Circuit breakers in a distribution system will trip in the event of a fault and protect the cables. Circuit breaker selectivity ensures coordination between protective devices so that when faults occur, power continues to all other parts of the electrical installation.
The use of circuit breakers has grown as they enable swift identification of the faulty circuit and allow fast reconnection. Industrial plants, hospitals, data centres and any type of facility cannot tolerate extended downtimes due to electrical problems. They affect productivity and profitability and can also have a negative impact on customer satisfaction.
Electrical system design and the selection of protective devices contribute directly to ensuring power availability. Achieving coordination is through circuit breaker selectivity or discrimination of protective devices. Careful device selection ensures they work with other devices in the system, including switches, contactors, circuit breakers, and RCDs.
Why is selectivity important?
This article from Schneider Electric considers the advantages of coordination between circuit breakers. There are several ways to achieve coordination in electrical systems, depending on the requirements. One is known as selectivity, but cascading is another method. Circuit breaker standard IEC 60947-2, Appendix A addresses both types of coordination.
Firstly, IEC 60364 makes selectivity mandatory for installations supplying safety services. For facilities like hospitals, data centres, and airports it is important to maintain up-time for all critical loads. Moreover, for continuous applications like industrial processes or food production, power loss can result in lost raw materials, products, and time.
Circuit breaker selectivity means that when a fault condition occurs on a circuit, the circuit breaker closest to the fault will trip. The circuit breakers upstream from the tripped breaker remain unaffected, so power remains available to all other circuits and loads. Additionally, it is fast for the facility team to locate and fix the source of the fault by identifying the one breaker has tripped. Conversely, a tripped upstream breaker can result from many downstream distribution circuits, so would take longer to locate.
Multiple levels of circuit breaker selectivity
It is important that circuit breakers work together. In commercial buildings for example, the function and rating of a circuit breaker depends on its position in the electrical architecture. For example, ACBs or high rating MCCB as incomer, with MCCBs middle level and MCBs for final circuits.
For complex installations, the quality of the installation will depend on how the system designer considered product coordination to manage the short circuit. Yet, this is difficult to confirm when using several brands of products together. Choosing products from a single manufacturer helps to ensure the best coordination.
In the case of a short-circuit at one point of the installation, all the circuit-breakers between the power source and the fault will see an overcurrent. Delayed tripping on an ACB or high-rating MCCB main incomer may achieve “time-based selectivity.” The challenge is to define the right settings. For current limiting circuit breakers, including most MCCBs on feeders and MCBs in final distribution circuits, achieving selectivity is trickier. It relies on limiting the let-through energy of all circuit-breakers involved and the non-tripping energy of the upstream circuit breaker. This needs consideration during the design of breaking characteristics, and the tripping characteristics of the full range.
The Schneider Electric range of selective MCBs, MCCBs and ACBs allow architectures with several intermediate switchboards to optimise cable lengths.
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