FACTORS TO CONSIDER WHEN DETERMINING MAINTENANCE INTERVALS James R. White Senior Member IEEE Shermco Industries, Inc.2425 E. Pioneer Drive Irving, Texas 75061
[email protected]
Abstract – This paper reviews factors that should be considered when determining maintenance intervals for overcurrent protective devices and switchgear. Performing maintenance on a regularly scheduled basis is good, but how do you determine those intervals? This paper provides some guidance on that question.
Ron Widup Member IEEE Shermco Industries, Inc.2425 E. Pioneer Drive Irving, Texas 75061
[email protected]
Maintenance frequencies for electrical equipment are in Annex K, “Long Term Maintenance Guidelines.” Fig. 1 is excerpted from the 2013 edition of NFPA 70B as an example.
Index Terms – Preventive Maintenance (PM), maintenance intervals, ANSI/NETA MTS-2011, NFPA 70B. I. INTRODUCTION Electrical power systems that are properly maintained are safer to operate, suffer fewer unscheduled outages and operate more efficiently. The 2012 edition of NFPA 70E(1) has requirements for the testing and maintenance of electrical power systems and overcurrent protective devices (OCPD): 205.3 General Maintenance Requirements. Electrical equipment shall be maintained in accordance with manufacturers’ instructions or industry consensus standards to reduce the risk of failure and the subsequent exposure of employees to electrical hazards. 205.4 Overcurrent Protective Devices. Overcurrent protective devices shall be maintained in accordance with the manufacturers’ instructions or industry consensus standards. Maintenance, tests, and inspections shall be documented.
Fig. 1 Annex K from NFPA 70B, Partial Annex L, “Maintenance Intervals” has guidelines for electrical equipment that can be shut down more frequently. Fig. 2 is an example of the information in Annex L.
Some direction is provided as to which industry consensus standards are to be utilized in section 200.1 (NFPA 70E), “Informational Note: Refer to NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, and ANSI/NETA MTS-2011, Standard for Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems, for guidance on maintenance frequency, methods, and tests.” This paper discusses these two standards and their benefits. Note that a company’s in-house experience may be the best guide for determining maintenance frequency. II. NFPA 70B (2) NFPA 70B is like the book, “Everything You Wanted to Know About … But Were Afraid to Ask”. In this case, it’s everything about preventive and predictive maintenance. Maintenance philosophies, maintenance recommendations, personnel safety, system studies and power quality are just a few of the 31 chapters, plus annexes. There is a chapter on virtually every piece of equipment in the electrical system.
Fig. 2 Annex L from NFPA 70B, Partial NFPA 70B is a very complete guide for setting up and administering a PM program, as well as Reliability Centered Maintenance (RCM), forms, diagrams and even information on conducting a walk-through inspection. Every maintenance manager, engineer and supervisor should have a copy in their office.
II. ANSI/NETA MTS-2011 ANSI/NETA MTS-2011(3) differs from NFPA 70B, in that it doesn’t cover anything but what to do and what the results should be. ANSI/NETA MTS-2011 (the current edition) contains no maintenance philosophies and provides no guidance on differing types of maintenance programs. ANSI/NETA MTS provides a listing of virtually every major piece of electrical equipment in a power system, what visual/mechanical inspections and electrical tests should be performed and what the results should be. The chapter marked “Tables” contains several tables with recommended test values covering all types of electrical tests, while Annex B covers “Frequency of Maintenance Tests”. This section provides guidance on how often electrical equipment should be maintained based on two factors; equipment condition and its criticality. Using molded-case circuit breakers as an example, Fig. 3 shows the Maintenance Frequency Matrix. This matrix takes the two factors and assigns a multiplier. If the equipment is in average condition and has a medium importance in the electrical power system, it would be assigned a multiplier of “1”. In Fig. 3 the circuit breaker involved has an average condition and a high need for reliability.
every six months and Visual & Mechanical & Electrical maintenance would be required every 18 months. This simplified concept works well when considering only those two factors, and certainly assists in determining maintenance frequency, but is that adequate? IV. OTHER FACTORS TO CONSIDER It would be very difficult to create a usable matrix that takes all needed factors into consideration. The two aforementioned guides should be used, but these other factors also need to be considered: A. Equipment Criticality and Device Significance B. Current Condition C. Lubrication Life Initial Installation D. Maintenance History E. Operational History F. Industry Experience Maintenance Philosophy G. Operating Environment H. Time Allowed for Maintenance Manufacturer’s Recommendations Having an understanding of these eleven factors, and using them as critical decision points, can greatly enhance your ability to accurately judge the frequency at which you maintain your equipment. These eleven factors are discussed in detail below. A. Equipment Criticality and Device Significance
EQUIPMENT RELIABILITY REQUIREMENT
Fig. 3 ANSI/NETA MTS-2011 Maintenance Frequency Matrix Example The multiplier derived from Fig. 3 is used with the second table in Annex B, which shows the frequency of maintenance tests in months. In our molded-case circuit breaker example, the maintenance requirements are shown in Fig. 4.
Fig. 4 ANSI/NETA MTS-2011 Frequency of Maintenance Tests for Molded-Case Circuit Breakers The 0.5 multiplier from the Maintenance Frequency Matrix will effectively cut the indicated frequency in half, so the Visual & Mechanical maintenance would now be required
How important is the device under consideration? The importance of the device and its relevance to your operation weigh heavily on factoring the type, frequency, and magnitude of the testing. Questions to ask about the device significance would be: o How important is this to my operations? o Does it have safety significance? o What is the economic impact of the device? o Will this affect other areas of the operation? How critical a device is to your operation should play a significant role in the overall assessment of maintenance needs. This factor should be placed high on the relevance scale. As an example, a circuit breaker protecting the main power transformer for the facility would be extremely critical, as operations would come to a standstill if it were lost. A circuit breaker protecting a water fountain would not have that same criticality, and would not require the same level of maintenance and attention. That being said, don’t judge criticality based only on the cost of the protective device. The following true event illustrates this point: A company installed a new 13.8kV transformer, switchgear and battery bank to support expanded production. The manufacturer of the equipment provided the acceptance testing in accordance with the purchasing company’s specifications mutually agreed to in their contract. The molded-case circuit breakers in the outdoor substation were not included in the acceptance testing specifications, as it
would cost more to test them than it would to be to replace them. A few months later one of the underground feeder cables failed, sending fault current through the transformer and reactor, destroying both. The brand new 13.8kV switchgear failed to operate and an upstream main circuit breaker had to clear the fault, causing a plant-wide outage. The problem was traced back to a $100 100A two-pole molded-case circuit breaker that when tested, tripped in 70 seconds with only 45A of current flowing through it. This circuit breaker fed the battery bank that supplied the dc tripping power for the 13.8kV protective system. With no dc power available, the circuit breakers were unable to trip. The situation could have been caught before the fault and corrected, but since it was a new installation and just tested, no one at the company bothered to check the status of the battery bank. When the circuit breaker feeding the battery bank tripped, the batteries were depleted and could not be recharged. Figs. 5, 6 and 7 illustrate the equipment involved and the associated damage. A simple low-voltage alarm would have prevented this from occurring, but are often not installed for substation battery banks, such as this one.
Fig. 7 Transformer (Foreground) and Reactor Destroyed by Through-Fault Current Moral – It’s just not the cost of the testing that is important, but also the criticality of the equipment to be tested. Critical devices must be tested on a regular basis, even if it exceeds the replacement cost. This failure cost over $5,200,000, not to mention legal fees. B. Current Condition What condition the equipment is currently in has obvious implications as to both the frequency at which it should be tested as well as its remaining useful life. Electrical equipment begins to deteriorate from the time of manufacture until being removed from service. The current condition of that equipment, determined through as-found testing and evaluation, provides critical data points to evaluate.
Fig. 5 The Offending Circuit Breaker
Fig. 6 The Circuit Breaker Fed this Battery Bank, Which Lost all Power
For example, if you have a situation where an electric motor shows signs of decreasing insulation resistance, this current condition factor of declining insulation health would have a greater impact to your maintenance decision process than some of the other listed factors that would weigh in as less relevant. Another, related factor is the rate of decline. If a sudden decrease in insulation resistance (or decrease in insulation power factor) is recorded during maintenance testing, that device may be in danger of imminent failure. Even though the test value is satisfactory the rate of decline may be such that the device fails before the next scheduled test interval. In this next example, a maintenance assessment was initiated at a facility, Fig. 8 illustrates the as-found condition of a 13.8 kV substation that was in a less than desirable state. There were obvious environmental problems as well as the presence of tracking across the insulation system. The bus insulation’s current condition would factor in to the overall needs assessment of the testing frequency as well as the overall expected service life.
old 2500kVA transformer blew the terminal chamber cover through a double-thickness concrete block wall, ripping out 23 one-inch studs holding the cover on. The culprit turned out to be a bad termination made during the initial installation. Figs. 9 and 10 show examples of improper installation. Fig. 9 is 4.16kV unshielded power cable installed on CPTs. The cable began to deteriorate almost immediately and was close to failure when found. Fig. 10 is an installation of a 480-volt molded-case circuit breaker. Apparently there was an issue with nuisance tripping and the contractor found a fool-proof method of fixing it. Fig. 8 Tracking on Redboard Insulation C. Lubrication Life Lubrication, especially as it applies to power circuit breakers, is a huge consideration when determining overall (4) reliability and equipment health . What type of lubrication was used at the time of manufacture and what is the expected service life of the lubrication must be taken into account when determining maintenance frequencies. A grease cannot be expected to function properly 20 years after being applied in a factory, and therefore an electrical device cannot be expected to operate properly when called upon if the original grease specifications are not met. The greases used in the current path of circuit breakers will often last just three to five years before needing replacement. This is due to the heat generated by the current flow through the breaker 2 (I R losses) and is true even if the circuit breaker is rarely operated. In fact, circuit breakers that sit for extended periods of time without operating are more prone to lubrication issues than those that are operated frequently. Grease compatibility is another significant area of concern and is part of the overall lubrication assessment. Has the original grease been replaced, and if so was it replaced with grease that is compatible to the original? Also, has “technician in a can”, or spray-on penetrating fluids been used on critical mechanical and electrical components? The use of such spray penetrating lubricants actually speeds the deterioration of breakers. If there was any original lubricant remaining, it would be flushed out by the spray. Also, these spray lubricants will only last a few weeks at best, then its metal-to-metal wear.
Fig 9 Unshielded Cable Improperly Installed
Fig. 10 Dangerous Installation
Companies have to have some level of trust in the expertise and integrity of the contractors they hire to perform work. That may mean not using the cheapest guy that shows up with a pickup truck and tools. In all three of the illustrations worker safety was put at risk due to the lack of care and integrity of the contractor who installed the equipment. E. Maintenance History How well the equipment has been maintained can have farreaching effects on the overall health of the equipment. Just as a used car that “has all service records” is more desirable than one that does not; electrical equipment that has been properly maintained with a regular regimen of maintenance testing is a more desirable (and reliable) product. As stated in NFPA 70E section 205.4, “Maintenance, tests, and inspections shall be documented”. If a worker is injured while working on electrical equipment, its state of maintenance would probably be a matter of interest for any good attorney.
In addition to the electrically-operated devices that have lubrication concerns, mechanical components must be evaluated as well. Components such as cell interlocks, racking mechanisms, jack screws, bearings, hinges, etc. must all be evaluated for their lubrication effectiveness.
The specific maintenance that has been performed, what the lubrication practices were during that maintenance activity, the environment the maintenance activity was performed in, and the maintenance interval are all important points to consider when factoring in the maintenance history.
D. Initial Installation
F. Operational History
This is one factor that is frequently overlooked. It takes years to overcome someone’s carelessness when electrical power system equipment is initially installed. Usually the issue is most often seen at cable terminations, but can impact almost any electrical device. At one power plant a 35 year-
When evaluating electrical equipment, the severity of service a piece of equipment has been subjected to can be very important when making maintenance decisions. Items to consider when looking at operational history are:
o Number of maintenance work orders (problems) o Number of operations since the last maintenance activity o Duty cycle (heavy load or light load) o Number of faults seen by the device o Severity of faults seen by the device o Number of times the device has been racked in or out o Exposure to transients or switching surges Including these operational elements into your overall maintenance assessment will help to identify those components that are called upon to work harder. Those devices with above-listed factors will likely have a greater degree of “wear and tear” which ultimately affects overall reliability. G. Industry Experience A company’s experience with maintenance activities on equipment with similar design, age, lubrication, environment, maintenance, and operational history has invaluable benefit when assessing test frequency. Having experience with a particular type or model of equipment, including failure history, reliability, and overall performance can greatly increase one’s ability to make sound judgments on maintenance activities. For example, a 30-year old low-voltage power circuit breaker with series trip units cannot be relied upon to operate within the manufacturer’s specifications, and may not operate at all when needed. History has shown a high failure rate for series trip units, and experience with these types of circuit breakers can assist in making experience-based decisions about the device.
H. Maintenance Philosophy Do you work in a facility that has a proactive and thorough maintenance philosophy, or do you work in a facility that has a run to failure maintenance mentality? Lack of maintenance or poor maintenance practices affect the overall health and condition of electrical power equipment. Just as it is detrimental to personal health not to see a physician on a regular basis for a health checkup, electrical equipment requires much of the same probing analysis as to overall health and physical condition. Fig. 12 illustrates another problem. When it is 2013 and there is a piece of masking tape with the words “PM-ED 7-1199”, does this give you any indication as to the maintenance philosophy and likely skill set of the “PM’ers”? Is it possible that Homer Simpson runs an electrical maintenance company? It is almost certain that this piece of equipment has not had proper maintenance in more than ten years; well beyond any recommendations of the manufacturer or of industry. As an additional note (under the Industry Experience factor), is it acceptable industry practice to allow the 600-volt control wire on the transducer to hang down below the front of the medium-voltage circuit breaker – the circuit breaker that will be racked in to the cell and likely pull the 600-volt wires with it? That actually happened in this facility, seriously injuring two of their employees and causing this company to abandon a circuit breaker cubicle. Luckily they had a spare.
Another example is shown in Fig. 11. Would industry experience indicate certain things about the overall condition and needs of this particular device? Note the corrosion within the mechanism and obvious signs of neglect. Industry experience with this type of equipment will guide you in making smart and informed decisions on what to do next.
Fig. 12 Carelessness Shows Up In Many Difference Ways I. Environment One of the quickest ways to cause failure of electrical equipment is to place the equipment in a poor environment. Whether it is subjected to moisture, corrosive atmosphere, physical abuse, or general dirt and debris environment is one factor that has a high degree of relevance.
Fig. 11 Rust and Corrosion Render Circuit Breakers Unreliable
When environmental conditions come into play on frequency of test assessment, a poor environmental factor can weigh heavily on the assessment, especially with older equipment. Equipment in a positive-pressure filtered indoor environment will require much less maintenance than equipment located
outdoors, using in-door filters to catch debris. When was the last time those filters were actually changed out? J. Time Allowed for Maintenance The equipment is finally scheduled for maintenance – but how much time is allowed for the maintenance personnel to perform the critical maintenance activities? The amount of time made available for the equipment shutdown can greatly impact both the quality and the quantity of required maintenance. Outage schedules should be well thought out with regards to comprehensive maintenance tasks, especially if the equipment is critical to plant operations or has known issues affecting performance.
being least weighted and ten being highest weighted. What you end up with is a number that takes into account eleven factors associated with the device, and from that number you should be able to better establish a priority list of importance and relevance for maintenance testing and frequency. Use your best judgment and apply numerical values to these considerations while also using the ANSI/NETA Maintenance Testing Specifications, and your overall quest for power system reliability should become a better defined reality. VI. REFERENCES 1.
Often times it will be required to hire outside maintenance personnel to meet the time constraints of an outage and to bring in the required expertise for the equipment being tested and maintained, and it makes economic sense to use personnel specifically trained and certified for these types of activities to ensure the tasks proceed safely and effectively.
2.
3.
K. Manufacturer’s Recommendations Another important aspect to any equipment maintenance schedule is the manufacturer’s recommendations for service, even if the manufacturer is no longer in business. At the onset of the equipment’s life, the manufacturer most likely presented the owner with a plan for maintenance testing and frequency. While the manufacturer’s data does not take in to account the other factors presented in this paper, they do typically offer up a conservative guide to maintenance needs. Another advantage to the manufacturer’s guidance is the fact that they quite often point out special needs or requirements for equipment specific to that particular device; information that might otherwise not be available from any other source. Manufacturers also produce service advisories and product recalls. It is important to be aware of such advisories and recalls, as quite often they are linked to equipment performance and/or operation, most of which will affect the serviceability, or continued serviceability, of that particular device. V. SUMMARY Take these maintenance factors and apply a relevance number to each factor from one to eleven (or one to one hundred, or one to one thousand, etc.), with one being the least relevant and the highest number being most relevant. Score your most important factors the highest, and the lesser aspects at a lower numerical value. Then take the relevance factor and multiply it by a range, again with a weighted number from one to eleven, with one
4.
National Fire Protection Association, NFPA 70E, “Standard for Electrical Safety in the Workplace”, 2012 edition. National Fire Protection Association, NFPA 70B, “Recommended Practice for Electrical Equipment Maintenance”, 2013 edition American National Standards Institute and interNational Electrical Testing Association, ANSI/NETA MTS-2011, “Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems”, 2011 edition White, James R., “Safety Maintenance Requirements for Power Circuit Breakers”, 2012 PPIC Conference paper. VII. VITA
Ron Widup is president of Shermco Industries, Inc. located in Irving, Texas. Ron is a member of IEEE, is the Primary interNational Electrical Testing Association (NETA) representative on NFPA 70E®, the Alternate representative on the NFPA 70B committee, represents NETA on NEC Code Making Panels 5 and 11, is a member of the NETA Standards Review Council, Chairman of NETA’s Training Committee and is on the NETA Board of Directors. Ron is also a NETAcertified Level IV Senior Substation Technician. James White is the Training Director for Shermco Industries, Inc. located in Irving, Texas. He is a Senior member of the IEEE, the recipient of the 2011 IEEE/PCIC Electrical Safety Excellence Award, the 2008 IEEE Electrical Safety Workshop Chairman, Alternate interNational Electrical ® Testing Association (NETA) representative on NFPA 70E , ® Primary representative on NFPA 70B , is the NETA ® representative to ASTM F18 and represents NETA on NEC CMP-13. James is the recipient of the 2013 NETA Outstanding Achievement Award. James is also a certified Level IV Senior Substation Technician with NETA, an inspector member of IAEI and serves on the NETA Safety and Training Committees. James is the author of Electrical Safety, A Practical Guide to OSHA and NFPA 70E and Significant Changes to NFPA 70E – 2012 Edition both published by American Technical Publishers.
