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.
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TC Code
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UL Definition (at 77°F)
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Comments
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General Importance of TC Rating
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The TC Code defines a protectors ability to clear a low level of (“slow –cook”, potentially long term damaging) overload.
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0
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Tripping current is less than 125% of amp rating
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TC0 devices have been confirmed to trip within 1 hour with 125% of rated value applied.
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1
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Tripping current is in the range of 125% to 135% of amp rating
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TC1 devices have been confirmed to trip within one hour within 125% – 135% of rated value
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2
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Tripping current is more than 135% of amp rating
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TC2 devices are considered the least regulated in demonstrating protection against low-level overloads. TC2 devices have been confirmed to trip within 1 hour at a manufacturer’s defined percentage of overload >135%. For example competitive devices carrying a TC2 rating may have been tested at very differing levels of overload – one may have been tested at 140%, while another may have been tested at 175%
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3
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Tripping current and time is standardized at 135% and 200% of amp rating
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TC3 devices are considered by most engineers to represent the highest measure of protection against low level overloads. TC3 devices have been confirmed to trip within 1 hour with 135% of rated value applied and within a manufacturer’s defined trip time range with 200% of rated value applied.
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Note: Mechanical Products manufactures a wide variety of supplementary protectors of the TC1 – TC3 types, with all protectors “TC” tested = < 150% of rated value.