Initial symmetrical short-circuit current at the fault point (IEC 60909-0): the rms current the network drives into a bolted three-phase fault, with the voltage source replaced by the c·Un/√3 equivalent.

Why it matters: It is the number every device rating is compared with — Icw, breaking capacity and the cable thermal check all read against Ik. Under-estimate Ik and the panel you quote will be destroyed on the first fault; over-estimate it and the client pays for switchgear he does not need.

Single-phase-to-earth short-circuit current (IEC 60909-0), calculated from the zero-sequence impedance Z0 in addition to Z1 and Z2.

Why it matters: It sizes the earth-fault protection and the rated withstand of the neutral / earthing path. Depending on the transformer vector group and the earthing arrangement Ik1 can be smaller or larger than the three-phase Ik, so it must never be assumed equal to it.

Grading (coordination) time interval Δt: the intentional time difference between two cascaded overcurrent devices so that the downstream device alone clears the fault. Standard practice is of the order of 0.2–0.4 s depending on relay and breaker type — an engineering practice, not a standard-mandated figure, so the value used must be stated in the study.

Why it matters: Too small and both devices trip (loss of selectivity, the whole plant goes down on one branch fault); too large and every fault lasts longer, raising the arc-flash energy and the PPE requirement. It is a trade-off the report must show explicitly.

Zone selective interlocking: overcurrent relays in adjacent protection zones exchange a blocking / pilot signal, so a fault inside their own zone trips instantly while a downstream fault is left to the downstream device (IEC 60255 relay functions).

Why it matters: It buys back the time delay that selectivity normally costs: faults are cleared faster, the arc duration shortens and the incident energy drops. Because it changes the relay wiring and the communication scheme, it must be declared in the specification and priced.

Time multiplier setting of an overcurrent relay: the scale factor applied to the standard inverse-time characteristic defined in IEC 60255-151, moving the whole curve up or down in time.

Why it matters: It is the single number that positions each relay curve on the time axis during grading. Raising the TMS up the chain buys selectivity but lengthens fault duration — and therefore the arc energy the site has to survive.

Rated short-time withstand current: the rms current a switchgear assembly can carry for a stated short time (commonly 1 s or 3 s) without damage (IEC 61439-1 / IEC 62271-1).

Why it matters: It is the acceptance criterion for the panel: the design rule is Icw ≥ Ik at the same point. A panel offered with Icw below the calculated Ik simply cannot be used, whatever the price.

Rated peak withstand current: the peak current the assembly and its busbars withstand without deformation (IEC 61439-1). The standard relates Ipk to Icw by a multiplier that depends on the rated short-time current — use the value from the standard table, not a guess.

Why it matters: It is the dynamic counterpart of Icw and must be ≥ ip. Because the two are quoted together and are easy to confuse, a quotation that gives only one of them is incomplete.

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.

Ratio of the equivalent reactance to the equivalent resistance seen from the fault point (IEC 60909-0).

Why it matters: It fixes two things at once: the peak factor κ (hence ip) and the decay rate of the DC component. A high X/R (inductive LV feeds, generator sources) gives both a higher peak and a longer DC transient, which the breaker must interrupt.

Per-unit value: a quantity expressed as a fraction of a chosen base value (1.00 pu = nominal voltage or rated power), the standard language of power-system studies.

Why it matters: It lets a 0.4 kV LV board and a 110 kV network be compared on one axis and one chart. Voltage 0.95–1.05 pu is the usual acceptance band, so any per-unit figure in a report can be read directly against the criterion.