Short-circuit & equipment verification
IEC 60909 fault levels, per-node grading and withstand checks
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10 kV incoming (200 MVA short-circuit level) → 1250 kVA transformer (Dyn11) → LV main switchboard → busbar → 5 feeders plus a fire/emergency ATS branch. 1000 kW calculated load, cosφ 0.85, power-factor target 0.95.
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Calculator mode computes the few outputs that only depend on parameters you type in yourself. Anything that genuinely needs the whole system (full-network load flow, coupling with other devices) is shown as “needs a scheme” and no number is given.
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Note: every number comes from the engine. With no scheme, nothing is invented here.
Definitions, units and why the number matters for selection and quotation. Searchable, grouped by topic, collapsed by default.
Every ? mark on this page opens the same explanation in place — no page change.
15 term(s) shown of 15
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.
Peak short-circuit current ip = κ·√2·I″k (IEC 60909-0): the highest instantaneous value of the fault current, reached about half a cycle after inception.
Why it matters: It is the number that generates the electrodynamic force on busbars, cable cleats and device terminals. Compare it with the rated peak withstand Ipk — quoting only Ik leaves the mechanical strength of the switchboard unchecked.
Peak factor κ = ip / (√2·I″k): the ratio of the first peak to the rms value, calculated in IEC 60909-0 from the R/X ratio of the equivalent impedance.
Why it matters: It is what converts a short-circuit current into a mechanical force. An inductive network (LV cables, transformers) pushes κ towards 2.0 and raises the peak force, which is why ip cannot be derived from Ik alone.
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.
Voltage factor c for short-circuit calculation (IEC 60909-0 Table 1): the ratio between the voltage assumed at the fault location and the nominal voltage — c > 1 for maximum currents, c < 1 for minimum currents (for LV: 1.05 / 0.95; for HV: 1.10 / 1.00).
Why it matters: It is the explicit safety margin of the calculation. The maximum-current factor drives the device ratings you quote; the minimum-current factor drives protection sensitivity. Using the wrong one shifts Ik by several percent in a way that is invisible in the report unless the factor is printed.
Aperiodic (DC) component of the short-circuit current, i_dc = √2·I″k·e^(−ωt·R/X) (IEC 60909-0): the decaying offset that rides on top of the symmetrical component.
Why it matters: It is the extra current the breaker has contact to interrupt at the moment of contact parting, and it is why the required breaking capacity is higher than I″k for fast (generator-close) faults. In a quotation it appears as the rated breaking capacity at that % DC component.
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 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.
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.
Two different withstand ratings of the same switchboard. Icw is a THERMAL rating: the rms current the assembly carries for 1 s (or 3 s) without overheating. Ipk is a MECHANICAL rating: the peak current the busbars and supports survive without being bent apart.
Why it matters: They are compared with two different calculated quantities — Icw against Ik, Ipk against ip. Comparing Icw with ip (or Ipk with Ik) is a frequent and expensive mistake: it either over-sizes the board or leaves it under-rated.
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.
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.
Voltage unbalance: how far a three-phase voltage set departs from symmetry, expressed as the ratio of the negative-sequence (or zero-sequence) component to the positive-sequence component.
Why it matters: Motors and converters derate and overheat on unbalanced supply — limits are set by standard or by the machine vendor (for reference: IEC 61000-2-2 gives 2 % for LV networks), so the applicable limit must be named. Reducing it usually means redistributing single-phase loads or resizing the neutral.
Frequently asked questions
- What can this calculator do?
- Enter the external conditions (Ssc and X/R), the transformer Sn and uk, the length, section and material of every branch from the source to the fault point, the motor capacity (largest motor and total motor), the fault type and the clearing time. The page runs the IEC 60909 short-circuit study of the scheme — equivalent voltage source method, engine engines/shortcircuit-60909.js — and turns the per-node result into selection verdicts for four fault types (three-phase, two-phase, two-phase-to-earth, single-phase-to-earth) on both the MV and the LV side. Typical uses: fixing the rated short-time withstand and breaking capacity to order, finding the minimum cable section that survives the thermal duty until the protection clears, and checking that the protection settings can still be selective. Missing data is shown as missing, never as zero.
- How is the network impedance built up?
- Z_Q comes from the external short-circuit capacity Ssc with the X/R ratio split into R and X, then folded to the LV side through the transformer Sn and uk, and the branches on the path to the fault point add their own impedance from length, section and material. The motor contribution (largest motor and total motor) is included, which is what makes the value different from a simple transformer-only estimate.
- What do the four fault types change?
- They change both the magnitude and which selection decision is bound: the three-phase fault sets the breaking duty on switchgear, the two-phase cases give the comparison the standards ask for, and the single-phase-to-earth case is the one that sizes the earth-fault protection and the neutral path. The per-node table is what lets each verdict be traced back to the fault case that produced it.