How The Eyes (and IR camera) Can Be Misled
By Joe Gierlach,
Manager - Technical Training & Support,
The IR camera is a great tool used in our everyday predictive
maintenance endeavors, but it can play tricks on our eyes if
we do not investigate beyond what we are observing. Things
truly are not always as they seem, here's an example:
1) As indicated in the images below, we “appear” to
have a hot spot and differential in temperature on the “C” phase
of the breaker, just to the bottom of the actuator handle.
2) The operating
parameters at the time of this visit were as follows:
||“A” – 8 amps
||“B” – 48 amps
||“C” – 24 amps
||“A” – <2%
||“B” – <2%
||“C” – <2%
||“A” – .001
||“B” – .001
||“C” – .001
Relatively normal operating conditions in a generally climate
controlled room, with no evidence of any reflective source
of infrared radiation noted.
3) So why is there a temperature of nearly 40° C (104° F
for those who relate to Fahrenheit)? With a high emissivity
on the breaker itself, reflective effects would need to originate
from a much higher heat source to have an effect on the observed
actual temperature on the breaker.
4) How could the operating parameters be within guidelines,
no apparent resistance increase was measured across the breaker
contacts via milli-volt observation, harmonic distortion levels
were deemed negligible, and the highest load demand on “B” phase
DID NOT have the highest temperature rise?
The answer to this was revealed when the breaker manufacturer,
Cutler-Hammer, assisted in the dilemma resolution. The source
of the heat was determined to be an under voltage relay coil
used in conjunction with the electronic sensing and trip unit,
mounted under the case at the location illustrated above. The
heat associated with electromagnetic induction properties or
transformers and coils radiated to the breaker case and were
detected by the IR camera.
The unit in question is illustrated in the blow up view, showing
this can be either left or right hand mount.
Below is an assembled view illustrating physical location
of where “heat” may be observed.
Another good example is illustrated below. A transformer with
balanced loading well below its rating, low harmonic content,
and no extreme conditions present in the surrounding environment “appears” to
have a coil winding that has a temperature difference of nearly
10° C between “B” and “C” phase
windings, as indicated by the arrow. The “B” phase
winding was approximately 48° C and the “C” phase
was approximately 41° C.
Based on those operating parameters, this is a problem as
defined by NETA guidelines that is near the “Severe” range
as related to our severity criteria levels. In fact, closer
quantitative analysis of the thermogram reveals the reasons
for the “apparent” differential temperature.
1) The “B” phase winding does not have the volumetric
area to allow for natural convection cooling of the heat, thus
it cannot dissipate heat as efficiently.
2) The core temperature is much higher as compared to the
windings and the heat generated in this area will influence
the winding temperatures if its effects are not compensated
for. This causes a radiating of heat to impinge on the winding
and affect the observed temperature.
3) The actual measured temperature does not exceed the manufacturer
specified guidelines with respect to temperature.
Again this example illustrates the absolute need to provide
quantitative analysis of any “apparent” thermal
anomalies. Other surrounding factors had an effect on what
we were observing and they had to be taken into consideration.
With the advent of solid-state protection in devices of this
nature, this scenario is going to become more common. We cannot
rely on what we see alone, and sometimes even the analytical
tools at our disposal still fail to reveal the source of a
The first example aids in demonstrating the absolute need
to utilize milli-volt drop tests with respect to thermal anomalies,
as it will provide one of several methods in assisting us to
prove or disprove the presence of an actual problem, by effectively
measuring and quantifying the passive resistance of a contact
point using Ohms law, and allowing us to calculate I^2 R losses.
With the lack of voltage drop and harmonic currents in this
problem, we can safely determine the breaker contacts and terminations
are suitable and heating is not related to non-linear loading.
To rely solely on temperature difference from similar components
or ambient air is not enough for us to diagnose a problem such
as this. Even the generalized guidelines in the NETA specs
for electrical equipment cannot be relied upon as a rigid benchmark
in this respect. If the NETA standards were applied in the
above example, it would be approaching the “Critical” level,
as the temperature difference is compared to ambient is nearly
40° C. Is the case in the first example? Would you recommend
an immediate outage to service this component? Not likely with
the operating conditions noted. In the second example, the
untrained eye would most certainly identify this as a problem
based solely on differential temperature guidelines.
We must use all of our tools to draw accurate conclusions,
as many factors need to be taken into consideration with respect
to these types of situations. We cannot just assume that because
a certain temperature differential exists, apparent or true,
particularly with respect to direct measurement, that a problem
exists. Making qualitative and quantitative analysis is key
in keeping us one step above the rest. Sure a hot spot exists
in these examples, but your eyes can deceive you if you do
not give consideration to such variables as external influencing
factors and load demands, and not provide proper compensation
values to the camera for these factors.