电弧闪光分析（Arc Flash）

Short-circuit apparent power of the upstream network at the connection point, Ssc = √3·Un·I″k, or the equivalent source impedance behind it.

Why it matters: It is the single input that says how "stiff" the grid is. Without it no fault-current figure can be computed, so it must be requested from the utility (or taken from a stated assumption) before any switchgear can be quoted.

Protection clearing time: the total time from fault inception to arc extinction — relay detection + intentional delay + breaker operating time.

Why it matters: In an arc-flash study the incident energy is almost proportional to the clearing time, and the PPE category is selected from it. Halving the clearing time usually halves the energy and can drop the required PPE by one step — which is a direct operating-cost item.

Short-circuit impedance voltage of a transformer: the primary voltage, in percent of rated voltage, that drives rated current through the short-circuited secondary winding (IEC 60076-1).

Why it matters: It fixes the LV fault level (approximately I″k ≈ In / uk) and at the same time the voltage drop under load. This is a real quotation trade-off: a larger uk lowers the fault level (cheaper switchgear downstream) but increases voltage drop and losses. Always quote uk together with the transformer rating.

Point of common coupling (PCC): the interface in the network where the customer installation and the utility network are coupled, and therefore where the connection conditions apply.

Why it matters: It defines what the utility may complain about and what you must prove: voltage band, harmonic distortion, flicker and voltage-dip behaviour are all specified at the PCC, not at the equipment terminals. Studies or measurements made somewhere else do not demonstrate compliance.

Arcing current: the rms current that actually flows in the arc, predicted from the bolted-fault current by the IEEE 1584-2018 model using electrode configuration, gap and enclosure factors.

Why it matters: Everything in an arc-flash study — incident energy, boundary, PPE category — is driven by Iarc, not by Ik. Iarc is markedly lower than the bolted-fault current, so substituting Ik for it grossly over-estimates the energy and the required PPE.

Bolted-fault current Ibf: the three-phase short-circuit current at the equipment location, taken as the input current of the IEEE 1584-2018 arc-flash model.

Why it matters: It is where the arc-flash study starts, so it must be produced with the same voltage, transformer and cable assumptions as the short-circuit report. A mismatch between the two documents is one of the most common audit findings.

Incident energy: the thermal energy per unit area that would reach a surface at the working distance during an arc, as defined by IEEE 1584-2018.

Why it matters: It is the number that selects the PPE (NFPA 70E thresholds at 1.2 / 4 / 8 / 25 / 40 cal/cm²) and that must be printed on the equipment label. An under-estimated incident energy is a safety liability, not a commercial rounding difference.

Arc-flash boundary: the distance from a prospective arc at which the incident energy falls to 1.2 cal/cm², the level at which arc-rated PPE becomes necessary (IEEE 1584-2018 / NFPA 70E).

Why it matters: It defines the barricade and the approach boundary on site: a larger AFB means more barricading and heavier PPE for the same work. It is a safety deliverable that also has a direct operating cost.

Personal protective equipment category: the arc-rated clothing and equipment class required for a task, selected from the calculated incident energy according to NFPA 70E-2021.

Why it matters: It is a safety and legal deliverable: the category decides what the field team must wear, what the work permit says and whether work needs de-energisation. It follows directly from the arc-flash calculation, so it must be reproducible for the assumed upstream fault level.