Thermal overcurrent protection devices are a versatile, space saving and economical circuit protection option available to the application engineer – a.k.a. thermal circuit breakers and supplementary protectors.  Thermal circuit breakers and supplementary protectors are offered in a wide variety of standard “off the shelf” specified/qualified VAC and VDC ratings ranging from tenths of an Amp to more than 200 Amps.  And, for more unique/challenging applications, certain circuit breaker manufacturers will work closely with the OEM to provide engineered solutions to customize an overcurrent protection device specific to the application requirements.

Thermal circuit breakers are available in a variety of packages, bezel mounts (including snap in) and actuator configurations.  They are available in automatic reset configurations and in a wide selection of switch and/or indicator configurations including rockers, push/pull, toggle, illuminated and push to reset.  The usual number of poles are up to 2.  Terminals can be wires, quick connect blades, screw terminals, Edison base plug type, or fuse clip terminals.  Shunt trip, relay trip and alarm circuit devices are also available. 

Additional design flexibility is available because many thermal circuit breakers are offered compliant to a wide variety of agencies and requirements such as UL489, UL1077, UL1500, CCC, CSA 22.2 No.235-04, IEC, ABYC, SAE J553 and SAE J1625.   

Given the range of options offered in thermal overcurrent protectors, these robust, space saving, economical devices should warrant serious design consideration for the protection of numerous low voltage electrical circuits.

The essential specifications of overcurrent protection devices (commonly called  “Circuit Breakers”) are covered by Underwriters Laboratories standards UL 489 Molded-Case Circuit Breakers and UL 1077 Supplementary Protectors for Use in Electrical Equipment.

While both standards require that devices are investigated for continuing compliance, the standards differ in performance expectations and follow-up services. UL 489 and UL 1077 “Listing” and “Recognition” marks denote some of these differences.

Without a clear understanding of the requirement/performance differences between a “Circuit Breaker” and a “Supplementary Protector”, dangerous misapplications may result.

For detailed information on the differences in these performance requirements and UL follow up services please refer to MP Technical Brief, Article 2.0 UL Circuit Breaker Standards Comparison – UL 489 vs. UL 1077.