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 › Appendix 11-5-2-280.1-D

Current through June 25, 2024
GENERALIZED INTERPRETATION OF
							 STRUCTURE-TO-SOIL POTENTIAL MEASUREMENTS (VOLTAGES) OBTAINED ON IMPRESSED
							 CURRENT CATHODIC PROTECTION SYSTEMS
Listed in this table are some generalized
							 observations that can be applied to the interpretation of structure-to-soil
							 potentials. Depending on the specific site conditions and other factors,
							 differing interpretations are possible.
VOLTAGE (mV)
GENERALIZED INTERPRETATION
ANY POSITIVE VOLTAGE OR 0 TO -100 "ON" or
							 "OFF"
Can indicate that the structure you are attempting to
							 measure is not bonded to the impressed current system (conduct continuity
							 testing). Stray current could be affecting the protected structure (consult a
							 corrosion expert). Positive and negative wires could be reversed (negative must
							 be to protected structure and positive to anode). Test leads are reversed
							 (positive lead should contact structure and negative lead should be connected
							 to reference electrode). Could indicate that you are measuring the potential of
							 a copper wire.
-101 to -399 "ON" or "OFF"
Try again A reading in this range is not normally seen
							 on an underground steel structure. Could indicate that steel structure is
							 electrically connected to a significant amount of a more noble metal (e.g.
							 copper). Very corroded low carbon steel may also be indicated.
-400 to -599 "ON" or "OFF"
Usually means that the steel structure has no cathodic
							 protection. Existing impressed current anodes could be completely "burned-out".
							 Continuity of anode lead wires (positive circuit) could be broken. Negative
							 bonds on the protected structures may be broken or non-existent.
-600 to -849 "ON" or "OFF"
Usually means that the steel structure has some
							 protection but for whatever reason, something is causing a low reading that may
							 indicate adequate cathodic protection has not been provided. The impressed
							 current system may be trying to protect a structure that requires more current
							 than it can produce (rectifier output too small). The impressed current system
							 may not be capable of effectively distributing the required current to all
							 parts of the structure you are trying to protect (not enough anodes, anodes
							 improperly installed, soil resistivity too high). The steel structure that is
							 intended to be protected may not be electrically continuous with the other
							 metallic structures under protection (conduct continuity testing). The
							 environmental conditions may not be favorable at the time you are attempting to
							 obtain the reading. Retest during the next 90 days.
-850 or MORE NEGATIVE "ON"
Steel structure may or may not be adequately
							 protected. Usually indicates that the impressed current system is providing
							 current to the structure although the reading normally includes a large voltage
							 (IR) drop. Because the flow of current through the soil causes a voltage drop,
							 the on potential cannot be used to indicate that adequate cathodic protection
							 has been provided. Instant off potentials must be utilized to demonstrate
							 cathodic protection.
-850 or MORE NEGATIVE "OFF"
Steel structure protected by impressed current system
							 meets regulatory requirements and cathodic protection is judged to be adequate.
							 A potential measurement of -850 mV or more negative with the protective current
							 temporarily interrupted (850 off) is considered to be the best indicator that
							 adequate cathodic protection has been provided.
MORE NEGATIVE THAN -1220 mV "OFF"
Instant off potentials more negative than -1220 mV are
							 theoretically not possible. If you observe an instant off potential more
							 negative than -1220 mV, you should suspect stray current is affecting the
							 protected structure. Consult a corrosion expert immediately since stray current
							 can cause a rapid corrosion failure of the protected structure.
MORE NEGATIVE THAN -2000 "ON"
Usually means that a high resistance exists in the
							 ground bed that is causing a large voltage drop. This condition is normally
							 evident by checking the rectifier output since the voltage is very high but the
							 amperage is relatively low. However, you should be cautious when abnormally
							 high voltages are observed since this can have a detrimental effect on
							 cathodically protected structures or the anodes may be rapidly depleted. Stray
							 current may also be generated that can adversely affect other buried metallic
							 structures such as water lines and other utilities. Consult a corrosion expert
							 whenever it is suspected that too much voltage is being generated.
VARIABLE "ON" or "OFF"
If the voltmeter readings vary, you should suspect
							 that stray current may be affecting the cathodically protected structure.
							 Sometimes, the stray current can cause a pattern to develop that is
							 recognizable. An example would be the on/off pattern of a nearby DC powered
							 welding operation. A corrosion expert should be contacted immediately since
							 stray current can cause a corrosion failure in a relatively short period of
							 time.
RAPIDLY FLUCTUATING "ON" or "OFF"
If the voltmeter will not stabilize, it usually means
							 that there is a high electrical resistance somewhere. Check all lead wires and
							 connections and make sure that you are making a solid and clean metal to metal
							 connection. Soil where the reference electrode is placed could be too dry. Add
							 water to the soil or wait until a heavy rain occurs and try again. Petroleum
							 contaminated soils may cause a high contact resistance. The tip of the
							 reference electrode may need to be cleaned or replaced.