Minimum Ultimate Trip & Maximum Ultimate Trip

In recent articles, we explained the real, and many times critical, performance differences that exist between overcurrent devices qualified as Supplementary Protectors per UL1077.  As a further aid in the selection of a UL1077 device, over the course of the next several articles, we will define the key terminology common to the industry and agencies.

The comparison of performance data from one device to the next has relevance only if one clearly understands the underlying meaning behind the terminology associated with the “numbers”.  We must therefore first define what quantity the numbers represent.  Once a term is defined, one then may consider the ramifications of the differences between (seemingly) similar devices.

While our focus will be on terminology most commonly used in the thermal overcurrent protection industry, most of the terms are also common to the data sheets and agency requirements for magnetic-hydraulic and thermal-magnetic types of protectors as well.

For thermal overcurrent supplementary protectors, virtually all electrical performance and associated data, unless otherwise noted, is obtained from evaluating the device at 77?F (25?C).

Today we will consider two terms commonly known in the industry as “Minimum Ultimate Trip” and “Maximum Ultimate Trip”.

The “Minimum Ultimate Trip”, sometimes referred to as the “Hold Current”, is the amount of current the device will conduct indefinitely at a specified ambient temperature.

The “Maximum Ultimate Trip”, sometimes referred to as the “Must Trip” or “Tripping Current” (“TC” for UL1077 and CSA qualification purposes), is the current rating at which a device will trip (open the circuit) within a certain period of time at a specified temperature.

So, what real performance differences might we see through a close examination of the “Hold” and “Trip” currents of seemingly similar devices?

Assuming no differences in the ambient temperature at which the devices were evaluated or in the maximum voltage rating of the devices, a “Minimum Ultimate Trip” of 100%-of-rated-current is a straight forward definition with no real underlying comparative issues with which to be concerned.

Such is not the case with the “Must Trip”.  As you may recall, in an earlier article dealing with the differences in UL 1077 performance we reviewed “Tripping Current” (TC).  Under this UL 1077 and CSA category, the manufacturer of the device is allowed to select the TC level at which to test the device.  As part of the overall qualification testing to the standard, once the TC ( “Must Trip”) level is defined, the agency tests the device to verify that it “trips” within a given time (one or two hours, depending on the protector rating).

Under UL 1077, the “Maximum Ultimate Trip” categorized as Tripping Current (TC0 – TC3) defines a protector’s ability to clear a low level (“slow-cook”, potentially long term damaging) fault.

Here, the importance of understanding the underlying meaning of the data may best be explained by comparing the performance differences between a device rated at, say, TC2 and one rated at TC3.

In reviewing all of the different attributes between protective devices; mechanical, dimensional, environmental, agency qualification, electrical performance, etc., one could very easily overlook the importance of the difference between a TC2 and a TC3 rating. In fact, there is a great deal of difference in the protective performance of these two ratings.

As explained in our earlier article, a TC2 rated device is the least regulated in demonstrating protection against low-level overloads, while a TC3 rated device is considered by many to represent the highest measure of protection against these types of “slow-cook” faults.  This is because a TC2 rated device is tested at some level of tripping current greater than 135%, as selected by the manufacturer.  A TC3 rated device however, is standardized to trip at 135% and 200%-of-rated-current.  From this, it is easily seen that a TC2 rated device tested to trip at 175% within one hour offers far less protection than a TC3 rated device that trips at 135% in one hour and within very specific times at 200%-of-rated-current.

In conclusion, when comparing data between overcurrent protective devices, it is important to pay close attention to your design’s need for protection against “slow-cook” faults as measured by the “Maximum Ultimate Trip”, or “Tripping Current”, as defined by UL and CSA.

Our next article will consider the differences between “trip-free” and “cycling trip-free” overcurrent protection devices.