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Clearing up the Confusion

Electrical engineers do not want to see designs go up in smoke.  Naturally, engineers protect their equipment with what they believe to be appropriate circuit protection.  However, there is widespread misunderstanding of industry standards for circuit protection and the meaning of terms such as "circuit breakers", "supplementary protectors", "circuit breakers for equipment" and "branch circuit protection".  In some cases, this confusion results in the specification of the wrong type of circuit protection and increases the risk of overheating, premature failure and catastrophic faults.

To undertand the source of confusion and to learn how to specify circuit protection correctly, we need to review standards and how they apply.

Branch Circuit Protection:

The National Electrical Code (NEC) is primarily concerned with the safety of hard-wired branch circuits within a building.  Article 100 defines a branch circuit as "the circuit conductors between the final overcurrent device protecting the circuit and the outlet".

For over current protection devices in a branch circuit, the requirements are spelled out in stardard UL 489 "Standard for Molded-Case Circuit Breakers and Circuit Breaker Enclosures," published by Underwriters Laboratories, Inc.

UL 489

UL 489 encompasses circuit breakers "intended for installation in a circuit breaker enclosure or as parts of other devices, such as service entrance equipment and panelboards."  According to UL terminology, devices meeting this standard are considered listed products.

For approval, UL 489 requires the device pass a series of callibration, overload, endurance and short-circuit tests (Table. 1).  The minimum short-circuit test must be performed at 5000A.  Overload tests are performed at six times the current rating of the device or 150A minimum.  Devices rated up to 600V and 6000A are covered in this standard.  Additionally, most UL 489 devices are used in electrical distribution panels; therefore, the minimum current ratings available are seldom less than 15A.  During UL 489 testing, the device must survive short-circuit testing and continue to provide future overload protection.  In service entrance panels, available short-circuit fault currents measure 50,000A or greater.  However, as power is distributed throughout the building, the available short-circuit currents diminish.  If an electrical outlet is just 20 feet away from the power source, Ohm's law states even with limitless available short-circuit current AWG 14 copper wire limits the maximum available fault current at the outlet to no more than 1200A at 120V.

Supplementary Protection

Although the NEC recognizes "supplementary overcurrent protection used for lighting fixtures, appliances and other equipment or for internal circuits and components of equipment," it does not specifically define supplementary over current protection.  Nonethless, the NEC implies that it is used in conjunction with a branch circuit overcurrent device upstream of the equipment.  The requirements of supplementary protectors are described in UL 1077, "Standard for Supplementary Protectors for Use in Electrical Equipment".

UL 1077

UL 1077 defines supplementary protectors as devices intended for use as overcurrent, over-voltage or under-voltage protection within an appliance or other electrical equipment where branch circuit overvoltage protection is already provided or is not required.  In UL terms, UL 1077 compliant devices are labeled as recognized components.  

Similar to UL 489, UL 1077 supplementary protectors must pass a series of calibration, overload, endurance and short-circuit tests (Table 1).  Because most UL 1077 circuit breakers are rate 20A or less and are used in electrical appliances or other types of utilization equipment, the overload and short-circuit tests are generally performed at lower levels that those required by UL 489.  To pass the short-circuit test under UL 1077, the device must safely interrupt short-circuits at least one time without causing a fire hazard.  Unlike UL 489, it does not necessarily need to survive the test.

UL 1077 allows manufacturers to obtain approval for different circuit conditions.  For example, an overcurrent supplementary protector can be short-circuit tested with or without a backup fuse or circuit breaker.  A supplementary protector can be overload tested at 1.5 times its rating for general use or 6 times its rating for across-the-line motor starting.  It may trip at less than 125% of its rating or greater than 135%, etc. 

When UL 1077 was revised to meet changing market requirements and safety considerations, UL added a category of overcurrent supplementary protectors know as "recalibrated after short-circuit testing" which are also described as "fit for further use".

Overcurrent supplemnetary protectors rated "fit for further use" survive a three cycle short-circuit test and continue to provide overload and short-circuit protection in future operations.  Not all manufacturers of UL 1077 approved circuit breakers strive to meet this more stringent classification.  Before selecting a manufacturer, it is essential for the desing engineer to verify which tests the circuit breakers are subjected to.  If the datasheet provided by the manufacturer does not provide this information, the UL Recognized Component Directory for Supplementary Protectors can be used as a reference in determining the suitability of the protectors as "fit for further use" and/or for motor starting applications, etc.

It is the duty of the design engineer to select the proper supplementary protector for the application to ensure optimal safety and reliability.  See below a brief comparison of UL 489 and UL 1077 Test Standards and Requirements:


UL 1077

UL 489

Minimum Terminal Spacing

For commercial appliances 3/32 inch up to 300 V 1/4 inch at terminals

1/2 inch up to 130 V

3/4 inch up to 300 V

1 inch up to 600 V 

through air

Calibration Test

@ 300% and

@ trip current +5%

Varies with current rating

12 sec to 2 min @ 200%
<1 hour @ 135% (<50A)

<2 hours @135% (>50A)

Maximum Temperature Rise at Terminal

50°C / 122°F

50°C / 122°F

Overload Test

50 Cycles

@ 1.5 x lN (general use)

@ 6 x lN (motor starting)

50 cycles @ 6 x lN or

150 A minimum

Endurance Test

6000 cycles @ ln (S-type)

10,000 cycles

(6000 @ ln + 4000 mechanical (up to 100A))

Short-Circuit Test

1 - 3 times (C-0-0) at 5000 A or less, depends on current and voltage (may fail safe or recalibrate after short-circuit tests for "fit for further use")

Depends on ratings.

3 -7 times @ 5000A minimum and must operate @ 200% final test