|CAN I USE THE SAME SEVERITY CRITERIA ON INDIRECT MEASUREMENTS AS I USE ON DIRECT MEASUREMENTS? NO!
A direct measurement is one where there is little or no thermal insulation between the IR camera and the target of interest. An indirect measurement is where there is considerable thermal insulation between the target and the surface seen by the IR camera.
Examples of indirect measurements for electrical applications include: Oil filled circuit breakers, bus ducts, load tap changers, internal connections on transformers, underground switchgear, heavily insulated switchgear and other connections hidden from direct view due to inaccessibility.
The temperature rise indicating serious to critical problems for these indirectly view items is usually just a few degrees. An experiment we performed on a bus duct section indicated the internal temperature rise was about 25 times the bus duct surface temperature rise. As shown in Figure 4, for this bus duct, a 22º F rise seen on the duct would indicate about a 550º F rise on the internal bus! This multiplier should not be universally applied, but it is indicative of the serious nature of indirectly viewed targets.
Another potential problem with indirect targets is environmental effects3. Solar loading of previously shaded underground switchgear, exposed to the sun when the access cover is opened, can rapidly mask problems. Wind effects can also hide indirectly viewed problems. In general, environmental and other external influences can more readily mask the low temperature differences associated with indirect measurement, making them even more challenging to diagnose. One large organization thought they were being extremely conservative with their severity criteria by using a 10º C rise as the level to report any problem. They did not discriminate direct from indirect readings. Until an 8º C rise went unreported and a serious fire resulted three weeks later. Their severity criteria now incorporate direct and indirect readings.
From "Common misconceptions in infrared thermography condition based maintenance applications Robert P. Madding, Director Infrared Training Center, North Billerica, MA