Published In December 2001
A successful design/build project is completed on time and within budget, and meets the customer’s needs and expectations. Customers expect a reliable, maintainable system. These characteristics are not abstract terms—they are characteristics of a power distribution system that need to be considered in the design as much as load magnitude and other load characteristics. The term “reliability” as it is used in this article means a system’s ability to operate without failure for an acceptable period of time. The customer must define the degree of reliability required, consider the consequences of an outage, and work with the electrical contractor to select the system that provides the necessary reliability. “Availability” is the system’s ability to function when needed. The term is predictive and deals with the future ability of a power system, subsystem, or component to operate when needed. The term “reliability” is often used when “availability” would be more appropriate. For example, regarding the utility power supply to a facility, “reliability” would be the characteristic of interest to the customer because it refers to the likelihood of the primary utility power supply operating without interruption. But “availability” might more appropriately describe the likelihood of the emergency power system operating when the utility system did fail. Quantifying system reliability and availability Power system reliability and availability can be quantified through a risk assessment study that is typically called a “reliability study.” This study for a commercial, industrial, or institutional power system uses industry data on the failure rates of individual components such as switchgear, transfomers, and cable to estimate the owner’s risk of losing power. “Reliability” is quantified as the probability of failure when needed and “availability” is quantified as the probability that a system will function when needed. Using study results, the expected cost of power outage can be calculated and design alternatives compared, to balance the customers expected costs and benefits. In most cases, a reliability study that quantifies the risk of a power outage is unnecessary. Understanding the customer’s business and the consequences of a power outage will provide insight to the degree of reliability that is expected and the type of system needed. However, if the cost of a power outage is significant to the customer or if its design/build project performance requirements specify a minimum system reliability, then a study should be performed and the results reviewed and approved by the customer. Factors impacting reliability Power distribution systems’ reliability is impacted by the primary power source, system configuration, equipment operating environment, and other factors. Reliability of primary power source. The reliability of a facility’s primary power source is an important factor in the design of the customer’s power distribution system. Commercial, industrial, and institutional facilities almost always get their primary source of power from the local utility. The first step in designing for reliability is to evaluate the reliability of the utility supply at the facility’s location. This information can be obtained from the local utility as well as from discussions with the facility managers of surrounding properties. If a more reliable source of power is required than what can be provided by the local utility, then the installation of a standby power source may be required, which could include a second utility feed, on-site generation, or uninterruptible power supplies (UPS). System configuration. In a perfect world, a simple radial distribution system [Figure 1] would be adequate for all commercial, industrial, and institutional facilities. A radial distribution system is the most common, least expensive, and simplest configuration that can be installed. However, a radial system has only a single connection from the primary power source to the load, which may not provide the reliability needed for the customer’s loads. Redundancy in power sources, distribution equipment, and feeders will improve the reliability. A number of arrangements allow the load to be fed from multiple feeders or through multiple transformers. These arrangements include primary [Figure 2] and secondary selective [Figure 3] arrangements, as well as secondary spot networks [Figure 4] using specially matched transformers and network protectors on the transformer secondaries. For example, a double-ended substation is a secondary selective arrangement where the loads on the substation can be fed from either of two primary feeders or stepdown transformers. Equipment operating environment. Equipment and materials must be designed, tested, and approved for use in their intended thermal and physical environments. If they are unsuitable for these environments, they may fail prematurely and cause outages. Economics of reliability A balance between reliability and cost must be struck for the design to be successful. If the degree of reliability designed into the system cannot be justified by the customer’s needs, then the money spent achieving the excessive reliability is wasted. The contractor should use its collective knowledge and experience to provide the customer with a power distribution system that meets its needs as economically as possible. Maintainability A good design considers power distribution system maintainability. Otherwise, needed preventive maintenance may be unduly deferred or neglected, reducing system reliability. The system should be designed and equipment selected with ease of maintenance in mind. Adequate working space and access around equipment exceeding the minimums specified in Article 110 of the National Electrical Code (NEC) should be provided. Equipment should be arranged and located to facilitate maintenance activities. The ability to switch to an alternate feeder or an internal equipment bypass should be provided to allow critical loads to operate while distribution equipment such as UPS systems are taken out of service for preventive maintenance. Finally, electrical equipment needs to be specified with the necessary access and accessories to make it easy to maintain. Use your service advantage Service personnel who deal with reliability and maintainability problems every day should be involved in design/build projects, especially if the design/build project customer is a regular one. They know how the systems will be used, their expectations, customer preferences, the physical environment within the customer’s facility, the access required for regular maintenance, and the customer’s maintenance program, among other things. Acknowledgement This article is the result of ongoing research into the development of service contracting business by electrical contracting firms sponsored by the Electrical Contracting Foundation, Inc. The author would like to thank the foundation for its continuing support. GLAVINICH is Chair and Associate Professor of Architectural Engineering at The University of Kansas. He can be reached at (785) 864-3435 or email@example.com.