Thermal imaging is the cure to electrical distribution woes

Second, the electrical measurement safety standards as specified in NFPA 70E apply. Anyone standing in front of an open, live electrical panel must not be carrying jewelry, keys or a watch and must use proper personal protective equipment (PPE) (Figure 1).

Safety first

Depending on the situation and the incident energy level (bolted fault current) of the equipment being scanned, this might include:

• Flame-resistant, long-sleeved shirt and workpants
• Category-rated leather-over-rubber gloves
• Leather work boots
• Arc flash-rated face shield, hard hat and hearing protection
• Arc flash-rated suit, hood and gloves

Always make sure you know the proper PPE for the environment in which you will be working. For PPE guidelines, reference NFPA Standard 70E, Tables 130.7 (c)(9)(a), (c)(10) and (c)(11).

The third point is called emissivity, a concept that describes how well an object emits infrared energy, or heat. This variable affects how accurately a thermal imager can measure a surface temperature. Materials emit infrared energy in different ways, and their specific emissivities are ranked on a scale of 0.0 to 1.0. Greater emissivity yields more accurate temperature readings.

Objects with a high emissivity emit thermal energy well, but aren’t usually very reflective. Materials with low emissivity are usually fairly reflective, but they don’t emit thermal energy well. This can cause confusion and incorrect analysis if you’re not careful. A thermal imager reading is most accurate if the emissivity of the surface under test is relatively high and the imager’s emissivity setting is the same as the object’s actual emissivity.

Most painted objects have a high emissivity of about 0.90 to 0.98. Ceramic, rubber, most electrical tape and conductor insulation have relatively high emissivities as well. This makes them ideal for thermal imaging work. Aluminum bus, however, is reflective. So is copper and some types of stainless steel. It’s difficult to get accurate temperature readings on these materials, even with a proper imager emissivity setting.

The good news is that most thermal imaging performed for electrical inspection purposes is a comparative, or qualitative, process. Usually, you don’t need a precise temperature reading. Instead, you’ll be looking for some spot that’s hotter or cooler than similar equipment under the same load conditions – temperature anomalies you don’t expect to see. You might also look for a certain temperature difference over ambient temperature. Who cares about the precise temperature reading if you find a component that is 35°C, or more, above the ambient surroundings?

Noting the unexpected

Keep in mind that, through conduction, heat travels between objects that are in contact with each other, from the hotter object to the cooler. Even though you might not be able to get an accurate temperature reading on an aluminum lug or a bare conductor, you usually can get a good reading on the insulation around that conductor. Because the insulation is in contact with the conductor, and because the insulation is on the outside, you can say with confidence that the conductor is hotter than the insulation.

Another trick is to look for a cavity emitter, a small hole or crevice (such as a lug hole), which acts like a tiny thermal oven that effectively increases the effective emissivity of even poor-emitting materials. Temperature readings taken from cavity emitters are much more accurate than those from flat, reflective parts of a low-emissivity material.

Another option is to install high-emissivity “targets” on bus bars, tubular bus, large metallic electrical connectors and most unpainted metals. This dramatically improves measurement reliability. There are no standards that apply to such targets, but they must be installed while the equipment is de-energized, and they must be able to withstand high operating temperatures. Many plants have reported success using electrical tape and flat spray paint, especially brands designed for electronic components. Be careful not to use combustible materials such as black paper or plastic tape. Note: The clear glass, Lexan or Plexiglas “touch-safe” covers that are increasingly prevalent inside electrical control cabinets aren’t transparent to most infrared. Mid- and long-wave thermal imagers can’t “see” through them.

Troubleshooting electrical systems

Use a thermal imager as your first inspection method when chasing breaker or load performance problems in your electrical system. Once you’ve completed the repairs, take another thermal scan. If the repair was successful, the hot spot you first detected should have gone away. It’s a thermography myth that every electrical hot spot is the result of a loose connection. There are many causes for hot spots. That’s why it’s wise for a qualified electrician either to perform the thermal scan, or to be present while it’s being done. Here’s what to check.

Three-phase imbalance: Capture thermal images from inside electrical panels and other high-load connection points such as drives, disconnects and controls. Wherever you discover higher temperatures, follow that circuit to examine associated branches and loads.

Compare the three phases side by side and look for significant temperature differences. A cooler-than-normal circuit or leg might signal a failed component. More heavily loaded phases appear warmer. Hot conductors might be undersized or overloaded. However, an unbalanced load, an overload, a bad connection and harmonics can produce a similar pattern, so follow up with electrical or power quality measurements to diagnose the problem.

Voltage drops across fuses and switches also can appear as imbalance at the motor and excess heat at the root trouble spot. Before you assume the cause has been found, double-check with both the thermal imager and a multimeter or clamp meter current measurements.

Connections and wiring: Look for connections that have higher temperatures than other similar connections under similar loads. That could indicate a loose, over-tightened or corroded connection. Connection-related hot spots usually (but not always) appear warmest at the spot of high resistance, and they cool in relation to the distance from that spot.

In some cases, a cold component is abnormal because the current is being shunted away from the high-resistance connection. You might find broken or undersized wires, or defective insulation. The guidelines of the InterNational Electrical Testing Association (NETA} suggest that if the temperature difference between similar components under similar loads exceeds 15°C (~25°F), you should make repairs immediately.

Fuses: If a fuse shows up hot on a thermal scan, it might be at or near its current capacity. However, not every problem shows up as hot. A blown fuse, for example, exhibits a cooler than normal temperature.

Motor control centers: To evaluate a motor control center under load, open each compartment and compare the relative temperatures of key components: bus bars, controllers, starters, contactors, relays, fuses, breakers, disconnects, feeders and transformers. Use the guidelines above for inspecting connections and fuses and identifying phase imbalance. Measure the load at the time of each scan, so that you can trend and evaluate your measurements against normal operating conditions.

Transformers: If a thermal imager is to detect an internal transformer problem, the malfunction must generate enough heat to be detectable from the outside. That means that a malfunctioning bushing connection, for example, will be much hotter than the surface temperatures read by the imager.

For oil-filled transformers, use a thermal imager to examine high- and low-voltage external bushing connections, cooling tubes, and cooling fans and pumps, as well as the surfaces of critical transformers. This doesn’t apply to dry transformers, which have coil temperatures so much higher than ambient that it’s difficult for thermal imagery to detect problems.

Use the guidelines above for connections and imbalances. The cooling tubes should appear warm. If one or more tubes appear comparatively cool, oil flow is probably restricted. Keep in mind that like an electric motor, a transformer has a maximum operating temperature that represents the maximum allowable rise in temperature above ambient (typically 40°C). A 10°C rise above the nameplate operating temperature will probably reduce the transformer’s life by half.

Preventive maintenance

Thermal imagers also are an excellent tool for routine preventive maintenance. Technicians should measure and compare thermal signatures for each piece of equipment on the inspection route. If the temperature is markedly different from previous readings, the technician can then use other test tools (digital multimeter, power-quality analyzer, etc.) to investigate further.

Michael Stuart manages thermography products for the Fluke Corp., Everett, Wash. Contact him at [email protected] and (800) 760-4523.