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Maintenance
Circuit Breaker
Dynamic Testing
Sometimes in the course of breaker maintenance, a
technician opens a breaker but can’t close it again. Consequently,
the
loads on that breaker are no longer available for operation.
The typical production manager may react by saying something like,
"We were running before you fixed things. Now we can’t run." Although
this is technically a correct observation, it implies maintenance
created a breaker failure — not merely discovered it.
Consider the alternate scenario. You are able to close a faulty
breaker, and assume that means you can leave it in service. Then, a
fault occurs and the breaker fails to open. The reality is that not
being able to close a faulty breaker is much better than being able to
close it only to burn the plant down a few days later. And it’s
better
than finding yourself in court listening to a forensic engineer
describe
a problem you should have caught.
Therefore, don’t return a breaker to service just because you can.
You need assurance it will open on a fault. A key to making that happen
is to operate the breaker multiple times, both mechanically and
electrically. How many times is enough? That's a judgment call, but
certainly more than once or twice. Go with at least the minimum
recommended by the manufacturer, and test all methods of opening and
closing.
When operating the breaker manually (with no power applied to the
breaker), observe the mechanism for binding, and observe the contacts
for bounce.
If you outsource your breaker testing, hire only a qualified testing
firm. Before commissioning the work, review the testing plan against
the
manufacturer’s recommendations and resolve any differences.
Tie Breakers and
Maintenance
If you have a tie breaker between two main breakers (A
and B), on which breaker do you perform maintenance first? Answer: Do
maintenance on the tie breaker first.
There are several reasons for this. In a typical application, the
tie
breaker is open, and you can’t close it unless you have first closed
breaker A or B. So, the tie breaker isn’t subject to stress because
it
isn’t carrying any load. You already know breakers A and B can carry
the load because they are doing that now. You don’t know that the tie
breaker can carry the load, because it’s not doing so now and you
don’t have any test data to prove it can. Thus, you test the tie
breaker first.
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Repair
Electrical
Troubleshooting Quiz
Your facility has a large, multifunction machine.
It’s
fed by a roll of steel, and it produces stamped and welded panels.
Operators are complaining that:
- They get headaches and eyestrain (although the lights don’t
appear
to be dim).
- The stampings lack the proper definition, even though the dies meet
spec and the settings are correct.
- The welds don’t look uniform and don’t look right.
Can there possibly be a root cause common to these problems? If so, how
can you determine what it is?
The answers to these questions appear at the end of this
newsletter.
Motor Repair, Part
2
In Part 1, we said you need to be a bit of a forensic
examiner to do a proper motor repair. You should also tap the expertise
of another forensic examiner: your motor repair shop.
A repair isn’t complete just because you’re running again. A
repair is complete only when you’ve fixed the cause of the failure.
You can’t fix what you don’t know.
Send a failed motor to the motor shop for postmortem analysis, if
the
cause is unknown and any of the following apply:
- The application is critical.
- The application makes replacement difficult, dangerous, or
time-consuming.
- The motor is large (and, thus, expensive).
- A motor failed prematurely (regardless of cost or
criticality).
Important: Identification of a root cause issue with a
noncritical motor can help you prevent failure of a critical
one.
Fuses and Capacitors
You may have a capacitor bank for any number of
reasons.
For example, you may have one at your service entrance for power factor
correction. Or, you may have one because you need to draw high current
briefly for process reasons. Capacitors can solve a large number of
problems, but they can also add problems if you don’t fuse them
correctly. This is one reason there are fuses made specifically for
capacitor protection.
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Operation
NEC in the
Facility
Dry-type transformers don’t necessarily stay
dry. They are called dry-type because they aren’t intentionally
filled
with a liquid (e.g., oil). They are vented, and water can get in the
vents, soaking the paper inside them. When that happens, the safety and
performance of the transformer are severely compromised. This is why
the
NEC requires all dry-type transformers installed outdoors to be in a
weatherproof enclosure [450.22]. See Art. 100 for the definition of
weatherproof.
Why You Should
Report Any Injury, Part 2
By reporting your injury, you identify hazards that
could hurt others. This is true even of a very small injury. For
example, someone runs a portable cord across a stairway. You trip, but
catch yourself. Your only injury is a minor scrape on your elbow, so
you
just reroute the cord problem and don’t report the injury.
The next day, the same person drapes the same cord across the same
stairway. A coworker trips over it and breaks his arm. Had you reported
your injury (and the unsafe act), steps could have been taken to
prevent
your coworker’s injury.
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Quiz Answer
Answer to
Electrical Troubleshooting Quiz
The root cause appears to be a power problem. Low-level
lighting flicker could cause the eyestrain, which would cause the
headaches. Power distortion or low voltage could cause the tooling
problems and the flicker. Use a power analyzer to identify power
anomalies.
First steps in solving any “mysterious” power anomalies on this
machine:
- Have the machine’s capacitors properly tested.
- Inspect every connector and raceway on the machine for
“grounding” (bonding) path continuity.
- Inspect all related equipment for bonding deficiencies (per NEC
Art.
250 Part V and IEEE-142).
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