Goals of System Design

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

2. Regulatory Requirements

3. Minimum Initial Investments

4. Maximum Service Continuity

5. Maximum Flexibility and Expandability

6. Maximum Electrical Efficiency (Minimum Operating Costs)

7. Minimum Maintenance Cost

8. Maximum Power Quality

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 forthe people who are responsible for electrical equipment maintenance and upkeep.

The Occupational Safety and Health Administration (OSHA) is a federal agency whose “mission is to assure safe and healthful workplaces by setting and enforcing standards, and by providing training, outreach, education and assistance.” OSHA’s electrical requirements are covered under several categories, the broadest being 1910 Subpart 10 Electrical including references to the National Fire Protection Agency (NFPA) 70 and 70E.

To address the concerns for personnel safety from arc flash hazards, the 2014 Edition of the NEC as well as the 2015 Edition of NFPA 70E have enhanced the requirements for personnel protection when working on or near live equipment. The 2014 NEC introduces new arc flash labeling requirements.

 Additionally, Article 240.87 offers a number of prescriptive alternative methods for arc flash energy reduction; one of which must be provided, for speeding up the clearing time of a circuit breaker that can be set to trip at 1200 A or above. Eaton’s Arcflash Reduction Maintenance System is available in various electronic trip units for molded-case and power circuit breakers to improve clearing time and reduce the incident energy level.

The National Electrical CodeT (NECT), NFPAT 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 ofproviding 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 healthcare facilities, places of assembly, theaters and the like, as well as 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.

2.      Regulatory Requirements: Over the course of the past century, electrical product safety and performance standards have been developed in cooperation between various agencies such as: American National Standards Institute (ANSI) as well as industry groups such as the Institute of Electrical and Electronics Engineers (IEEE) and the National Electrical Manufacturers Association (NEMA). These are often referenced together with specific test standards developed in conjunction with Underwriters Laboratories (UL). As an example, low-voltage switchgear falls under ANSI C37.20.1 and is tested in compliance with UL 1558.

The 2014 National Electrical Code (NEC) Article 110.2 states that: “The conductors and equipment required or permitted by this Code shall be acceptable only if approved.” The informational note references the definitions in Article 100 for Approved, Identified, Labeled and Listed.

OSHA has qualified a number of Nationally Recognized Testing Laboratories (NRTL) to demonstrate and certify “product conformance to the applicable product safety test standards.” Among the oldest and most respected of these electrical product testing organizations is Underwriters Laboratories (UL), which was founded in 1894.

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 in order 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.

In addition to NFPA and NEC guidelines, the design professional must also consider International Building Code (IBC) and local municipality, state and federal requirements. The United States Department of Energy, for example, mandates minimum efficiencies for transformers and other equipment.

Many of these regulatory codes reference ANSI/ASHRAE/IES Standard 90.1-2013 “Energy Standard for Buildings Except Low-Rise Residential Buildings”. Section 8.1 covers power and includes receptacle load control. Subsection 8.4.3 is titled Electrical Energy Monitoring and covers metering and monitoring systems that notify building tenants and engineers of the increased use of electric power. Section 9.1 covers lighting and lighting control system requirements.

Finally, utility incoming service standards for customer interconnects are key elements in the selection of both the incoming service voltage and the protection required for this equipment. Knowledge of these standards is particularly important when incorporating renewable energy or distributed generation resources into a design.

3.      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 total cost of the installation. This includes reducing on-site assembly time and cost by prefabricating various electrical components into a single deliverable system and reducing floor space and possible extra cooling requirements.

4.      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 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 customerowned power sources such as generators or batteries supplying Energy Storage Systems or uninterruptable power supplies.

e. Selecting the highest quality electrical equipment and conductors.

f. Using the best installation methods, including proper system grounding design.

g. Designing appropriate system alarms, monitoring and diagnostics.

h. Selecting preventative maintenance systems or equipment to alarm before an outage occurs.

5.      Maximum Flexibility and Expandability: 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 loadcenters 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 busways 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 to assure future capacity for growth. Power monitoring communication systems connected to electronic metering can provide the trending and historical data necessary to ensure future capacity for growth.

6.      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 long-term utility cost for the losses inthe transformer or other equipment versus the initial budget and cost of money.

7.      Minimum Maintenance Cost: Usually the simpler the electrical system design and the simpler the electrical equipment, the lower the associated maintenance costs and operator errors. As electrical systems and equipment become more complicated to provide greater servicecontinuity or flexibility, the maintenance costs and chance for operator error increases.

When designing complex systems, the engineer should consider adding an alternate power circuit to take electrical equipment (requiring periodic maintenance) out of service without dropping essential loads. Use of drawout type protective devices such as breakers and combination starters can also minimize maintenance cost and out-of-service time. Utilizing sealed equipment in lieu of ventilated equipment may minimize maintenance costs and out-of-service time as well.

8.      Maximum Power Quality: The power input requirements of all utilization equipment has to be considered, including the acceptable operating range of the equipment. Consequently, 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 expandability, 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.