Letters: Reader Feedback

Breaking on series-rated breaker analysis Regarding the Specifier's Notebook on series-rated breakers "Comparing Series-Rated and Fully Rated Electrical Systems" (CSE 11/04 p. 62), I don't think it tells the full story. I especially take issue with the idea that the only way to provide selective coordination is by using fuses.

12/01/2004


Breaking on series-rated breaker analysis

Regarding the Specifier's Notebook on series-rated breakers "Comparing Series-Rated and Fully Rated Electrical Systems" ( CSE 11/04 p. 62 ), I don't think it tells the full story. I especially take issue with the idea that the only way to provide selective coordination is by using fuses. This is just not true. While the example the author provided is true, if the upstream breaker has only short-time protection, selective coordination could be achieved. Additionally, if zone-interlock protection is employed, selective coordination can be achieved even when the upstream breaker has instantaneous protection.

While I agree with the author that the series-rated option should be evaluated and discussed with the client, I feel that the full story should be presented. The fact of the matter is that when a high degree of reliability is needed, a fully rated, selectively coordinated system is the best solution, be it breakers or fuses. Additionally, while a series-rated system has a lower first cost, once the client realizes that they lose power over a greater portion of their system, instead of isolating just the faulted equipment, the client soon realizes they made a mistake. It is imperative that the engineer fully discloses the downside risks of a series-rated system instead of playing up the first-cost advantage.

Mark T. Turczynski, P.E., Midwest Engineering Consultants, Ltd., Moline, ILL.

More breaker trips

The author illustrates his point about selective coordination of fully rated breakers with trip curves for two molded-case breakers, both of which have instantaneous trips. If the 400-amp upstream breaker has adjustable long-time and short-time trips, but no instantaneous trip, selective coordination is simple. The key is proper selection and specification of the breakers. If the designer performs an overcurrent coordination study during the design process, identification of appropriate trip functions for each breaker will be evident.

The author does not illustrate or elaborate on his point about selective coordination being possible only when fuses are used. Fuse manufacturers provide tables that give minimum ratios between the ratings of upstream and downstream fuses for selective coordination. This is simple for designs in which there are few layers in the electrical system. On a complex design with many layers, maintaining the minimum fuse ratios becomes difficult and often impossible.

Christopher M. Johnston, P.E., EYP Mission Critical Facilities, Atlanta





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