Testing, Part 2
NEC in the Facility
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MRO Insider addresses topics such
Working with management and supervision
National Electrical Code® on the production floor
Safety procedures and programs
Equipment maintenance and testing tips
Managing motors and generators
Trends in training and education
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The designations "National Electrical Code" and "NEC" refer to the
National Electrical Code®, which is a registered
trademark of the National Fire Protection Association.
Resistance Testing, Part 2
Insulation resistance testers measure these three
- Absorption current starts high and then drops. At the same
time, voltage starts low and rises. What you're measuring is the rate
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
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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.
A repair that isn't done correctly is a failure waiting
to happen. Some common errors include:
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?
- Poor hygiene. Lay disassembled parts on a clean surface,
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.
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NEC in the
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
To read more on this story, visit EC&M's Web
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
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
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
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