Grounding 101

The idea of grounding is that the earth itself acts as the electrical ground, providing not only a common reference for all electrical devices, but also a standard between devices regardless of location. The earth is used as the common reference conductor.

The consideration that the earth acts as a constant conductor is the underlying principle, and in the larger sense, it does. However, the connection to the ground or “earthing” needs to have the lowest resistance possible. Any resistance due to improper or faulty connections will allow the presence of a current to flow. In the context of safety, current flowing through a vital organ is what causes injury or death. The frequency of the alternating current, the duration of contact, and the path of contact are all important factors in determining the severity of the shock.

The connection to the earth consists of two basic parts: the grounding electrode and the connection (or bonding) to that electrode. Any resistance introduced in the grounding path could lead to a damaging or dangerous condition.

The relationship between resistance, current, and voltage is revealed in Ohm’s Law1:
I  =  V / R
Where :
I  =  current flow
V  =  voltage
R  =  resistance2

Using this equation, if the resistance is 0, then the current flow will be maximized in the ground circuit. (This is what you want, not current flow in another path that does not include the ground, but may include YOU!)

The accepted National Electrical Code (NEC), standard for a grounding circuit is 25 ohms or less, but much lower values can be obtained using multiple grounding electrodes.

“A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2) through (A)(7). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8m (6ft) apart”. - Section 250.56 of the National Electrical Code.

Recently the following was added:

“In facilities with sensitive equipment it should be 5.0 ohms or less.”3

The Grounding Electrode

The usual form of grounding an electrode is a copper rod, sunk into the earth at a point near the device or electrical service to be protected. NEC calls for a minimum of 8 feet of electrode to be in contact with the earth. However, not all “earth” has the same electrical properties. (See Table 1).

To overcome this variable, sometimes multiple numbers of rods can be placed in an area to lower the resistance to ground.

When using multiple ground rods, they are all connected in a “daisy chain” and one conductor is used to connect to the service or device to be grounded. This effectively places them in parallel and the resulting resistance is lower. Although it would seem the resistance is in direct proportion to the number of rods, the calculation is a little more complicated.

The total resistance of a multiple rod system is calculated by using the formula:
Rt  =  (Rs/n) ´ F
Where:
Rt  =  combined ground resistance of the system
N  =  number of electrodes
Rs  =  typical resistance of one electrode
F  =  factor shown in table 4 for the number of rods

Testing for earth ground resistivity requires the use of an earth ground resistivity meter and some additional stakes and cabling to measure the resistance between the grounding conductor and earth. One of the most accepted tests uses the Wenner method developed by Dr. Frank Wenner of the U.S. Bureau of Standards (now known as NIST) in 1915.6

The Wenner method involves placing four probes in the earth at equal spacing. The probes are connected with wires to the ground resistance test set. The test set passes a known amount of current through the outer two probes and measures the voltage drop between the inner two probes. Using Ohm’s Law, it will output a resistance value which can then be converted to a resistivity value using the equation:
  =  2aR
Where:
P  =  soil resistivity in ohm-cm
a  =  spacing between probes in cm
  =  3.1416
R  =  resistance value measured by test set

Soil resistivity values vary depending on the soil type, temperature, and moisture content. Typically, data is collected to depths of one to ten meters with additional testing required for difficult (high or widely varying resistance) sites.

Finally, testing of the grounding system is important to determine whether ground resistance targets are met. Grounding professionals should be called to perform the ground resistance tests since each test must take into account on-site conditions, and it is very easy to get erroneous data. Testing can be accomplished using clamp-on resistance testers, Fall-of-Potential methods, or by simply calculating the probable resistance. Detailed procedures for accurate testing can be found in ANSI IEEE Standard 81.

The use of a good test set is the best way to not only design and test a new grounding system, but also to test existing systems for degradation and ensure they will perform properly in the event of a nearby lightning strike. Although the cost for these sets can range upwards of $3,000 (rentals are sometimes available for a fraction of the cost), the cost of a replacement scale system due to a noncompliant grounding system can be much more expensive, not to mention the priceless protection it provides to individuals.