Oil circuit breakers (OCBs) can be a real challenge for thermographers as they are an indirect measurement. Small temperature rises on the outside surface seen by the infrared camera, also known as a thermal imager can represent large temperature rises internally. This means great care must be taken, as a good understanding of surface emissivity and background temperature can be crucial when dealing with temperature rises of a few degrees C. Fortunately, most OCBs are painted with a matte finish, nonmetallic paint and have relatively high emissivities,
approaching 0.95.
This paper presents some OCB thermography data and examines problem severity using the watt loss calculation software developed by Dr. Madding and presented at InfraMation 2001. Post mortem photos of an OCB’s internals also lend strong validation to the calculation and the calculation software.
Using the calculation software is outlined and the procedures discussed, such as geometrical, thermographic and load measurements. Sample calculations are given to show the sensitivity of an OCB’s surface temperature rise to internal electrical resistance and other parameters. As a matter of further interest we add a lightning arrester power loss calculation.
Keywords: Infrared thermography, lightning arrester, heat transfer, convection, radiation, oil circuit breakers, watt
loss, power loss, internal electrical resistance, thermal balance, condition based maintenance, predictive
maintenance.
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