Plant Services - March 2008 - (Page 35)

Power distribution Safety well, but usually aren’t 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 accurate only 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 paints have a high emissivity of about 0.90 to 0.98. Ceramic, rubber and most electrical tape and conductor insulation have relatively high emissivities as well. They’re 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. Safety first Temperature readings taken from cavity emitters are much more accurate than those from flat, reflective parts of a low-emissivity material. 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 also might 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? 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 The thermal process • List the critical points in your electrical distribution system that are to be inspected, giving priority to essential and failure-prone equipment. • Develop an inspection schedule that details how often the points are to be inspected. • Use your thermal imager to capture baseline images of each piece of electrical equipment and note the electrical load and temperatures of key components for trending purposes. In complicated or large installations, such as a motor control center, capture separate thermal images of each key component or subsystem. • Download the baseline images into software and annotate them with location descriptions, inspection notes, temperatures and emissivity modifications, if appropriate. • At the next inspection, compare the previous inspection images to the new images and look for changes in operational temperatures. • Always capture a thermal image baseline of any new distribution equipment the first time it goes live. March 2008 Figure 1. Dress appropriately for the potential power inside the cabinet. good reading on the insulation around that component. Being in contact and having the insulation on the outside allows you to say with confidence that the conductor is hotter than the insulation surface temperature that you measured with the probe. Another trick is to look for a cavity emitter, a small hole or crevice (such as a lug hole), that acts like a tiny thermal oven that effectively increases the emissivity of even poorly 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 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 35 www.PLANTSERVICES.com http://www.PLANTSERVICES.com

Table of Contents Feed for the Digital Edition of Plant Services - March 2008

Plant Services - March 2008 Contents From The Editor Letters The PS Files Up and Running Crisis Corner What Works Asset Manager: Enterprise Thinking Technology Toolbox: Hydrogen In Action Cover Story: Get Real Electrical: Reliable Distribution Power Transmission: When the Belt Breaks Web Hunter: Tooting Horns In the Trenches Product Picks Classifieds Fast Facts Energy Expert: Energy versus Sustainability

Plant Services - March 2008

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