I continue to be surprised by how many equipment designers are confused by UL circuit protection standards. In trying to meet conflicting standards, engineers may actually decrease the level of circuit protection. The confusing terminology in the standards themselves doesn't help. Branch circuit protection refers
to circuit protection at the panel box. This is covered by UL 489, which requires higher short-circuit protection (interrupting capacity). UL 1077 covers supplementary protectors used in equipment. Generally, these protectors have lower short-circuit protection, as it's assumed that there's branch circuit protection upstream.
That's easy to understand. But then it gets tricky. Most supplementary protectors rated 20 A or less are used in equipment that plugs into an outlet. Thanks to Ohm's Law, wiring typically reduces the available fault current at the outlet to well under 1000 A. Yet when the continuous current rating of equipment exceeds 20 A, or if the equipment has a "convenience outlet," the meaning of "branch circuit protection" becomes blurred.
A few years ago, inspectors weren't sure that supplementary protectors in industrial or commercial equipment would adequately protect certain loads after short-circuit faults. As a result, many local governing authorities began to require "branch circuit protection" within utilization equipment.
Deciphering Standards Because engineers interpret the term "branch circuit protection" as requiring a UL-listed device, UL 489-rated circuit breakers are being used in equipment that would be better protected by certain UL 1077 supplementary protectors. Here's an example: To comply with the meaning "branch circuit protection," a manufacturer replaces an 8-A UL 1077 supplementary protector with a UL 489 circuit breaker. As most UL 489 breakers are rated 15 A and higher, the engineer uses a 15-A breaker. This increases short-circuit protection but decreases overload protection. This specification could lead to motor failure, overheating of wire insulation, fires, and (ironically) catastrophic short circuits.In 1999, the new term "fit for further use" was added for certain UL 1077 protectors that can withstand short circuits and keep working. A "fit-for-further-use" circuit breaker protects equipment during short circuits and common overloads. Moreover, UL 1077-recognized circuit breakers can be tested to various standards, so they offer different levels of protection.
Actually, circuit protection standards are not that hard. Overcurrent protection devices in a branch circuit are defined in UL 489, "Standard for Molded-Case Circuit Breakers and Circuit Breaker Enclosures." Minimum short-circuit tests are performed at 5000 A. Overload tests are performed at six times the current rating of the device, or 150 A minimum. The device must survive short-circuit testing and work during future overload operations.
The National Electric Code recognizes "supplementary overcurrent protection" used with a "branch circuit overcurrent device" upstream of the equipment. Requirements are in UL 1077, "Standard for Supplementary Protectors for Use in Electrical Equipment." Overload and short-circuit tests are at the same or lower levels than UL 489 requires. The device under test doesn't necessarily need to survive the test multiple times to pass the short-circuit test.
Not all UL 1077 supplementary protectors are alike. They can be overload tested at 1.5 times their rating for general use, or six times their rating for across-the-line motor starting. "Fit-for-further-use" supplementary protectors must survive a three-cycle short-circuit test and continue to provide overload and short-circuit protection. Not all manufacturers of UL 1077 supplementary protectors meet these standards.
Applying the right standard of protection isn't hard, if engineers have the right vocabulary. With a proper understanding of UL ratings and requirements, the equipment designer is ensured optimal performance and safety.