Understanding the Relationship Between Icu and Icw in Switchboard Design
RAJITHA UKWATTA C. Eng MIET
Chartered Engineer| LV/MEP design| Estimation|
Recently, I have noticed that many technical professionals ask whether the Icu rating of an MCCB/ACB must match the Icw rating of a switchboard. For example, if the main busbar of a switchboard is rated for 85kA for 1 second, does this mean that all outgoing MCCBs must have an Icu of 85kA?
The direct answer is yes or no, depending on proper coordination and fault current calculations.
This question often arises due to a lack of knowledge in electrical installation practices (e.g., BS 7671/IEC 60364). In standard design methodology:
? Breaker rated currents are determined from the load side to the transformer secondary (downstream to upstream).
? Short-circuit levels are calculated from the transformer secondary to the end load (upstream to downstream).
Simply put, the Icu rating of a circuit breaker should always be higher than the prospective short-circuit level at the installation point. This means that, in a switchboard, some outgoing MCCBs may have a lower Icu than the Icw rating of the main busbar, depending on the actual short-circuit levels at those points.
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However, due to uncertainty about prospective short-circuit levels at outgoing circuits, many switchboard builders standardize all MCCBs to have the same Icu rating as the main incoming breaker purely for safety. While this is a conservative approach, it may increase costs unnecessarily if proper fault current calculations are not performed.
It is important to remember that:
By understanding this distinction, engineers can optimize switchboard designs for both safety and cost-effectiveness while ensuring compliance with industry standards.
Higher National Diploma in Electrical Engineering
3 天前Useful.. thank you for sharing
LV design manager at Devash Control System
1 周if the incoming MCCB, we can minimize the ultimate short circuit because the incoming is current limiting but if the incoming is ACB, Outgoing feeders should be sized according to the main bus bar short circuit
MEP Engineer with a passion for the global I Reading MBA IBU, Toronto I B.Eng (Hons) Leicester, UK, UOM I AMIIESL
1 周The outgoing MCCBs do not all need to have an Icu of 85kA just because the busbar has an Icw rating of 85kA for 1 second. Instead, each MCCB’s Icu should be chosen based on the actual prospective short-circuit current at its location while ensuring proper discrimination and protection coordination.
International Professional Engineer & Chartered Engineer (Sri Lanka)
1 周Generally, we consider Icu rating. But, we must actually consider the Ics rating of the circuit breakers. This is because if the Ics value is greater than the prospective short circuit level, then the breakers can be able to clear the faults without any damages to them so that those can be safely reused. However, if the fault current exceeds the Icu rating, then the breaker can safely clear the fault but the breaker might have been damaged beyond reuse. That is why, it is recommended to consider the Ics rather than Icu. Most of the nowadays breakers have Icu equals to Icu. But, there are breaker is the market with higher Icu and lower Ics values. So, we need to be very careful.
Electrical Engineer - AMIE (SL) | AEng. EC (SL)
2 周Clearly structured problem breakdown!