Letters: Reader Feedback

Staff -- Consulting-Specifying Engineer, 7/1/2006

Electrical addendum

Editor's note: This past year, CSE has reissued George Farrell and Frank Valvoda's classic "Art of Protecting Electrical Systems" series in our monthly electrical e-newsletter, archived in the electrical community at csemag.com. In the March Editor's Viewpoint, it was noted that Mr. Farrell, who had been updating the series, passed away. To keep the articles current, readers were encouraged to comment on any changes to the codes or electrical systems since the time of the articles' original publication, which in some cases, was many years ago. The following letter addresses one such change.

I greatly enjoyed reading the "The Art of Protecting Electrical Systems, Part 6" by By George Farrell and Frank Valvoda. I've been involved with short-circuit and withstand testing of fire pump controllers for some time and am glad to see a cogent and well thought out article on these topics. I particularly noticed the "selective protective devices" subheading. It may be interesting to note that 2005 Edition of NFPA 70 requires selective coordination of overcurrent protective devices used in emergency power systems. See Articles 700.27 and 701.18. There is significant confusion and controversy over these two new clauses.

JIM NASBY, MASTER CONTROL SYSTEMS

Thank you once more for your straightforward and correct cautionary editorial ("Are we Overbuilt?" CSE 5/06, p.7). My sources tell me that construction costs are going up at 1.5% per month and that condo sales are bombing. Many cagey developers are selling buildings to institutions—who want the revenue stream—and placing their money only in very certain projects. They can do this because it is cheaper to buy a reasonably good building than to build a comparable new one. Thank you once again for your integrity.

DAVE THOMAS, FAIRFAX COUNTY, VA.

Editor's note: Jeff Miller, who leads ABB's HVAC market for low-voltage drives, responds to a letter from last month commenting on the smoke-removal/train tunnel ventilation scheduling program practiced at Atlanta's Hartsfield Airport. The writer suggested that stress to ventilating fans during start-up is great and that the fans would last longer if run over a monthly vs. weekly period.

Under normal, across-the-line applications, this would be true. Because VFDs provide a soft-start feature—ramping up the motor slowly—the soft start eliminates the mechanical shock, stress and noise associated with across-the-line starting. The time between alternation no longer becomes as important.