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CONTENTS
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You and IR, Part 7
Electrical Troubleshooting
Quiz
Troubleshooting PLC Analog
Output Modules, Part 1
NEC in the Facility
Safety
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About This Newsletter
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This twice-a-month
e-newsletter is brought to you from the
publisher of EC&M magazine. MRO Insider addresses topics such
as:
Working with management and supervision
National Electrical Code® on the production floor
Safety procedures and programs
Troubleshooting techniques
Equipment maintenance and testing tips
Managing motors and generators
Trends in training and education
Managing energy use
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Maintenance
You and IR, Part 7
It's great to have a quality thermographic camera and a
well-administered thermography program. But what do those images and
temperatures really mean?
Certain types of testing are more relative than absolute. Insulation
resistance testing is very much that way, and thermography is mostly
that way.
For example, does a 170°F reading on a motor vent mean the motor is
fine or that it's overheating? How do you know what this reading is
really telling you?
The key to making these kinds of tests useful is to establish
baseline readings when the equipment is first put in service. These
will
be your reference for judging whether future readings are abnormal.
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Repair
Electrical
Troubleshooting Quiz
Your plant has a process that uses treated water. Based
on the flow rate to the process, the PLC program calls for makeup water
to be delivered to the day tank. Recently, the day tank has repeatedly
run dry during production. The lack of treated water ruins the product.
It also forces production operators to shut down, perform purges and
blowdowns, and then restart the system.
Preliminary troubleshooting has established the input side of the
control loop is problem-free. In the notes, you read:
- There is more than enough water treatment capacity to satisfy
production demand.
- The flow meter has been recently calibrated (no adjustment needed).
- The makeup water valve works perfectly.
Where do you go from here?
Visit EC&M's
website to see the answer.
Troubleshooting
PLC
Analog Output Modules, Part 1
In our previous two issues, we discussed
troubleshooting
digital output modules. Troubleshooting analog modules
is
a bit more complicated, for two reasons:
- Analog modules aren't on/off, as digital ones are, so there won't
be
a status light showing the logic state. You have to (in nearly all
cases) use a DMM to see what the output level is.
- You need to know the range (e.g., 4mA to 20mA) and possibly the
scale (e.g., 1 to 1,600 on the PLC register) for this output.
Determining this for certain may require walking through a long
documentation trail, beginning with the P&IDs. You may need to resolve
multiple conflicts of specifications, if this loop has not worked
properly before.
To read more on this story, visit EC&M's website.
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Operation
NEC in the
Facility
As we noted in our previous issue, Annex D3(a) begins
with a description of an industrial multi-building application case.
Then, it poses this problem: “Determine the overcurrent protection
and
conductor size for the feeders in the common raceway, assuming the use
of XXHW-2 insulation (90°C).”
Annex D3(a) shows you how to solve this problem, beginning with how
to determine the calculated load. Then, using the values you determined
for the noncontinuous and continuous loads, you determine the total VA.
Convert this figure to 3-phase AC current, and you have the minimum
size
overcurrent protective device.
Next, Annex D3(a) illustrates how to determine the feeder conductor
sizes. You do some straightforward calculations, using numbers from the
75°C column of Table 310.16. However, there is a twist. Per
310.15(B)(4)(c), you must count the neutral conductors as
current-carrying conductors because the discharge “has a substantial
nonlinear content.”
This brings us to an even trickier twist. The example says to use
the
90°C column, not the 75°C column, for this neutral conductor. Why
would that be?
To read more on this story, visit EC&M's website.
Safety
One of the great innovations for the construction
industry is the “all-day” battery-powered tool. These typically
operate at 24VDC, but that doesn’t mean there aren’t any safety
issues.
Take the cordless drill for example. When that battery gets low
after, say, sawing holes all morning, the drill will turn more slowly.
Cutting ability depends not only on the sharpness of the hole saw, but
also on the speed of the drill. As cutting ability decreases, the
likelihood of slippage and injury increases.
The solution? Use an AC-powered drill and portable cord. Or, have a
second drill or spare battery charged and ready to go. And don't forget
to replace worn hole saws and drills.
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