Understanding Short-Circuit Breaking Capacities: Icu, Ics, and Icw

1. Fault Currents and What Breaking Capacities Mean

During a low-voltage short circuit, the current is limited mainly by transformer and cable impedance. Plants with large or paralleled transformers can see prospective fault currents of tens of kiloamperes at the main busbar. Every breaker installed there must have a breaking capacity no lower than the calculated prospective short-circuit current; otherwise it may fail to clear, be destroyed, and leave the fault uninterrupted.

IEC 60947-2 for industrial circuit breakers defines three ratings that must be understood precisely: Icu (ultimate breaking capacity) is the maximum fault current the breaker can successfully interrupt under the prescribed test sequence, with no guarantee of continued full performance; Ics (service breaking capacity) is the highest current after which the breaker remains fit for continued service, expressed as a percentage of Icu; and Icw (short-time withstand current) is the current the breaker can carry without tripping for a stated time, used for time-delayed protection coordination.

2. Icu versus Ics: The Difference That Affects Continuity

The practical difference between Icu and Ics lies in the breaker's condition after clearing the fault. The Icu test uses an O–t–CO sequence, while the Ics test extends it to O–t–CO–t–CO followed by verification that the breaker still withstands voltage, trips on its curve, and carries rated current. A breaker with Ics equal to 100 percent of Icu suits locations that must restore supply quickly after a fault, such as hospital mains or continuous production feeders.

Conversely, choosing a breaker on high Icu alone when its Ics is only 25 or 50 percent of Icu means that after a severe near-rating fault it may clear successfully yet require replacement or major inspection before returning to service, extending the outage unnecessarily. Selection should balance equipment cost against the outage cost of each circuit: mains and critical feeders warrant high Ics, while ordinary final circuits can reasonably be relaxed.

• Icu: clears the maximum fault but continued service is not guaranteed
• Ics: remains serviceable after clearing, stated as a % of Icu (25/50/75/100)
• Icw: withstands fault current without tripping for a stated time, used for coordination
• Mains and critical feeders should have Ics at or near 100% of Icu

3. Calculating Fault Current for Rating Selection

Selection starts by calculating the three-phase symmetrical fault current at each busbar level. The widely used approximation divides the transformer's rated current by its percentage impedance (%Z) — a low-%Z transformer yields a high fault current — then adds the effect of cable impedance from transformer to board, which reduces the fault level with distance. Complex systems with paralleled transformers, large motors feeding back, or generators should be calculated per IEC 60909 using power system analysis software.

With the prospective fault current known, choose breakers whose Icu (and Ics per criticality) is not less than that value at the actual service voltage, reading manufacturer tables at the matching voltage since breaking capacity falls as voltage rises. Where cost matters, cascading (back-up protection) lets an upstream current-limiting breaker assist a downstream one, but only manufacturer-certified device pairs verified by combined testing may be used.

4. Common Rating Selection Mistakes

Frequent mistakes include upsizing or paralleling transformers without revisiting existing breaker ratings — silently pushing the new fault level beyond every device in the board; reading Icu from catalogue columns at a lower voltage than the actual system; ignoring motor contribution in plants with large aggregate motor load; and improvising cascading combinations across brands without certified combined test tables.

Prevention means treating the short-circuit study as a controlled document updated whenever transformers, sources, or system topology change; labelling the prospective fault current on the MDB for future operators and designers; and, when purchasing replacement breakers, verifying Icu, Ics, and cascading compatibility with the existing devices every time — never selecting on ampere frame and trip rating alone.

• Update the short-circuit study whenever transformers or sources change
• Label the prospective fault current on the MDB
• Always read breaking capacity at the actual system voltage
• Use only manufacturer-certified cascading pairs