Determining hot spots in HT anD LT switchgear.
Infrared thermography is the ability to detect and measure thermal energy emitted from an object – all objects warmer than absolute zero or -275,15°C emit energy. As infrared is in the invisible portion of the electromagnetic spectrum extending from 0,75 to 1000 microns, an infrared camera is required to convert the energy represented as heat into an electronic signal, which is processed to produce a thermal image that is visible on a screen.
Why make use of infrared thermography to pinpoint hot spots? An infrared image visually assists the thermographer to detect a ‘hot spot’, a point that appears hotter in relation to the area around it. All surfaces emit energy, the difference between a cold or hot surface is the degree to which energy has been absorbed or emitted. If the surface absorbs more energy than it radiates, it is ‘seen’ as cold and appears as blue and green or black. If however the surface emits more energy than it absorbs, it is ‘seen’ as hot and appears as white, yellow, orange and red depending on chosen palettes.
In South Africa, although no regulatory standard appears to be in place, the science is readily recognised as a reputable and acceptable method of predictive maintenance in both mechanical and electrical applications, compliance testing in the building and construction industry, quality control in production processes, detection of burn-through and deterioration in the refractory industry, and medical conditions in not only humans but animals as well.
In the electrical field, it has become the norm to inspect HT and LT switchgear, transformers, distribution boards and busbars on at least an annual basis. As almost everything that draws or generates electricity gets hot before it fails, detecting a hot spot in any application inevitably indicates trouble. In the electrical field it can spell catastrophic failure, downtime, production loss, fires, power outages or even injury or death. Using a calibrated infrared camera, one is quickly able to detect temperature differences in objects. It is critical however that the image results are viewed holistically and that contributing factors such as emissivity, humidity and radiated heat from surrounding surfaces are taken into account.
In the USA, insurance statistics reveal that coupled with electrical equipment damage, loose connections cause 25% of all failures to switchgear, circuit breakers and cables.
Trending and quantifying faults over a period of time allows faults to be categorised and managed at shutdowns and not on a crisis basis.
But how safe is infrared thermography? The safety of infrared thermography depends on the user and although information is obtained without making contact with the surface, dangers are still present.
Hot spots can be detected through closed panels depending on the degree of conduction, which is the process of energy transfer from one molecule to the next; however precise location of the point of heat generation will be virtually impossible without opening the panels. By opening panel doors one is exposed to several dangers including Arc Flash, which is essentially an electrical explosion caused by electrical current flowing through an arc outside its normal path; it generates high thermal energy which can have devastating effects. It is advisable in instances of low, medium and high voltage equipment to make use of ‘Infrared Windows’, which are installed in the panel door and which allow for safe, regular infrared scanning.
As the appreciation of this diagnostic technology grows, many new condition monitoring applications are being discovered, and as technology advances, infrared cameras are becoming more affordable, empowering industry to adopt the technology ever more widely.
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