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October 21, 2008 A Penton Media Publication Vol. IV No. 20


CONTENTS
Circuit Breaker Injection Testing

Partial Discharge Testing

Electrical Troubleshooting Quiz

Sample Inspections Reduce Repair Demand

NEC in the Facility

Safety

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
    Circuit Breaker Injection Testing
    Because circuit breaker injection testing is so beneficial, many facilities incorporate it into their regular breaker maintenance program. That makes good sense, but only if a qualified person does the actual testing. Otherwise, you have accuracy and safety issues. You may need to outsource to an electrical testing firm to get such a person. This will be someone who:
    • Uses special test sets for this purpose.
    • Is trained to conduct this test on the specific breaker under test.
    • Has sufficient knowledge to be able to understand the equipment and systems related to that breaker.
    Injection testing has three variations:
    1. Primary injection, which is more thorough than secondary.
    2. Secondary injection, which is safer and faster than primary.
    3. Secondary injection with primary verification.
    Which variation is best? That depends on your circumstances. Discuss with your testing team the merits of the three variations for your specific application, based on present conditions. The biggest cost component of this testing is the shutdown that interrupts production. To limit this cost, you should plan well beyond the actual work.

    Plan for mobilization, setup, cleanup, and contingencies. Prepare for such things as temporary power, temporary light (don’t forget bathrooms), fuel, lifting gear, ladders, security, access, spare parts, test equipment, working clearances, testing areas, repair areas, hand tools, power tools, lockout/tagout, equipment manuals, system drawings, and first aid. Also, don't forget to coordinate with affected departments, contractors, and suppliers.


    Partial Discharge Testing
    Insulation “weak spots” progressively deteriorate until they are finally breached by a partial discharge (PD) of current from the conductor to the surrounding air. Although this discharge is invisible to the unaided human eye, with PD testing you can see where these weak spots are located. Regularly scheduled PD testing for medium-voltage (MV) and high-voltage (HV) power distribution lines allows you to make repairs before those lines fault.

    PD testing also is worthwhile for equipment operating at those voltages, such as transformers and switchgear. It’s especially valuable for motors or generators operating at MV or HV levels. Eventually, the insulation in this equipment will exhibit partial discharge. You can’t prevent it.

    PD in motors is a bit like harmonics in that you can tolerate some, and it’s not cost-effective to try to eliminate all of it. Nevertheless, you can keep things from lurching into the disaster zone if you combine regularly scheduled PD testing with long-term trending (on the order of years).

    For cabling, switchgear, or transformers, PD tolerance is different. In fact, it’s zero. Once you can detect PD with those items, outright failure is in the near future.


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    Repair
    Electrical Troubleshooting Quiz
    Small motors don’t seem to last in your facility. Exhaust fans, HVAC blowers, mixers, conveyors, and door openers need a new motor within just a couple of years.

    You read that sending a small motor to a motor shop for forensic examination can help identify a root failure cause when failures are widespread. So, you send a couple of failed ones out.

    The motor shop informs you the bearings are severely pitted. What problem does this indicate, and what condition is most likely causing it?

    The answers to these questions appear at the end of this newsletter.


    Sample Inspections Reduce Repair Demand
    Nobody questions the wisdom of having a motor shop perform a forensic examination of motors that are expensive or used in critical applications. In many facilities, however, the policy is to regard failed non-critical motors below a certain size as “throwaways.” On a per-motor basis, the cost justification considerations make sense — but not on a facility-wide basis.

    If motors are failing due to some unknown root cause, you’re replacing motors unnecessarily. When you do ROI calculations across 10 motors instead of one, the cost of that forensic examination is peanuts.

    Look at your motor population, and determine a good sampling rate for sending “throwaways” out for forensic examination. If any particular failure cause is a mystery, then send that motor out regardless of the sampling rate — the inevitable second mysterious failure will double replacement costs.


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    Operation
    NEC in the Facility
    Although the demand for stationary batteries has exploded in recent years, you don’t want your batteries to do likewise. Conducting maintenance per the manufacturer’s recommendations is essential for preventing this.

    So is adequate ventilation, as required by 480.9. The NEC, not being a design guide [90.1], doesn’t define “adequate.” However, other references do. Contact your battery manufacturer if you need to identify the applicable installation standards.


    Safety
    You’ve heard the expression, “Dress for success.” One of its meanings is that shabby shoes say volumes about you. The same goes for a faded shirt or tattered jeans. But when you’re in the gritty, “get-dirty” environment of facility maintenance, does your dress code really matter to your success? The simple answer is yes.

    More importantly, clothes take on added significance when performing electrical work.

    • Clothing that is torn can create an easy snagging hazard. Clothing that is thin offers little or no skin protection.
    • Unserviceable shoes can cause problems with your back, knees, and even your neck.
    • Synthetic materials can carry a burn deep into tissue. To avoid inadvertently wearing synthetics on the job, eliminate polyester and other synthetics from your wardrobe; read the labels of all of your shirts, pants, jackets, and underclothes.



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    Quiz Answer
    Answer to Electrical Troubleshooting Quiz
    Severe pitting in bearings is a classic sign of undesired current flow through them, which in turn indicates a bonding deficiency. The undesired current doesn’t have the required low resistance path back to its source [250 Part V], so an inordinate amount flows through the bearings.

    Solve the pitting problem, and the subsequent motor failures, by applying Art. 250, Part V to all electrical equipment. Here’s a “find-it-fast” clue. Look for a driven rod on the load side. That location is probably missing a proper bonding jumper.


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