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CONTENTS
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Insulation Resistance
Testing, Part 2
Electrical Troubleshooting
Quiz
Failed Repairs
NEC in the Facility
Safety
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About This Newsletter
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This twice-a-month
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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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The designations "National Electrical Code" and "NEC" refer to the
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Maintenance
Insulation
Resistance Testing, Part 2
Insulation resistance testers measure these three
quantities:
- 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.
- 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.
- 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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Repair
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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Operation
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.
Safety
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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