Industrial plants, hospitals, data centers, and virtually every type of facility or campus cannot afford any downtime due to electrical system problems. Downtime negatively impacts customer satisfaction and the bottom line. Additionally, the IEC 60364 standard mandates selectivity for installations supplying safety services, with local regulations often requiring it for other specific applications.
The design of electrical systems, including the selection of protection devices, directly ensures power availability. Achieving this availability involves optimizing device coordination. Devices must be carefully chosen to function correctly with other components in the system, including switches, contactors, circuit breakers, and residual current devices (RCDs) within assemblies like switchboards.
Understanding SelectivityFor facilities such as hospitals, data centers, and airports, maintaining uptime for all critical loads is crucial. In continuous industrial processes or food refrigeration applications, power loss can result in costly damage to raw materials, products, and time. When an overload, short circuit, or ground fault occurs on a distribution circuit, energy availability should continue for all other parts of the electrical installation.
Selectivity, sometimes called discrimination, is one solution. When a fault condition occurs on a circuit, the circuit breaker closest to the fault trips. The upstream breakers remain unaffected, ensuring power remains available to other circuits and loads. This also allows the facility team to quickly locate and fix the fault source, as they only need to identify the tripped breaker. If an upstream breaker had tripped, locating the fault would be more time-consuming as it could be on any downstream circuit.
Multiple Levels of SelectivityCircuit breakers must be designed to work together. In commercial buildings, for example, the function and rating of a circuit breaker depend on its position in the electrical architecture: air circuit breakers (ACB) or high-rating molded case circuit breakers (MCCB) as incomers, with MCCBs in the middle level and miniature circuit breakers (MCB) for final circuits.
The quality of the installation depends on how well the products are coordinated to manage short circuits. This coordination is challenging when mixing different brands of products. Choosing products from a single manufacturer with closely collaborating engineering teams ensures better coordination.
In a short-circuit event, all circuit breakers between the power supply (e.g., utility grid) and the fault will experience overcurrent. An ACB or high-rating MCCB main incomer may be delayed to achieve “time-based selectivity,” requiring precise settings. Achieving selectivity with current-limiting circuit breakers, including most MCCBs on feeders and MCBs in final distribution circuits, is even trickier. It depends on the limitation of the let-through energy of all involved circuit breakers and the non-tripping energy of the upstream circuit breaker, which must be considered during the design of breaking and tripping characteristics.
Schneider Electric's close collaboration between MCB, MCCB, and ACB design teams results in an unparalleled range of selective products. This allows for architectures with several intermediate switchboards, optimizing cable lengths.
Selecting Circuit Breakers for SelectivityHow do you choose the right combination of circuit breakers and ratings for reliable selectivity? Schneider Electric offers dedicated software (EcoStruxure Power Design), online tools, and a guide (Selectivity, Cascading, and Coordination Guide) to support the design of low-voltage installations considering selectivity. The correct products, such as the MasterPact, ComPact, and Acti9 series circuit breakers, have a limited number of frame sizes and models, simplifying the process. These breakers are designed and tested for selective coordination – from ACB to MCCB to MCB, as well as motor starters and motor circuit breakers – ensuring reliable selectivity from mains to feeders to final distribution.
