Reference
Engineering Glossary
Plain-language definitions of the terms and standards that come up in our electrical power, corrosion and forensic work. Each entry links to the page or article that covers it in more depth, and to a calculator where one applies.
Entries for standards describe what the document covers. They are not the standard’s wording, so check the current edition before you rely on a requirement.
Terms A to Z
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0-9
- 100 mV polarization criterion
Corrosion and CP
An AMPP SP0169 criterion for steel: at least 100 mV of cathodic polarization, the shift from the native or depolarized potential to the polarized potential. It helps where −850 mV is hard to reach, but does not suit every structure, such as mixed metals.
- −850 mV criterion
Corrosion and CP
An AMPP SP0169 criterion for steel: a polarized potential of −850 mV or more negative, measured against a copper/copper sulfate reference electrode (CSE). It is judged with the IR drop removed, usually on instant-off readings.
A
- ACVG survey
Corrosion and CP
Alternating current voltage gradient survey. A transmitter puts an AC signal on the pipe, and a receiver with two ground probes finds coating defects where the signal leaks into the soil.
- AMPP SP0169
Corrosion and CP
AMPP SP0169 (formerly NACE SP0169), Control of External Corrosion on Underground or Submerged Metallic Piping Systems
The main industry practice for protecting buried and submerged pipe. It covers coatings, CP design, and the criteria used to judge protection, such as −850 mV and 100 mV polarization, with how to measure them.
Plain-language summary, not the standard’s wording.
- ANSI/NETA ATS and MTS
Grounding
InterNational Electrical Testing Association standards for acceptance testing of new equipment (ATS) and maintenance testing of equipment in service (MTS). They list the tests and the expected results for each type of equipment, including grounding systems.
Plain-language summary, not the standard’s wording.
- Service Ground Testing and Commissioning
- Arc flash boundary
Electrical power
The distance from a possible arc at which the incident energy falls to 1.2 cal/cm², about where a second-degree burn to bare skin can begin. Anyone closer needs arc-rated PPE (NFPA 70E).
- Arc mapping
Forensic
Locating and plotting every arc site on the circuits in a burned area. Once an arc severs a conductor, the circuit beyond it goes dead, so the arc site farthest along a circuit is usually the earliest. That helps narrow the area of origin.
- Area of origin
Forensic
The room or space where the fire began, narrowed down from fire patterns, witness accounts, the timeline and, where wiring is present, arc mapping.
- Service Fire & Explosion
- ASTM E1188
Forensic
ASTM E1188, Standard Practice for Collection and Preservation of Information and Physical Items by a Technical Investigator
How an investigator documents, collects, labels and stores evidence, so its condition and chain of custody can be shown later.
Plain-language summary, not the standard’s wording.
- Service Forensic Consulting
- ASTM E860
Forensic
ASTM E860, Standard Practice for Examining and Preparing Items That Are or May Become Involved in Litigation
Before any test that could change or destroy evidence, the other interested parties are told what is planned and given the chance to attend.
Plain-language summary, not the standard’s wording.
- Service Forensic Consulting
- ASTM G57
Corrosion and CP
ASTM G57, Standard Test Method for Measurement of Soil Resistivity Using the Wenner Four-Electrode Method
The standard procedure for measuring soil resistivity with four equally spaced pins in the field, or with a soil box on samples in the lab.
Plain-language summary, not the standard’s wording.
- Service Soil Corrosivity Analysis
- Authority having jurisdiction (AHJ)
Electrical power
The organization, office or person who enforces a code and approves equipment and installations, such as a city or county building official, a fire marshal or a state agency.
- Service Protection & Coordination
- Available fault current (AFC)
Electrical power
The most current that could flow at a point in the system during a short circuit, set by the utility source, transformers, cables and running motors. Equipment there must be rated to interrupt or withstand it.
B
- Bolted fault
Electrical power
A short circuit with almost no resistance at the fault, as if the conductors were bolted together. It gives the highest fault current, the starting value for equipment ratings and arc flash calculations.
C
- CFEI
Forensic
Certified Fire and Explosion Investigator, a certification from the National Association of Fire Investigators (NAFI). Candidates qualify through training and experience and pass an exam based on NFPA 921 and NFPA 1033.
- Service Forensic Electrical Engineering
- Clearing time
Electrical power
The time from the start of a fault until a protective device has fully interrupted it: the relay or trip unit’s operating time plus the breaker’s opening time. A longer clearing time lets an arc deliver more incident energy.
- Close interval survey (CIS)
Corrosion and CP
A walking survey that reads pipe-to-soil potentials every few feet along a pipeline, usually with the CP current switching on and off, to find stretches where protection falls short.
- Coating holiday
Corrosion and CP
A defect in a coating that leaves metal exposed to the soil or water, from a pinhole to a large gouge. CP current collects at holidays, and DCVG and ACVG surveys find them.
- CVFI
Forensic
Certified Vehicle Fire Investigator, a NAFI certification for investigating fires in cars, trucks and heavy equipment. Candidates qualify through training and experience and pass an exam.
- Service Fire & Explosion
D
- DCVG survey
Corrosion and CP
Direct current voltage gradient survey. The CP current is pulsed, and an operator with two probes follows the voltage gradients in the soil to coating defects, where current flows to bare steel. The size of the gradient helps grade each defect.
F
- Fall-of-potential method
Grounding
The three-point test in IEEE Std 81 for a ground electrode’s resistance to earth. Current flows to a remote probe, and voltage is read at a second probe stepped between them; where the readings level off, that plateau is the resistance.
- Fire patterns
Forensic
The marks a fire leaves, such as char depth, smoke lines and V-shaped patterns on walls. Investigators read them to trace how the fire spread back toward where it started.
- Service Fire & Explosion
G
- Galvanic (sacrificial) anode
Corrosion and CP
An anode of a metal more active than steel, such as magnesium, zinc or aluminum, connected to the structure so the anode corrodes instead of the steel. It needs no outside power.
- Groundbed
Corrosion and CP
The anodes of an impressed current system, buried together in the earth, usually in carbon backfill. A deep anode groundbed places them in a drilled hole well below the surface soil.
I
- IEEE 1584-2018
Electrical power
IEEE 1584-2018, IEEE Guide for Performing Arc-Flash Hazard Calculations
The calculation method behind most arc flash studies. The 2018 edition replaced the 2002 model with one based on many more arc tests. It covers three-phase AC systems from 208 V to 15 kV and accounts for how the conductors are arranged in the enclosure.
Plain-language summary, not the standard’s wording.
- IEEE Std 81
Grounding
IEEE Std 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System
Describes how to test grounds in the field, including the fall-of-potential and four-pin soil resistivity methods, where to place the probes, which instruments to use and the usual sources of error.
Plain-language summary, not the standard’s wording.
- Impressed current cathodic protection (ICCP)
Corrosion and CP
A CP system that uses a rectifier to drive DC current from long-life anodes through the soil to the structure. The output can be adjusted, which suits long pipelines, bare steel and high current demand.
- Incident energy
Electrical power
The heat an arc flash could deliver to a worker’s face and chest at the working distance, in calories per square centimeter (cal/cm²). Arc-rated clothing and face protection must be rated at or above it.
- Service Arc Flash Studies
- Tool Arc flash calculator
- Instant-off potential
Corrosion and CP
A structure-to-soil potential read a fraction of a second after the CP current is interrupted, before the steel starts to depolarize. It removes most of the IR drop and is used as the polarized potential.
- Interference
Corrosion and CP
Current from another source, such as a foreign pipeline’s CP system, DC rail transit or a parallel power line, that flows onto a structure and corrodes it where the current leaves.
- Interrupting rating (AIC)
Electrical power
The highest fault current a breaker or fuse can safely interrupt at its rated voltage, marked in kA or as AIC (amperes interrupting capacity). NEC 110.9 requires it to be at least the fault current available where the device is installed.
- IR drop
Corrosion and CP
The voltage error in a potential reading caused by CP current (I) flowing through the resistance (R) of the soil and coating. It makes on potentials look more protective than they are, so criteria are judged with it removed.
M
- Maintenance switching mode
Electrical power
A setting switched on while people work at the equipment that makes a breaker trip faster than its normal coordinated setting, lowering incident energy. It is one of the arc energy reduction methods in NEC 240.87.
- Article NEC 240.87: Arc Energy Reduction
N
- Native potential
Corrosion and CP
The structure-to-soil potential before any CP current is applied, or after the structure has fully depolarized. It is the baseline for the 100 mV polarization criterion.
- NFPA 70E
Electrical power
NFPA 70E, Standard for Electrical Safety in the Workplace, 2024 edition
The consensus standard US employers use to protect workers from shock and arc flash. It covers the electrical safety program, risk assessments, approach boundaries, choosing PPE, and when equipment must be put in an electrically safe work condition.
Plain-language summary, not the standard’s wording.
- NFPA 921
Forensic
NFPA 921, Guide for Fire and Explosion Investigations, 2024 edition
The reference most fire investigators follow. It is a guide, not a code. It lays out a systematic approach based on the scientific method: gather data, form hypotheses about origin and cause, and test each against the evidence.
Plain-language summary, not the standard’s wording.
- Service Fire & Explosion
O
- Origin and cause
Forensic
Origin is where the fire started. Cause is the ignition source, the first fuel ignited and the circumstances that brought them together. NFPA 921 classifies a cause as accidental, natural, incendiary or undetermined.
- Article How a Fire Investigation Works
P
- Polarized potential
Corrosion and CP
The structure’s potential with the IR drop removed, read as the instant-off potential. It shows how far the CP current has actually shifted the steel surface.
R
- Rectifier
Corrosion and CP
The power unit of an impressed current system. It converts AC power to adjustable DC current and drives it through the anodes to the structure.
- Reference electrode (CSE)
Corrosion and CP
A copper rod in saturated copper sulfate solution with a porous tip, set on the soil as a stable reference for potential readings. Pipeline criteria such as −850 mV are stated against it (CSE).
S
- Selective coordination
Electrical power
Coordination that holds for every overcurrent up to the available fault current, so only the device protecting the faulted circuit opens. The NEC requires it for emergency, legally required standby and critical operations power systems and some elevator circuits.
- Short-circuit current rating (SCCR)
Electrical power
The fault current an assembly such as a panelboard, switchboard or control panel can be connected to without damage beyond set limits. Like an interrupting rating, it must not be lower than the available fault current where the assembly is installed.
- Soil resistivity
Corrosion and CP
How strongly soil resists electric current, in ohm-centimeters (ohm-cm). Low resistivity usually means more corrosive soil. It also sets how much current CP anodes can deliver and how a groundbed is sized.
- Spoliation
Forensic
The loss, destruction or alteration of evidence before every interested party could examine it. Courts can sanction it, which is why the scene is documented first and other parties are notified before evidence is moved.
- Service Fire & Explosion
- Structure-to-soil potential
Corrosion and CP
The voltage between a buried metal structure and a reference electrode on the soil above it, the basic reading of a CP survey. Readings are compared with criteria such as −850 mV.
- Subrogation
Forensic
An insurer’s right, after paying a claim, to recover the payment from the party responsible for the loss, such as a manufacturer, an installer or a contractor.
- Service Fire & Explosion
- Sulfate-reducing bacteria (SRB)
Corrosion and CP
Bacteria that live in oxygen-poor soil and water and produce hydrogen sulfide, which speeds up corrosion of steel and iron. Their activity is one form of microbiologically influenced corrosion (MIC).
T
- Test station
Corrosion and CP
A post or flush box at the structure with wires bonded to it, where readings of potential and current are taken without digging.
- Service Cathodic Protection Design
- Time-current curve (TCC)
Electrical power
A log-log plot of how long a breaker, fuse or relay takes to operate at each level of current. Plotting the devices in series on one sheet shows whether the one nearest a fault will clear it first.
- Service Protection & Coordination
- Touch voltage
Grounding
The voltage between a hand on grounded metal and the feet on the ground while fault current flows. Bonding and grounding keep it low enough to limit the shock to anyone touching the equipment.
- Service Ground Testing and Commissioning
U
- Ufer ground
Grounding
A concrete-encased electrode: at least 20 feet of steel reinforcing bar or bare copper conductor in a footing or foundation that is in contact with the earth (NEC 250.52(A)(3)). Named for Herbert Ufer, who developed it during World War II.
W
- Wenner four-pin method
Grounding
A soil resistivity test with four pins in a straight line at equal spacing. Current flows between the outer pins and voltage is read between the inner two; the pin spacing sets roughly how deep the reading looks (ASTM G57, IEEE Std 81).
- Service Soil Corrosivity Analysis
- Working distance
Electrical power
The distance from a possible arc to the worker’s face and chest, where incident energy is calculated. IEEE 1584-2018 gives typical values from 18 inches for panelboards and motor control centers to 36 inches for medium-voltage switchgear.
- Service Arc Flash Studies
- Tool Arc flash calculator
X
- X/R ratio
Electrical power
The ratio of the system’s reactance (X) to its resistance (R) up to the fault. A higher ratio gives a larger, more offset current in the first cycles of a fault, which breakers and fuses must be rated for.
Z
- Zone-selective interlocking (ZSI)
Electrical power
Breakers that signal each other over a control wire, so the breaker nearest a fault trips without its intentional delay while the upstream breakers wait. It keeps selectivity and lowers incident energy.
- Article NEC 240.87: Arc Energy Reduction
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