Power Distribution and Management

Circuit breaker selectivity for power availability

Industrial plants, hospitals, data centers and, in fact, every type of facility or campus, can’t afford any amount of downtime due to electrical system problems. Downtime also negatively impacts customer satisfaction and the bottom line. Furthermore, the IEC 60364 standard makes selectivity mandatory for installations supplying safety services, while local regulations may also require it for other specific applications.

Electrical system design, including chosen protection devices, contributes directly to ensuring power availability. Part of achieving availability is optimizing how devices are coordinated. Devices should be carefully selected to work properly in conjunction with other devices in an electrical system, including switches, contactors, circuit breakers, and residual current devices (RCDs) inside an assembly like a switchboard.

In this blog series, we’ll look at the benefits of the coordination of circuit breakers. There are a few types of coordination that can be used in electrical systems, depending on the requirements. In this post, we’ll look at ‘selectivity’, while my next post will discuss ‘cascading’. Both methods are covered by circuit breaker standard IEC 60947-2, Appendix A.

How does selectivity work?

Clearly, for facilities like hospitals, data centers, and airports it’s important to maintain up-time for all critical loads. But for applications like continuous industrial processes or food refrigeration, loss of power can result in costly damages in 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.

One solution is applying selectivity – sometimes called discrimination – between circuits. How does it work? If 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 will be much faster for the facility team to locate and fix the source of the fault, as they simply need to identify the circuit where the one breaker has tripped. In contrast, if an upstream breaker had tripped, the fault could have happened any one of a number of downstream distribution circuits, so would take longer to locate.

Multiple levels of selectivity

It’s important that circuit breakers are 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 incomer, with MCCBs middle level and miniature circuit breakers (MCB) for final circuits.

When considering the multiple levels, the quality of the installation will depend on how the products are designed to be coordinated together to manage the short circuit.  This is difficult to assure when different brands of products are mixed together. Choosing products from a single manufacturer that has engineering teams working closely together can help ensure the best coordination.

In the case of a short-circuit at one point of the installation, we should keep in mind that all the circuit-breakers between the power supply (e.g. utility grid) and the fault will see an overcurrent. An ACB or high-rating MCCB main incomer may be delayed to achieve “time-based selectivity.” The challenge here is to define the right setting. For current limiting circuit breakers – this includes the majority of MCCBs on feeders and MCB in final distribution circuits – achieving selectivity is even trickier. It relies on the limitation of the let-through energy of all circuit-breakers involved as well as the non-tripping energy of the upstream circuit breaker. This needs to be considered during the design of breaking characteristics, and the tripping characteristics of the full range.

Thanks to close collaboration between our MCB, MCCB, and ACB design teams, Schneider Electric can offer an incomparable range of selective products, allowing architectures with several intermediate switchboards in order to optimize cable lengths.

Selectivity

Choosing circuit breakers for selectivity

More importantly, how do choose the right combination of circuits breakers and ratings so that selectivity works reliably?

Schneider Electric provides dedicated software (EcoStruxure Power Design), online tools, and a guide (Selectivity, Cascading and Coordination Guide) to support the design of a low voltage installation taking into consideration selectivity. Furthermore, having the correct products, such as MasterPact, ComPact, and Acti9 series circuit breakers offer a limited number of frame sizes and models to make this process even simpler. These breaker ranges are also designed and tested for selective coordination – from ACB to MCCB to MCB, as well as motor starters and motor circuit breakers – giving you the peace of mind that selectivity will perform, from mains to feeders to final distribution.

What are your thoughts? Continue the discussion on the Power Availability Forum


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