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.