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September 7, 2010 A Penton Media Publication Vol. VI No. 17



CONTENTS
You and CMMS, Part 2

Electrical Troubleshooting Quiz

Troubleshooting Thumb Switches, Part 1

NEC in the Facility

Safety


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This twice-a-month
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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
  • Troubleshooting techniques
  • Equipment maintenance and testing tips
  • Managing motors and generators
  • Trends in training and education
  • Managing energy use


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    Maintenance
    You and CMMS, Part 2
    One feature of a computerized maintenance management system (CMMS) is the ability to track material usage and labor costs. Some maintenance departments rely on the techs to record time and materials. Usually, techs “fill out paperwork” several hours after work completion, guesstimating everything. A “solution” to this is the work order writer guesstimates the job in advance. Another approach eliminates guesswork. The clock starts when a tech “checks out” a work order and stops when the tech turns it in. For materials, the tech scans barcodes and enters the work order number. With this third approach, you have efficiently obtained accurate information you can analyze to see where your costs actually are.

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    Repair
    Electrical Troubleshooting Quiz
    The office workstations have surge strips. Users plug their computers and monitors into these, and occasionally use the extra receptacles for various other small loads. Around the first of the year, there was a rash of computer failures. The vendor that does your computer support took a random sampling of these surge strips and found the metal oxide varistors (MOVs) were all blown. The strips were still supplying power, but they weren't providing any surge suppression. Further investigation revealed nearly every surge strip had this same kind of damage, and they were all replaced.

    Recently, computer failures began happening again. This vendor did another sampling of the surge strips and found the blown MOV problem had returned. How do you solve this problem and prevent another recurrence?

    Visit EC&M's website to see the answer.
    Troubleshooting Thumb Switches, Part 1
    We're accustomed to touch screens, which are a part of nearly all mobile devices. Touch screens have been in use for a long time. By the mid-1980s, they were fairly standard on digital control systems (DCS), such as those used for refineries and coal power-generating stations.

    Small applications have few inputs to change, so the need for a touch screen and its added complications is much less. Thus, thumb switches are widely used because of their simplicity and ruggedness.

    What if your thumb switch controls appear not to work? Fortunately, these are easy to troubleshoot.

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

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    Operation
    NEC in the Facility
    Here's a question for you. What NEC Articles are the most important for proper surge protection? If you answered, “Art. 250, 280, and 285,” then you're correct. But do you know where these apply, and do you know what other areas of the NEC also have bearing on this issue?

    The answer to the second part of that question is “Chapters 1-4, and Art. 90.” In Chapter 1, for example, people tend not to pay much attention to Art. 100. This single fact explains the existence of many power quality problems. You understand why, when you read the Art. 100 definitions of grounding and bonding. Mix these up, and your surge protection devices act more like surge recirculation devices.

    Now, where do these three Articles apply?

    Visit EC&M's website to see the answer.
    Safety
    Decide whether the following statement is true or false: A motor and its 480V supply transformer are each provided with a ground rod, so it's safe to put one foot on each ground rod. Now, give the reason for your answer.

    Many people will answer “true,” and their reason will be that the two rods are at equipotential because both are grounded. The correct answer is actually “false.” Earth has significant resistance, and thus there are differences of potential between two grounding points. This is why, for example, we use the fall of potential method in grounding work.

    But doesn't electricity take the path of least resistance? Draw a parallel circuit with a 100Ω resistor and a 200Ω resistor, and solve for current in each path. As you can see, electricity doesn't take the path of least resistance. Grounding does not remove potentially lethal shock hazards.

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