A maintenance electrician asked MIDWEST what he should do about a 200 amp Square D Bus Plug that was sagging several inches down from the overhead bus duct. He said the conduit from the bus plug was poorly supported and was pulling it down. His supervisor wanted him to try and fix it on the fly. That made him really uncomfortable and he wanted MIDWEST’s recommendations.
This problem has nothing to do with the manufacturer. It could have been a General Electric or Westinghouse bus plug just as easy. The problem is the installation, not the equipment. We have seen this all too often when doing Infrared Scans of overhead bus duct and bus plugs.
First of all we recommended in the strongest terms that no one try to fix this problem without shutting the power off. We mean shut off the power to the bus duct, not just shut off the bus plug. The connection of the disconnect fingers on the back of the bus plug to the bus duct is the most vulnerable to failure and will do the most damage if it faults while the system is on. Besides possibly getting someone seriously injured or killed, you could lose a section of bus duct and have a protracted power outage. The danger to the maintenance electrician is too great to even consider adjusting the bus plug energized.
As far as just leaving well enough alone, we don’t buy that either. This is what MIDWEST calls an incipient failure, a failure that will happen sometime in the future. When, we are not sure. But it will certainly happen. Our experience is these things happen on July 4th or Thanksgiving Day. That’s just the way things seem to go sometimes.
The danger of catastrophic damage to equipment and, more important, the danger to a maintenance electrician trying to adjust the bus plug while it is hot, is just too great. We would categorize the idea of doing this hot as “Crazy.” Besides, you are not allowed to do something that has the danger of serious injury or death.
Turn all the power off and properly plumb and support the bus plug. We also recommend removing the bus plug and checking it for damage, before reinstalling it correctly. Do it right and no one gets hurt, always a good idea.
For sake of convenience, economy of effort, or sheer tempting of fate, some mortal souls choose to swap out a bus plug on an energized bus duct. Imagine being up 20 feet in the air on a scissors lift and grabbing hold of a 200 amp bus plug and trying to yank the thing out of a hot bus duct, wearing no face shield, no arc flash or shock protection, and on top of all that, being under the gun to get it done before you go on your coffee break. No, this is not the Moscow circus. Some folks still do this.
Bus plugs connect to the bus bar in bus duct using spring loaded finger clusters. The bus plug is attached and removed from the bus bar by force of movement (F = MA, plus the added force needed to overcome the spring loaded tension of the finger clusters). The bus plug enclosure proper is mounted to the hanging bus duct enclosure by clamps holding the bus plug in place. The old bus plug is removed, the bus duct is inspected to make sure everything is copasetic and the new bus plug is put in place. The entire routine is fairly straightforward, on a de-energized and locked out circuit. On a live circuit, ladies and gentlemen hold on to your hats. Things unexpected can occur. Assumptions made may prove untrue. Working on a live circuit without proper arc flash or shock protection puts you in a place you really don’t want to be. Questions we’d like to ask: Is a person working under such live conditions aware of how much arcing fault current can take place? Are they aware that a fault might last much longer than a fault on a normal feeder? Are they aware that the blast could blow them off the lift; Are they aware that they have both hands in contact with a potential shock hazard and a shock could “cross their heart;” Are they aware that an arcing fault on many bus ducts is too dangerous even with the maximum protective clothing under NFPA 70E? Be safe, don’t rush, and MIDWEST recommends turning off the bus duct.
10.0 Why look down when you can look up.
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Two of the strangest locations we have seen with bus duct and bus plugs were in former manufacturing facilities that had been converted to very nice office spaces. One facility converted a building to their executive offices. Very fancy with lots of wood, glass and dramatic lighting. Had plush carpeting everywhere and actually very nice wall paintings and art. You would never guess the six feet above the ceiling was just jam packed with extremely old building infrastructure. Old and new heating ducts, sprinkler system, communication wires, conduits, a mass of abandoned pipes and cables, abandoned light fixtures, old dirty wood structure supporting an old wood roof, and a complete bus duct distribution system chuck full of old and obsolete bus plugs and just a few new bus plugs. There were two totally different worlds separated by fancy ceiling tile. It was like standing in 2010 and looking up into 1940.
Another facility was set up the same way, but this facility was only about 25 years old. The most dramatic feature of this facility was that the space above the office ceiling was about 20 feet to the roof. When you looked above the ceiling tile, you found a massive open area. The bus duct in this area was about midway between the roof and the office ceiling tile. From one vantage point, you could just about see the entire bus duct system and all the bus plugs. In addition there were no really old bus plugs. An eerie feature was all the threaded rod hanging from the roof supports down to the ceiling below. It was a massive lost space hidden above a normal office environment.
These were two strange places for bus duct and bus plugs, which are usually found in exposed manufacturing or commercial buildings.