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August 10, 2010 A Penton Media Publication Vol. VI No. 15



CONTENTS
You and IR, Part 7

Electrical Troubleshooting Quiz

Troubleshooting PLC Analog Output Modules, Part 1

NEC in the Facility

Safety



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MRO Insider addresses topics such as:

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  • Safety procedures and programs
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    Maintenance
    You and IR, Part 7
    It's great to have a quality thermographic camera and a well-administered thermography program. But what do those images and temperatures really mean?

    Certain types of testing are more relative than absolute. Insulation resistance testing is very much that way, and thermography is mostly that way. For example, does a 170°F reading on a motor vent mean the motor is fine or that it's overheating? How do you know what this reading is really telling you?

    The key to making these kinds of tests useful is to establish baseline readings when the equipment is first put in service. These will be your reference for judging whether future readings are abnormal.


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    Repair
    Electrical Troubleshooting Quiz
    Your plant has a process that uses treated water. Based on the flow rate to the process, the PLC program calls for makeup water to be delivered to the day tank. Recently, the day tank has repeatedly run dry during production. The lack of treated water ruins the product. It also forces production operators to shut down, perform purges and blowdowns, and then restart the system.

    Preliminary troubleshooting has established the input side of the control loop is problem-free. In the notes, you read:

    • There is more than enough water treatment capacity to satisfy production demand.
    • The flow meter has been recently calibrated (no adjustment needed).
    • The makeup water valve works perfectly.
    Where do you go from here?

    Visit EC&M's website to see the answer.


    Troubleshooting PLC Analog Output Modules, Part 1
    In our previous two issues, we discussed troubleshooting digital output modules. Troubleshooting analog modules is a bit more complicated, for two reasons:

    1. Analog modules aren't on/off, as digital ones are, so there won't be a status light showing the logic state. You have to (in nearly all cases) use a DMM to see what the output level is.
    2. You need to know the range (e.g., 4mA to 20mA) and possibly the scale (e.g., 1 to 1,600 on the PLC register) for this output. Determining this for certain may require walking through a long documentation trail, beginning with the P&IDs. You may need to resolve multiple conflicts of specifications, if this loop has not worked properly before.

    To read more on this story, visit EC&M's website.


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    Operation
    NEC in the Facility
    As we noted in our previous issue, Annex D3(a) begins with a description of an industrial multi-building application case. Then, it poses this problem: “Determine the overcurrent protection and conductor size for the feeders in the common raceway, assuming the use of XXHW-2 insulation (90°C).”

    Annex D3(a) shows you how to solve this problem, beginning with how to determine the calculated load. Then, using the values you determined for the noncontinuous and continuous loads, you determine the total VA. Convert this figure to 3-phase AC current, and you have the minimum size overcurrent protective device.

    Next, Annex D3(a) illustrates how to determine the feeder conductor sizes. You do some straightforward calculations, using numbers from the 75°C column of Table 310.16. However, there is a twist. Per 310.15(B)(4)(c), you must count the neutral conductors as current-carrying conductors because the discharge “has a substantial nonlinear content.”

    This brings us to an even trickier twist. The example says to use the 90°C column, not the 75°C column, for this neutral conductor. Why would that be?

    To read more on this story, visit EC&M's website.


    Safety
    One of the great innovations for the construction industry is the “all-day” battery-powered tool. These typically operate at 24VDC, but that doesn’t mean there aren’t any safety issues.

    Take the cordless drill for example. When that battery gets low after, say, sawing holes all morning, the drill will turn more slowly. Cutting ability depends not only on the sharpness of the hole saw, but also on the speed of the drill. As cutting ability decreases, the likelihood of slippage and injury increases.

    The solution? Use an AC-powered drill and portable cord. Or, have a second drill or spare battery charged and ready to go. And don't forget to replace worn hole saws and drills.


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