Thermal imaging is simply the process of converting infrared (IR) into visible images that depict the temperature differences of the scene under view by using a thermal camera. Using an IR camera, images are taken of the electrical panel board, concentrating on the equipment and electrical connections therein.
The camera can therefore judge the temperature of the focused items and, by way of colour, they can be indicated differentially.
With the cost of thermal imagers dropping over the years, thermal imaging of electrical panels has become very popular. Insurance companies, acknowledging that the cost of electrical fires has a negative impact on their bottom lines, have shown support of thermal imaging being undertaken regularly on larger installations. This approach adds value from a preventative maintenance perspective, which is good engineering practice.
The science: This is how/why it works
When current (I) flows in a conductor there is a loss seen as heat due to the resistance (R) of the conductor material, for example, loss is I²R current seen then to play the most active role. Therefore, all conductors will show a heating effect when carrying a load current. How do we use this IR to find a problem?
The idea here is to look for areas where the heating shows far higher than the general trend. A marginal variation is probably no cause for concern. A current balancing should always be done in all conductors so the thermal stresses are balanced; for this reason the load should be balanced, as best possible, across the three phases.
Interpretation
If all that has been said above is good, when would one start to have concerns? Interpretation is where things can get a bit tricky, for instance, where does one start to object to the level of the heat?
From basics we now know that all conductors under load current display heat, so the panel being tested should be in the run state and not OFF or just switched on. Allow for 2-3 hours prior to taking readings.
Conductors are also connected in series with the cables, lugs, busbars, circuit breakers and contactors. The circuit breakers, contactors and similar equipment will have been designed for the maximum load that can be safely applied. As the panel loads are never at maximum levels the heat temperature obtained has to be lower if the correct cable size has been used.
For circuit breakers, the allowable temperature rise at the point of connection is 70°C, i.e. ambient +70°C.
Unless the circuit breaker/contactor has been damaged or worn out at end of life it should not be subject for concern and the power connections should rather be the point of scrutiny.
Finally, do not anticipate issues with every connection, only those that have additional indication, e.g., insulation damage, heat discolouring, etc.
Not unlike x-rays, IR scans produce internal conditions within the circuit breakers and contactors. Note also that the correct cable size removes internal heat from these devices, so cable sizing will become important. Hot contact tips on their own are to be expected and should not ordinarily be of concern. However, there is no harm in erring on the side of caution. A lot will be learned from practice over time.
Kevin Flack has retired from industry and believes, with the wide experience gained in his 45+ years working career, he still has valuable in-depth knowledge of the South African electrical landscape. In these trying COVID-19 times, Flack can provide online training to keep your staff up to date on all aspects of circuit breaker deployment. Email kevin.flack@outlook.com
