When a problem occurs in an electrical distribution system, the first question usually is, “Where?” Which panel, what circuit, what component? A thermal imager probably can provide the fastest answer to this question. Scan the electrical components, look for abnormal temperatures and zero in. The follow-up question, “What happened?” is answered by a combination of technician and electrician expertise, and various electrical tests and measurements. But, that’s another story.
First, though, you need to identify problems within the electrical distribution system, both when trouble has hit, and in advance, through routine preventive maintenance.
Loading, safety, emissivity
View more content on PlantServices.com
Modern thermal imagers are rugged, easy to use and more affordable than models from even just a few years ago. They’re a realistic solution for routine electrical maintenance. A qualified technician or electrician points the device at the equipment in question and scans the immediate area, looking for temperature anomalies that appear on a live, false-color image that reveals where the equipment is emitting heat. Specific thermal images can be captured and uploaded to a computer for closer analysis, reporting and future trending. Imagers are easy to use, but they’re most effective in the hands of qualified technicians who understand both electrical measurement and the equipment being inspected. The following three points are especially important.
First, the electrical equipment being inspected must be under at least 40% of its nominal load if a thermal imager is to detect problems effectively. If possible, test under maximum load conditions for best results.
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).
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?
Thermal PM inspection process