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August 5, 2008 A Penton Media Publication Vol. IV No. 15

Circuit Breakers

Motor Maintenance Tip, Part 18

Electrical Troubleshooting Quiz

Wringing Revenue from Repairs

Fuses for Multiple Motors

NEC in the Facility


Answer to Electrical Troubleshooting Quiz

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MRO Insider addresses topics such as:

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  • National Electrical Code® on the production floor
  • Safety procedures and programs
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  • Equipment maintenance and testing tips
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    Circuit Breakers
    Circuit breaker maintenance falls into two general categories:
    1. Routine. Detection of problems via routine maintenance is preferable to detection by any other means. The basic methods are automated monitoring and scheduled PdM/PM.
    2. Major. When maintenance requires interfering with production, it’s “major.” So, you try to avoid doing it. When routine maintenance reveals a problem or it’s simply the recommended time, you can no longer avoid major maintenance.
    Examples of routine maintenance:
    1. Quarterly, you conduct infrared analysis and visual inspections of your feeder panels.
    2. Annually, you take voltage measurements at your feeder breakers.
    3. Automatic ground current leakage monitoring systems activate an alarm when ground leakage current reaches some predetermined threshold.
    4. Periodic reports from your power monitoring service provide recommendations for additional maintenance based on expert analysis of trends or events.
    Routine maintenance doesn’t reduce the likelihood of major maintenance. The real value of routine maintenance is that early detection allows you to schedule corrective actions to prevent serious deterioration or catastrophic failure.

    Circuit breaker manufacturers provide recommended frequencies and methods for routine maintenance. They also provide major maintenance recommendations that address parts, procedures, frequency, and decisions to repair or replace. Ensure this information is referenced from (or embedded into) your CMMS, procedures, and training.

    Motor Maintenance Tip, Part 18
    Voltage imbalance kills motors. Therefore, you need to detect and correct three types of imbalance. One type is a system-wide voltage imbalance (we’ll look at the other two types in a future issue).

    The most common cause of system-wide voltage imbalance is unbalanced loading. Examine all 3-phase panels to see that the phases are wired correctly — what may appear correct on the panel may not be the actual A-B-C phase arrangement. Although it may take considerable effort to track down and redistribute problem loads, it’s a worthwhile effort.

    A less common cause is the transformer taps are set wrong, resulting in different voltages on different phases coming right out of the transformer. The solution is to reset the taps on the transformer until all 3-phase voltages are identical.

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    Electrical Troubleshooting Quiz
    Occasionally when the big mixer starts, the branch-circuit fuses blow. Yet, there are no shorts or other obvious problems. If you replace the fuses, things run fine for several days.

    The production superintendent wants the fuses replaced with breakers so operators can just reset the breaker. What should you do?

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

    Wringing Revenue from Repairs
    Are you leaving free money on the table? Competent production managers know the cost of each minute of downtime on each piece of major equipment. Usually, lost revenue is the primary driver of that cost. Ask your production managers which equipment incurs the highest downtime costs. Focus on speeding up repair time on that equipment, making sure you tap a valuable resource: the manufacturer.

    Manufacturers often provide downtime-reducing products. For example:

    • Assemblies. Rather than dismantle assemblies to replace a component, you can pop out the old assembly and pop in the new one. If an assembly isn’t presently offered, discuss this with the manufacturer.
    • Special components. See if a particular part now has an improved version, such as “rough service” or “quick change.” Or, perhaps there’s a version more applicable to your application. If these aren’t offered, discuss your problem with the manufacturer to see what’s feasible.
    • Special tools. Race car mechanics have special tools you don’t find at any hardware store, and so should you. Manufacturers of industrial equipment often have special jigs, fixtures, removers, installers, and other tools that save time. If such items don’t exist, contact a reputable machinist to observe a repair in progress and propose such items. You may recoup the cost of fabrication in the next downtime event.

    Fuses for Multiple Motors
    Where you have more than one motor on a branch circuit, the maximum rating of the fuses (or breakers) protecting that branch circuit is “the largest motor load plus.” The plus part is the sum of the full-load current ratings of all other motors on that circuit and the ratings of other loads on that circuit [430.53(C)(4)].

    NEC in the Facility
    During your next outage, measure the resistance between your service transformer case(s) and any metallic items (e.g., fencing) nearby. In each instance, you should measure the resistance of a bonding jumper (which is nearly zero).

    If that outage is a ways off, visually check for bonding jumpers between all metallic objects in your transformer pens. A driven rod isn’t a bonding jumper. You want to ensure the bonding is per 250 Part V, and the grounding is per 250 Part VI and VII [450.10].

    Who should reset or replace overcurrent protection devices (OCPDs)? It may seem harmless to let operators reset the breaker and call maintenance only if it trips again or won’t reset. However, it’s not harmless. First of all, operators aren’t trained in operation of an OCPD or disconnect. They invariably stand in the blast path — without PPE.

    If that OCPD has tripped due to a fault and the fault is still present, anyone operating that OCPD is probably going to be severely injured. Only a person who is qualified to test for faults and then clear those faults should reset or replace an OCPD.

    Quiz Answer
    Answer to Electrical Troubleshooting Quiz
    Operators are not “qualified personnel” to work on OCPDs — this includes resetting breakers. What the superintendent really wants is uptime. To provide that, you need to determine the cause of the downtime rather than be complicit in a practice that is both unsafe and illegal.

    Are those fuses sized for the largest load plus? If not, then recalculate for the correct size. This alone may not solve the problem. Closely scrutinize the operation itself. Are the operators overloading that mixer before starting it? Because the problem occurs when starting the mixer, the solution may be as simple as starting the mixer with less material (or before adding material). Monitor the current draw of the mixer under various loading scenarios before reaching any conclusions.

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