Finding the Hot Spots
By Carroll MccormickMachinery and Equipment Maintenance
A rattlesnake can see the heat of its prey in the infrared spectrum, while a human can feel some infrared radiation as heat. To detect the heat given off at the location of a poor electrical connectio...
A rattlesnake can see the heat of its prey in the infrared spectrum, while a human can feel some infrared radiation as heat. To detect the heat given off at the location of a poor electrical connection or defective electrical component, however, requires a thermographic inspection.
Thermography is a way of measuring the temperature of objects without touching them, by using infrared cameras. Unlike single-point temperature reading devices, infrared cameras, such as those manufactured by Flir Systems, Fluke Electronics, Micron Infrared or Infrared Solutions, take pictures that simultaneously record thousands of temperature points to a resolution of about 0.06 degrees Celsius. A single photo creates a temperature map of equipment ranging in size from simple circuit breakers to huge transformers.
Hot spots in electrical equipment are caused by loose connections that increase electrical resistance, that is, make it harder for electrical current to pass; the best-known normal expression of a product of resistance is the light and heat produced as current pushes through the filament in an incandescent light bulb.
High-risk points in a plant’s electrical system include cable connectors, contacts and fuse clips — any place where a connection in an electrical system could become loose. Motor control centres, with their many points of contact, are common places to find problems.
Poor connections are, however, rarely caused by dirt or humidity, according to Michel Blec, co-owner of the Brossard, Que., company Simac Electrotechnique, founded in 1996. The company specializes in engineering, testing, maintenance, repairs, modifications and reconditioning of low-, medium- and high-voltage electrical equipment.
Rather than the environment, says Blec, “It is the equipment itself that is defective; for example, loose connections and vibration-caused loosening of screws or connections.”
Plant-wide, there could be tens of thousands of points where poor connections can develop. “In just one small junction box there could be 100 possible places where there could be a bad connection,” says Blec. Worst of all, it is usually impossible to see problems with the naked eye. “If the problem is very bad you might see discolouration around the fuse clips, but that is very rare,” he notes.
Problems are not only found in older plants; Simac technicians are just as likely to find hot spots in brand new plants and equipment. Thermographic inspections should be ordered once a year, Blec advises. “There are almost always problems to find in a plant. There are plants in which, every year, we find 20-30 problem areas. In the majority of plants — over 90% — we find problems every year.”
Insurance companies can demand thermographic inspections, but even without that incentive, they are economical compared to the cost of downtime and replacing equipment damaged by an electrical fire.
“It may take two hours to repair a breaker, but perhaps 125 hours to replace it, plus maybe $10,000 for the new breaker. In some cases it may take two minutes to change a $5 part, but it could cost $50,000 to repair equipment after a fire,” says Blec.
The cost of an inspection varies according to plant size, but a technician may be able to cover a 20,000 square-metre plant in a day, at a cost of between $700 and $1,000. Travel time and other variables will add or subtract from the bill.
The technicians at Simac are certified by the Academy of Infrared Thermography. Blec suggests that it is a rare company that has its own infrared camera, which can easily cost $40,000.
In preparation for a thermographic inspection, it is helpful for the maintenance department to number its electrical equipment. Referring to “that switch just to the left, inside the door to the whatsit room,” can be confusing when the time comes to carry out repairs.
During an inspection, the technician will point the camera at a piece of electrical equipment and look at the real-time image on the camera screen for problems. If he sees nothing, he moves on to the next piece of equipment. If, however, he sees a hot spot, which shows as a white patch on the camera’s screen, he captures and stores the image. He also takes a regular photo so plant personnel can better match the infrared image in the report Simac prepares about the problem equipment.
After taking a photo, the technician takes a measure of the current, in part to understand the relationship between the colours produced by the camera and the amperage of the current; the higher the amperage, the hotter the colour.
In the report that Simac prepares, the actual temperature of the hot spot is indicated, and is assigned a rating of light, medium or serious to indicate the gravity of the situation.
Light is when the temperature difference between the reference temperature (roughly speaking, the ambient or normal temperature inside the piece of equipment) and the hot spot is 11 degrees Celsius or less; medium indicates a temperature difference from 12-25 degrees and serious is when the difference is 35 degrees or more.
For a switch with three fuse holders, for example, the report will detail which phase is faulty, the current, the increase in temperature above the reference temperature, and the gravity of the problem. The recommendation might read, “Check and clean the mobile and fixed contact surfaces. Check that contact pressure is acceptable.”
In a breaker panel the recommendation might read, “The breakers are overloaded. Check if it is possible to redistribute the load charges. Changing the breakers should be a solution to consider.”
Those changes — corrections to serious but previously unseen problems — can save thousands of dollars worth of trouble and downtime.
Carroll McCormick is a senior contributing editor for Machinery & Equipment MRO. He is based in Montreal and can be reached at email@example.com.