Reclosing study — fault → protection clear → reclose
Three quasi-steady operating stages (fault, protection clearing, reclosure). Each stage runs the real load-flow solver, and the fault stage runs the real short-circuit engine. Both branches of the reclosure — a transient fault (reclose succeeds) and a permanent fault (reclose fails and trips again) — are checked for branch overload and bus voltage violations.
Pick a scheme first — then this column can calculate
This column runs the real engine on your project scheme, and there is none yet — that is why no calculation buttons are shown here (they are not broken). Start with any of the ways below; the fastest is the built-in example (one click, real parameters).
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.
Load the real example scheme and start calculating →The scheme and its parameters go straight into your browser; replace the data with your own at any time.
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.
Calculator mode is not available for this column yet — use one of the ways below.
Nothing is pre-filled and nothing is invented: leave a required field empty and the result area says “unavailable”.
In the studio, place devices, wire them and pick models — that becomes your project scheme (drawings / BOM / report can be exported).
Open the studio →Use the chat bubble bottom-right, or Contact us — we will set the example to your site parameters.
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.
8 term(s) shown of 8
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.
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.
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.
Per-unit bus voltage in a load-flow result: the node voltage divided by its base voltage, so 1.00 pu is nominal.
Why it matters: It is the acceptance number of every voltage-drop check (commonly 0.95–1.05 pu). A node below the band means equipment malfunction and a grid-code breach, and the answer is a larger cable, a different transformer tap or a compensation device — a quotation change, not a note.
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.
Loading: the current (or power) carried by a branch expressed as a percentage of its rated capacity.
Why it matters: It is the acceptance number for cables and transformers. Above 100 % the element will trip or have to be derated, and the margin left at the design case is exactly what the customer is paying for — so it must be shown with the assumption (load factor, ambient temperature) that produced it.
N-1 (single-contingency) criterion: the network must remain within its operating limits with any one single element (line, transformer, generator) out of service.
Why it matters: It decides how much redundancy — parallel feeders, duplicated transformers, busbar sections — goes into the quotation. If it is not stated explicitly, the design tends to be single-radial and the customer finds out during commissioning.
Frequently asked questions
- What can this calculator do?
- Pick the fault type (three-phase or single-phase-to-ground), optionally name the fault branch or node, and press Run. The study splits one line fault into three quasi-steady operating stages — fault, protection clearing and reclosure — solving the real load flow in each stage and the real short-circuit engine in the fault stage, and checks both the transient branch (reclose succeeds) and the permanent branch (reclose fails and trips again) for branch overload and bus voltage violation. You get the fault current Ik, the fault point that was chosen, the load lost and the minimum bus voltage and maximum branch loading per stage, plus the violation count. Typical uses: checking that a downstream load survives a reclose attempt, and documenting which stage or branch is the binding constraint. Nothing is computed until you press Run.
- What method is behind the three stages?
- A quasi-steady snapshot method: three separate load-flow solutions with the operating point switched between stages, the fault stage additionally running the short-circuit engine. Millisecond transient waveforms are not simulated — no EMTP-level reclose transient, no protection timing, no breaker arcing or pole disagreement — and the page states this on the result card.
- Why are both the transient and the permanent branch reported?
- Because they are the two different outcomes of the same event: on the transient branch the reclose succeeds and the load comes back, on the permanent branch it fails and the line trips again — and that second case is the one that decides how much load is lost and which branch or voltage limit is breached. Reporting both keeps a scheme from being judged on the optimistic branch alone.