Reference
Glossary
Every technical term used in the arc-flash and TCC tutorials, with a one-paragraph definition and links to related concepts. Filter by term name or definition text; clicking a related-term chip jumps to that entry within this page.
35 of 35 terms
| Term | Definition | Related |
|---|---|---|
| ANSI transformer damage curve | A through-fault withstand envelope published by ANSI/IEEE C57.109. Defines how long a transformer can safely carry a given multiple of its FLA before mechanical or thermal damage. Plotted on a TCC, upstream protective devices must clear below this envelope. | |
| Arc rating | A clothing system's tested ability (in cal/cm²) to attenuate an arc-flash thermal exposure before second-degree skin burns occur underneath. Determined by ASTM F1959 testing. Must equal or exceed the worker's incident-energy exposure at the working distance. | |
| Arc-flash boundary (AFB) | The distance from the prospective arc source at which incident energy drops to 1.2 cal/cm² — the onset threshold for second-degree skin burns. Inside the AFB, anyone present must wear arc-rated PPE or be physically excluded. | |
| Arcing current (Iarc) | The actual RMS current that flows when an arcing fault is in progress, lower than Ibf because the arc plasma adds impedance. Arc-flash energy is driven by Iarc, not Ibf — and Iarc tracks Ibf nonlinearly, so a lower bus Ibf can still produce damaging energy. | |
| ARMS / RELT (maintenance switching) | Vendor names for energy-reducing maintenance switching: a switch on an upstream breaker that temporarily makes its instantaneous element fast (or activates a lower pickup) while a worker is in front of downstream equipment. Method 3 of the six in NEC 240.87. | |
| Bolted-fault current (Ibf) | The symmetric RMS short-circuit current that would flow if the fault were a zero-impedance metallic connection between conductors. The worst-case bus fault current — used to size breaker AIC ratings and as the starting input to arc-flash math. | |
| Breaker AIC rating | Ampere Interrupting Capacity — the maximum symmetrical RMS fault current a breaker is rated to interrupt without mechanical damage or loss of insulation. Standard ratings: 14, 22, 35, 42, 65, 100, 200 kAIC at 480 V. The available Ibf at the breaker location must be below its AIC. | |
| Current-limiting fuse | A fuse class designed to interrupt within the first sub-cycle of a high-magnitude fault, truncating the peak current before it reaches the prospective Ibf. Class J and Class L fuses are current-limiting; RK5 is not. Sub-8 ms clearing in the current-limiting region. | |
| Differential relaying | A protection scheme that compares current entering a zone (transformer, bus, generator) with current leaving. If the difference exceeds a threshold, the relay trips on the assumption that the missing current is being drawn by an in-zone fault. Method 2 of NEC 240.87. | |
| Electrode configuration (VCB / VCBB / HCB) | How the arcing electrodes are oriented inside the enclosure, controlling how much arc energy bounces back toward the worker. VCB = vertical conductors in a box; VCBB = vertical conductors with a barrier behind; HCB = horizontal conductors in a box. | |
| Fuse class (RK5 / J / L / T) | A UL-defined family of low-voltage fuse types with distinct physical dimensions, interrupting ratings, and time-current characteristics. RK5 is the legacy general-purpose class; Class J is faster and more current-limiting; Class L covers 601–6000 A; Class T is compact and ultra-fast. | |
| ICEA cable damage curve | An I²t-based withstand curve for insulated power cable, derived from ICEA P-32-382. Defines how long the cable can carry a given fault current before its insulation reaches a temperature that permanently damages it. | |
| IEEE 1584 | The empirical regression standard that converts system parameters (voltage, bolted Isc, gap, electrode geometry, working distance, clearing time, grounding) into incident energy and arc-flash boundary. The 2002 and 2018 editions are both in use; this tutorial implements 2002. | |
| Incident energy (IE) | The thermal energy per unit area that a worker at the working distance would receive during an arc-flash event, expressed in cal/cm². Drives the arc-rating requirement of the PPE the worker must wear. | |
| Inrush current | The high-magnitude, short-duration current that flows when a transformer or motor is first energized. Transformer magnetizing inrush peaks at 8–12× FLA and decays over a few cycles; motor starting inrush is 5–7× FLA and lasts seconds. | |
| Instantaneous trip (INST) | A high-current pickup setting that trips a breaker with no intentional delay — typically one cycle (~16 ms) plus mechanical opening time. Provides fast arc-flash energy reduction at high fault currents but limits selective coordination with downstream devices. | |
| Long-time pickup (L) | The current threshold above which a breaker's long-time delay region starts timing toward a trip. Usually expressed as a multiple of sensor rating (e.g. 0.9× = 90 % of sensor). Sets where the inverse-time region of the TCC begins. | |
| LVPCB-LSI | Low-Voltage Power Circuit Breaker with Long-time, Short-time, and Instantaneous trip elements — the three-region adjustable protection used at service mains and large feeders in switchgear. LSIG adds a ground-fault element. | |
| Motor contribution | The fault current that running motors deliver into a downstream fault during the first few cycles. Motors act as generators behind their subtransient reactance, pumping current backward into the fault until the rotor decays. Standard lumped estimate: 4× connected motor FLA. | |
| NEC 110.16 | The label-content section of the National Electrical Code. Requires field-marked arc-flash warning labels on switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers likely to be examined or maintained while energized. | |
| NEC 240.67 | The fuse-side companion to NEC 240.87. Requires arc-flash energy reduction methods on fuses rated 1200 A or larger. Acceptable methods include current-limiting fuse selection or differential / arc-energy reduction relaying. | |
| NEC 240.87 | A National Electrical Code requirement that breakers 1200 A or larger with adjustable trip have at least one of six listed arc-flash reduction features active, addressing the long clearing times of high-AIC LVPCBs. Adopted in the 2014 NEC, expanded since. | |
| NFPA 70E | The U.S. consensus standard for electrical-safety work practices, including the PPE category table and the requirement that arc-flash hazards be analyzed and labeled. NEC 110.16 references it for label content. | |
| OSHA 1910.269 | OSHA's electrical safety regulation for electric power generation, transmission, and distribution. Cites NFPA 70E for general industry labels, energized-work permits, and PPE. The "compliance backbone" arc-flash labels rest on, even though the technical content is in NFPA 70E. | |
| Per-unit method | A normalization scheme for power-system impedances. Pick a base MVA and base voltage; convert every transformer, cable, and source impedance into a dimensionless ratio relative to that base. Series impedances add as scalars; voltage-level transitions wash out automatically. | |
| PPE category | A 1–4 banding of incident-energy levels that maps to a prescribed clothing system per NFPA 70E. Higher categories require more layers of arc-rated fabric, ranging from a single arc-rated shirt-and-pants (Cat 1) to a full hood-equipped arc-flash suit (Cat 4). | |
| PPE category method (NFPA 70E) | A table-driven shortcut for picking PPE that doesn't require an incident-energy study. Tables 130.7(C)(15)(a) and (b) prescribe a category based on equipment type and "task likelihood." Easier than an IE study; conservative for some equipment and dangerously under-conservative for others. | |
| Selective coordination | A protection-design property: at any fault current the system can deliver, only the device closest to the fault clears, leaving the rest of the system in service. Required by code for emergency, legally-required-standby, and elevator branches per NEC 700/701/620. | |
| Series reactor | A deliberately-installed inductance in a feeder or bus tie used to limit available fault current at a downstream point. Common in industrial substations where raising transformer %Z would impair motor starting. | |
| Short-time delay (S) | On an LSI-equipped breaker, a deliberate hold-off time during higher fault currents to let a downstream device clear first. The "S" in LSI. Adjustable by both pickup multiple and delay time. | |
| Thermal-magnetic MCCB | A Molded-Case Circuit Breaker with two trip elements: a thermal (bimetal) element for overload protection and a magnetic (electromagnet) element for instantaneous fault clearing. The default residential and light-commercial breaker. Limited adjustability — typically only the magnetic pickup, on commercial models. | |
| Time-current curve (TCC) | A log-log plot of a protective device's clearing time as a function of current. Reading current on the X axis up to the curve gives the time the device takes to trip at that current. | |
| Working distance (WD) | The distance from the prospective arc source to the worker's face and chest, where incident energy is evaluated. NFPA 70E standardizes WD by equipment class — 18 in for LV panelboards and MCCs, 24 in for LV switchgear, 36 in for MV switchgear. | |
| X/R ratio | The ratio of reactance to resistance at a fault point, controlling how much DC offset and asymmetrical peak current appears in the first cycles of a fault. Higher X/R → larger asymmetric peak, longer-lived DC tail. | |
| Zone-selective interlocking (ZSI) | A coordination scheme where each breaker asserts a signal output when its short-time element picks up, and reads inputs from breakers below. Upstream waits its full delay only if downstream is picked up; otherwise it trips with no intentional delay. Method 1 of NEC 240.87. |