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Figure 7.12 An infrared image of a circuit breaker
7.5.3
Inductive heating
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Figure 7.13 An infrared image of hot stabilizing weights
Eddy currents can cause a hot spot in the current path. In cases of very high currents
and close proximity of other metals, this has in some cases caused serious fires. This
type of heating occurs in magnetic material around the current path, such as metallic
bottom plates for bushing insulators. In the image above, there are stabilizing weights,
through which a high current is running. These metal weights, which are made of a
slightly magnetic material, will not conduct any current but are exposed to the alter-
nating magnetic fields, which will eventually heat up the weight. The overheating in
the image is less than +5°C (+9°F). This, however, need not necessarily always be
the case.
7.5.4
Load variations
3-phase systems are the norm in electric utilities. When looking for overheated places,
it is easy to compare the three phases directly with each other, for example, cables,
breakers, insulators. An even load per phase should result in a uniform temperature
pattern for all three phases. A fault may be suspected in cases where the temperature
of one phase differs considerably from the remaining two. However, you should always
make sure that the load is indeed evenly distributed. Looking at fixed ampere meters
or using a clip-on ampere meter (up to 600 A) will tell you.
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Publ. No. 1558240 Rev. a156 – ENGLISH (EN) – March 1, 2006
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7 – Introduction to thermographic inspections of electrical installations
