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.