A one-line diagram for an electric power distribution system is an electrical drawing that uses single lines and graphic symbols to illustrate the current path, voltage values, circuit disconnects, fuses, circuit breakers, transformers, and panelboards.
One-line diagrams use the most basic symbols because the intent of the drawing is to illustrate as clearly as possible the flow of current throughout the building distribution system and where each component or device connects into the system.
One-line diagrams are also used when designing large commercial and industrial installations to show the path of electric power throughout a building.
One-line diagrams are also used when troubleshooting distribution system problems such as loss of electric power, low voltage, blown fuses, tripped circuit breakers, and poor power quality.
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They are also used to determine electric power shut-off points, future expansion capacity, and where emergency backup generators or secondary power systems are connected into the system.
A one-line diagram is helpful when troubleshooting an electric power system and can show the entire distribution system or specific parts of a system. See Figure 1.
Figure 1. One-line diagrams use single lines and symbols to show system components and operation.
For example, a one-line diagram may show a 13.8 kV feed into a building and the transformers used for the distribution of specific voltages.
High voltages are used for the distribution of large amounts of electric power using small conductor sizes. The high voltage is then stepped down to low voltage levels and delivered to distribution panels. The distribution panels route power to individual loads such as industrial equipment, motors, lamps, and computers.
Feeders and Busways
The electrical distribution system of a building must transport electric power from the source of power to the loads. In large buildings, the distribution may be over large areas with many different electrical requirements throughout the building. See Figure 2.
In many cases where it is common to shift production machinery, the distribution system must be changed from time to time.
A busway is a metal-enclosed distribution system of busbars available in prefabricated sections. Prefabricated fittings, tees, elbows, and crosses simplify the connection and reconnection of the distribution system.
When sections are bolted together, electric power is available at many locations and throughout the system.
A busway does not have exposed conductors. This is because the power in a plant distribution system is at a high level. To offer protection from the high voltage, the conductors of a busway are supported with insulating blocks and covered with an enclosure to prevent accidental contact.
A typical busway distribution system provides for fast connection and disconnection of machinery. Busways enable manufacturing plants to be retooled or re-engineered without major changes in the distribution system.
Figure 2. The electrical distribution system in a plant must transport the electric power from the source of supply to the loads.
The most common length of busways is 10′. Shorter lengths are used as needed. Prefabricated elbows, tees, and crosses make it possible for the electric power to run up, down, and around corners and to be tapped off from the distribution system. This allows the distribution system to have maximum flexibility with simple and easy connections as work is performed on installations.
The two basic types of busways are feeder and plug-in busways. See Figure 3.
Feeder busways deliver the power from the source to a load-consuming device. Plug-in busways serve the same function as feeder busways, but they also allow load-consuming devices to be conveniently added along the bus structure.
A plug-in power panel is used on a plug-in busway system. The three general types of plug-in power panels used with busways are fusible switches, circuit breakers, and specialty plugs such as duplex receptacles with circuit breakers and twist-lock receptacles.
The conduit and wire are run to a machine or load from the fusible switches and circuit breaker plug-in panels. Generally, power cords may be used only for portable equipment.
Figure 3. The two basic types of busways are feeder and plug-in busways.
The loads connected to the electric power distribution system are often portable or unknown at the time of installation. For this reason, the power distribution system must often terminate in such a manner as to provide for a quick connection of a load in the future. To accomplish this, an electrician installs receptacles throughout the building or plant to serve the loads as required. With these receptacles, different loads can be connected easily.
Because the distribution system wiring and protective devices determine the size of the load that can be connected to it, a method is required for distinguishing the rating in voltage and current of each termination. This is especially true in industrial applications that require a variety of different currents, voltages, and phases.
Equipment grounding is required throughout an entire electric power distribution system. All non-current-carrying metal parts including conduit, raceways, transformer cases, and switchgear enclosures must be connected to ground.
The objective of grounding is to limit the voltage of all metal parts to the ground and establish an effective ground-fault current path. See Figure 4.
Figure 4. Electronic grounding, equipment grounding, and building grounding are the three types of grounding needed to create a safe work environment for individuals.
Grounding is accomplished by connecting the non-current-carrying metal to a ground with an approved grounding conductor and fitting.
A ground bus is a network that ties solidly to grounding electrodes. A grounding electrode is a conducting object through which a direct connection to earth is established.
The ground bus must be connected to the grounding electrodes in several spots. The size of the ground bus is determined by the amount of current that flows through the grounding system and the length of time the current flows.
In addition to grounding all non-current-carrying metal, lightning arresters may be needed. A lightning arrester is a device that protects transformers and other electrical equipment from voltage surges caused by lightning.
A lightning arrester provides a path over which the surge can pass to the ground before it has a chance to damage electrical equipment.
Never assume that a metal enclosure is properly grounded. An electrical shock can be caused by ungrounded enclosures when the metal handle is touched to turn off a circuit at a disconnect switch.