Mississippi Administrative Code
Title 11 - Mississippi Department of Environmental Quality
Part 5 - Underground Storage Tank Regulations
Chapter 2 - Mississippi Commission on Environmental Quality Underground Storage Tanks Regulations Technical Standards and Corrective Action Requirements for Owners and Operators of Underground Storage Tanks (UST) (Adopted March 22, 1989; Amended August 25, 2011, Last Amended August 23, 2018)
Appendix 11-5-2-280.1 - GUIDELINES FOR THE EVALUATION OF UNDERGROUND STORAGE TANK CATHODIC PROTECTION SYSTEMS
Section 11-5-2-280.1-6 - CATHODIC PROTECTION TESTING
Section 11-5-2-280.1-6.3 - Voltage (IR) Drops
The effect voltage drops have must be considered whenever structure-to-soil potentials are obtained during the survey of a cathodic protection system. The concept of voltage drops is a difficult and controversial subject and a full discussion is beyond the scope of this document. However, stated in the simplest terms, a voltage drop may be thought of as any component of the total voltage measurement (potential) that causes an error.
The term IR drop is sometimes used and it is equivalent to voltage drop. IR drop is derived from Ohm's Law which states that V = I R. In this equation, V stands for voltage, I represents current (amperage) and R stands for resistance. Because the observed voltage is equal to the amperage (I) multiplied by the resistance (R) a voltage drop is commonly referred to as an IR drop. There are various sources of voltage drops and two of the more common are discussed below.
Current Flow - Whenever a current flows through a resistance, a voltage drop is necessarily created and will be included whenever a measurement of the electrical circuit is conducted. In order to effectively eliminate this voltage drop when testing impressed current systems, it is necessary to interrupt the protective current. The magnitude of the voltage drop obtained on impressed current systems is evaluated by conducting both on and instant off potential measurements.
To illustrate how this type of voltage drop contributes to the potential observed when measuring impressed current systems consider the following example. A potential of-950 mV is observed when the rectifier is on. A potential of -700mV is observed when the power is interrupted. Taking the absolute values (negative is dropped), the voltage drop component of the on potential is 250 mV (950 - 700 = 250). Figure 2 is a graphical representation of this voltage drop and also shows how the instant off potential will degrade over time until the native potential is reached.
Raised Earth - All active anodes will have a voltage gradient present in the soil around them producing a "raised earth effect". An abnormally high (more negative) potential will be observed if the reference electrode is within the voltage gradient of an active anode. The magnitude or area of influence of the voltage gradient is dependent predominantly on the voltage output of the anode and the resistance of the soil. Unfortunately, there is no "rule of thumb" guidance that can be given to determine how far away you must be from an anode in order to be outside the voltage gradient. If you suspect the potential you obtain may be affected by raised earth, you should take a remote reading and compare the two.
Because of the raised earth effect, it is necessary to place the reference electrode as far away from any active anode (and still be directly over the structure) when obtaining local potentials on galvanic systems. Since the protective current can not typically be interrupted in galvanic systems, any effect this type of voltage drop may have is evaluated by placing the reference electrode remote. Placement of the reference electrode remote ensures that the reference electrode is not within the voltage gradient of an active anode. Since it is desirable to eliminate any effect voltage drops may have, it is necessary to obtain both local and remote structure-to-soil potentials on galvanic systems. Any effect raised earth may have when testing impressed current systems is eliminated by temporarily interrupting the power.
