Infrared Inspection of Electrical Equipment
Infrared (IR) thermographic scanning is commonly used as a part of the elec- trical preventive maintenance program. IR scanning can be used easily and safely on energized electrical equipment and apparatus to diagnose electrical problems in their early stages, which then can be corrected before they become major problems. IR scanning is most useful when equipment is under heavy load because certain types of hot spots cannot be detected while the equipment is at partial load or light load. Also, many maintenance personnel conduct an IR scanning inspection before a major preventive maintenance shutdown to identify electrical problems that can be then corrected during the shutdown.
Types of Thermographic Scanners
A variety of IR thermographic systems are available for conducting IR scan- ning to identify heat-related problems in energized electrical equipment. The instruments range from hand-held radiometers to imaging systems or scan- ners. These instruments are very compact and portable. These instruments detect radiation in a narrow section of the IR band at a frequency just below the visible light given out by heat. The radiometers are noncontact IR ther- mometers which have temperature readouts and are usually mounted on gun-stock. These instruments are ideal for spot checking of heated electrical equipment, lines, wires, and the like. The scanners use optical components and use different detectors as noted below.
The thermal detector responds to temperature change in the detector and displays actual scene with temperature superimposed as a red graph. Coolest ambient temperature is graph baseline, and the height of trace line indicates warmer temperature. The temperature range of the thermal detector is 10°C–1000°C and it can detect temperature differences of 0.5°C. It has a slow response. The main advantage of this unit is that it requires no external cooling.
In these detectors, a germanium lens focuses radiation from an object on thin membrane coated with gold black. The black surface is in vacuum with a few drops of oil. The oil evaporates, and condenses on the membrane forming a thermal image. It is very slow but provides quantitative judgment. There is no cooling required for this unit.
Photon detectors (argon cooled)
These types of detectors are completely self-contained and contain argon gasfilled cylinders used for the detector coolant. The view is red on black, the brighter the red, the hotter the object. This detector uses six IR photo detectors, six LEDs, and a 10-segment, double-sided rotating mirror which converts the LED output into 60-line scan image. The temperature resolution is 0.1°C at 22°C between closely adjacent objects and between objects and their background.
Photon detector (nitrogen cooled)
These detectors use liquid nitrogen to cool the detector. The view is white on black, the brighter the white, the hotter the object. Also, these detectors come where the view is in color. The temperature resolution is 0.2°C. These units use camera scanner and video display. The brightness can be referenced to actual temperature.
These units are similar to the above units, but the image is green on black. These units do not have reference graph between brightness and temperature.
These units do not require external cooling.