There is a great deal of difference in the types of overcurrent circuit protectors available to eliminate electrical circuit overloads.  To this end, the following article link is offered to assist one in understanding the differences in the requirements and test parameters of UL489 – the standard for Molded-Case Circuit Breakers and UL1077 – the standard for Supplementary Protectors for Use in Electrical Equipment (aka CBEs – Circuit Breakers for Equipment).

http://www.mechprod.com/circuit-breakers---ul-standards/

The article provides a solid background in UL’s qualification requirements for switching and push to reset circuit protectors.  Both trip free and cycling trip free devices are reviewed for such performance characteristics as overload and short-circuit conditions, endurance, resistive and motor start applications, and level of agency oversight.

This means that the “Hold” and “Trip” characteristics defined on manufacturers’ data sheet (also known as the time/current curve) indicate the performance of the supplementary protector when operating at an ambient temperature of 77°F.

For example, at 77°F a 10 Ampere rated breaker will “Hold” 10 amps indefinitely and “Trip” within the specified time windows for given percentages of overload.  At an operating ambient of 77°F, a typical data sheet might indicate:

Thermal circuit breakers react to the effects of heating.  Therefore, it is important to keep in mind that performance characteristics will differ with changes in ambient temperature.

To achieve the desired “Hold” and “Trip” performance (to avoid nuisance tripping on a circuit at elevated ambient temperatures for example) the ampere rating of the thermal circuit breaker may need to differ from what would be used on the circuit at a nominal 77°F.   This adjustment to the selected CBE’s rating is typically determined by multiplying the rating of the circuit breaker that would be required at the nominal 77°F by an Ambient Temperature Correction Factor (aka Derating Factor) to arrive at the correct circuit breaker rating for the given ambient temperature.

For the 10 ampere circuit protector in our example above, a manufacturer’s typical Ambient Temperature Correction Factor table may indicate the following multipliers:

From the table, for an ambient temperature of 122°F, the 10 ampere nominal circuit breaker in our example would be replaced by a 12.5 – 13 ampere rated supplementary protector to avoid nuisance trips (10 x 1.25).

The selection of the correct circuit breaker rating for a given ambient condition is very simple.  Giving this a few minutes of consideration will help assure your circuit is adequately protected.

Continuing our review of Thermal Circuit Breaker terminology, we will now consider Overload Rating, which should not be confused with Short Circuit Interrupt Capacity (planned for future review).

While testing varies slightly from agency to agency, overload tests are basically fifty “on-off” manual or automatic trip cycles, at a defined multiple of the device’s rated current and at a specific power factor. This testing confirms a device’s capability for either general use (on a resistive load) or in a motor starting application (an inductive load).

Specifically under UL1077, the Overload Rating of a supplementary protector is determined by the device manufacturer from the table below:

Test Current for Overload Tests

From the table, it is easy to see that supplementary protectors qualified for use “Across the line motor starting” must be significantly more robust than the “General use” types.  Within UL1077 this is the difference between Overload Ratings of OL1 and OL0, respectively.

A single device may also have a variety of Overload Ratings at various voltages. In other words, one supplementary protector used in many different applications may be rated at OL1 at 125 Volts AC and OL1 at 30 Volts DC, while at the same time having an OL0 rating at 250 Volts AC.

Caution should be taken when comparing data sheets of various supplementary protectors.  Reference to the thermal circuit breaker’s actual Overload Rating is frequently omitted by manufacturers.  Details on this and other UL1077 qualification categories may be found by visiting the UL Online Certification Directory at, http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.html.

Although the above discussion has been primarily focused on UL1077, another widely used North American standard is C22.2 No. 235-04 published by CSA International. Within this standard, Overload is described in much the same terms as in UL1077, including similar Overload Ratings of OL0 and OL1.  Details may be found within the CSA Certified Product Listings at  http://directories.csa-international.org/