A Practical Guide for Electrical ReliabilityOct 1, 2004 12:00 PM, By Wally Vahlstrom, Emerson Process Management 0 Comments Reliability analysis often only implies an assessment of design, but system operations and maintenance typically have a more significant effect on system reliability A short outage may not cause much trouble for a refrigerated storage facility, but unplanned shutdowns may cost millions of dollars and cause a drop in share price for data centers, microchip manufacturers, or e-commerce-related companies. In addition, performance appraisals and salaries of facility engineers and plant managers may be negatively influenced by the extent of downtime of their electrical systems. In fact, system reliability is often a critical performance measure for facility engineers and plant managers, yet many of them misunderstand important concepts about system reliability and how to improve it. These people, who get paid to know their facilities' systems inside and out, often have trouble answering questions like, What is the total cost of an outage at the facility?, Does the facility meet “six nines” availability criteria?, or What would a probabilistic risk assessment (PRA) of the electrical system tell about downtime? Photo 1. A high-rise was designed with this indoor rated bus duct placed in an area that was washed down periodically. This eventually led to a fault on the system. Also, terms like “N+2,” “MTBF,” “failure rate,” and “high nines of availability” are often misused or misunderstood. Although PRA techniques have been applied for many years and can now be applied using off-the-shelf software, the details of this technique can easily become overwhelming. To provide continuous operation under all foreseeable circumstances. Describing and detailing PRA would take a book's worth of pages to do justice. A listing of most commonly used reliability terms and equations. can be used to predict . including utility outages and equipment breakdown. design will have an effect on system availability and on the length of shutdowns when they do occur. Investigating the number of redundancies designed into the electrical system is one of the common analytical approaches. Table 1 provides a list of some of the terms and formulae used in analyzing reliability. you must design reliability into an electrical system (Photo 1). the predicted reliability and availability of your electrical system can be calculated. Performing a probabilistic risk assessment (PRA) is another way to look at system reliability. Even though system design is typically not the direct cause of equipment failure or system shutdown. If the system is designed with a normal path and two alternate paths (N+2 design). Using common reliability analysis tools. Using the typical failure rate for a given type of equipment and the mean time necessary to repair it. a failure could occur in a second path. If the system has been designed with multiple redundancies. and ideally. However. A system with one redundant path would be termed an N+1 design. allowing maintenance without system shutdown. IEEE Standard 493. depending on the number of redundancies built into system design. An N+1 or N+2 assessment of a system can reveal single points of failure within the system. Table 1. PRA looks at the probability of failure of each type of electrical power equipment and. it can allow for maintenance outages and can ride through equipment failures without resulting in an unplanned shutdown.Reliability through good design. Reliability through probabilistic risk assessment. This would allow for one of the paths to be de-energized for maintenance while the other is still energized. Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems (the Gold Book) does provide data and describe a process for assessing system performance based on PRA principals. It identifies the normal source (N) and any redundant circuits/sources or equipment that would provide alternate paths for electrical power to flow. the third path would supply power to the load without interruption. one path could be down for maintenance. if the data used for such a study isn't representative. even a five. Quantifying system reliability. Henley (ISBN: 0-7803-6017-6). Books are also available to help explain this concept. but it may not be very representative of failure rates and repair times for your facility. such as Probabilistic Risk Assessment and Management for Engineers and Scientists. Maintenance clearly affects reliability. Before performing a reliability analysis. Software is commercially available to perform PRA calculations on electrical distribution systems.availability. if such data is available. and annual downtime. it's necessary to first define the term “loss of power. Reliability through proper maintenance. you would need to modify this data by including more site-specific information or substituting better data. it's possible (by multiplying the probability of failure and cost of failure. the results will be unreliable. As such. While this may be an acceptable level of availability for many facilities. You can then use this information to evaluate return on investment (ROI). Table 3. intensive care units. number of failures per year. Some don't keep records of those interruptions shorter than five minutes. Table 3 shows the relationship between downtime and availability. or nine nines of availability. by Hiromitsu Kumamoto and Ernest J. you must understand and agree on the circumstances that qualify as a power failure. Note that six nines of availability represents an average annual downtime of more than 30 seconds.” Many utilities don't keep records of service interruptions shorter than one minute.S. If the cost of outages and estimated costs of various improvement projects are known. and other critical facilities that may expect seven. the results of such reliability studies can only predict performance based on available data. The IEEE 493 Standard provides data showing that failures increase when maintenance is deferred or done .or 10-second outage would qualify as loss of power. Listing of respective relationships between downtime and availability. And obviously. Table 2 (click here) provides a small sample of the type of data available from IEEE 493. Of course. and then subtracting that cost from the cost of each of the improvement projects) to compare the relative merits of the current system and each of the alternatives. 2nd Edition. eight. industrial plants over the past several years. Data provided by IEEE 493 is based on failure rates and repair time information gathered from U. To quantify system reliability. But for many critical facilities. it would be completely unacceptable for many data centers. “As soon as new equipment is installed. it's important to establish an ongoing program designed to maintain an acceptable level of reliability for the facility.poorly. Recommended Practice for Electrical Equipment Maintenance. Unchecked. Loose connections (Photo 2) or insulation damage may not show up until more equipment comes online and electrical loads increase.) Several good methods exist for establishing maintenance programs designed to maximize reliability: NETA Maintenance Testing Standard (MTS) recommendations National Fire Protection Association (NFPA) 70B Standard recommendations . In other cases. refer to the recommendations provided by the InterNational Electrical Testing Association (NETA) Acceptance Testing Standards (ATS). or other equipment can be “smoked” due to relatively small installation errors. Simple thermographic scanning of this panel would probably have detected the loose connection and prevented this electrical failure. Perfectly good switchgear. Photo 2. (See the comments below about trending of test data. according to NFPA Standard 70B. When normal acceptance and start-up testing isn't performed (usually to save a few dollars).” With this in mind. transformers. the results can be disastrous. Acceptance and start-up testing also provides valuable baseline or benchmark information that can be used later. Also. the failures don't occur until months after the facility has gone into operation and the warranties have expired. starting with effective system startup and acceptance testing. a process of normal deterioration begins. the deterioration process can cause malfunction or an electrical failure. You can greatly improve reliability of electrical systems and equipment through proper maintenance practices and procedures. To implement effective acceptance testing procedures. and even injuries. yet a surprisingly low number of facilities rigorously maintain these crucial documents in an accurate and up-to-date condition. temperature. Maintaining a spare parts inventory for emergencies requires the implementation of a program that identifies which equipment is critical and which spare parts are . It has been estimated that 70% to 80% of all unplanned shutdowns are due to human error. insulation resistance. meaning that only 20% to 30% of unplanned shutdowns are due to equipment malfunction or poor design. Reliability through proper operations. Many operational errors cause shutdowns but don’t result in electrical equipment failure. Such documents should include an up-to-date single-line diagram of the system and a list of emergency contact numbers. you should trend the results. A reliability centered maintenance (RCM) assessment. which rigorously reviews critical system and equipment failure effects and establishes appropriate condition assessment tasks and maintenance activities for facilities or systems where reliability is critical To get the full benefit of condition assessment and maintenance testing. and other indicators will warn of deterioration and often provides an opportunity for a planned shutdown for correction of the problem before failure. While the relationship between quality of maintenance and resulting system reliability may be clear. equipment failures. Trending contact resistance. Recognizing that equipment may fail or human error may occur. An extensive shutdown and expensive failure resulted. Out-of-date or nonexistent drawings and instruction manuals can result in unnecessary shutdowns. it's important that you have documents and procedures in place to quickly enable recovery actions and minimize the length of the shutdown. This area of human interaction and its effect on the electrical system is considered the main source of unavailability (Photo 3). Photo 3. The condition and availability of facility records also influences reliability. the effect on reliability due to operations and other actions of personnel may be less obvious. This wasn’t the case when a worker shorted two phases while working on an energized switchboard. Having at least a minimal number of spare parts for critical components is also essential to system availability. Procedures at many data centers and other similar “critical facilities” or facilities with “critical environments” require very detailed work procedures or scripts. To effectively examine the overall picture. Table 4 (click here) provides a basic checklist that can help identify areas that need to be evaluated. and approved by all the appropriate stakeholders. and maintenance. When considering the implications of reliability. you must remember that reliability analysis should examine all three pillars of system reliability: design. information technology. Calif. including utility outages and equipment breakdown. . You must design reliability into an electrical system to provide continuous operation under all foreseeable circumstances. and operations and procurement. before any physical work begins. Such a program should also involve periodic condition assessment of each of these spare parts and regular updating of the inventory. maintenance. Analyzing the number of redundancies designed into the electrical system and conducting a PRA are two methods of looking at system reliability. Emerson Process Management. technical services. This checklist serves as a basic blueprint to identify areas in need of evaluation. try using an electrical operations and maintenance checklist like the one shown in Table 4.needed for emergency conditions. One step ahead. These scripts must be written and then reviewed. Personnel training and detailed procedures for operation are essential. construction. Electrical Reliability Services. The step-by-step work procedures must be followed without exception. In a typical facility. operations usually have a larger effect on system reliability than maintenance or system design. Vahlstrom is director. including engineering. and when used in an actual working environment it should be written and reviewed by all appropriate personnel in the facility. San Ramon. operations. revised if necessary.
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