SQUARE D SD78419

Bus Duct and Bus Plug Basics

Electrical bus duct and bus plugs are an effective way to distribute power about a building that feeds many loads.  Visualize bus duct as a metal enclosure that might be 6" deep, 8" high, and 100 ft. to 600 ft. long, hanging from the ceiling of a large manufacturing building.  Inside the metal enclosure are copper or aluminum rods or bars that carry the current.  They might carry 600 amps to 4000 amps of power.  You may have looked right at a bus duct in a manufacturing plant and not really noticed it.  But it gets noticed by everyone very quickly if it is disturbed by something, say a platform lift, that results in a horrific arcing fireball.

Electrical bus duct is manufactured for three applications, (1) Service Bus Duct, (2) Feeder Bus Duct, and (3) Plug In Bus Duct.  The basic difference is that a bus plug or bus plugs are installed on Plug In Bus Duct only.  Service Bus Duct takes power from a source outside a building and carries it to a main electrical distribution center, switchboard, or panel board inside the building.  Feeder Bus Duct takes power from the main electrical distribution center inside the building and carries it to another large electrical distribution center further inside the building.  Both Service Bus Duct and Feeder Bus Duct take power from one point directly to another.  But Plug In Bus Duct is the real work horse.  Plug In Bus Duct has removable metal plates on its sides all along the length of the bus.  A plate is removed and a bus plug inserted into the side of the bus duct.  Many bus plugs can be inserted into a single bus duct.  Typically a bus plug is installed near a machine, or other load.  A conduit or cable assembly is dropped from the bus plug to the machine.  The bus plug is a protective power interrupting device.  It typically has a fused switch or circuit breaker inside.  It is sized for the load to be connected to the bus plug.  Plug In Bus Duct may have many bus plugs installed on it.  Plug In Bus Duct is a very effective way to distribute power in an area that has many machines and other loads and that may require frequent relocation of some machines.  It is a very simple task to remove and relocate a bus plug or even a group of bus plugs.

MIDWEST must advise following an extremely important "Safe Work Practice."  MIDWEST strongly recommends a "Safety Policy" of always turning off the power to the bus duct before removing or installing a bus plug.  We recommend this Safe Work Practice for the following reasons:

1. Probably very high arc flash hazard levels,
2. The electrician typically is very close to the probable source of the arc.  The electrician basically has to 'hug' the bus plug to remove or install it.
3. The protective device for the bus duct may be sized for continuity of service instead of fast response to a fault.
4. The protective device for the bus duct and in the bus plugs may be a circuit breaker.  Under a fault condition, breakers can be slow and fuses can be fast.  The fuse may be your friend.  The breaker your enemy.
5. The electrician can not escape the hazard area.  He/she may actually fall into the arc source.
6. The electrician typically is working at a dangerous elevation.

In manufacturing facilities, bus duct and bus plugs are common.  In most facilities the bus plug is switched from the floor, using an insulated 'hot' stick.  The bus plug operating handle would have an opening that permits inserting the stud at the end of the hot stick such that the handle can be pulled or pushed to safely turn the bus plug off and on.