Printed from Kezuriba (kezuriba.net/en/maintenance/thermography/)

Inspection with a thermal camera

Cautions on emissivity, what to look at in electrical equipment and motors, guide values for judging temperature differences, and how to compare with images from normal operation.

Shiny metal
has low emissivity,
so be careful!

What a thermal camera measures

Every object emits infrared radiation according to its temperature, and the amount increases with the fourth power of temperature. The camera emits nothing; it receives the infrared radiation coming off the surface and converts it to temperature. The amount depends on the surface temperature and the emissivity. It sees only surfaces; it cannot see through to the back or the inside.^[1][2]

Cautions on emissivity

Painted metal, wood, water, skin, and cloth have an emissivity of 0.9 or higher and are easy to measure. Unpainted metal is low, below 0.6, and can read lower than the actual temperature or read higher by picking up reflected heat from the surroundings. For shiny areas, apply black tape or paint and measure there for an accurate reading (Fluke). Nippon Avionics also cautions about the influence of reflection from the surroundings.^[2][3][1]

Covers and panels

Infrared does not pass through panel doors or metal busducts. If it can be done safely, open the cover and look directly. If the outside surface is hot, assume the inside is much hotter.^[4][1]

Load during measurement

Measure in a steady state after warming up fully, at 40% or more of normal load (per NFPA 70B). Heat from a poor contact increases with the square of the current, so the higher the load, the easier it is to find. Also note that wind or fans cooling the surface lowers its temperature.^[4](1 source, for reference)

What to look for in electrical equipment

When cables deteriorate or terminals loosen, resistance rises and abnormal heat is generated while energized, which can lead to fire if left alone. Heat shows up in many kinds of faults, so inspection can be done safely without shutting down. Under the same load, the three phases should be at the same temperature; if only one phase is hot, possible causes include imbalance, overload, and poor contact, and identifying the cause requires measuring the current. If only one side of a fuse is hot, suspect contact resistance at that end; if both ends are hot, suspect overload or insufficient capacity.^[5][6][4]

Guide values for judging temperature differences

According to Fluke, the guidance of NETA (InterNational Electrical Testing Association) says to act immediately when the temperature difference between similar electrical components under the same load exceeds 15 °C, or when the difference between a component and the ambient air exceeds 40 °C. Think of repair priority in this order: safety hazard, importance of the equipment, size of the temperature rise.^[4](1 source, for reference)

What to look for in motors

Measure at 40% or more of design load. When measuring at low load, note that a small temperature difference grows as the load rises (Fluke).^[3](1 source, for reference)

Keep images of normal condition and compare

Set an inspection route and take images regularly, and comparing them with normal-condition images lets you notice abnormalities early. Take images after a repair too to confirm it is fixed, and use them as the new baseline.^[6][4][3]

📚Sources

Items without a mark are those on which two or more materials from different publishers agree. For product values, each manufacturer's documentation takes precedence. The text is summarized in Kezuriba's own words.