Prospective short-circuit current — from Ik'' at the point of connection to the required Icu of switchgear

The MCCB versus MCB guide explains how the short-circuit withstand of switchgear is designated (Icn for MCBs, Icu/Ics for MCCBs). This article covers the question that precedes it: which short-circuit current does that Icu/Icn actually need to control, and where does that number come from?

What Ik'' actually is

The prospective short-circuit current (Ik''), as defined in IEC 60909-0, is the current that would flow for a short circuit with negligible impedance at a given point in the network — so it is not the actually measured current during a real fault (which is always limited by residual impedance), but a theoretical, calculated limit value that serves as a design starting point. Ik'' is a calculated quantity, not an equipment specification: the switchgear itself does not have an "Ik''" — it has an Icu/Icn that must be compared against the calculated Ik'' at the location where the device is applied.

Why this number comes from the grid operator

For a new or modified connection, the grid operator provides, on request, the prospective short-circuit current at the point of connection (the point where the customer's installation connects to the public grid, often the secondary side of the transformer or the meter cabinet). This value is the starting point of the design calculation, but it is not the value that applies everywhere in the installation:

The further a breaker or MCB is from the point of connection, the more cable and transformer impedance sits between them, and the lower the actual prospective short-circuit current at that position becomes.
The Icu (or Icn) of each switching device must be equal to or greater than the actual short-circuit current at its own position in the installation — not the Ik'' at the point of connection itself, which for a breaker deep in the installation would often be an unnecessarily strict (and therefore unnecessarily expensive) requirement.

Note: this is a different kind of calculation than the adiabatic equation from the short-circuit protection §434 guide — that one determines whether the cable can thermally withstand the short circuit until disconnection; this article determines whether the switching device may and can interrupt the short circuit without failing itself. Both calculations are needed and complement each other, but answer a different question.

Practical steps

Request the prospective short-circuit current (Ik'') at the point of connection from the grid operator (or, for an own transformer, calculate it from the transformer's power rating and short-circuit voltage uk%).
For every position in the installation where switchgear is applied, calculate the actual short-circuit current at that position (accounting for the impedance of cables and any transformers between the point of connection and that position).
For every position, select switchgear with an Icu/Icn equal to or greater than the calculated value at that specific position — not universally based on the Ik'' at the point of connection.

Practical relevance

When designing or extending an installation, it must be explicitly recorded which prospective short-circuit current served as the basis for each breaker selection, and at which position in the installation that value was calculated — a circuit-breaker sized for the Ik'' at the point of connection while actually located deep in the installation is functionally safe but potentially significantly oversized (and thus unnecessarily expensive); a breaker sized for a too-low, outdated Ik'' value (for example after the grid operator upgraded the connection) is instead undersized and thus a safety risk.

Common mistakes

Applying the same Icu/Icn requirement throughout the installation, based on the Ik'' at the point of connection — this is safe but leads to unnecessarily heavy (and expensive) switchgear further into the installation, where the actual short-circuit current is already considerably lower due to cable impedance.
Continuing to use an outdated Ik'' value after a connection upgrade — a larger connection capacity is often accompanied by a higher prospective short-circuit current from the grid operator, which can render existing downstream switchgear undersized.
Confusing Ik'' with the adiabatic cable calculation from §434 — both use a short-circuit current as input, but assess a different property (cable thermal withstand versus the switching device's breaking capacity).