Motor Maintenance Tip, Part
Motor Repair, Part 1
Fuses with Breakers
NEC in the Facility
Why You Should Report Any
Answer to Electrical
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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.
If routine circuit breaker maintenance reveals a
with a 400A breaker supplying critical equipment, are you sure you can
install the spare you keep in the stockroom? If you need to make that
installation at the start of a holiday weekend, can you do so with
what’s on hand?
To ensure you can use spare breakers when needed, answer the following
three questions for each one you have on hand:
- Is it the right breaker? Check ratings, settings, frame size, and
compatibility with the intended application.
- Can you install it? Review the manual to make sure you understand
everything related to installation, and have on hand everything
recommended to complete that installation.
- How do you know it functions properly?
Tip, Part 20
When one phase supplying a 3-phase motor opens, the
motor will usually continue to run as a single-phase motor. That’s
we say “single-phasing” even though two phases are available.
The bad news is single-phasing causes the motor to draw more current
in the remaining two phases than what the windings can handle.
Interestingly, that’s also the good news because of how we can use
strength of that current against itself.
Since the 1971 edition, the National Electrical Code (NEC) has
required an overload current-sensing device within each of the 3-phase
conductors supplying each motor (previously, the protection was
only in two phases). You’ll find the requirements in Art. 230, Part
The overload current “trip” settings for these devices are 140%
or less. The exact setting depends on the type of motor, its service
factor, and the type of sensing device. If you properly select and set
the overload devices, then they will shut the motor down when
But what about restarting after the shutdown? Unless you have a
restart interval explicitly for restart after overload, assume any
recommended interval is based on normal on/off control and thus too
short for restart after overload. Before resetting (or replacing) motor
overload protection devices, allow for enough ventilation (forced or
otherwise) and time to cool sufficiently for a safe restart. You can
avoid guesswork by using a thermal camera or IR gun to check winding
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A large production line has about 50 motors. These
operate a mixing vat, a sprayer rack, a roller press, an oven blower,
some small doors, ventilation fans, and other equipment required to
Production stops once or twice a week due to some motor-related
problem or another. You’re almost happy when it’s “just an
overload trip,” because you’ve had to replace as many as three
motors in one day. How can you reduce this motor mayhem?
The answer to this question appears at the end of this
Motor Repair, Part
You may not be Quincy, but you still need to be a bit
a forensic examiner to do a proper motor repair. The more you know
why a particular motor failed, the better you can prevent the failure
its replacement (and other motors in your facility).
For example, suppose your motor shop says your last dead motor had
scorched windings and clogged vents. You make a note to add vent
cleaning to the PM. This is a good heat reduction measure, but will it
reduce heat enough in that application to protect windings from
overheating? Maybe not, because other factors probably
to excess heat in that particular application.
A good physical inspection may reveal several small problems that
to an overheating condition. For instance, you find a damaged coupling,
a broken angle iron, and a shield that blocks airflow to the motor
(those first two problems are factors in excess vibration, which
contributes to overheating).
You can use a series-rated fuse and circuit breaker
combination (fuse on line side, breaker on load side) on a circuit in
which the available fault current exceeds the marked interrupting
of the breaker. The requirements are in 240.86.
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NEC in the
For any indoor dry-type transformer over 112.5kVA,
ensure it’s in a transformer room that has a fire rating of at least
hr [450.21(B)]. The NEC allows exceptions to this requirement for
transformers with Class 155 or higher insulation systems, under
Why You Should
Report Any Injury, Part 1
An injury may be serious, even if no outward signs are
evident to you. This is especially true of a blow to the head (a
hardhat, while life-saving, doesn’t offer 100% protection). You need
qualified person to conduct a proper examination. You wouldn’t hand a
nurse a screwdriver and ask him or her to change a 480V breaker unless
you knew that nurse was also an electrician qualified to perform that
work. Don’t assume you’re qualified to do a medical
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Electrical Troubleshooting Quiz
First, look for a root cause. Start with a power
analysis that includes checking for voltage imbalance. After you solve
any problems you find, failure rates should drop.
However, your job isn’t done. When widespread failures exist,
may be multiple root causes. If, for example, maintenance of the
power distribution system has been insufficient to ensure voltage
balance on the phases, then chances are maintenance is also
in other ways. So, look for and solve all possible root causes.
Your job still isn’t done. These failures may have compound
causes — a mix of different failure contributors that, by
themselves, don’t cause failure. When compounded on top of each
the result is failure. The only way to find these is to methodically
examine one motor at a time. Check the entire system of each
motor. Key areas include:
- Input. Use a power analyzer to detect problems with power
factor, waveform quality, harmonics, and voltage imbalance.
- Load. Check alignment and lubrication.
- Mounting. Use a vibration monitor to determine if it’s
adequate and serviceable.
- Bonding. Ensure metal parts of the motor system are bonded,
not grounded (See NEC Art. 100 definitions).
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