Consulting-Specifying Engineer - August 2008 - (Page 34) M/E Roundtable not known how that equipment will perform in a failure mode. Similarly, if system level testing is not done in a rigorous manner with detailed up-front planning, what happens during an incident will be unpredictable, possibly causing a major failure. Finally, debugging the interdependencies between the power, cooling, and control systems during integrated systems testing is really the culmination of all the efforts in making the facility as resilient and robust as it was meant to be. RENER: A very important role, perhaps the single most cost-effective part in improving reliability in today’s mission critical facilities. The two really are separate but connected services. Acceptance testing using InterNational Electrical Testing Assn. standards and qualified technicians ensures that the various components in an electrical system are independently and rigorously tested in the field. Commissioning is a systems process using National Environmental Balancing Bureau standards and independent thirdparty professionals to review everything— from design, to components, to installation, and testing—to ensure that the entire electrical and mechanical system operates exactly the way it was intended. These services may only add 1% to 2% of upfront costs, but result in significant reductions in change orders, time lost, operational problems, and outages. Commissioning also can include gathering and providing a substantial amount of documentation on the equipment and operating sequences for the facility, especially the HVAC and electrical system. This makes it much easier for the on-site O&M staff to better understand the systems and respond more effectively in the event of problems. COTTULI: It is incredibly important to conduct a full commissioning in as close to a real operating condition as possible. Commissioning comes as the last item in a long, expensive journey as it faces pressures to shorten and expedite. This is a critical mistake. The commission phase needs to be one of the most comprehensive to ensure operability of equipment, familiarization with operating personnel, and final acceptance of a long expense capital expenditure project. In other words, did you get what you paid for from your vendor? SPEARS: Commissioning and acceptance testing are extremely important to ensuring the successful operation during a powering disturbance. All too often, due to cost, logistical complexities, availability of test equipment, or just plain negligence, this testing is not done comprehensively, or not done completely. This presents a significant risk for the end user, who may not be aware that mission a critical power system has not been tested for every contingency. The participation of experts, whether they be consultants or vendors, should be a part of every installation. The critical equipment should not be turned over until this testing has been completed and reviewed, with all outstanding issues addressed. WALTER: Commissioning and acceptance testing are crucial in validating the overall design; ensuring that various pieces of equipment, as installed, function as intended; and ensuring that the equipment and subsystems integrate properly to accomplish the desired functionality. These activities span everything from the obvious, such as breaker coordination and correct wiring of control interfaces, to more subtle issues of timing, threshold settings, and dynamic response to transient conditions. Documented test results also provide a benchmark against which future performance may be compared to highlight potential problems that may develop before they increase risk to the critical mission. Ask the experts: overcurrent devices Every month, Consulting-Specifying Engineer editors ask a distinguished panel of experts for information about how to best solve your problems, challenges, and new engineering issues. At www.csemag.com/asktheexperts, CSE gives its readers and Web visitors the opportunity to pose questions directly to the panelists. Below is a question for August’s topic, specifically about NEC 700.27 overcurrent devices. “ How does NEC article 700.27: overcurrent devices affect the selectively coordinated system using circuit breakers? Is it possible to use circuit breakers in a selectively coordinated system under NEC article 700.27? ” —Donald Zasada, Construction Official—Electrical, New Jersey Dept. of Community Affairs, Trenton, N.J. BRIAN RENER: It depends. There are a wide variety of circuit breaker types. Specifically, many engineers overlook selecting breakers with various adjustable tip features. Advanced features such as zone interlocking also are available on larger circuit breakers. Further, how a system is designed can help or hinder the coordination processes. If properly designed and specified, circuit breakers are an excellent choice for a selectively coordinated system. Another factor is selective coordination of ground fault protection. NEC requires ground fault protection for 277 V/480 V circuits 1000 amps and larger. There are many instances where a ground fault on a branch circuit has resulted in tripping the 1,000 amp feeder, thus affecting a significant portion of a facility rather than isolating the ground fault closer to the fault. It may be necessary to apply ground fault protection to lower amperage feeders and branch circuits to provide effective selectivity for ground faults. 34 Consulting-Specifying Engineer • AUGUST 2008 http://www.csemag.com/asktheexperts
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.