Letters: Reader Feedback
Turn off the lights! Regarding your Viewpoint on the challenges of LEED and sustainable design (CSE 12/04 p. 5), a question your readers should be asking is, How do we broaden the green building concept to airports? The good news, is it's been done. Where? At Salt Lake City International Airport, where my colleagues and I implemented a system that saves more than $100,000 annually.
Turn off the lights!
Regarding your Viewpoint on the challenges of LEED and sustainable design ( CSE 12/04 p. 5) , a question your readers should be asking is, How do we broaden the green building concept to airports? The good news, is it's been done.
Where? At Salt Lake City International Airport, where my colleagues and I implemented a system that saves more than $100,000 annually.
How? By simply turning off the unneeded airside lighting late at night. In this case the airport had two parallel runways, where after peak hours, only one was operated.
Think of the savings if such a plan were installed nationwide, or even internationally! There are 125 U.S. airports with parallel runway lighting systems, most of which remain on all night under control-tower supervision.
Yet despite success in Salt Lake this plan was rejected in California and Florida. FAA tower management remains uneducated on cost and pollution savings potential and even refuses to direct operators to learn and manage existing Surface Movement Guidance and Control systems.
So what can be done? The FAA must be the directive source. So how do we change their mind? Perhaps we need to lobby President Bush; I've already written to [Energy] Secretary Sam] Bodman. But based on history, they'd probably ignore even the president. So I ask the readers of CSE for suggestions on breaking this logjam. Because if we just turn off unfunded airside lighting, we can do a lot to make airports green.
Charles Fehner, P.E., Fehner and Assocs.
Editor's note. Mr. Fehner expounds on this idea in Specifier's Notebook, p. 54.
More breaker bits
The December Letters had two comments on breaker selective coordination to which I would like to offer some additional input. I agree that in order to have an electrical distribution system that possesses functional operating integrity, a fully rated and selective coordinated system is required. Proper selection and specification based on system study is part of this process and is a necessity for any engineered power system.
Where I have some difficulty in the discussion is that most coordination software and manufacturer's literature does not clearly point out some of the operating characteristics of molded and insulated case breakers. All of these devices have an instantaneous trip threshold point (override). Regardless of whether the engineer's specification calls for no upstream instantaneous trip function or asks for zone-interlock systems, the breakers have to open to self-protect against damaging peak fault levels if this threshold is reached. UL listing for interrupting rating is based on this threshold-opening ability. There is no trip lockout when threshold current is reached. Normally, this threshold point starts at the eight-times frame size, and can even go to 18 times or greater at a +/-20% trip tolerance in very high-cost insulated case breakers. This means the breaker's self-protecting instantaneous trip override function—an integral present mechanism—will sense the ramp-up of the first peak cycle of a fault above the threshold level, and begin to open.
I would also like to offer that specifying time delay beyond three to six cycles could place electrical system components at risk of damage, since the UL testing on most of these devices does not exceed this exposure time duration to the fault. Metal-clad switchgear and bus duct do have reduced fault ratings for longer fault durations. In addition to the possible voiding of the UL listing on electrical components to be protected, using the circuit-breaker short-time-delay trip function for intentional time delay can greatly increase the arc flash hazard for electrical personnel working on or near the equipment. This increases not only worker exposure to higher arc energy hazards, but adds to equipment downtime and lost production cost as well as possible litigation.
The design engineer has tools to overcome many functional traits of circuit breakers in his application of overcurrent protective devices and avoid NEC violations of equipment damage due to time delay stressing electrical components beyond their capacity. Motors, transformers, motor controllers, lighting panels and critical feeders should be protected against damaging fault levels through limiting current in the first cycle of exposure, and isolating them from the rest of the feeder circuits so that true selective coordination can be obtained. This is easily accomplished with overcurrent protective devices that can operate in the first half cycle. Consideration should be given to applying current limiting overcurrent protective devices at the electrical component to be protected. Proper selection of these devices will insure component protection and fault isolation (selective coordination). It should also be noted that a high percentage of electrical failures occur at the controller, motor, transformer and panel level of the distribution system, not at upstream mains and feeders.
Terry L. Macalady, P.E.
November Specifier's Notebook author Keith Lane responds:
I appreciate the fact that my short article on series-rated breakers created so much interest. The intent of the article was to illustrate the differences between series- and fully rated breakers for small- to medium-sized, general-purpose electrical distribution systems, not large critical facilities. The example of a 400-amp main breaker and 100-amp feeder breaker was to illustrate a small electrical distribution system.
At Sasco, we have designed and built many large critical facilities where fully rated and fully selectable systems are required. In these facilities we have utilized breakers that do not have the instantaneous trip function to allow the system to be fully selectively coordinated. The expense of the large electronic breakers with these settings is prudent in large critical systems.
It must be understood that the short time delay from not having an instantaneous trip can allow fault currents to flow for several cycles, which can subject the electrical system to high mechanical and thermal stresses. The system equipment must also be coordinated to ensure it can withstand this tripping time delay. In addition, if the breaker utilizes an instantaneous override function for high fault current levels, selective coordination can be lost. For these reasons, as well as the fact that many small breakers do not have the option of no instantaneous trip setting, breakers without instantaneous tripping are more suited for larger systems.
I do agree that facilities that require a high degree of site availability should have full overcurrent protective device selectability. As always, it is the engineer's job to present the cost and benefit analysis of a fully selectable system compared to a system that is not fully selectable.
That being said, I probably should have noted the exceptions that would allow for circuit breakers to be fully selectable.