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March 9, 2010 A Penton Media Publication Vol. VI No. 5

Insulation Resistance Testing, Part 2

Electrical Troubleshooting Quiz

Failed Repairs

NEC in the Facility


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    Insulation Resistance Testing, Part 2
    Insulation resistance testers measure these three quantities:
    1. Absorption current starts high and then drops. At the same time, voltage starts low and rises. What you're measuring is the rate of storage (absorption) of potential energy in and on insulation. Understanding this process is important to understanding the time resistance method of insulation testing.
    2. Capacitive current is the initial surge of current that occurs when you apply voltage to a conductor. The conductor acts like a capacitor in that it "charges up." As with absorption current, capacitive current starts high and then drops.
    3. Leakage current, also called "conduction current," is the steady flow of current through and on the insulation. If you graph this on a time trend, you should notice a gradual sloping of the line. When that slope suddenly changes, the insulation is undergoing accelerated deterioration.

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    Electrical Troubleshooting Quiz
    A rash of motor failures has afflicted your plant. Your plant engineer read some application notes from the Web site of a test instrument manufacturer and subsequently used a power analyzer on the feeders of these motors. There were some anomalies, but nothing that explains these failures.

    These failures are occurring in a few unrelated systems: the plant HVAC, a process exhaust hood, a scrap grinder, a conveyor system, and a wash tank. These same systems keep having motor failures. What should you look at to get to the bottom of this problem?

    Visit EC&M's Web site to see the answer.

    Failed Repairs
    A repair that isn't done correctly is a failure waiting to happen. Some common errors include:

    • Poor hygiene. Lay disassembled parts on a clean surface, such as cloth for that purpose. At every step, prevent the introduction of grit, chemical contaminants, and water. Even finger oil on contact surfaces can create problems later.
    • Misapplication of lubricants. No, it's not okay to spray breakers with a handy can of spray lubricant. A spray lubricant doesn't have the "body" to adhere to high pressure points of contact. Use only the lubricant specified by the manufacturer. Don't over lubricate.
    • Re-using old fasteners. For control wiring and other low-torque applications, this is usually acceptable. If you need more than a screwdriver to tighten the connection, replace the hardware.
    • Misapplication of spring-type fasteners. Belleville washers are commonly misapplied. The errors are usually over-tightening or use of the wrong size. Replace after each use with the correct size, and follow the installation instructions for a reliable connection.
    • Improper torqueing. Improper tightening of motor mounting bolts is a leading cause of replacement motor vibration problems. Use the torque value specified for the fasteners you’re using, not the value from a generic table.
    You can prevent most "failed repair" problems by addressing potential failure causes in your repair procedures. Ask a few key questions, such as: For breaker X, what lubricants do we use? For feeder bus 06E, which Belleville washers do we use?

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    NEC in the Facility
    Most facilities today have surge arresters on the building supply (service). These are typically bought on a budget mentality that results in a waste of the entire investment. It doesn't have to be that way. In fact, correct specification and installation will provide a very high return on investment (ROI).

    The first thing to consider is that these systems need to be part of a larger surge protection strategy. They will protect only against the larger spikes (reducing them to a lower voltage) from outside your facility.

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

    The best protection against arc blast is "don’t be there." This, however, isn't always possible. The next best precaution is to minimize your exposure (do as much as possible outside the hazardous area) and wear the proper PPE.

    You may need to wear protective sleeves, a blast suit, or other protective clothing. Don't fall into the trap of asking, "What can I get by with?" Instead, ask what you must do to protect yourself against the highest energy hazard available in this location.

    • Thoroughly assess the situation before making PPE decisions.
    • Avoid "calculation obsession." Protection calculations determine the minimum protection. You aren't required to downgrade protection to fit those calculations.
    • Remember that there is always enough time to perform the proper safety analysis prior to commencing work. You may not get a second chance.

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