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April 22, 2008 A Penton Media Publication Vol. IV No. 8

Pareto Chart Basics

Motor Maintenance Tip, Part 11

Electrical Troubleshooting Quiz

Modify to Speed Repairs

Clear Up Current-Limiting Con-fuse-ion

NEC in the Facility


Answer to Electrical Troubleshooting Quiz

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This twice-a-month
e-newsletter is brought to you from the publisher of EC&M magazine.

MRO Insider addresses topics such as:

  • Working with management and supervision
  • National Electrical Code® on the production floor
  • Safety procedures and programs
  • Troubleshooting techniques
  • Equipment maintenance and testing tips
  • Managing motors and generators
  • Trends in training and education
  • Managing energy use

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    Pareto Chart Basics
    Pareto's 80/20 rule means 80% of your results come from 20% of your efforts. The trick in making this work to your advantage is determining which 20% to do. A Pareto analysis will tell you this. A Pareto chart will show you this.

    Creating a Pareto chart is easy. For example, suppose you want to show the top three downtime causes in terms of minutes lost:

    • From your CMMS, generate a report of downtime minutes per cause over the past six months.
    • Import the numbers to a spreadsheet.
    • Sort the causes by the number of minutes in decreasing order from left to right.
    • Choose a simple bar chart from the selections offered, and let your spreadsheet create a chart.
    • You now have a Pareto chart. The three bars on the left are your top three downtime causes (in terms of minutes).
    • Too much information defeats the point of doing this, so use minimal tags and titles.
    This is especially useful if you can do it by "revenue lost" or some other cost factor, rather than just minutes. Consider limiting your data set to the most critical equipment.

    Motor Maintenance Tip, Part 11
    Avoid short cycling. The inrush current required to start a motor is normally about five to six times the running current. Inrush current means extra heat, but motors dissipate it as they run. If you repeatedly start the motor without sufficient time for heat dissipation between starts, you overheat the motor. If you aren't sure how long to wait between starts, contact the manufacturer.

    Large motors usually have restart information on their nameplates. You may find two starting frequencies for the same motor (usually in a 3:1 ratio). If so, use the one that requires the greater time (or fewer starts per hour). The only time to use the other one (fewer minutes or more starts per hour) is if you ran the motor continuously since its last starting.

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    Electrical Troubleshooting Quiz
    Sometimes, you have to troubleshoot your methods, not just the equipment. Let's illustrate with an example. Since obtaining an infrared camera six months ago, you've stepped up your PdM program. The reduction in downtime has delighted the production managers. Even the division VP has taken note.

    However, opening the various covers takes more labor hours than you have available, primarily because of the NFPA 70E issues. Cutting back on the PdM work and then explaining the increase in downtime doesn't strike you as a particularly brilliant career move. Despite your success, there's no additional budget for more overtime or more people.

    To compound the problem, people are either dropping cover screws and not replacing them, or deliberately leaving them out to save time.

    One manufacturer told you that replacing the screws with flip-catches would cause loss of the testing lab approval, violating NEC requirements in 90.7, 110.2, and 110.3. You decide that making your plant uninsurable is probably not a good idea.

    How can you get this highly beneficial PdM work done in less time while maintaining enclosure integrity?

    The answer to this question appears at the end of this newsletter.

    Modify to Speed Repairs
    A textbook example of production process improvement is the practice of replacing the screws in a jig (assembly used for holding a part) with flip levers. Rather than turn a screw 16 times, you flip a lever once. This is a huge time-saver.

    Some of the access panels you encounter during a repair use screws to hold their covers in place. Replacing these with flip levers can reduce downtime and possibly improve the integrity of the enclosure. Examine equipment to see where doing this might make sense.

    Note that arc blast and other considerations rule out doing this on power distribution equipment. In fact, you should inspect covers on distribution panels for missing screws. If a cover has 18 screw holes, leaving in only six screws will save time during repair or maintenance. However, that cover could become a deadly missile during a fault, instead of serving to protect people and property from the blast.

    Clear Up Current-Limiting Con-fuse-ion
    A semiconductor fuse is not a special fuse made of silicon. It's a very fast-acting fuse designed to protect power semiconductors.

    NEC in the Facility
    The maximum rating of an overcurrent protective device (OCPD) for a motor depends on the type of OCPD and the type of motor. Taking these variables into account, Table 430.52 provides four columns of multipliers to use on the Full Load Current (FLC) so you arrive at the correct maximum rating.

    When safety may be in doubt, do you voice your concerns? If so, great. But are you looking for someone to tell you it's OK, or are you trying to ensure you've identified the possible safety problems? Don't accept an answer just because it makes things easier or conforms to your earlier idea of what conditions probably are. Accept the explanation only if it comes with verification.

    A good answer to a safety question involves proving that something is safe. And that begins with asking the right question. "Did you lock it out?" is not the right question. Instead, ask, "Can you walk me through the system, and show me how you identified all possible energy sources?"

    Quiz Answer
    Answer to Electrical Troubleshooting Quiz
    You know safety can't be compromised, and you don't want to be the one responsible for increasing downtime. It appears, however, that you must make a trade-off between job efficiency, safety, and uptime. Fortunately, things aren't always as they appear.

    One solution is to reduce the scope of your department's responsibilities. For example, outsource certain tests. Or, outsource some of the construction work that maintenance gets saddled with. But expenditure approvals and other obstacles might be in the way.

    Now, revisit the fastener issue. Would replacing those slotted head screws with Phillips-head or Torx screws void the equipment listing or approval? Probably not, but run this by the manufacturer to be sure. Then, invest in the industrial-grade, battery-powered screwdrivers with industrial bits. Instead of twisting each screw by hand, you can zip them in and out almost instantly.

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