Bus Plugs Blog – Expert Safety and Usage Information

PBQ3640G Square D Bus Plug – Available at www.swgr.com

During a routine Infrared Scan of a large manufacturing facility, our Thermographer found an interesting, but not uncommon, problem. A 400 amp Square D bus plug, PBQ3640G, was lit up like a dim light bulb. That means the entire bus plug looked warm, not just one area of the bus plug. Note, when viewed with infrared, it does not take much of a temperature difference for an object to look dramatically warmer than the area around it. But this was hot enough to indicate there was a problem, an overheating problem that was not localized inside the bus plug. While trying to angle for another view of the bus plug, our Thermographer discovered three different feeder conduits coming out of the 400 amp bus plug. One went to a 400 amp panel board, one went to a 225 amp lighting panel, and one went to a molding machine.  As it turned out, there was a lot of load diversity, but the electric heaters on the molding machine were just too much. There was not a problem yet. There were no poor connections causing the heat. But the total load on the 400 amp bus plug was high enough to make the entire bus plug look warm under Infrared. If they continued to add load to the panel board or to the lighting panel, the fuses in the bus plug would have blown. Ignoring the fact not all the individual feeders out of the bus plug were protected, sooner or later they were going to have a problem with the PBQ3640G bus plug. Their solution was to put the molding machine on a new separate bus plug.

General Electric AC323R Bus Plug Available from MIDWEST

Sometimes you just can not safely perform an electrical maintenance service per the written specification. For example, in today’s world of real concern for arc flash hazards, safely opening the cover of an energized bus plug, to perform an Infrared Scan, may not be possible. It may not be safely justified because of the high risk. So one argument is whether or not one can actually tell if a bus plug is hot because of overloading or because of a poor connection that is overheating. Is it a load problem or a connection problem?  Is there a problem at all?

Because our Thermographers are all required to work on bus plugs in our switchgear shop, they are very familiar with the interior of most bus plugs. They may have totally reconditioned a specific Square D or Bull Dog 100 amp bus plug. Or they may have repaired many ITE, GE General Electric or Cutler Hammer 60 amp bus plugs. Therefore they know where the interior switch or breaker is located. They may know exactly where the conductors connect to the bus plug feeder terminals.  This knowledge and experience is a big advantage, but the Infrared task is still difficult. The indications of overheating, whether load or connection related, may be very subtle because the interior heat must be re-emitted by the bus plug enclosure, greatly dampening the heat pattern.

Again, knowledge of the bus plug allows the Thermographer to turn art into science, at least to some degree. The thermographer may compare a suspect old bus plug to an identical bus plug nearby. Comparing the feeder conduit helps identify the possible ampacity of an old bus plug.  Comparing the heat pattern of the different plane surfaces of a suspect old or new Cutler Hammer or Square D bus plug may give additional information to help determine the existence and cause of heating.

Our thermographers have special safe ammeters to measure the load on a bus plug feeder, where practical and safe. Knowing the actual load, say on a feeder from an old Federal Pacific bus plug, is extremely helpful in determining if the suspected heating is load related or connection related. It is extremely difficult, if even possible, to determine the presence of overheating, much less the cause, by just looking at one plane of the energized bus plug with even a very sensitive Infrared Scanner.

It is the combination of the Thermographer’s experience and equipment knowledge that makes scanning energized bus plugs still very useful.  This same experience and knowledge aid in determining the possible cause of a suspected problem manifested by heat.

MIDWEST frequently is asked how we can tell if an overheating problem in an electrical bus plug, found using Infrared Thermography, is a connection problem or a load problem. Especially when the bus plug cover can not safely be opened.  Overheating from a load problem usually displays a paintbrush effect on the bus plug enclosure. Large and continuous areas may display a higher temperature pattern. Looking at the electrical bus plug from different angles may still display a wide pattern of overheating and no indication of a spot source. This is difficult, even for an experienced Thermographer. MIDWEST’s Thermographers have the advantage of extensive training and experience reconditioning, maintaining, repairing and testing bus plugs in our switchgear shop. With this experience, they know the location of the internal components of the various bus plugs, whether Square D, Cutler Hammer, GE General Electric, Westinghouse, ITE Bulldog, or Federal Pacific. The combination of this knowledge and experience and their understanding of Infrared Thermography, gives them the tools they need to best differentiate between overheating due to load and overheating due to a poor connection or contact.

A connection problem may display a more localized heat pattern that sometimes can be confirmed by Infrared Scanning the bus plug from different angles.  The heat pattern at each angle may confirm the heat is coming from the same somewhat specific location.  With the cover closed, it is very difficult to identify the exact location. It might be the fuse clip or the lug for a feeder cable or the switch or breaker contact.

Finally, when possible, measuring the load on the bus plug feeder, when safe, and comparing it to the bus plug rating can help validate whether the problem is load or connection. Only when safe, the bus plug cover may be opened and an accurate scan of the inside of the bus plug performed.  It is not always possible to determine the exact cause of overheating in a bus plug, but these are some of the tools that increase our success.

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.