North American low-voltage switchgear varies by application, with different products and standards. The typical standard for metal-clad low-voltage power switchgear is UL1558, while the standard for general user-side low-voltage distribution panels is UL891. Both standards pertain to low-voltage switchgear, and their application scenarios are similar to those of domestic/IEC low-voltage switchgear—specifically primary main distribution and secondary distribution switchgear—though there are differences. In China, primary main distribution switchgear includes models such as OKKEN and MNS, while secondary distribution switchgear includes PRISMA and ArTu, with noticeable differences in rated current and other parameters. A key distinction lies in the parameter ratings: both UL1558 and UL891 have high specifications, with UL891 reaching a maximum rated current of 6000A and short-time withstand current up to 100kA, which is not significantly different from UL1558‘s 7000A and 150kA ratings.
Since the power and user sides differ, they naturally have different characteristics. UL1558 is a switchgear for the power system level, providing system protection, while UL891 is switchgear for the user terminal side.
UL1558 requires complex control and protection circuits, similar to medium-voltage switchgear that uses separate circuit breakers, current transformers, and microprocessor-based protection. Many low-voltage molded-case circuit breakers do not have built-in trip units; instead, like medium-voltage circuit breaker switchgear, they rely on current transformers and integrated protection relays to achieve protection, especially system-level protection.
Each circuit breaker in the switchgear is equipped with a dedicated electronic interface (the molded-case circuit breaker itself does not have an electronic trip unit). The electronic interfaces between breakers enable communication of raw data to the system‘s central processor, allowing the receipt of operational commands from the central processor and their execution on the respective breakers. The electronic interface contains two CPUs, which serve as backup for instantaneous and overload protection in the rare event of communication failure. Powered by current sensors, it enables automatic tripping. This intelligent solution is particularly suitable for industrial applications, offering high reliability, durability, and meeting the requirement for 50 years of service life.
UL1558 uses only draw-out type frame circuit breakers. It features a standard four-compartment design with a separate instrument compartment, and compartments separated by 3 mm thick metal plates. It includes a dedicated cable compartment for outgoing lines, secondary wiring channel, and top-mounted circuit breaker lifting device. The switchgear meets internal arc fault requirements and must pass the internal arc fault test according to IEEE C37.20.7 standards. The cabinet has a large depth, typically over 1.6 meters, and high structural strength.
UL891, as a switchgear for end users, is simple to operate. The traditional 891 switchgear has almost no secondary circuits and consists entirely of fixed circuit breakers operated manually.
The busbars, cables, and switches are installed within a single enclosure without compartmentalization. The dead front design features a fixed front panel that allows breaker operation without opening the panel, enabling high installation density. Internal arc fault protection is not required, with NEMA 1 (IP2X) protection rating and high ventilation efficiency. The cabinet has a compact depth, typically around 1 meter.
The distribution panel can handle a maximum current of up to 6000A, with switchgear capable of interrupting short-circuit currents up to 100kA. Unlike switchgear cabinets, distribution panels offer high installation density, simplicity, and efficiency. They do not require high protection ratings, internal arc resistance ratings, short-time withstand current capability, compartmentalization between functional units, extensive secondary control and protection systems, complex operating mechanisms, or withdrawable trolley designs. This structure ensures excellent thermal performance even at high installation densities, enables convenient and simple cable connections, simplifies maintenance and operation, and significantly enhances reliability.
The North American standard UL891 switchgear has unique design requirements, clearly defining structural specifications with horizontal busbars positioned at the center of the rear cabinet, typically in two sets vertically, capable of handling up to 6000A and 150kA.
The UL891 molded case circuit breaker can handle output currents up to 1200A, and a single distribution panel can accommodate four 1200A molded case circuit breakers for output circuits.
UL891 distribution panels typically use copper busbars to directly secure vertical busbars to the line terminals of molded case circuit breakers. On the load side, cable terminations are provided, allowing stripped copper cores of cables to be directly inserted into the outgoing cable terminals of the molded case circuit breakers. For high current applications, up to four 300 mm² cables can be connected per phase.
For incoming and bus-tie switchgear, molded-case circuit breakers are typically fixed-type. "Dead front" means the front is completely secured and generally not allowed to be opened; therefore, using withdrawable circuit breakers would be meaningless. However, some customers insist on using withdrawable frame circuit breakers with door panels instead of permanently fixed covers. In such cases, UL1558 compartmentalization requirements for barriers must be applied to prevent contact with live parts such as busbars and cables when the door panel is opened. This design has implications for temperature rise, switchgear dimensions, and other factors.