By Bennie Kennedy, Instructor, Curriculum Developer Prairie State College
A major contribution to my successes over the years in resolving problems with electrical power distribution systems is my ability to use test equipment and evaluate the results. One of the most valuable tools in my arsenal is a thermal imager or infrared camera. I attended a course by The Snell Group in the early 90's. Since then, I have used infrared to detect:
Tracing a blown fuse
I responded to a call from an electrical contractor. A 600 hundred amp 480 volt fused disconnect feeding a transformer was blowing a fuse randomly. The customer had used a chart recorder to record the current on the load cables. The current measured never exceeded the fuse's long time or instantaneous ratings. Several expensive 450 amp fuses had been replaced in the course of trouble-shooting the problem. Fuse list price $198.00 each.
Any time I encounter a protective device issue I perform an infrared evaluation. The goal is to eliminate abnormal heating as the cause or contributing factor. An infrared scan will identify the issue at hand and predict future failures.
I start with a comparative analysis. All connections should be approximately the same temperature. I expect a normal connection to measure ambient plus less than 20 degrees Fahrenheit. Note: B phase may be higher due to its location between A and C phase. All three fuses should be the same temperature, provided all the fuses are the same manufacturer and type. When the line side fuse connection temperature on any phase is higher than the load side connection, the higher temperature connection is likely the problem. Keep in mind the current flowing should be the same on all three phases in motor and transformer applications.
The 450 amp fuse blowing above was the result of a bad connection at a fuse clip. The thermal image clearly showed abnormal heating at the fuse connection. To confirm my diagnosis I carefully cut a blown fuse open. I found the element intact but disconnected at one end of the fuse. The condition of the fuse element typically indicates the cause of the fuse failure.
I regularly use comparative analysis to evaluate circuit breakers as well. Again the line and load connection temperatures should be the reasonably close. A loose connection could result in nuisance tripping of the breaker. In a matter of minutes, I can evaluate all the fuse disconnects and /or breakers in a switch board in a matter of minutes. Depending on severity, a high resistance connection internal to a breaker will be visible through the case via infrared.
Always follow the appropriate, recommended safety and inspection standards for your company and country, including OSHA, NFPA 70B and NFPA 70E. The 2009 edition of NFPA Standard 70E Electrical Safety in the Workplace makes PPE requirements for thermographers more straight-forward and easier to follow. For a description of the thermal-specific guidelines, visit www.fluke.com/FN-thermal-safety.
I use infrared to evaluate bolted connections, breakers, switches, and bus and cable connections. I have found connections that were mechanically tight but over heat when current is applied. Because the manufacturer recommends using a torque wrench on current carrying connections, the connection should be tightened to the specified value. If the hardware used is bad, no torque wrench is used or the wrench is out of calibration the wrench may indicate proper torque prematurely. An over torque connection can stretch the bolt when current is applied. The result can be abnormal heating of the connection when the current is reduced.
I worked at a customer site where checking switch-board connection hardware for proper torque was part of their annual preventative maintenance procedure. I recommended performing an infrared evaluation and documenting any connections issues. Any abnormal heating would be addressed during the preventative maintenance shut down. Connection issues that would not have been identified using a torque wrench were found. Because we did not have to check all the connections, the down time and labor hours required to perform the maintenance was substantially reduced.
Infrared safety tips
I cannot over-emphasize wearing the proper personal protective equipment (PPE) while conducting an infrared inspection. When you conduct an infrared evaluation, you are looking for a problem, and you need to expect to find one.
Always follow the appropriate, recommended safety and inspection standards for your company and country, including OSHA, NFPA 70B and NFPA 70E. The 2009 edition of NFPA Standard 70E Electrical Safety in the Workplace makes PPE requirements for thermoraphers more straight-forward and easier to follow. For a description of the thermal-specific guidelines, visit FN Thermal Safety.
Power distribution equipment is most dangerous when there is something wrong with it. If the equipment appears to lack regular maintenance or is old, be especially careful. Leave the room while the covers are being removed and replaced. Survey the equipment prior to opening doors or removing covers.
Abnormal heating discovered with the covers in place could indicate a very dangerous condition. Never break the plane of the equipment. Keep both feet on the floor. Ultra violet light could reveal evidence of rodents living in electrical equipment, opening covers could cause them to move resulting in an arc-flash. Examine bus-way from the floor; if there is abnormal heating you can see it.
Concentrate on the most critical equipment and where failures have occurred in the past. And above all if it is ever questionable as to whether you can do it safely don't do it. Some time the safest procedure is not to proceed.
Bennie Kennedy is a licensed electrical contractor. He also serves as an OSHA authorized general industry outreach safety trainer and instructor/curriculum developer for Prairie State College, Lakeland College, Joliet Junior College and the Indiana Safety Council.