and for the climatic zones. The measurement periods may also differ depending on
the type of plant to be inspected, whether they are hydroelectric, nuclear, coal-based
or oil-based plants.
In the industry the inspections are—at least in Nordic countries with clear seasonal
differences—carried out during spring or autumn or before longer stops in the oper-
ation. Thus, repairs are made when the operation is stopped anyway. However, this
seems to be the rule less and less, which has led to inspections of the plants under
varying load and operating conditions.
7.2.2
General equipment data
The equipment to be inspected has a certain temperature behavior that should be
known to the thermographer before the inspection takes place. In the case of electrical
equipment, the physical principle of why faults show a different temperature pattern
because of increased resistance or increased electrical current is well known.
However, it is useful to remember that, in some cases, for example solenoids, ‘over-
heating’ is natural and does not correspond to a developing defect. In other cases,
like the connections in electrical motors, the overheating might depend on the fact
that the healthy part is taking the entire load and therefore becomes overheated. A
similar example is shown in section 7.5.7 – Overheating in one part as a result of a
fault in another on page 33.
Defective parts of electrical equipment can therefore both indicate overheating and
be cooler than the normal ‘healthy’ components. It is necessary to be aware of what
to expect by getting as much information as possible about the equipment before it
is inspected.
The general rule is, however, that a hot spot is caused by a probable defect. The
temperature and the load of that specific component at the moment of inspection will
give an indication of how serious the fault is and can become in other conditions.
Correct assessment in each specific case demands detailed information about the
thermal behavior of the components, that is, we need to know the maximum allowed
temperature of the materials involved and the role the component plays in the system.
Cable insulations, for example, lose their insulation properties above a certain tem-
perature, which increases the risk of fire.
In the case of breakers, where the temperature is too high, parts can melt and make
it impossible to open the breaker, thereby destroying its functionality.
7
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Publ. No. 1558146 Rev. a156 – ENGLISH (EN) – February 28, 2006
7 – Introduction to thermographic inspections of electrical installations
