Short-Circuit Current Calculation (Impedance Method)
Design method per IEC 60909-0 in which the prospective short-circuit current at a point in the installation is calculated by vectorially summing the impedances of all upstream components (transformer, cables, protective devices) into a total impedance Zk, after which Ik = U / (√3 × Zk) for a three-phase fault. This calculation determines the required breaking capacity (Icn) of the protective device to be selected and the thermal/dynamic withstand of cables and busbars, and is re-performed both at design stage and whenever the supply structure changes (e.g. a new, larger transformer).
Bij het ontwerp van een hoofdverdeelinrichting berekent de ontwerper Zk als de som van de transformatorimpedantie (uit Uk%) en de kabelimpedantie tot de HVI, om vast te stellen of een vermogensschakelaar met Icn 25 kA of 36 kA nodig is.
Bij de impedantieberekening de weerstand van de kabels verwaarlozen en alleen met de transformatorimpedantie rekenen — dit overschat de werkelijke kortsluitstroom verderop in de installatie en kan leiden tot een onnodig duur, te zwaar gedimensioneerd beveiligingstoestel.
See also
- → IEC 60909-0 Short-circuit current calculation per IEC 60909-0 — the equivalent voltage source method
- → §434 Short-circuit protection & the adiabatic equation — §434
- → §551 / IEC 60364-5-55 Fault-current capability of a standby generator — why protective devices can behave differently on generator power
- → §434 / IEC 60909-0 Prospective short-circuit current — from Ik'' at the point of connection to the required Icu of switchgear
- → IEC 62020 Residual current monitoring (RCM) versus RCD — continuous measurement instead of tripping
- → §433.3 / §434.3 When overload or short-circuit protection may be omitted (§433.3/§434.3)