When considering the design of an electrical distribution system for a given customer and facility, the electrical engineer must consider alternate design approaches that best fit the following overall goals.
1. SAFETY
The No. 1 goal is to design a power system that will not present any electrical hazard to the people who use the facility, and/or the utilization equipment fed from the electrical system.It is also important to design a system that is inherently safe for the people who are responsible for electrical equipment maintenance and upkeep.
The National Electrical Code® (NEC®), NFPA® 70 and NFPA 70E, as well as local electrical codes, provide minimum standards and requirements in the area of wiring design and protection, wiring methods and materials, as well as equipment for general use with the overall goal of providing safe electrical distribution systems and equipment.
The NEC also covers minimum requirements for special occupancies including hazardous locations and special use type facilities such as health care facilities, places of assembly, theaters and the like, and the equipment and systems located in these facilities. Special equipment and special conditions such as emergency systems, standby systems and communication systems are also covered in the code.
It is the responsibility of the design engineer to be familiar with the NFPA and NEC code requirements as well as the customer’s facility, process and operating procedures; to design a system that protects personnel from live electrical conductors and uses adequate circuit protective devices that will selectively isolate overloaded or faulted circuits or equipment as quickly as possible.
2. MINIMUM INITIAL INVESTMENT
The owner’s overall budget for first cost purchase and installation of the electrical distribution system and electrical utilization equipment will be a key factor in determining which of various alternate system designs are to be selected. When trying to minimize initial investment for electrical equipment, consideration should be given to the cost of installation, floor space requirements and possible extra cooling requirements as well as the initial purchase price.3. MAXIMUM SERVICE CONTINUITY
The degree of service continuity and reliability needed will vary depending on the type and use of the facility as well as the loads or processes being supplied by the electrical distribution system.For example, for a smaller commercial office building, a power outage of considerable time, say several hours, may be acceptable, whereas in a larger commercial building or industrial plant only a few minutes may be acceptable. In other facilities such as hospitals, many critical loads permit a maximum of 10 seconds outage and certain loads, such as real-time computers, cannot tolerate a loss of power for even a few cycles.
Typically, service continuity and reliability can be increased by:
A. Supplying multiple utility power sources or services.
B. Supplying multiple connection paths to the loads served.
C. Using short-time rated power circuit breakers.
D. Providing alternate customer owned power sources such as generators or batteries supplying un-interrupt-able power supplies.
E. Selecting the highest quality electrical equipment and conductors.
F. Using the best installation methods.
G. Designing appropriate system alarms, monitoring and diagnostics.
H. Selecting preventative maintenance systems or equipment to alarm before an outage occurs.
4. MAXIMUM FLEXIBILITY AND EXPEND-ABILITY
In many industrial manufacturing plants, electrical utilization loads are periodically relocated or changed requiring changes in the electrical distribution system. Consideration of the layout and design of the electrical distribution system to accommodate these changes must be considered. For example, providing many smaller transformers or load centers associated with a given area or specific groups of machinery may lend more flexibility for future changes than one large transformer; the use of plug-in bus ways to feed selected equipment in lieu of conduit and wire may facilitate future revised equipment layouts.In addition, consideration must be given to future building expansion, and/or increased load requirements due to added utilization equipment when designing the electrical distribution system. In many cases considering transformers with increased capacity or fan cooling to serve unexpected loads as well as including spare additional protective devices and/ or provision for future addition of these devices may be desirable. Also to be considered is increasing appropriate circuit capacities or quantities for future growth.
Power monitoring communication systems connected to electronic metering can provide the trending and historical data necessary for future capacity growth.
5. MAXIMUM ELECTRICAL EFFICIENCY (MINIMUM OPERATING COSTS)
Electrical efficiency can generally be maximized by designing systems that minimize the losses in conductors, transformers and utilization equipment. Proper voltage level selection plays a key factor in this area and will be discussed later. Selecting equipment, such as transformers, with lower operating losses, generally means higher first cost and increased floor space requirements; thus, there is a balance to be considered between the owner’s utility energy changes for the losses in the transformer or other equipment versus the owner’s first cost budget and cost of money.6. MINIMUM MAINTENANCE COST
Usually the simpler the electrical system design and the simpler the electrical equipment, the less the associated maintenance costs and operator errors. As electrical systems and equipment become more complicated to provide greater service continuity or flexibility, the maintenance costs and chance for operator error increases. The systems should be designed with an alternate power circuit to take electrical equipment (requiring periodic maintenance) out of service without dropping essential loads. Use of draw-out type protective devices such as breakers and combination starters can also minimize maintenance cost and out-of-service time.7. MAXIMUM POWER QUALITY
The power input requirements of all utilization equipment has to be considered including the acceptable operating range of the equipment and the electrical distribution system has to be designed to meet these needs. For example, what is the required input voltage, current, power factor requirement? Consideration to whether the loads are affected by harmonics (multiples of the basic 60 Hz sine wave) or generate harmonics must be taken into account as well as transient voltage phenomena.The above goals are interrelated and in some ways contradictory. As more redundancy is added to the electrical system design along with the best quality equipment to maximize service continuity, flexibility and expand-ability, and power quality, the more initial investment and maintenance are increased. Thus, the designer must weigh each factor based on the type of facility, the loads to be served, the owner’s past experience and criteria.
Summary
It is to be expected that the engineer will never have complete load information available when the system is designed. The engineer will have to expand the information made available to him on the basis of experience with similar problems. Of course, it is desirable that the engineer has as much definite information as possible concerning the function, requirements, and characteristics of the utilization devices. The engineer should know whether certain loads function separately or together as a unit, the magnitude of the demand of the loads viewed separately and as units, the rated voltage and frequency of the devices, their physical location with respect to each other and with respect to the source and the probability and possibility of the relocation of load devices and addition of loads in the future.Coupled with this information, a knowledge of the major types of electric power distribution systems equips the engineers to arrive at the best system design for the particular building.
It is beyond the scope of this guide to present a detailed discussion of loads that might be found in each of several types of buildings. Assuming that the design engineer has assembled the necessary load data, the following pages discuss some of the various types of electrical distribution systems that can be used. The description of types of systems, and the diagrams used to explain the types of systems on the following pages omits the location of utility revenue metering equipment for clarity. A discussion of short-circuit calculations, coordination, voltage selection, voltage drop, ground fault protection, motor protection and other specific equipment protection is also presented.