Infrared Scanning Enclosed Bus Duct and Bus Plugs
We were asked by a maintenance supervisor if it made sense to infrared scan the bus duct in his manufacturing plant. He suggested it was a waste of time because the conductors in his bus duct were totally enclosed. MIDWEST hears this question a lot. He had a mixture of old bus duct and bus plugs and new. He was especially concerned with the new bus duct and bus plugs because he was adding more and more load and they had a lot of trouble adding some of the new bus plugs because of their location above production machines.
As we have said many times, infrared scanning is the best service possible for finding current related problems in bus duct and attached bus plugs. The Infrared Scanner is so sensitive, it can find warm bus splices and overheating bus plug connections long before they turn into a full blown critical hot connection or arcing failure. This is true for obsolete bus plugs and duct, even when it has been operating for decades without any problem. The real challenge with infrared scanning is when the bus is over production equipment and in very had to see locations. Still, for an experienced Thermographer, these challenges are easily overcome.
So, whether vented or enclosed, feeder or plug in bus duct, Infrared Scanning is the least expensive and most useful maintenance service available.
Recently at one of our factories, we had a failure of the insulation in an indoor metal enclosed bus duct (GE Spectra Series 600V Class). This is the only bus duct failure we have seen in the past several years. Now we are wondering if bus duct insulation failures are common, since most people seemed concerned about overheating failures related to poor connections.
Roughly what percentage of bus duct failures are due to insulation failures vs. connection failures?
What type inspection/testing do you recommend for preventing this type failure? What type inspection/testing do you recommend if we are unable to take the bus duct out of service for long periods of time?
MIDWEST seldom sees insulation failure in bus duct. We believe most bus duct insulation failures have a mechanical root cause, such as vibration and other mechanical movement from manufacturing processes that eventually physically damage the insulation. There is the occasional water or steam cause. And foundries and some chemical processes are the exception. But in a stable and clean environment, we would expect bus duct to be very reliable.
There are many reasons for mechanical failure of bus duct, from fork trucks to extreme load cycles. A guess at the ratio of insulation failure to mechanical failure might be 1 to 50. MIDWEST suggests Infrared (Thermographic) Scanning bus duct regularly, once a year. And make a hard focused inspection of the entire bus duct run, looking for anything out of the ordinary, vibration, subtle physical movement, sagging, airborne contaminants or overspray from equipment in the area, rodents, birds, roof leaks. Anything that looks out of place, even if you are not sure why.
@Roger Wood
Roger,
Do you know if that Bus Failure was due to the Bolts that go Through the Bus ? Any way possible you could call me 513.266.0882 or email me melam@isoc.net I had a couple ??
Insulation failure in 480 volt and 208 volt bus duct can be difficult to detect during the incipient stage. We would expect electrical insulation failure to actually be failure over the surface of the insulation and not ‘through’ the insulation. To be caused by contamination or physical breakage of the insulation due to constant vibration or impact or huge load cycling. Failure at ‘through bolts’ might be caused by constant vibration. If the failure was at the end splice plates, one would suspect wrong bolt torque values. It is a challenge to maintenance test bus duct for ‘insulation resistance’ because it is hard to isolate the bus duct proper. If you have a failure, it is important to save the failed equipment and analyze it for ‘failure mode.’ Use this analysis to determine how to avoid possible chronic failures. These are easy words, but hard to do in the real world.