Icw and Icm of a circuit breaker — short-time current and making capacity alongside Icu/Ics

The guide on circuit-breaker settings (LSI) covers the trip parameters a breaker itself uses to decide when to trip. The guide on prospective short-circuit current and Icu sizing covers how a breaker's Icu (ultimate breaking capacity) must match the actual fault current at its position. This article covers two additional, often overlooked short-circuit characteristics from the same standard — Icw and Icm — which become especially relevant once a breaker is applied in a time-graded (selective) protection scheme.

Icw — the RMS short-time withstand current

Icw (rated short-time withstand current) is the RMS value of the short-circuit current that a circuit breaker can withstand for a specified short duration — typically somewhere between 0.05 and 1 second — without its properties being impaired, while the breaker itself does not trip.

This is directly relevant in a time-graded selective protection scheme: to achieve selectivity between an upstream and a downstream breaker, the upstream breaker is intentionally delayed (see the guide on LSI settings), so that the downstream breaker, closer to the fault, trips first. During that delay, however, the upstream breaker remains closed and carries the full fault current. If that breaker cannot withstand the fault current for the set delay time without damage — in other words, if its Icw for that duration is too low — then the selectivity scheme is not valid, even though the same breaker's Icu is ample to ultimately interrupt the fault.

Icm — the peak-value making capacity

Icm (rated short-circuit making capacity) is the peak value of the current onto which a circuit breaker may close an already-present short circuit, without the contacts welding shut or being mechanically damaged by the electrodynamic forces of that peak current. Unlike Icu and Ics (which are expressed as RMS values), Icm is a peak value, and in IEC 60947-2 it is derived from Icu via a multiplication factor k that depends on the Icu range and the associated standardized power factor of the test current:

Icm = k × Icu

where k typically lies on the order of 1.5 to 2.2, increasing as Icu increases (a lower power factor at higher short-circuit currents gives a higher ratio between peak and RMS value).

Note: the exact k-factor values per Icu range and the associated standardized power factor are given in a table in IEC 60947-2; this article covers the principle behind the derivation, not the full table.

Why these are two separate characteristics, distinct from Icu/Ics

Icu and Ics describe a breaker's ability to interrupt a short circuit (the ultimate value and the value intended for repeated use, respectively). Icw and Icm describe two entirely different moments in the life of a fault at that breaker:

Icw: can the breaker withstand the fault current for a while without tripping itself (relevant for an intentional delay used for selectivity)?
Icm: can the breaker close onto an already-existing fault without mechanical damage from the peak current?

A breaker can have an ample Icu for its position in the installation, and yet be unsuitable for a specific selective scheme because the specified Icw for the required delay time is too low, or because the chance of closing onto an existing fault (for example when re-closing after an unknown fault elsewhere in the installation) requires an Icm that the breaker does not achieve.

Practical relevance

When designing or reviewing a time-graded selective protection scheme between two or more series-connected circuit breakers, it is not enough to compare only each breaker's Icu with the prospective short-circuit current at its position — the upstream breaker's Icw must also be checked against the actually configured delay time (tsd) from the LSI settings. When selecting a breaker for a position where there is a real risk of closing onto an existing fault (for example on re-energizing after an unknown disturbance), the specified Icm is likewise a relevant selection criterion.

Common mistakes

Checking only Icu when setting up a time-graded selectivity scheme, without verifying whether the upstream breaker's Icw is sufficient for the configured delay time.
Confusing Icm with Icu — Icm is a peak value and typically considerably higher than the RMS value of Icu; the two are not directly comparable without the k-factor.
Using a specified Icw value without checking the associated duration — an Icw specified for 0.05 s says nothing about whether the breaker can also withstand the same current for 1 s.
Assuming a higher Icu automatically implies a higher Icw and Icm — these are separately tested and specified properties that can vary by breaker type and manufacturer.