MIDWEST Infrared Scans miles of bus duct and thousands of bus plugs every year. By using our large real time Infrared (Thermographic) Scanners, this is a very practical and valuable service. Our Thermographers are frequently asked what the most common serious problem they find on bus plugs. The answer is heating at the disconnect fingers. The disconnect fingers are spring loaded contact clips located on the back side of the bus plug. Depending on the size of the bus plug, for example a Square D PQ3603 (30 amp bus plug), the disconnect fingers may be the size and appearance of small clothes pins or as big as your fist for a large bus plug, like a Square D PTQ36120 (1200 amp bus plug). If the mechanical connection of the disconnect fingers to the bus bar in the bus duct is deficient or outright defective, the heating at the connection can be enough for our experienced Thermographers to detect. The same for heat due to poor contact surfaces between the bus bar and contact tips of the disconnect fingers. Sometimes the disconnect fingers are not even clipped on to the bus bar, but are bent under or over the bus bar. These defects may be present for years and not manifest themselves until the circuit becomes more heavily loaded. MIDWEST’s Thermographers are trained and experienced in reconditioning and testing bus plugs in our Switchgear Service Shop. They may have even melted a few disconnect fingers on one of our high current test sets, just for fun of course, when no one was looking. Actually that is what we call ‘real world’ training.

Locating these problems on energized bus plugs is extremely important because of the damage that can occur to the bus duct itself. We can repair or recondition a 400 amp Cutler Hammer ITAP 365R bus plug, but the bus bar in bus duct is more costly and time consuming.  So finding this common problem with in-service bus plugs is also finding a more serious problem involving the bus duct. Both very important.