By Buddy Hutson

This information is intended to assist students of the NACE Cathodic Protection Training and Certification Program prior to class attendance. Other students may also find this information useful as a starting point for understanding the fundamental principles underlying cathodic protection technology.

When you attend the NACE Cathodic Protection Tester course, one of the first things you will be taught is basic electricity. Whether your plans are to be an electrician, electronics technician or work in corrosion control, you will need to have a good working knowledge of basic electrical fundamentals. One of the things we have learned from cathodic protection courses taught through the years is that many students struggle with the math involved with some of the electrical fundamentals. With this in mind, we have put together a short introduction that will help you be better prepared when you come to the course. Please read this carefully, before coming to class. These concepts will be thoroughly reviewed at the beginning of class; however, students may have difficulty understanding cathodic protection practices that are demonstrated in class if they are not familiar with these fundamentals.

One of the most basic concepts of electrical fundamentals has to do with Ohms Law. Back in 1828, George Simon Ohm discovered some facts involving voltage, current and resistance. Before we get into those facts that led to “Ohms Law”, let’s look at the elements involved. Where do we find voltage, current and resistance? How about in your car, TV, flashlight, and even your pacemaker? Any time there is an active electrical circuit; these basic elements will be there. To make sure everyone knows what we are talking about here, let’s define the three elements of Ohms Law. Keep in mind that these terms will be expounded upon during the actual course. For simplicity sake, we will be talking about Direct Current (DC) throughout this discussion. During the actual course, we will get into (AC) Alternating Current as well. Keep in mind that batteries are DC and power from the wall socket in your home is AC.

Voltage

Voltage can be defined as a difference of potential. Potential refers to the possibility of doing work. When we use a voltmeter to read across the terminals of a battery, we are reading the difference in potential between one terminal and the other. Voltage can be called an electromotive force, indicating the ability to do the work of forcing electrons to move. The symbol for voltage is the letter E and this comes from the term electromotive force. The unit of measure of voltage is the “Volt ”. In a gas system, voltage compares to pressure. Gas flows through a pipeline as a result of the pressure on the system.

Current

Current can be defined as the flow of electrons. When the difference of potential between two charges forces a third charge to move, the charge in motion is an electrical current. To produce current charge must be moved by a potential difference. The unit of measure for current is the “Ampere”. We often just say Amps. The symbol for current is the letter I. This comes from the term Intensity since current is a measure of how intense or concentrated the electron flow is. Current compares to gas flow in a gas system. The pressure causes the gas to flow in the pipe just as voltage caused electrons to flow in an electrical circuit.

Resistance

Resistance can be defined as the opposition to current flow. Depending on the atomic structure of a material, it may be termed a conductor, semi-conductor or an insulator. These terms are determined by how many free electrons are available to allow the flow of current when a difference of potential is applied across it. Conductors have many free electrons and therefore, current flows quite easily. Semi-conductors have less free electrons and provide more opposition to the flow of current. Insulators have very few free electrons and offer extremely high opposition. Practically speaking, extremely good insulators do not allow the flow of current. The measure of resistance is the “Ohm” (I suppose that George Simon Ohm didn’t want to be forgotten). The symbol for the Ohm is the letter R from the word resistance.

We also see the Greek letter Omega used as well.

In a gas or liquid piping system, resistance can be compared to the orifice effect or the restriction to the flow provided by the inside diameter of the pipe.

Stop here and answer a few questions without looking back at the text. If you can’t answer the questions read the material again before proceeding.

1. Voltage can be defined as:

A. Electrodynamic force
B. Electropotential force
C. Electrochemical force
D. Electromotive force

2. Current is the

A. Build up of electrons
B. Flow of electrons
C. Division of electrons
D. Passing of electrons

3. Resistance is the ___________ of/to current flow.

A. Opposite
B. Opposition
C. Result
D. Reverse

4. Voltage compares to __________ in a gas system.

A. Flow of gas
B. Restriction of gas flow
C. Pressure
D. Orifice effect

5. Current compares to ________ in a liquid piping system.

A. Orifice effect
B. Flow of product
C. Pressure pushing the product
D. Friction of the inside pipe wall

6. Resistance compares to ___________ in a gas or liquid piping system.

A. Build up of pressure
B. Flow of product
C. Orifice effect
D. Pipe’s outside diameter Text.

Hopefully, you marked 1. (D), 2. (B), 3. (B), 4. (C), 5. (B), 6. (C)

Ohm’s Law

Now that we have defined the elements of “Ohm’s Law” let’s look at the discoveries that George made back in 1828. He found that, in a simple circuit like the one shown in Figure 1, when the voltage is held constant, the current and resistance will vary inversely. This means that when the current in the circuit goes up, the value of resistance decreases. Figure 2 show this relationship.

Figure 1. Simple Electrical Circuit

Figure 2. Relationship between Current and Resistance

The next thing that George observed was that when the Resistance is held constant, the Voltage and Current values are directly proportional. This simply means that if one increases the other increases and if one decreases the other decreases and at the same proportions. Figure 3 shows the relationship.

Figure 3. Relationship between Current and Voltage

He established the fact that these relationships never change, so now they can be expressed by math. Thus, Ohm’s Law became the formula E = IR or Voltage equals Current multiplied by Resistance. Lets see if the relationship holds true if we put in values. If we had a current of 2 amps and a resistance of 10 ohms, we would have E = 2 x 10 or a Voltage equal to 20 volts. If we used only half the current (1 amp), now the voltage would be E = 1 x 10 or 10 volts, also half of the first. This proves the second fact that voltage and current are directly proportional, current went down by 50% and the voltage went down by 50%.

The following are excerpts from the slide presentation I use when teaching Ohm’s Law. This should help you understand how the basic formula can be changed with a little algebra for finding current and resistance.

There is an easy way to remember the Ohm’s Law Formula. It is called the Magic Circle or sometimes it’s a Magic Triangle.

Simply cover up the symbol of the factor that you are looking for, and the formula for calculating that factor is revealed by the position of the two remaining symbols.

   Finding Voltage

   Finding Current

   Finding Resistance

One important thing to note about Ohm’s Law: in order to find an unknown value, you must know the two other values. The math is impossible to solve without at least two values.

This completes this short introduction to Ohm’s Law. We look forward to seeing you in class!

Other Resources

If you need more information and do not plan to attend the NACE CP Tester Course, then here are some other options for learning more about cathodic protection:

1. For CP beginners, I recommend Peabody’s Control of Pipeline Corrosion. See the book. It also is the reference book given to students in the NACE CP Tester course.
   
   
2. NACE offers short 1 to 2 day Cathodic Protection tutorials that are sponsored by NACE Areas and Sections. For additional information on the CP Tutorials, please e-mail .
   
   
3. If you have a technical question about CP, remember that you can use the NACE Corrosion Network list serve to post technical questions. (http://www.nace.org/nace/content/Discussion/ncnindex.asp).
   
   
4. If you have additional questions about the NACE Cathodic Protection Training and Certification program, please e-mail .
   
   
5. Here are some other very good training programs that teach substantially on cathodic protection technology:

   Kilgore College (Kilgore, Texas) offers a 2-year Associates degree program in corrosion technology with semester courses devoted to cathodic protection technology.
   (See http://www.kilgore.edu/corrosion.html)

   The Appalachian Underground Corrosion Short Course (AUCSC) is a week-long annual event that provides seminars and other topical presentations on cathodic protection.
   (See http://www.aucsc.com)

   The University of Oklahoma also sponsors a short course in conjunction with several NACE sections: go to http://www.occe.ou.edu/engr/corrosion/