Circuit-breaker selectivity is the coordination between protective devices installed in series so that, in the event of a fault, the device closest to the faulty point operates preferentially, keeping the unaffected parts of the installation energized.<\/p>\n\n\n\n
The direct answer is: electrical selectivity exists to prevent a localized fault from unnecessarily disconnecting a larger area of the installation<\/strong>. Instead of a short circuit in a final circuit tripping the panel main breaker, selectivity seeks to make only that circuit breaker operate, keeping the remaining circuits in service.<\/p>\n\n\n\n In low-voltage installations, selectivity depends on time-current curves, thermal and magnetic settings, prospective short-circuit current, breaking capacity, let-through energy, breaker type, system position, manufacturer tables and continuity-of-service requirements. Therefore, it should not be treated as a simple amperage choice.<\/p>\n\n\n\n Electrical selectivity is the principle of coordinating protective devices to limit disconnection to the smallest possible part of the installation when a fault occurs.<\/p>\n\n\n\n The concept is simple: when a fault occurs in a given circuit, the device responsible for that section should operate before upstream devices. This prevents a localized problem from shutting down an entire board, a whole sector, a process panel or the building’s main supply.<\/p>\n\n\n\n Selectivity becomes especially important in commercial, industrial, healthcare, educational and condominium installations, supermarkets, data centers, building systems and any environment in which continuity of service matters.<\/p>\n\n\n\n\n Circuit-breaker selectivity is the specific coordination between two or more circuit breakers installed in series. The downstream breaker protects the circuit closest to the load. The upstream breaker protects the feeder, panel or installation at a higher level.<\/p>\n\n\n\n When a downstream fault occurs, the desired behavior is for the downstream breaker to operate first. The upstream breaker should remain closed, provided the fault is within the downstream device’s protection zone and safety criteria are satisfied.<\/p>\n\n\n\n Without selectivity, a fault in a final circuit may trip the main breaker. This reduces continuity of service and may cause process stoppage, loss of operation, shutdown of critical loads and difficulty in fault diagnosis.<\/p>\n\n\n\n NBR 5410 addresses coordination between protective devices and establishes that, when safety or utilization reasons require service continuity to be affected as little as possible, series-connected devices should have operating characteristics selected so that only the device responsible for the circuit where the fault occurs operates.<\/p>\n\n\n\n The standard also requires coordination between overload protection and short-circuit protection. When these functions are performed by different devices, their characteristics must be coordinated so that the energy let through during a short circuit does not exceed the withstand of the protected device and conductors.<\/p>\n\n\n\n Therefore, selectivity is not merely a good operational practice. In certain installations, it becomes part of the safety, operation and technical-documentation criteria.<\/p>\n\n\n\n\n Breaking capacity is the circuit breaker’s ability to interrupt the prospective short-circuit current at the point where it is installed. Selectivity is the coordination used to define which device should operate first when a fault occurs.<\/p>\n\n\n\n A breaker may have adequate breaking capacity and still not be selective with the downstream breaker. Likewise, it makes no sense to discuss selectivity if the device breaking capacity is inadequate for the installation point.<\/p>\n\n\n\n The correct analysis considers both subjects together: first, each device must withstand and interrupt the expected fault; then the operating characteristics must be coordinated to limit disconnection to the smallest possible part of the installation.<\/p>\n\n\n\n Selectivity only makes sense after breaking capacity has been verified<\/strong><\/p>\n\n\n\n Before coordinating which breaker should operate first, it is necessary to confirm that each device can interrupt the short circuit at its installation point. Also see Circuit-Breaker Breaking Capacity: what it is, Icu, Ics and prospective short-circuit current<\/a>.<\/p>\n\n<\/div>\n\n\n\n Total selectivity occurs when, across the entire expected fault-current range, only the downstream device operates before the upstream device. It is the ideal scenario when continuity of service is critical.<\/p>\n\n\n\n Partial selectivity occurs when selectivity is guaranteed only up to a certain short-circuit current. Above that value, both devices may operate, or the upstream device may operate before the downstream device.<\/p>\n\n\n\n In low-voltage systems, total selectivity is not always feasible with every breaker combination. It depends on device characteristics, prospective short-circuit current and manufacturer coordination tables.<\/p>\n\n\n\n\n Current-based selectivity is achieved through different operating-current levels of devices connected in series. The downstream device operates at lower currents, while the upstream device is set to operate only at higher currents.<\/p>\n\n\n\n This type of selectivity is common for overloads and in some short-circuit ranges. However, it can be limited when fault current is very high because the devices may enter their instantaneous operating region.<\/p>\n\n\n\n In adjustable molded-case circuit breakers, current-based selectivity can be obtained through rated current, thermal setting, magnetic setting and trip-unit configuration.<\/p>\n\n\n\n Time-based selectivity is achieved through a difference in operating time. The downstream device operates faster, while the upstream device has an intentional delay over a given current range.<\/p>\n\n\n\n This strategy is common in systems using adjustable circuit breakers or electronic trip units, especially in main low-voltage switchboards and larger panels. The delay allows the device closest to the fault to clear it before the upstream device disconnects.<\/p>\n\n\n\n Time-based selectivity requires care because increasing operating time can also increase let-through energy. It must therefore be coordinated with the withstand of conductors, busbars and devices.<\/p>\n\n\n\n Energy-based selectivity considers the energy let through by devices during a short circuit. It is especially relevant for high-current faults where operating time is very short and current\/time analysis alone may not be sufficient.<\/p>\n\n\n\n Let-through energy, often associated with the Joule integral, must be compatible with downstream conductors and devices. NBR 5410 addresses the need to verify the energy let through by the protective device against conductor thermal withstand.<\/p>\n\n\n\n In practice, energy selectivity depends heavily on manufacturer data. Curves, selectivity tables and energy-limitation information are essential.<\/p>\n\n\n\n\n Time-current curves show how a protective device behaves for different current levels. The current axis represents the overcurrent magnitude; the time axis indicates how long the device takes to operate.<\/p>\n\n\n\n To assess selectivity, the curves of series-connected devices are superimposed. The objective is to verify whether the downstream device curve operates before the upstream device curve over the relevant current ranges.<\/p>\n\n\n\n For modular circuit breakers, B, C and D curves explain part of the magnetic operating behavior. For molded-case and larger breakers, settings and electronic trip units can change the operating curve.<\/p>\n\n\n\n Time-current curves are the basis of selectivity analysis<\/strong><\/p>\n\n\n\n To understand why one breaker operates before another, it is necessary to interpret curves, operating ranges and settings. To review this basis, see Circuit-Breaker Trip Curves: Type B, Type C and Type D explained<\/a>.<\/p>\n\n<\/div>\n\n\n\n Manufacturer selectivity tables identify tested or calculated combinations between devices from the same line or family. They help verify whether two breakers in series provide total or partial selectivity up to a given short-circuit current.<\/p>\n\n\n\n These tables are important because actual selectivity cannot always be defined by a simple visual comparison of curves. At high short-circuit currents, current limitation, let-through energy and internal device behavior influence operation.<\/p>\n\n\n\n Using the tables requires attention to manufacturer, model, rated current, trip unit, setting, breaking capacity, voltage and system position. Mixing devices without coordination data may invalidate the analysis.<\/p>\n\n\n\n\n Circuit breakers in series should be coordinated according to the function of each device. The upstream breaker may protect a feeder or panel. The downstream breaker may protect a final circuit, load, specific panel or subpanel.<\/p>\n\n\n\n The coordination analysis must answer questions such as:<\/p>\n\n\n\n Without this analysis, an installation may operate normally for years but fail when a significant fault actually occurs.<\/p>\n\n\n\n In smaller boards, a main breaker commonly supplies several final-circuit breakers. Without coordination, a fault in one final circuit can trip the main breaker and disconnect every circuit in the board.<\/p>\n\n\n\n In small residential installations this may be inconvenient. In commercial and industrial installations it may be critical. A general shutdown can affect lighting, socket-outlets, equipment, servers, refrigeration, pumps, access control and safety systems.<\/p>\n\n\n\n Selectivity between the main breaker and final-circuit breakers depends on panel short-circuit current, breaker curves, rated currents, settings and breaking capacities.<\/p>\n\n\n\n\n In a main low-voltage switchboard, selectivity is one of the most important electrical-protection topics. The main board distributes power to subpanels, panels and significant loads. Unwanted tripping at the main board can interrupt a large portion of the installation.<\/p>\n\n\n\n Selectivity in a main low-voltage switchboard should consider the main breaker, outgoing breakers, feeders, prospective short-circuit current, busbars, breaking capacity, trip units and operational continuity.<\/p>\n\n\n\n In many cases, molded-case and air circuit breakers are used because they allow more suitable settings and coordination. Selectivity should be addressed in the protection study and represented consistently in the single-line diagram.<\/p>\n\n\n\n Main switchboards and panels require documented selectivity<\/strong><\/p>\n\n\n\n In main boards and industrial panels, coordination among the main breaker, outgoing circuits and feeders must be defined before devices are purchased and assembled. See Molded-Case Circuit Breaker: what it is, applications and specification criteria<\/a>.<\/p>\n\n<\/div>\n\n\n\n Industrial panels may supply motors, drives, resistive loads, machines, compressors, pumps, automation systems and process loads. In these environments, nuisance operation can stop production, damage processes or make operation more difficult.<\/p>\n\n\n\n Selectivity should consider the hierarchy of protections: panel incomer, internal feeders, control circuits, motor starters, motor circuit breakers, fuses, contactors, thermal overload relays and electronic devices.<\/p>\n\n\n\n It is also necessary to assess the risk of disconnecting equipment that must remain operational. NBR 5410 refers to situations in which unexpected disconnection may create danger or disable essential equipment, requiring specific protection and signaling solutions.<\/p>\n\n\n\n\n Cascading, backup or back-up protection are concepts related to coordination between upstream and downstream devices. In some cases, an upstream device with a higher breaking capacity can allow a downstream device to be applied at a short-circuit level higher than it could withstand on its own, provided the combination is validated by the manufacturer.<\/p>\n\n\n\n This is not the same as selectivity. Cascading can improve the short-circuit withstand of the assembly, but it may reduce or alter selectivity over certain current ranges.<\/p>\n\n\n\n Therefore, cascading and selectivity should be analyzed together. A solution may be adequate in terms of breaking capacity while failing to meet the desired continuity of service.<\/p>\n\n\n\n Selectivity depends on breaking capacity because each device must be able to interrupt the short-circuit current at the point where it is installed, or be technically coordinated with an upstream device.<\/p>\n\n\n\n Breaking capacity defines whether the device can withstand and interrupt the fault. Selectivity defines which device should operate first. Without adequate breaking capacity, the discussion of selectivity loses its technical basis.<\/p>\n\n\n\n In low-voltage designs, these subjects should appear together: prospective short-circuit current, breaking capacity, let-through energy, selectivity and coordination.<\/p>\n\n\n\n Selectivity may also involve residual-current devices. NBR 5410 addresses selectivity between RCDs connected in series and establishes criteria so that the upstream device does not operate before the downstream device.<\/p>\n\n\n\n In practical terms, an upstream RCD should be coordinated with downstream RCDs by rated residual operating current and time-current characteristic. In many cases, a selective or time-delayed device is used upstream and general-purpose devices downstream.<\/p>\n\n\n\n When an RCBO is used, the analysis involves both residual-current protection and overcurrent protection. The assembly must comply with the circuit-protection criteria.<\/p>\n\n\n\n\n An SPD is not a selectivity device, but its application can affect protection coordination. An SPD may require an overcurrent protective device to clear an internal fault, and that device must be compatible with the prospective short-circuit current.<\/p>\n\n\n\n NBR 5410 also addresses coordination between SPDs installed at different points of the installation. Although this topic differs from circuit-breaker selectivity, it reinforces the same principle: protective devices must be analyzed as a system.<\/p>\n\n\n\n In main boards and panels, SPDs, circuit breakers, RCDs and RCBOs must be coordinated to avoid loss of protection, unwanted disconnections or coordination failures.<\/p>\n\n\n\n The most common mistakes are:<\/p>\n\n\n\n These mistakes can cause widespread outages, poor continuity of service and difficulty in diagnosing faults.<\/p>\n\n\n\n\n Selectivity specification should begin by defining the protection hierarchy. The main device, outgoing devices, subpanels, final circuits and critical loads must be identified.<\/p>\n\n\n\n Next, prospective short-circuit current at relevant points must be determined, devices with adequate breaking capacity selected, time-current curves compared, selectivity tables consulted and settings defined.<\/p>\n\n\n\n The technical documentation should record:<\/p>\n\n\n\n A specification that indicates only circuit-breaker amperage does not demonstrate selectivity.<\/p>\n\n\n\n Selectivity should be in the electrical design, not only in the catalog<\/strong><\/p>\n\n\n\n The study should record the devices analyzed, short-circuit current, settings, total or partial selectivity and coordination limitations. To connect this subject with technical documentation, see Low-Voltage Electrical Designs<\/a>.<\/p>\n\n<\/div>\n\n\n\n\n Specialized engineering is recommended when there are main low-voltage switchboards, industrial panels, feeders, subpanels, an on-site transformer, molded-case circuit breakers, air circuit breakers, critical loads, motors, variable-frequency drives, SPDs, RCDs, recurring trips or modifications to existing boards.<\/p>\n\n\n\n It is also recommended when the installation must demonstrate compliance with NBR 5410 and NR-10, when operational continuity is required or when the single-line diagram is outdated.<\/p>\n\n\n\n Technical analysis makes it possible to define whether selectivity will be total or partial, which devices must be adjusted, which combinations are valid and which limitations must be documented.<\/p>\n\n\n\n Deepen your knowledge of circuit-breaker coordination and selectivity<\/strong><\/p>\n\n\n\n Selectivity should be analyzed together with design current, breaking capacity, settings, RCDs, SPDs and electrical-system documentation. See the white paper Method for Specification and Sizing of Circuit Breakers in Low-Voltage Electrical Installations<\/a> and complement the analysis with the eBook Electrical Grounding: Fundamentals, Design and Standardization<\/a>.<\/p>\n\n<\/div>\n\n\n\n\n Circuit-breaker selectivity is the coordination of protective devices so that a fault is isolated by the device closest to the faulty point. It reduces unnecessary outages, improves continuity of service and makes fault diagnosis easier.<\/p>\n\n\n\n In low-voltage systems, selectivity depends on time-current curves, settings, breaking capacity, prospective short-circuit current, let-through energy, manufacturer tables, RCDs, SPDs and the electrical-system architecture.<\/p>\n\n\n\n In simple installations, selectivity may be limited. In larger commercial, industrial and building installations, it should be treated as a design, documentation, operation and maintenance criterion.<\/p>\n\n\n\n [1] ABNT. NBR 5410:2004 \u2014 Low-voltage electrical installations<\/a>.<\/p>\n\n\n\n [2] IEC. IEC 60947-2:2024 \u2014 Low-voltage switchgear and controlgear \u2014 Part 2: Circuit-breakers<\/a>.<\/p>\n\n\n\n [3] IEC. IEC 60898-1:2015+AMD1:2019 \u2014 Circuit-breakers for overcurrent protection for household and similar installations<\/a>.<\/p>\n\n\n\n [4] ABNT. NBR IEC 61439-1 \u2014 Low-voltage switchgear and controlgear assemblies \u2014 General rules.<\/p>\n<\/details>\n\n\n\n Electrical selectivity is the coordination of protective devices so that a fault disconnects only the smallest possible part of the installation.<\/p><\/div> It is the coordination between circuit breakers in series so that the breaker closest to the fault operates preferentially.<\/p><\/div> Because it prevents a localized fault from unnecessarily disconnecting entire boards, sectors or systems, improving continuity of service.<\/p><\/div> No. Breaking capacity indicates whether the breaker can interrupt the short circuit. Selectivity indicates which device should operate first.<\/p><\/div> It is when coordination between devices is guaranteed over the entire expected fault-current range.<\/p><\/div> It is when selectivity is guaranteed only up to a certain short-circuit current.<\/p><\/div> It is selectivity obtained through different operating-current thresholds of devices in series.<\/p><\/div> It is selectivity obtained through different operating times between protective devices.<\/p><\/div> It is selectivity related to let-through energy during a short circuit, often associated with the Joule integral.<\/p><\/div> Time-current curves, settings, prospective short-circuit current and manufacturer selectivity tables must be compared.<\/p><\/div> They can, but selectivity must be demonstrated by technical data. It should not be assumed without reliable curves, tests or tables.<\/p><\/div> It is a manufacturer table that identifies device combinations and the expected level of selectivity between them.<\/p><\/div> Cascading, backup or back-up protection is coordination in which an upstream device contributes to the downstream device’s short-circuit withstand.<\/p><\/div> No. Cascading concerns coordination for short-circuit withstand. Selectivity concerns preferential operation of the device closest to the fault.<\/p><\/div> NBR 5410 requires series-connected devices to be selected to limit operation to the affected circuit when safety or utilization requires continuity of service.<\/p><\/div> The study should be performed by a qualified professional based on the electrical design, manufacturer data and prospective short-circuit current.<\/p><\/div><\/div>\n<\/details>\n\n\n\n Understand circuit-breaker selectivity, how protection coordination works in low-voltage systems and why it matters in main switchboards, panels and feeders.<\/p>\n","protected":false},"author":1,"featured_media":78670,"parent":0,"template":"","meta":{"_a3a_global_related_solutions":[],"_a3a_global_related_services":[],"_a3a_global_related_materials":[],"_a3a_post_lang":"en-us","_a3a_translation_group_id":"b01d107d-c983-4638-9339-d1e6eb10b651","_a3a_i18n_canonical_slug":"circuit-breaker-selectivity-how-it-works-low-voltage","_a3a_prod_post_id":"","_a3a_lang_url_en-us":"","_a3a_lang_url_es-es":""},"categories":[],"segments":[],"mercados":[],"etapas":[],"class_list":["post-83323","articles","type-articles","status-publish","has-post-thumbnail","hentry"],"_links":{"self":[{"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/articles\/83323","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/articles"}],"about":[{"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/types\/articles"}],"author":[{"embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/users\/1"}],"version-history":[{"count":1,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/articles\/83323\/revisions"}],"predecessor-version":[{"id":83329,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/articles\/83323\/revisions\/83329"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/media\/78670"}],"wp:attachment":[{"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/media?parent=83323"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/categories?post=83323"},{"taxonomy":"segments","embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/segments?post=83323"},{"taxonomy":"mercados","embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/mercados?post=83323"},{"taxonomy":"etapas","embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/etapas?post=83323"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}What is electrical selectivity?<\/h2>\n\n\n\n
What is circuit-breaker selectivity?<\/h2>\n\n\n\n
What does NBR 5410 say about selectivity?<\/h2>\n\n\n\n
Selectivity is not the same as breaking capacity<\/h2>\n\n\n\n
Total selectivity and partial selectivity<\/h2>\n\n\n\n
Current-based selectivity<\/h2>\n\n\n\n
Time-based selectivity<\/h2>\n\n\n\n
Energy-based selectivity<\/h2>\n\n\n\n
Circuit-breaker time-current curves<\/h2>\n\n\n\n
Manufacturer selectivity tables<\/h2>\n\n\n\n
Coordination between circuit breakers in series<\/h2>\n\n\n\n
Coordination between the main breaker and final circuits<\/h2>\n\n\n\n
Selectivity in main low-voltage switchboards<\/h2>\n\n\n\n
Selectivity in industrial panels<\/h2>\n\n\n\n
Cascading, backup and back-up protection<\/h2>\n\n\n\n
Relationship with breaking capacity<\/h2>\n\n\n\n
Relationship with RCDs and RCBOs<\/h2>\n\n\n\n
Relationship with SPDs<\/h2>\n\n\n\n
Common mistakes in circuit-breaker selectivity<\/h2>\n\n\n\n
How to specify selectivity in electrical design<\/h2>\n\n\n\n
When should specialized engineering be engaged?<\/h2>\n\n\n\n
Conclusion<\/h2>\n\n\n\n
Technical references<\/summary>\n
Frequently asked questions<\/summary>\n
Additional technical materials<\/summary>\n
Related solutions<\/h3>\n
Related engineering services<\/h3>\n
Related technical content<\/h3>\n
Guides and references<\/h3>\n