Schneider Electric Singapore Pte Ltd

Introduction The performance of low voltage circuit breakers are characterized by a few key parameters: Rated current Rated insulation voltage Rated operational voltage Rated breaking capacities The most interesting and usually drawing the most interest are the circuit breakers’ short-circuit breaking capacities. Most often than not, we assume that a circuit breaker has just one rated breaking capacity and ignore the significance of all the other rated breaking performances. This document is aimed at clarifying this critical point in the specifications and selection of Low Voltage Circuit Breakers, which is normally neglected. Definition of Icu & Ics The key performances and testing methods for Low Voltage Circuit Breakers are defined in IEC60947-2. Icu is the abbreviation for Rated Ultimate Short-circuit breaking capacity. This is the current for which the prescribe conditions according to a specified tests sequence do not include the capability of the circuit breaker to carry its rated current continuously after the tests. It is the maximum short-circuit current that the circuit breaker can break and it is checked by the following sequence:

O – t – CO O - refers to a breaking operation CO - refers to a making operation followed by a breaking operation t - refers to the time separating two operations, equal to 3 minutes or the length of time needed to reset the breaker, whichever is longer

After the tests, the circuit breaker is simply tested to show that it is electrically safe. This is done by a dielectric test under a testing voltage equal to twice the rated service voltage but at least equal to 1000V. Ics is the abbreviation for Rated Service Short-circuit breaking capacity. This is the current for which the prescribe conditions according to a specified test sequence include the capability of the circuit breaker to carry its normal rated current continuously after the test. The tests are conducted in the following sequence: O – t – CO – t – CO Following the tests, the circuit breaker undergoes a temperature rise test, a dielectric test and a tripping test to verify that the breaker is qualified to be returned to service. Ics is expressed as a percentage of Icu (eg 25%, 50%, 75% or 100% of Icu). In a nutshell, this is the maximum current that the breaker can break for 3 times and yet returned to service with its operational integrity intact.

THE CONTROVERSY OF ICU AND ICS

Vol 2 2005

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Schneider Electric Singapore Pte Ltd

Selection of Low Voltage Circuit Breakers Circuit breakers are usually selected based on their ability to break the prospective short-circuit currents. The reference prospective short-circuit currents are based on the worst-case scenario of 3-phase fault. The conventional approach to circuit breakers selection has been based on the value of Icu. This does not fully address the expectation of users who want continuity-of-service. Icu only guarantees the fault interrupting capability of the breakers. It does not guarantee that the breakers can be returned to service without overheating after clearing the fault. This is so because the test procedures stipulated in IEC60947-2 do not require that the breaker be subjected to temperature rise checks after clearing of faults. On the other hand, Ics, besides guarantees interruption of fault, it also guarantees that the breaker can carry it full rated current continuously without excessive temperature rise after the fault current interruption. Purely from a user’s point of view, in order to build an electrical distribution system with good continuity-of-service, it is desirable to select breakers with due consideration for their performance at Ics level. Specification of Low Voltage Circuit Breakers The suggested approach to ensuring the right selection of protection devices is first to address the need to specify the required performance criteria explicitly. The suggested specifications may be: “The Ics rating of all Low Voltage Circuit Breakers must at least be equal to the prospective short-circuit current at the point of installation”. This will ensure that in the worst-case scenario, the breaker is able to clear the full magnitude of fault current 3 times and remains operational until further service action.

Conclusion As with any other protection devices, circuit breakers are never built equal. A well-written technical specification and right selection of circuit breakers will certainly go a long way in ensuring a robust system. The difference between Icu and Ics should be recognized. The rated breaking capacity is commonly specified by many M&E engineers, however, the requirement for Ics is seldom mentioned explicitly. The need to evaluate and select circuit breakers based on Ics can be significant.

Suggested Specification Notes: The Ics rating of all Low Voltage Circuit Breakers must at least be equal to the prospective short-circuit current at the point of installation.

Edited by DNP, February 28, 2005 Should you require more information, please call Daniel PANG 98713579 or Phil CHAN 96973644 Email: daniel.pang@sg.schneider-electric.com