Bus Plugs Blog – Expert Safety and Usage Information

XLVB321 ITE Bus Plug

Having spent forty years of an Electrical Engineering career in Industrial Research and Development, I feel extremely comfortable in an Electrical Engineering Power Laboratory.  There were few pieces of electrical test equipment that I did not know intimately and use on a regular basis.  Working with 240 and 480 Volt, three phase equipment was quite normal.  And in a Research and Development laboratory, the power wiring was always in a state of flux.  Wiring could be changed minute to minute, hour to hour.  Virtually all power connections were considered transient. 

 So, it came as a surprise when someone used the term “bus plug” to me.  At first, it sounded like somebody from the transit company had an all electric powered school bus, and they had to find a suitable wall outlet to plug into.  Close, but no cigar?   Actually, this wasn’t even close.

Embarrassingly, I had to ask what a bus plug was.   Well, the guru from MIDWEST asked me where did I get my power?  Well, I said, there usually was a distribution network of rectangular gray metal conduits that contained the three phase power busbars that spidered to every lab bench.  I knew these were called bus ducts.  Then the guru asked how did I connect to it?  Well, I said, there are these boxes that attach to the bus ducts.  Each bench’s power comes from these boxes.  Inside, there are usually fuses, and a switch.  The switch usually was just a metal arm with a hole in it that came out of the box.  To turn them on and off, we had long poles with a hook on the end that captured the hole in the arm.  I used these all the time.  Most of the lab benches had these long poles right next to them.  But, the only time I ever saw the interior was when a fuse had been blown inside the box; even then, it was usually a technician’s job to climb up a twelve foot ladder and replace the fuse.  But, a few times, usually at 3 am, I was the one that climbed the ladder.

And the MIDWEST guru said, those boxes are called bus plugs.  Eureka!!!  I’d been using them forever, and just didn’t know what they are named.  It was explained that in order to tap the power from the bus duct conductors, spring loaded fingers touch the internal conductors.  It was also explained that these bus plugs could more or less be snapped onto the bus ducts most anywhere.  

And I said, “Well, now I know exactly what a bus plug is.“   

MIDWEST was asked why the total capacity of all the bus plugs connected to a 1600 amp bus duct was over 2500 amps. The particular facility had one 400 amp bus plug, four 200 amp, eight 100 amp, and twelve 30 to 60 amp bus plugs on the old bus duct.  These were mostly very old bus plugs and a few reconditioned or new bus plugs. Purchasing wanted to order two new 60 amp bus plugs but didn’t know how it was possible to have over 2500 amps connected to only a 1600 amp bus duct and then add more. Actually the equipment he wanted was no longer manufactured, so he would have to use old reconditioned or refurbished bus plugs. It was a maintenance man asking purchasing the question.  This was a case were the maintenance man was doing production equipment repair one day and electrical maintenance the next. But he was not an electrician by trade.  MIDWEST explained.  The size of the bus plugs is nominal. The actual fuse or breaker in the bus plug may have a lower rating. For example, a 100 amp bus plug may have a 60 amp fuse. Or 250 amp fuses in a 400 amp bus plug.  In addition, all connected load is not on all at the same time. There is a diversity factor. The diversity of an old or new bus duct system might result in only 50% loading. It would not be unusual for a 2000 amp bus duct to have less than 1200 amps on it. We get extremely nervous if the load approaches 80%. We see a huge increase in problems when this equipment is operating anywhere near full rating.  When we infrared scan a bus duct that is heavily loaded, the whole bus duct and many of the bus plugs just seem to light up under infrared. So usually old and new bus plugs and bus ducts are not loaded to full rating, but when they are, bad things start to happen. If the load is not known, it may be easy to spot measure the load on the bus plug feeders. Do this safely at the load. Do not measure load at the bus plugs. That would be extremely dangerous.  Always, safety first.