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Old February 17th, 2017, 07:09 AM   #1

catalinoac is offline
Join Date: Feb 2013
Location: Cebu
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Door simulation lower limit switch (ls2) wiring configuration

  • The original limit switch wiring configuration on Door Simulation are
    LS1 = N.O.
    LS2 = N.O.

    Upon reading on the exercise, there is a "For Discussion" in which the
    the LS2 (lower) may be rewired to N.C. for "fail safe design" operation.

  • Upper Limit switch wired N.O., Lower limit switch rewired N.C. resulting in:
    Upper limit switch (door open) goes False(0) when Door reaches the fully open position.
    Lower limit switch (door closed) goes False(0) when door reaches the fully closed position.
Task> Explain why this last rewiring option should result in a safer, more fault tolerant installation.
  • </u>With the wiring configuration of LS2 change from N.O. to N.C. would result in a fail-safe (OPEN CIRCUIT) protection. (As the motor will not run down if the circuit on limit switch is open)

    My question is, how about "SHORT CIRCUIT"? If LS2 will be shorted, then motor will run down (when PB closed is pressed) and will NOT STOP down to LS2 (as LS2 is shorted)
Please enlighten. Thanks

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Old February 17th, 2017, 07:22 AM   #2
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Mispeld is offline
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Location: VA
Posts: 282
The task asks why the configuration is "more fault tolerant." Focus on the failure modes handled by one N.O. and one N.C. switch.
Your question is valid and goes down the path of performance levels in safety designs. There are devices on the market which can detect shorted wiring and faulty devices, but are more complicated and expensive than basic limit switches.
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Old February 17th, 2017, 07:28 AM   #3

gclshortt is offline
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The above information should help you out. The top link will also discuss "Short Circuits".
We now have "Safety Relays" that will prevent all errors on the circuit.

Hope this helps you out.
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Old February 17th, 2017, 07:42 AM   #4

catalinoac is offline
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Thanks you so much guys. Much appreciated.
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