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October 7, 2008 A Penton Media Publication Vol. IV No. 19


CONTENTS
Maintaining Low-Voltage Circuit Breakers

Two Tests for Every Breaker

Coordination

Electrical Troubleshooting Quiz

Simple Test Prevents Re-Repair

Fuses and Transformers

NEC in the Facility

Why You Should Report Any Injury, Part 3

Answer to Electrical Troubleshooting Quiz


About This Newsletter
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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    Maintenance
    Maintaining Low-Voltage Circuit Breakers
    In general, you can choose from one of three strategies when servicing breakers.
    1. Corrective-only. This is the cheapest strategy. However, it’s also the most costly, because (in defiance of logic) it relies on failure instead of preventing it. When a breaker fails open, production stops. That’s expensive enough, but things can get far more costly than that. If a breaker fails (doesn’t open) during a fault, the consequences can include loss of the entire facility. Putting failure in charge won’t guarantee success.
    2. Interval-based. This is a step up from corrective-only. With this strategy, you try to time your maintenance against wear and tear. The underlying assumption is that breakdowns occur in predictable patterns, and you just have to get there first. But the schedule can't account for the condition of the breaker. If a power event happens immediately after a PM, you won’t correct the damage until either the next scheduled PM or a failure occurs. Because this strategy doesn't match limited resources against actual conditions, it creates “maintenance holes” in some areas and unnecessary maintenance in others.
    3. Condition-based. With this strategy, you rely on real information rather than "best guess" and gambles. Using a combination of information sources, you perform the right amount of maintenance. Thus, you optimize performance, safety, and reliability at the lowest cost. Information sources include infrared imaging, power monitoring, statistical data, maintenance testing, and visual inspections.
    With the condition-based strategy, you rely on diagnostics rather than a calendar and with the interval-based strategy, you rely on a calendar instead of failure.


    Two Tests for Every Breaker
    Conduct these tests with the breaker isolated from its power source.
    1. Contact resistance. Close the breaker. Test contact resistance on the line-to-load terminals of each phase. The values should not vary by more than 50%. If they do, you may have a problem. If the equipment manual doesn't specify the contact resistance, find out from the manufacturer what it should be.
    2. Insulation resistance. Close the breaker. Test insulation resistance phase-to-phase and phase-to-ground (equipment ground, not earth ground). In addition, measure across the open poles. If the equipment manual doesn't specify the minimum acceptable value, find out from the manufacturer what it should be.
    Coordination
    Most facilities undergo changes in equipment and power distribution over the years. When was your last circuit protection coordination study? If it’s been a while, don’t be surprised when (not if) a problem in a minor branch circuit causes a wide area shutdown. To make best use of limited time and money, focus on critical branch and feeder circuits first.


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    Repair
    Electrical Troubleshooting Quiz
    You’ve just completed a work order for a replacement motor on a hopper feeder. The work order instructed you to “test motor for rotation, wire it in, and return to service.” When you apply power, however, the motor shakes violently. What are some possible explanations, and how do you isolate the cause?

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


    Simple Test Prevents Re-Repair
    Your crew worked hard to remove the old motor and install the new one while production was down. The location made the work dangerous and difficult, so everyone’s glad it’s over. But when you apply power, the new motor shakes fiercely. So now you have to replace the new motor.

    How could you have prevented this? When the motor arrives, inspect it. Rotate the shaft and listen for the grinding sound a bent shaft makes. That’s a qualitative test. You can also perform the quantitative test of measuring bearing runout with a dial indicator. If the shaft rotates eccentrically (more than 0.004 to 0.009 or so of runout), then you have a bent shaft, bad bearings, or both. Don’t install that motor, because you’ll just have to yank it out again.


    Fuses and Transformers
    A transformer has high inrush current, because its core (typically) has to reach saturation. Fuses used with transformers typically need to have time withstand values that are 25 times the transformer primary rated current — but for only 0.01 second. Then, another value comes into play: The withstand can be “only” 12 times the transformer primary rated current for 0.1 second. The reason for the second value is inrush current decreases as core saturation increases.


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    Operation
    NEC in the Facility
    Oil-filled transformers installed outdoors have special requirements [450.27]. One critical requirement is providing for containment of the oil, in the unlikely event of a rupture or spill. The gravel bed around the typical outdoor transformer is there to reduce defoliation requirements and assist with water removal. It doesn’t “contain” the oil. If oil spills into this bed, the EPA requires excavating all the way down to uncontaminated soil.


    Why You Should Report Any Injury, Part 3
    Prompt reporting establishes a “time of incident.” This is important because, if complications arise later, you have a valid claim against your employer, and your employer has a valid claim against its insurer.
    For example, the load shifts while you’re installing a motor. You feel a sharp twinge in your back. You walk around a bit and the pain disappears. So, you don’t report this.
    Undiagnosed disk damage deteriorates over several days, until, in agony, you finally report the incident. By this time, however, it’s too late to say for sure that the motor installation incident caused the injury.



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    Quiz Answer
    Answer to Electrical Troubleshooting Quiz
    Something is bent or out of alignment. You came in after the mechanicals were done, so you’ll need to walk through each of those steps to see what was done wrong.

    Check motor/load alignment, then decouple the motor from the load and run the motor:

    • If the motor doesn’t vibrate and the motor/load alignment was good, there’s a problem in the load itself.
    • If the motor does vibrate, check the motor shaft runout with a dial indicator. If the runout is within tolerance, check the motor mounting. The most common error in motor mounting is overtightening, which bends the motor feet. Also look for loose bolts. Tighten to spec, not any tighter. Inspect the base and pedestal for problem such as cracks or twisting.


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