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A breaker can be correctly sized for the load yet still be unsuitable for the fault current at the board. This can become apparent after a transformer is added or the supply arrangement changes. The load-current rating may stay appropriate while the required interruption duty increases.<\/p>\n

Circuit breaker coordination<\/strong> deals with both fault interruption and how the protective devices operate together. Start with the available fault current, then assess each breaker\u2019s ratings and the required selective behaviour. Icu, Ics and Icw answer different questions, as do selectivity and backup protection.<\/p>\n

Fault-Current Studies for Circuit Breaker Selection<\/h2>\n

The short-circuit study estimates fault current at defined points in the network. It accounts for source characteristics, transformer impedance, cables or busbars, connected generation and the network arrangement. Maximum current matters for breaking and withstand ratings; minimum current can matter for achieving the required protective operation.<\/p>\n

Include the required normal and alternative supply modes. Parallel transformers can raise the maximum fault current. A generator or inverter-supported source may supply a different current for a different duration, which can change when protection operates. A lower available fault current does not necessarily mean faster clearing.<\/p>\n

Read the circuit breaker short-circuit rating<\/strong> at the study\u2019s operating voltage. A 400 V rating cannot simply be applied at 690 V, and AC data does not establish DC performance. If backup protection is used, the exact device combination and application conditions must be covered by the manufacturer\u2019s data.<\/p>\n

Breaking Capacity and Short-Time Withstand Ratings<\/h2>\n\n\n\n\n\n\n\n\n
IEC quantity<\/th>\nMeaning for selection<\/th>\nWhat it does not prove<\/th>\n<\/tr>\n<\/thead>\n
Icu<\/td>\nUltimate short-circuit breaking capacity under the specified test duty<\/td>\nUnrestricted continued service after any fault<\/td>\n<\/tr>\n
Ics<\/td>\nService short-circuit breaking capacity under its specified test duty<\/td>\nSelective operation with another breaker<\/td>\n<\/tr>\n
Icw<\/td>\nShort-time withstand current for a stated duration<\/td>\nPermission to use any desired time delay<\/td>\n<\/tr>\n
Icm<\/td>\nRated short-circuit making capacity, expressed as peak current<\/td>\nA direct RMS interruption comparison<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

IEC 60947-2 defines ratings and test duties for industrial circuit breakers. Other standards use different ratings and conditions; for example, IEC 60898-1 uses Icn for household and similar AC breakers. Compare like duties before treating two kA figures as equivalent.<\/p>\n

As an illustrative first check, take a 400 V board with 25 kA prospective RMS fault current and a breaker rated at 36 kA Icu at that voltage. The Icu figure exceeds the calculated current. You still need to assess Ics, any required withstand duty, making capacity, protection settings and assembly requirements before accepting the device.<\/p>\n

\"Icu
Compare like duties at the specified voltage, duration and test conditions.<\/figcaption><\/figure>\n

Circuit Breaker Selectivity vs Backup Protection<\/h2>\n

Selective coordination<\/strong> concerns which breaker opens. Total selectivity and partial selectivity describe different ranges of operation. If the manufacturer gives a current limit for a pair, record that limit in the study; behaviour above it may differ.<\/p>\n

Backup protection, often called cascading in manufacturer literature, uses an upstream device\u2019s protective contribution in a specified downstream combination. It may allow that tested combination to meet a fault duty, but the upstream breaker may also open. Backup data therefore does not, on its own, demonstrate selective operation.<\/p>\n

Time-current curves help assess overloads and time delays. At high fault currents, current limitation and interaction between devices can also matter, so visually separated curves may not tell the whole story. Use data for the actual models, releases, voltage and settings. The ACB rating and trip-unit guide<\/a> explains why the release is part of the selection.<\/p>\n\n\n\n\n\n\n\n\n
Design objective<\/th>\nRequired evidence<\/th>\nLimit to document<\/th>\n<\/tr>\n<\/thead>\n
Interrupt the maximum fault<\/td>\nFault study and device rating at system voltage<\/td>\nApplicable voltage and test conditions<\/td>\n<\/tr>\n
Keep upstream supply energized<\/td>\nSelectivity data for the device pair and settings<\/td>\nTotal or partial selectivity and its current limit<\/td>\n<\/tr>\n
Use an approved backup combination<\/td>\nManufacturer\u2019s tested backup table<\/td>\nExact upstream and downstream configurations<\/td>\n<\/tr>\n
Apply a short-time delay<\/td>\nWithstand duty, settings and assembly assessment<\/td>\nCurrent, duration and clearing requirements<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Breaker Trip Settings and System Withstand Limits<\/h2>\n

A longer short-time delay gives a downstream device more time to clear some faults, but also exposes the breaker, cables and assembly to fault current for longer. Assess those withstand limits alongside clearing time and the relevant electrical safety or arc-flash requirements.<\/p>\n

Identify manufacturer-verified device combinations<\/strong> by model, release version and critical settings in both the study and the order. If a replacement changes one of those items, review the affected result. An equal current rating does not preserve the original coordination automatically.<\/p>\n

For W9\u2019s WLM6E electronic MCCB<\/a>, request the proposed trip unit, time-current data, voltage-dependent ratings and any available tested coordination information. The engineering review should identify which required results are supported by that data and which remain to be established for the project.<\/p>\n

\"Longer
Review the clearing-time benefit alongside the equipment's withstand limits.<\/figcaption><\/figure>\n

Circuit Breaker Coordination FAQs<\/h2>\n

Is circuit breaker breaking capacity the same as rated current?<\/h3>\n

No. Rated current relates to normal operation and protection selection. Breaking capacity describes the fault current the device can interrupt under specified conditions. Both must suit the circuit.<\/p>\n

Does a larger upstream breaker guarantee selectivity?<\/h3>\n

No. Selectivity depends on the pair\u2019s characteristics, settings and fault current. Check the manufacturer\u2019s tables and protection study rather than using the ampere ratio alone.<\/p>\n

Does backup protection keep the upstream breaker closed?<\/h3>\n

Not necessarily. Backup protection establishes a specified combination\u2019s fault-protection duty. Keeping the upstream supply available requires a separate selectivity assessment.<\/p>\n

Does replacing an MCCB affect coordination?<\/h3>\n

It can. A different release or breaker characteristic may change the coordination even when the current rating matches. Review the proposed replacement against the study and assembly requirements.<\/p>\n

What data is needed for an ACB coordination study?<\/h3>\n

Obtain ratings at the operating voltage, time-current curves, trip-unit details, short-time withstand data and the relevant coordination tables. The study also needs each required supply mode and the assembly limits.<\/p>\n

Does earth-leakage protection replace short-circuit coordination?<\/h3>\n

No. Residual-current or earth-fault functions address particular faults and may need their own coordination review. They do not replace adequate overcurrent interruption and conductor protection.<\/p>\n

Circuit Breaker Coordination References<\/h2>\n