Letters: Reader Feedback
Editor's note: From August through October we ran a three-part analysis of the 2005 revision of the National Electrical Code in our Codes and Standards column. In its conclusion, author Keith Lane made some observations on fused distribution. In the December column, reader and fellow author George Farrell expressed his opinions on the subject in a rebuttal.
In reading George Farrell's response to your series on the National Electrical Code, "The Debate Over Circuit Breakers vs. Fused Solutions Continues" ( CSE 12/05, p.31 ), I was not surprised to see the market tug of war between fuse and breaker vendors continue, as it has been going on for many years. By and large, there is some element of technical truth on both sides as they write articles or papers or product literature. But I was surprised, appalled even, by what I consider to be outright fabrications. Some specifics:
"For any facility requiring an 800-amp circuit breaker... it is reasonable ... to assume 2.5% impedance..." This is blatantly untrue. In over thousands and thousands of designs I've reviewed for architectural/engineering firms, I have never come across such a specified rating for impedance for a 3-phase transformer of this size. ANSI standards require 5.75% over 500 kVa—period. I can only guess Mr. Farrell's motive is to make fault currents appear much larger than they are for that size transformer since fuses protect very well at very high currents.
"They may actually take up less space than CB switchgear..." I have laid out thousands of fusible and breaker type switchboards. Fused switch switchboards never take less space than molded case type.
"The use of Class J fuses... may reduce ...equipment and size." For the major switchboard manufacturers, the space a fusible switch takes up in panelboards or switchboards is dependent on the switch amp rating, not the fuse clips. They do not reduce space.
In scanning the rest of the article, I continued to come across distortions and outright fantasy. In honesty, I couldn't read in detail past the opening. But the danger is that A/Es who may not know these untruths can be duped into believing them. Your magazine is probably the best in my industry—power system design in facilities—and I ask you to keep up your normally great work.
BOB GUSTIN, P.E., EATON ELECTRICAL, NOVI, MICH.
NEC help needed
I have enjoyed your recent analysis of the NEC, but I have a question about the code that was not addressed, specifically Article 230.71 and the maximum number of disconnects allowed.
I'm presently working on a project involving electrical metering of a commerical development involving 10 meters. It's my contention that electrical metering equipment, when developed as part of a "meter pack"—with each meter located in close proximity to another and each protected by a circuit breaker—do not fall under the six-disconnecting device rule.
The code states that "disconnecting means used solely for power monitoring equipment ... installed as part of the listed equipment, shall not be considered a service-disconnecting means."
Does power metering equipment fall under the classification of power monitoring? Requiring a main disconnect device for electrical metering equipment seems a waste of money. Any advice from your experts or readership would be appreciated.
WILLIAM BUCKNER, P.E., BUCKNER ENGINEERING, BARTLESVILLE, OKLA.
Not sold on DCV
I recently read the report "Title 24 Gives DCV Some Validation" ( CSE 11/05, p. 11 ). I'm concerned, from a purely design standpoint, that this article presents a somewhat skewed analysis of the use of demand-control ventilation (DCV) as related to Title 24 Section 121 (c), Operation and Control Requirement for Minimum Quantities of Outdoor Air . Readers could easily come to several inaccurate conclusions, although the essential content of the article is correct.
While valid techniques exist to relate indoor CO 2 concentrations to building ventilation—e.g., ASTM Standard D6245-98, Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation —the calculated energy savings available is primarily determined by subjective and unrealistic assumptions. The actual application of CO 2 -based DCV is not as effortless as suggested in the news report. What subchapter 3 Section 121 does say is that occupied spaces "...shall be ventilated with a mechanical system capable of providing an outdoor air rate no less than that total required." This means that the outside air should be designed for the worst possible case or be directly measured and controlled sufficiently to prevent any factor from forcing the total below the minimum requirements for compliance. When energy considerations are included, operation assuming full occupancy at all times is not first on our list of options. It is apparent that any dynamic system will require some sort of dynamic control and response for optimal energy usage. This is the reason CO 2 -based DCV initially gained attention. It was an attempt to respond to dynamically changing occupancy levels.
While true, recent changes in Title 24 do allow CO 2 -based DCV; it is only in very specific situations, i.e., intermittently occupied, single-zone HVAC systems, with an operational economizer section and having an occupant density greater than or equal to 25 people per 1,000 sq. ft. In fact, there are only a few exceptions, and it does not infer or endorse CO 2 -based DCV as an acceptable procedure for general use. The article implies that CO 2 -based DCV is adequate for other applications in saying, "As awareness of the benefits of DCV grows, so will its use." Here, I think you are advocating a risky position.
CO 2 -based DCV has a number of performance characteristics and limitations that must be carefully considered before it can effectively be applied in any HVAC system:
The basis for CO 2 -based DCV on concentration balance formulas has not been shown to reliably indicate total occupancy for a range of conditions. It tends to overventilate in most models and can never insure that underventilation does not occur.
Most codes make no provision for an initial operational verification of performance, which could provide some level of security in the implementation of the design. Regardless, the risk of nonperformance is always with the owner/operator and designer.
Nonperformance may not be detected until years after installation, the damage from which may not be readily detected until it is too late to correct cost-effectively.
The inability of CO 2 -based DCV measurements to economically or effectively provide ventilation rates that relate to the dilution of building-generated contaminants. Typically, most proposed DVC schemes are so overventilated that the energy saving aspect of the method are negated.
If occupancy can be reasonably predetermined, as in daily classroom usage, time schedule operation is usually more cost-effective and infinitely more healthful due to the significant time lag required for CO 2 -based DCV to respond to changes in occupancy.
The questions surrounding the basic accuracy, precision and stability of CO 2 sensors to perform as theory proposes has persisted. Overnight reset is not so much a product feature as it is an attempt to overcome the significant potential for drift.
CO 2 appears to stratify significantly and doesn't disperse quickly or evenly. This calls into question the use of single sensor(s) in a space or in a return duct and sensor placement requirements in general. This is based on undocumented observations in our test facility and during testing for ASHRAE Guideline 16P - Selecting Outdoor, Return and Relief Dampers for Air-side Economizer Systems.
I don't want to be construed as critical of CO 2 sensors. I'm sure the products have been improved over the years, and there are many valid applications for their use. Title 24 subsection F tries to address sensor performance verification, but there are inherent limitations to the technology, the proposed methods of control and the general assumptions of CO 2 generation.
It's no secret that there can be a wide discrepancy in CO 2 production in humans, which leads to huge variations in the calculated dynamic intake rates. Add to that the effects of the overall ventilation system operation, and CO 2 measurements should be considered, at best, suspect. Also, to effectively use a single CO 2 measurement as a representation of the actual occupant totals, the space needs to reach a time-related "steady-state" before the calculations have any real meaning. The time required can vary from 12 hours to three or less, depending on the air change rates.
Another serious issue is positive building pressure flow. By not maintaining proper positive building pressure control, there can be a number of issues for a CO 2 -based system. First, there is no relationship between CO 2 and intake rates. Therefore, CO 2 -based DCV systems are inherently prone to pressure instability, without providing any indication that control has been lost. Secondly, by allowing a building to have a negative pressure flow, there is always the potential for a variety of building envelope problems ranging from mold growth to compromise of the actual building structure.
Lastly, a typical building has two primary contaminant sources that can lead to "unsatisfactory" indoor air quality, including the building itself, which can contribute more than 50% of the pollutants. A second source is body odor produced by occupants. Since a CO 2 -based DCV system suggests there are energy dollars to be saved by reducing the outside air rate, in reality we are only talking about potential savings for the occupancy component of the dilution intake rate—roughly 50% of the total outside air rate depending on the building. The dilution rates specified in ventilation codes may reference "cfm per person" as a convenient method of determining total requirements for the space, but the ventilation tables were determined as the "minimums" needed to dilute pollutants from both primary sources.
If I were asked to advise an owner who wanted to enhance their HVAC energy efficiency, I would advocate measuring airflows using a quality airflow measurement product, in both the supply and return air, plus directly measuring the outside air. By measuring primary air system variables continuously, first, you're going to keep the air-handling system within it's design performance envelope, and second, make it pressure-independent, dynamically allowing the equipment to compensate for internal and external changes. By measuring the supply and return fans, the owners will know exactly how much fan HP vs. energy they're using all the time. I don't need to remind readers that the fan energy used is the cube of the fan horsepower. From that point, the supply fan performance and energy use can be minimized by running the system at a minimum performance duct pressure set point, tracking the return fan to make sure the building always has a positive pressure flow, and knowing exactly when to change loading filters by comparing supply fan performance to the VFD speed. Dynamically bringing in only the outside air that the building needs to maintain both indoor air quality and a net positive building pressure, one would also save energy.
If all of these performance benchmarks were met, only then might it be worthwhile to consider DCV for unpredictably variable and intermittent occupancy spaces.
DAVID W. DAY, EBTRON, INC., LAKE OSWEGO, ORE.
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