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September 9, 2008 A Penton Media Publication Vol. IV No. 17


CONTENTS
Spare Breakers

Motor Maintenance Tip, Part 20

Electrical Troubleshooting Quiz

Motor Repair, Part 1

Fuses with Breakers

NEC in the Facility

Why You Should Report Any Injury, Part 1

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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    The designations "National Electrical Code" and "NEC" refer to the National Electrical Code®, which is a registered trademark of the National Fire Protection Association.
     
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    Maintenance
    Spare Breakers
    If routine circuit breaker maintenance reveals a problem with a 400A breaker supplying critical equipment, are you sure you can install the spare you keep in the stockroom? If you need to make that installation at the start of a holiday weekend, can you do so with what’s on hand? To ensure you can use spare breakers when needed, answer the following three questions for each one you have on hand:
    1. Is it the right breaker? Check ratings, settings, frame size, and compatibility with the intended application.
    2. Can you install it? Review the manual to make sure you understand everything related to installation, and have on hand everything that’s recommended to complete that installation.
    3. How do you know it functions properly?
    Motor Maintenance Tip, Part 20
    When one phase supplying a 3-phase motor opens, the motor will usually continue to run as a single-phase motor. That’s why we say “single-phasing” even though two phases are available.

    The bad news is single-phasing causes the motor to draw more current in the remaining two phases than what the windings can handle. Interestingly, that’s also the good news because of how we can use the strength of that current against itself.

    Since the 1971 edition, the National Electrical Code (NEC) has required an overload current-sensing device within each of the 3-phase conductors supplying each motor (previously, the protection was required only in two phases). You’ll find the requirements in Art. 230, Part III.

    The overload current “trip” settings for these devices are 140% or less. The exact setting depends on the type of motor, its service factor, and the type of sensing device. If you properly select and set the overload devices, then they will shut the motor down when single-phasing occurs.

    But what about restarting after the shutdown? Unless you have a restart interval explicitly for restart after overload, assume any recommended interval is based on normal on/off control and thus too short for restart after overload. Before resetting (or replacing) motor overload protection devices, allow for enough ventilation (forced or otherwise) and time to cool sufficiently for a safe restart. You can avoid guesswork by using a thermal camera or IR gun to check winding temperature.

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    Repair
    Electrical Troubleshooting Quiz
    A large production line has about 50 motors. These operate a mixing vat, a sprayer rack, a roller press, an oven blower, some small doors, ventilation fans, and other equipment required to make the product.

    Production stops once or twice a week due to some motor-related problem or another. You’re almost happy when it’s “just an overload trip,” because you’ve had to replace as many as three motors in one day. How can you reduce this motor mayhem?

    The answer to this question appears at the end of this newsletter.
    Motor Repair, Part 1
    You may not be Quincy, but you still need to be a bit of a forensic examiner to do a proper motor repair. The more you know about why a particular motor failed, the better you can prevent the failure of its replacement (and other motors in your facility).

    For example, suppose your motor shop says your last dead motor had scorched windings and clogged vents. You make a note to add vent cleaning to the PM. This is a good heat reduction measure, but will it reduce heat enough in that application to protect windings from overheating? Maybe not, because other factors probably contribute to excess heat in that particular application.

    A good physical inspection may reveal several small problems that add to an overheating condition. For instance, you find a damaged coupling, a broken angle iron, and a shield that blocks airflow to the motor (those first two problems are factors in excess vibration, which contributes to overheating).
    Fuses with Breakers
    You can use a series-rated fuse and circuit breaker combination (fuse on line side, breaker on load side) on a circuit in which the available fault current exceeds the marked interrupting rating of the breaker. The requirements are in 240.86.

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    Operation
    NEC in the Facility
    For any indoor dry-type transformer over 112.5kVA, ensure it’s in a transformer room that has a fire rating of at least 1 hr [450.21(B)]. The NEC allows exceptions to this requirement for transformers with Class 155 or higher insulation systems, under specific conditions.
    Why You Should Report Any Injury, Part 1
    An injury may be serious, even if no outward signs are evident to you. This is especially true of a blow to the head (a hardhat, while life-saving, doesn’t offer 100% protection). You need a qualified person to conduct a proper examination. You wouldn’t hand a nurse a screwdriver and ask him or her to change a 480V breaker unless you knew that nurse was also an electrician qualified to perform that work. Don’t assume you’re qualified to do a medical examination.

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    Quiz Answer
    Answer to Electrical Troubleshooting Quiz
    First, look for a root cause. Start with a power analysis that includes checking for voltage imbalance. After you solve any problems you find, failure rates should drop.

    However, your job isn’t done. When widespread failures exist, there may be multiple root causes. If, for example, maintenance of the power distribution system has been insufficient to ensure voltage balance on the phases, then chances are maintenance is also insufficient in other ways. So, look for and solve all possible root causes.

    Your job still isn’t done. These failures may have compound causes — a mix of different failure contributors that, by themselves, don’t cause failure. When compounded on top of each other, the result is failure. The only way to find these is to methodically examine one motor at a time. Check the entire system of each motor. Key areas include:

    • Input. Use a power analyzer to detect problems with power factor, waveform quality, harmonics, and voltage imbalance.
    • Load. Check alignment and lubrication.
    • Mounting. Use a vibration monitor to determine if it’s adequate and serviceable.
    • Bonding. Ensure metal parts of the motor system are bonded, not grounded (See NEC Art. 100 definitions).
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