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January 25, 2007 A Prism Business Media Publication Vol. III No. 2



CONTENTS
Power Factor Correction

Electrical Troubleshooting Quiz

Planning Plant Maintenance Repairs

NEC at the Facility

Bonding vs. Grounding

It's Time to Hit the Beach

Answer to Electrical Troubleshooting Quiz


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    Maintenance
    Power Factor Correction
    If your facility has power factor (PF) correction capacitors at its service entrance, those capacitors are probably there to prevent a utility penalty. What they don't do is improve power factor at the load, which means your actual energy consumption is higher than it needs to be. This, of course, means your company pays for electricity it doesn't use.

    Have you been unable to get approval to pay for a site power survey to see where your energy losses are? Try shrinking the scope. Start with a one-line diagram of your distribution system, and limit the survey so it goes no further than the major loads. If you identify energy savings with this limited survey, the money you save can help you justify a more detailed follow-up survey.

    Even with PF correction at the service entrance, you may be surprised to find that you have unacceptably low PF at some loads. But don't just go slapping PF correction capacitors on your large motors, if they have variable-frequency drives. Consult the drive manufacturer for recommendations.

    Before implementing any load-level PF correction, determine the effects on your entire distribution system. Corrections at the load will change the PF capacitor size you need at the service. Make sure you regularly inspect PF correction capacitors. Power events can damage these capacitors -- and that damage may not be readily apparent. If you can't remove individual capacitors for testing or replacement because the lid of your PF unit interferes with this, consider replacing that unit with one that has a maintenance-friendly lid design.


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    Repair
    Electrical Troubleshooting Quiz
    The breakers supplying large loads have been tripping on overcurrent. You used the recording feature of your DMM and verified the high current levels, yet voltage is normal. The problem is so widespread that you doubt it's due to problems with any individual load. What is a likely cause?

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

    Planning Plant Maintenance Repairs
    How is this for embarrassing? You arrange for a two-hour shutdown to replace a nuisance breaker that supplies a main feeder circuit. After closing the breaker and restarting production, everything works fine on that feeder. But another breaker in that same panel now won't reclose.

    All kinds of similar situations have made sharp maintenance people look like chumps. Why does this happen?

    We've all been taught, "If it works, don't fix it." We have a tendency to limit the scope of a repair to the item that caused us to fight for every hour of downtime to make the repair. This doesn't always work in our favor.

    If you need to replace a breaker in Panel A, it usually does not make sense to start tearing into Panel B also. But if you have to shut down Panel A to replace a breaker, logic dictates that Panel A may have other problems. The cost and hassle of multiple shutdowns is far greater than just extending the Panel A shutdown for a complete checkup and preventive maintenance.

    But you can take "full maintenance" too far. Trying to cram every conceivable test and task into a limited time slot by piling people and equipment into a limited space is asking for trouble. Instead, take these steps:

    • Assess your time constraints. If the shutdown is occurring on systems that don't run 24x7, timing the shutdown to coincide with normal downtime or even shift changes may give you a larger window. Consider also any outages that are already scheduled.
    • Assess the maximum crew size. This is a critical step many planners overlook. There is only so much space in a given work area, and you can't have people working on top of each other.
    • Assess what is most critical. Yes, it would be nice to check off the box that you cleaned the cabinets. But breaker testing may be a more productive use of what little time and space you have.
    • Consider the next downtime window. This is a subset of the previous step. Assuming any repairs will need another downtime window, what tests can and should you perform to identify problems to fix then? Factor in any lead times for ordering parts.

    Operation
    NEC at the Facility
    An example of where the language of the NEC creates confusion between grounding and bonding is 314.4: "All metal boxes shall be grounded with the provisions of Article 250." The NEC does not require you to ground metal boxes -- it requires you to bond them. When you dig into the Article 250 provisions, this becomes apparent. It's just not obvious from the wording in 314.4.

    Bonding vs. Grounding
    Article 100 of the NEC clearly defines grounding (earthing) and bonding (metallic connection). Yet, the NEC frequently uses "grounding" where it actually means "bonding." Unfortunately, OSHA follows this same misuse of terminology. How can you know when to bond rather then ground?

    Keep in mind that, because the earth is a high-resistance path, grounding does not eliminate differences of potential. Any time the goal is to prevent shock or flashover, grounding isn't going to help you. To achieve those goals, you must bond.

    One trick experts use is to draw the circuit on paper and apply Ohm's Law. Because voltage is current times resistance, the low resistance you get from bonding will result in a far lower voltage than the high resistance you get from grounding.

    But what if your drawing ends up being a parallel circuit? Simply apply Kirchoff's Law. In essence, it says that electricity will flow in inverse proportion to the paths presented to it. To shunt more undesired current, bond to get that lower resistance path in parallel with the equipment or people you are trying to protect.


    Show & Events
    It's Time to Hit the Beach
    If it's your job to make sure all systems are "go," you need to go to Electric West. This show and conference offers the right information and product mix to meet all of your information needs. Do you maintain and operate electrical systems in a facility? If so, you have to make plans to attend the Electric West conference program in Long Beach, Calif. Check out this event's 40+ seminars in the areas of power quality, safety, Code changes, and industrial applications, and make plans to meet 200+ leading suppliers. Or register now.


    Quiz Answers
    Answer to Electrical Troubleshooting Quiz
    The fact the loads are all drawing too much current is a classic sign of low power factor, although high harmonics may be the culprit. You need more information before changing anything. Use a power analyzer to determine the actual power factor and harmonics level on each feeder and at the service. If the problem is recent and seems to have occurred all on its own, suspect failed PF correction capacitors.


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