Using Tmax Corrected Formula to Prioritize Electrical Exceptions
Tip written by: Infraspection Institute
For decades, temperature measurement has been used to gauge the operating condition of electrical components. With this Tip, we explore an alternative to the traditional Delta T method of prioritizing exceptions during infrared inspections of electrical distribution systems.
Thermographers have long used temperature differentials or Delta T measurements as a means of prioritizing electrical exceptions. Typically, Delta T values are calculated by comparing the temperature of an exception to similar components under similar load or to ambient air temperature. Although they work well in many circumstances, Delta T readings are not applicable for components that do not qualitatively manifest themselves as an exception.
An alternative to Delta T calculations is a formula known as Tmax Corrected. This formula is based upon an IEEE formula and calculates pass/fail criteria based upon several factors including equipment type, ambient air temperature, and circuit load. The Tmax Corrected formula looks like this:
TmaxCorr = {(A meas ÷ A rated )2 (T rated rise)} + Ambient
Where:
- Tmax Corr = corrected maximum allowable temperature
- A meas = measured load, in amperes
- A rated = rated load, in amperes
- T rated rise = rated temp rise for component
- Ambient = measured ambient temp
It should be noted that the exponent can vary between 1.6 to 2.0. In this Tip, we have shown an exponent of 2 for simplicity.
Despite taking a little more time to apply than Delta T calculations, Tmax Corrected allows one to determine if a component of interest is running within specification for any load or ambient temperature. Tmax Corrected is especially useful for equipment that is not manifesting itself as an exception. In particular, Tmax Corrected can be an invaluable tool for those who perform infrared inspections as part of commissioning studies or use thermography for acceptance testing of new installations, repairs, or retrofits.
Proper use of the Tmax Corrected formula is just one of the many topics covered in all Infraspection Institute Level II Certified Infrared Thermographer® training courses. The proper application of Tmax Corrected along with a comprehensive table of rated temperature rises is also detailed in the Standard for Infrared Inspection of Electrical Systems and Rotating Equipment. Copies of this Standard may be purchased by calling 609-239-4788 or through the Infraspection Online Store.
