In considering the most critical UL 1077 performance categories, we first reviewed the Tripping Current (TC) and then Overload (OL) performance ratings supplementary protectors may qualify to.

While we consider the TC and OL ratings equally as critical, the third of our examples, Short-circuit (SC) performance rating, may have  the greatest potential for causing application mistakes.  This is because under the SC performance category,  the circuit protector manufacturer is allowed the greatest latitude in how it may qualify its device to UL 1077.

For surety of protection, the protector selected needs to be capable of  clearing the maximum fault potential of the application.  The OEM’s design/application need for the reusability of the protector after clearing a short circuit also must be considered.  Reusability of the circuit protector after clearing a fault can be very beneficial in limiting an OEM’s future expenses for service calls and warranty claims, and could be considered an essential requirement in critical applications, like medical equipment.

In qualifying a device to UL 1077 for Short-circuit (SC) performance, the supplementary protector manufacturer:

• Defines the test’s:
• Maximum AC and/or DC Voltage.
• Short-circuit (kilo-amperes) current.

• Chooses 1 of 7 UL predetermined SC test set-ups that demonstrate increasing degrees of a protector’s design robustness/capability/survivability.
• See table below for further explanation.  The table defines the various SC codes and offers comments on the implications of each.

By UL definition, the short-circuit current rating in kilo-amperes (kA) is followed by a letter and number designating the test conditions and any recalibration following the short-circuit test as follows:

* The “C” prefix designates the short circuit tests were conducted with a backup device, usually a fuse to simulate upstream/cascaded protection that is present in certain applications.

**  The “U” prefix designates that the short circuit tests were conducted without a backup device, clearly demonstrating the protector’s ability to eliminate the defined fault.

From this summary of UL1077 Short circuit (SC) options,  it is clear that the potential exists for significant performance differences between devices carrying the same “UL Mark”.  For a given level of overload, a protector compliant to an SC rating of U3 offers much greater surety of protection and durability than one merely qualified as a C1 device.   Close attention to the OEM design needs relative to  this particular attribute is essential for maximizing the safety and durability of the OEM’s product.  Needless to say, applications assistance from the supplementary protection manufacturer may prove quite helpful in selecting the protector that best suits a design’s requirements.

In considering the most critical UL 1077 performance categories, we first reviewed the UL Tripping Current codes (TC).  The Overload Rating (OL) is another performance category we consider to be very critical in selecting the correct UL 1077 supplementary protector  for an application.

The Overload Rating (OL) designates whether the protector has been tested for general use or for motor starting applications:

• OL0 =  The device was qualified/tested at 1.5 times the ampere rating for general use.
• OL1 = The device was qualified/tested for motor starting applications
• 6 times the ampere rating for AC applications, and/or
• 10 times the ampere rating for DC applications.

In MP’s experience, the mistaken use of “general use” qualified protectors in “motor starting” applications happens all too frequently.   At start-up and during “locked rotor” faults the inrush current can range from five to ten times the steady state current, depending on the motor design.

If the supplementary device is to protect against potential motor faults, it is very important that the UL 1077 device chosen has an OL1 rating at the applicable voltage.  Suffice it to say, protectors are qualified as ”general use” type because they are incapable of meeting the testing demanded of the “motor starting” requirement(s).  Typically, this is because “general use” qualified protectors use inferior  (less expensive) contacts instead of the silver metal oxide types that are required to achieve the best performance.  In a worst case scenario, under a start-up inrush current or severe overload, a  “general use” protector (misapplied) in a motor starting application could result in a very hazardous fire situation.  Under start-up inrush current or a severe overload, the incapable contact mechanism may result in contacts welding together, creating a permanent conductive path with virtually no protection, unless the device reaches a catastrophic state and the protector actually fuses.

Further to the performance categories we consider to be among the most critical in selecting the correct UL 1077 device for an application, today Tripping Current (TC) performance will be examined,  with reviews of Overload rating (OL); and Short-circuit Current Rating (SC) considered in future articles.

According to UL, the Tripping Current (TC) is coded as a percentage of the amp rating  and is the percentage of the device’s rated value at which it will trip within a specified time(s).  The table defines the various TC codes and offers comments on the implications of each.

Note:  Mechanical Products manufactures a wide variety of supplementary protectors of the TC1 – TC3 types, with all protectors “TC” tested = < 150% of rated value.

We last discussed the differences between UL 489 Circuit Breakers and UL 1077 Supplementary Protectors.  From an understanding of these differences, if one determines that a UL1077 approved device is appropriate for an application, further research may still be required.

While all UL1077 devices carry the same UL “Mark”, one needs to recognize that all Supplementary Protectors are not created equal.  A cursory review of  the basic performance of competing UL 1077 qualified devices does not necessarily indicate that they have the identical performance levels of UL agency qualifications.  This is because, under the same UL 1077 “Mark”, manufacturers are allowed a great deal of latitude in how they qualify their devices.  Individual performance/qualification categories range all the way from those that are “manufacturer defined”, through categories that have various UL predefined testing criteria levels (degrees of difficulty) that the manufacturer may select from to qualify their device, and those that are strictly defined.  Given the number of categories and the degrees of difficulty a manufacturer may choose from, one needs to be cautious not to select a minimally qualified device for use in a demanding application.

Here, we will not undertake a comprehensive review of UL 1077, but rather, by way of examples,  try to bring to the Engineer’s attention the need for critical performance comparisons when selecting a supplementary protector.  Further limiting the scope of our discussion, while there are four different “Types” of Supplementary Protectors recognized under UL 1077, we will focus our testing category examples to those specific to “Overcurrent” type devices.  We will then review those performance categories we consider to be among the most critical in assuring the proper selection of a UL 1077 device.