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Previously, we said motors are special victims of power
quality problems. It's also true that motors are special contributors of
these problems. With their inrush currents, inductive load
characteristics, and electronic drives (if not harmonics corrected and
power factor corrected), motors place heavy burdens on a power
distribution system. Even the physical connections require extra care
and attention beyond that afforded to "normal" distribution system
loads. This is why you can buy a roll of rubber "motor lead tape." How
many other loads can you name that have a special lead tape named for
them?
Article 430 ("the Motor Article") is the longest Article in the
entire National Electrical Code, and yet other Articles of the Code
still deal with motors. Part of the solution to that part of the motor
maintenance puzzle dealing with the power distribution system is in Art.
430. If your motor supply doesn't conform with Art. 430, then you’re
going to have a "high maintenance" situation. If you are in that
situation, review Art. 430. You might just find that someone installed
it improperly.
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Over the past few years, equipment failure rates have been
steadily rising, and you've been called in to find out why. The plant
manager called you aside and told you: - All PMs have been done
on schedule; no deferments.
- The PM frequency on critical equipment has been doubled, but
this didn't change the failure rate.
- The PM procedures were all entered into the CMMS last
September.
- Each technician has a bound copy of the PM procedures.
- Everyone in maintenance has been multi-craft
trained.
What the plant manager does not talk about provides good clues. What
might those be?
Visit EC&M's website
to see the answer.
In Part 7, we started looking at the line-to-neutral (L-N)
and neutral-to-ground (N-G) voltage measurements at a convenience
receptacle in the cabinet where circuit boards were turning to toast.
Why are these measurements useful?
As the circuit load increases, so does the current (per Ohm's
Law). Voltage drop increases as current increases. So if a branch
circuit is overloaded, voltage drop increases. Voltage drop is something
you calculate based on circuit current and voltage, and conductor length
and gage size.
Using a DMM, you can also measure the voltage drop. If
your system is wired and bonded correctly (and connections are good),
then your N-G measurement will be the voltage drop. Subtract this from
your nominal voltage, and the result should match your L-N measurement.
If it does not, you have current flowing in the ground conductor.
To read more on this story, visit EC&M's website.
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Commonly misunderstood/misapplied terms, part 4.
Pop quiz: Do explosionproof enclosures protect components from
explosions?
Answer: By "explosionproof," the NEC means the enclosure serves
as a containment for the explosion. It confines an explosion to its
inside rather than protects against one from the outside.
Some other commonly misunderstood terms
include: - Fitting. A termination isn't a fitting. In a
wiring system, a fitting primarily serves a mechanical function rather
than an electrical one. A bushing is an example of a fitting. It may
also serve an electrical function by preserving electrical continuity of
metallic raceway.
- Garage. It isn't necessarily a place where vehicles are
stored. To be designated a garage, it merely needs the realistic
potential of serving that purpose (or of being a place where vehicles
are sold, rented, repair, exhibited, or demonstrated).
- Ground. This term has been a source of confusion and
friction in the NEC for decades. The NEC definition now is very simply
"the earth." The NEC still occasionally uses "grounding" to mean
"bonding," but that practice is giving way to this definition.
- In sight from. This has more meaning than its words
indicate. In NEC parlance, it is also 50 ft away or closer.
You use various chemicals in your work. Your supervisor's
probably told you to read the label on a container before using what's
in it. But if you've read the MSDS, why read the label? True, labels
repeat what's in the MSDS. For example, a label may tell you what
precautions to take when handling the material. However, the label will
also tell you how to dispose of the very container you're holding in
your hand.
Rock climbing is a sport that many people consider dangerous. To
reduce the danger, safe climbers check their harness before each climb
— even if they've already checked it multiple times. This redundant
checking is one principle behind reading the label, even though you may
have read the same information on the MSDS less than an hour ago.
What if a container doesn't have a label? Then don't use it! Even
if you "know" what's in it, a mistake could prove fatal.
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