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Maintenance
Maintaining
Low-Voltage
Circuit Breakers
In general, you can choose from one of three strategies
when servicing breakers.
- Corrective-only. This is the cheapest strategy. However,
it’s also the most costly, because (in defiance of logic) it relies
on
failure instead of preventing it. When a breaker fails open, production
stops. That’s expensive enough, but things can get far more costly
than that. If a breaker fails (doesn’t open) during a fault, the
consequences can include loss of the entire facility. Putting failure
in
charge won’t guarantee success.
- Interval-based. This is a step up from corrective-only. With
this strategy, you try to time your maintenance against wear and tear.
The underlying assumption is that breakdowns occur in predictable
patterns, and you just have to get there first. But the schedule can't
account for the condition of the breaker. If a power event happens
immediately after a PM, you won’t correct the damage until either the
next scheduled PM or a failure occurs. Because this strategy doesn't
match limited resources against actual conditions, it creates
“maintenance holes” in some areas and unnecessary maintenance in
others.
- Condition-based. With this strategy, you rely on real
information rather than "best guess" and gambles. Using a combination
of
information sources, you perform the right amount of maintenance. Thus,
you optimize performance, safety, and reliability at the lowest cost.
Information sources include infrared imaging, power monitoring,
statistical data, maintenance testing, and visual
inspections.
With the condition-based strategy, you rely on diagnostics rather than
a
calendar and with the interval-based strategy, you rely on a calendar
instead of failure.
Two Tests for
Every Breaker
Conduct these tests with the breaker isolated from its
power source.
- Contact resistance. Close the breaker. Test contact
resistance on the line-to-load terminals of each phase. The values
should not vary by more than 50%. If they do, you may have a problem.
If
the equipment manual doesn't specify the contact resistance, find out
from the manufacturer what it should be.
- Insulation resistance. Close the breaker. Test insulation
resistance phase-to-phase and phase-to-ground (equipment ground, not
earth ground). In addition, measure across the open poles. If the
equipment manual doesn't specify the minimum acceptable value, find out
from the manufacturer what it should be.
Coordination
Most facilities undergo changes in equipment
and power distribution over the years. When was your last circuit
protection coordination study? If it’s been a while, don’t be
surprised when (not if) a problem in a minor branch circuit causes a
wide area shutdown. To make best use of limited time and money, focus
on
critical branch and feeder circuits first.
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Repair
Electrical
Troubleshooting Quiz
You’ve just completed a work order for a replacement
motor on a hopper feeder. The work order instructed you to “test
motor
for rotation, wire it in, and return to service.” When you apply
power, however, the motor shakes violently. What are some possible
explanations, and how do you isolate the cause?
The answer to this question appears at the end of this
newsletter.
Simple Test
Prevents Re-Repair
Your crew worked hard to remove the old motor and
install the new one while production was down. The location made the
work dangerous and difficult, so everyone’s glad it’s over. But
when
you apply power, the new motor shakes fiercely. So now you have to
replace the new motor.
How could you have prevented this? When the motor arrives, inspect
it. Rotate the shaft and listen for the grinding sound a bent shaft
makes. That’s a qualitative test. You can also perform the
quantitative test of measuring bearing runout with a dial
indicator. If the shaft rotates eccentrically (more than 0.004 to 0.009
or so of runout), then you have a bent shaft, bad bearings, or both.
Don’t install that motor, because you’ll just have to yank it out
again.
Fuses and
Transformers
A transformer has high inrush current, because its core
(typically) has to reach saturation. Fuses used with transformers
typically need to have time withstand values that are 25 times the
transformer primary rated current — but for only 0.01 second. Then,
another value comes into play: The withstand can be “only” 12 times
the transformer primary rated current for 0.1 second. The reason for
the
second value is inrush current decreases as core saturation increases.
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Operation
NEC in the
Facility
Oil-filled transformers installed outdoors have special
requirements [450.27]. One critical requirement is providing for
containment of the oil, in the unlikely event of a rupture or spill.
The
gravel bed around the typical outdoor transformer is there to reduce
defoliation requirements and assist with water removal. It doesn’t
“contain” the oil. If oil spills into this bed, the EPA requires
excavating all the way down to uncontaminated soil.
Why You Should
Report Any Injury, Part 3
Prompt reporting establishes a “time of incident.”
This is important because, if complications arise later, you have a
valid claim against your employer, and your employer has a valid claim
against its insurer.
For example, the load shifts while you’re installing a motor. You
feel
a sharp twinge in your back. You walk around a bit and the pain
disappears. So, you don’t report this.
Undiagnosed disk damage deteriorates over several days, until, in
agony,
you finally report the incident. By this time, however, it’s too late
to say for sure that the motor installation incident caused the injury.
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Quiz Answer
Answer to
Electrical Troubleshooting Quiz
Something is bent or out of alignment. You came in
after
the mechanicals were done, so you’ll need to walk through each of
those steps to see what was done wrong.
Check motor/load alignment, then decouple the motor from the load
and
run the motor:
- If the motor doesn’t vibrate and the motor/load alignment was
good, there’s a problem in the load itself.
- If the motor does vibrate, check the motor shaft runout with a dial
indicator. If the runout is within tolerance, check the motor mounting.
The most common error in motor mounting is overtightening, which bends
the motor feet. Also look for loose bolts. Tighten to spec, not any
tighter. Inspect the base and pedestal for problem such as cracks or
twisting.
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