Letters to the editor.
In the cases described in the “Be prepared: Hospital protection for catastrophic events” article, June 2008 (page 32), the hospital's emergency power supply (EPS) was a medium voltage bank of generators. While the choice of medium over low voltage has its advantages for large systems, this design always leaves transformers normally de-energized, as is the case with the described systems.
The insulation system of dry type transformers is hygroscopic and the manufacturer's instructions recommend an insulation resistance measurement be taken when the unit is allowed to cool to ambient. ANSI/IEEE C57.94 paragraph 7.2, “IEEE Recommended Practice for Installation, Application, Operation, and Maintenance of Dry-Type General Purpose, Distribution, and Power Transformers” also makes the same recommendation. Both the instructions and the standard both recommend a drying out procedure should the measurement not meet specifications.
I would like to know of a transformer specification, preferably dry-type, which allows for a normally de-energized state, then a sudden application of full voltage and load.
Jim Hall , PE Electrical Engineer AKF Engineers Philadelphia
This is definitely one of the pro/con discussions for medium-voltage systems. There are a few ways we address this concern:
Use medium-voltage transfer switches where possible, allowing the step down transformers to be constantly energized. This is typically used for non-essential services, large equipment loads such as chillers, and for service loops, such as exists at Queens.
Manufacturers typically recommend the use of cast coil transformers instead of dry type when periods of no-load conditions exist. These are designed to minimize moisture in the coils.
Specify space heaters to minimize moisture buildup inside the transformer. In addition, you will need power from a separate source.
In a healthcare facility code requires that the emergency system is tested weekly, which will energize all transformers at least once per week.
Systems of this size will typically partake in utility load sharing programs, which will also increase the frequency that these transformers are energized.
Typically, the size of these facilities and the emergency power supply system (EPSS) require the use of medium-voltage systems. A 480-V system is limited from combined generator fault output, automatic transfer swtich size, cable quantity, and maximum switchgear current ratings.
James Ferris, PE Associate Electrical Project Engineer, TLC Engineering for Architecture Olando, FLA.
“Selecting & maintaining smoke detectors” July 2008 (page 60) correction
In the article, I believe they got the theory of operation of photo and ion detectors reversed. The difference of operation in a life safety situation, when installed according to their manufacturers' instructions, is non-consequential. My professional preference is to use Ionization unless problems with false alarming crops up after the installation. Then I replace those Ion detectors that are being problematic with photo detectors.
I would never use a heat detector in place of a smoke detector that is to be used for detection of fire in a life safety situation The only time a heat detector is applicable is when the heat detector is used in conjunction with a sprinkler fire protection system, i.e. detection for a pre-action style fire protection system, for property protection.
Joseph M. Crowley , Electrical Engineer Jaco Systems Stoddard, WIS
Your reader was correct. The functional uses of the two detector types were inadvertently switched
A.J. Sevast , Set Consultant RJA Group Greenwood Village, COLO.
I enjoyed the July 2008 issue of Consulting-Specifying Engineer, on page 52, there is a list of “Additional Reading” for grounding and bonding. Widely considered the most definitive work on the topic was omitted. “The Soares Book on Grounding and Bonding” by the International Assn. of Electrical Inspectors is used by inspectors and should be a reference on the bookshelf of every electrical engineer working with power.
Michael Beanland , PETriAxis Engineering Inc.Vancouver, Wash
Send your letters to Michael Ivanovich, editor-in-chief, Consulting-Specifying Engineer , 2000 Clearwater Drive, Oak Brook, IL 60523, or via e-mail email@example.com.
Letters should be no longer than 200 words, and may be edited for space, style, spelling, and grammar.