Article and photos by Gregory Havel
In most commercial buildings, electricity is distributed by means of insulated electrical conductors inside steel or plastic conduit or raceways. These conduits or raceways originate at switchgear (circuit breakers or fusible switches) and end with outlet boxes recessed into or mounted on the surface of walls. The outlet boxes contain switches for lighting circuits or electrical receptacles for connecting appliances equipped with cords.
In industrial and manufacturing occupancies requiring higher voltages and large amounts of three-phase electrical current, “busways” (sometimes called “bus ducts”) are often used. They are rectangular, painted, steel enclosures hung near the ceiling that contain rectangular or round copper or aluminum alloy conductors called “bus bars.”
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Photo 1 shows the following:
- A: Four parallel conduits containing the insulated conductors for an 800-ampere, 480-volt, three-phase, four-wire power system.
- B: The terminal box where the wires from the conduits are connected to the bus bars in the busway.
- C: The busway enclosure.
- D: The joint between two sections of the busway and the bolt used to compress the layers of bus bar separated by insulators to ensure a good electrical connection. (In some types of busways, especially those at higher voltages, bus bar connections are made.
- E: A door that can be opened to plug in another fusible switch to power another piece of equipment. There are similar doors on the opposite side of the busway.
- F: A plug-in fusible three-pole switch installed on the busway.
- G: The conduit carrying insulated wires from the switch to the machine’s control panel.
Bus bars rated for 600 volts are usually bare metal; those rated for higher voltages (5,000 to 34,500) usually have an epoxy or polymer insulating coating. The bus bars are supported and separated from each other inside the busway enclosure on insulators made of polymer, glass, or porcelain.
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The most common type of busway is a modular unit rated for 600 volts and 600 to 800 amperes (photo 1). These modules assemble easily end-to-end (photo 2), are usually supported from the roof beams, and are equipped with doors (photo1-E, photo 3, and photo 4) so that “plug-in” fusible switch modules (photo1-F) may be easily attached for connection of manufacturing equipment. In photo 4, the door is open, exposing the edges of the bus bars onto which the plug-in switch will be installed.
- The contacts labeled “L1”, L2”, and “L3” are the energized or “hot” conductors.
- The contact labeled “N” is the grounded or neutral conductor.
- The fifth contact at the upper right is the equipment grounding conductor, to ensure that the equipment ground is continuous between the machine and the source of electrical current.
- In this system, there are 480 volts between any two of the “L” contacts.
- In this system, there are 277 volts between any one of the “L” contacts and the “N” contact or the system ground.
How to use and install busways are discussed in detail in National Fire Protection Association (NFPA) 70, National Electrical Code®, 2008 edition, Chapter 3, Article 368.
Maintenance of busways is discussed in NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, 2010 edition, Chapter 20.
High electrical currents require air circulation to keep the conductors cool, so most busways have ventilation openings. Some high-voltage busway enclosures are manufactured with louvers in addition to the usual ventilation openings.
Please note that most busway enclosures are not water-tight. Although they will successfully shed water from rain (if rated and listed for outdoor use), roof leaks, or activated fire sprinkler heads, water from fire hose streams operated from below can penetrate them.
Do not use hose streams in manufacturing occupancies with energized busways; the risk for electrocution is high. Protect yourself and the exposures until the power supply to the busway has been deenergized and locked out, as required in Occupational Safety and Health Administration’s 29 CFR 1910.147. Note: In large manufacturing facilities, there may be more than one busway installation, and each run of busway must be de-energized and locked out separately. (The plug-in switches on the busway can be operated from the floor with a hook stick to isolate individual pieces of manufacturing equipment. The busway itself will remain energized.)
If there is a fire in a manufacturing facility that is not controlled by the automatic fire sprinkler system and that will require offensive operations, work with the plant manager to deenergize and lock out the complete electrical system for that section of the facility. This may require deenergizing several power sources, such as lighting, housekeeping, and maintenance power circuits, and the circuits supplying power to busways for manufacturing equipment.
Although noting the location of every switch for every type of circuit in a manufacturing plant will make your prefire plan so bulky as to make it useless, the presence of electrical busways in the building is a hazard that should be noted on the preplan for your protection.
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Subjects: Building construction for firefighters
