![]() With automatic throw-over equipment at the primary switches, the number of failures per year is reduced by a factor of 6. IEEE 493-2007 compares the reliability of a simple radial system (one source), a primary selective system with manual throw over (9 min switchover), and a primary selective system with automatic selective throw over (5 sec switchover) (see Table 1). The examples given conclude that increased reliability is obtained with a dual utility source arranged in a primary selective configuration. This standard provides valuable examples that prove useful in determining the reliability of single- and dual-utility sources, and in comparing various campus distribution techniques. Whilst the start and finish ends of the ring are at the same location, power is delivered by both pathways of the ring into substations located around the ring.To analyze the reliability of the electric utility service, which, according to IEEE, is the largest contributor to both the failure rate and the forced-hours downtime per year at the 480 V point of use, engineers should refer to IEEE Standard 493-2007: Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems. ![]() Figure 3 – Parallel feeder systemĪ similar level of system reliability to that of the parallel arrangement can be achieved by using ring main feeders. This usually results from the growth of load supplied by a parallel feeder where the cabling has been installed along different routes. These are most common in urban and industrial environments. Parallel feeders are more common in urban areas or for feeders to large single customers, where load shedding in an emergency may be possible. To improve the reliability factor it may be possible to have the separate sets of cables follow different routes. In this case the capital cost is double that of a radial feeder but there is a greater reliability factor for the line. This may be justified if the load is higher, more customers are being supplied, or there are loads such as hospitals which require high levels of reliability. In the event of a line fault only one of the feeder sets of cables will be affected, thus allowing the remaining parallel feeder to continue to supply the load. ![]()
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