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

Current through June 25, 2024
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.
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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.