.

Ultimately the engineer must determine two (2) things: The site-specific maximum allowable voltage that a person can safely withstand and the actual voltages that will be experienced at the site during a fault.

Each site will have different levels of voltages for both of the above. Unfortunately, we cannot simply say that a human being can withstand X-level of voltages and use that value all the time, since this voltage is determined by the surface layer resistivity, the fault duration and the subtransient X/R ratio. Additionally as each site has different fault durations and different soil conditions, it is critical that calculations be made for each and every possible fault location.

Step potential is the step voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the electrode. A person could be at risk of injury during a fault simply by standing near the grounding point.

Touch potential is the touch voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object and a point some distance away. The touch potential or touch voltage could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it. For example, a crane that was grounded to the system neutral and that contacted an energized line would expose any person in contact with the crane or its uninsulated load line to a touch potential nearly equal to the full fault voltage.

Reducing resistance to ground (RTG) of the site is often the best way to reduce the negative effects of any Ground Potential Rise event, where practical. The Ground Potential Rise is the product of the fault current flowing into the grounding system times the resistance to ground of the grounding system. Thus, reducing the Ground Potential Rise will reduce the Ground Potential Rise to the degree that the fault current flowing into the grounding system does increase in response to the reduced Ground Potential Rise.

Now that we have a better understand of small details about what electricity can do to us and what  is required to properly engineer a grounding system, we can progress into the design and do’s and don’ts of installing grounding for our projects. But as always “A Qualified Engineer, who understands grounding!” should be consulted before design, install and operation, should begin… More to follow in Grounding Basics Part 2...