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Friday, August 28, 2015

PLC Troubleshooting

PLC Troubleshooting - Failure Probability

PLC MTBF does not apply, so much.

(Right click, "Open Image in New Tab", click on image in new tab to view full size)

Failure probability is categorized like this (in order of most likely to least likely).

  1. Mechanical
  2. Electro-Mechanical
  3. High Current
  4. Low Current
  5. Solid State


Environment can be an exception to the general rule.
(Relay card most likely, and MTBF is useless unless solid state, as frequency PLC program activates, dictates failure rate as well as the environment does.)

The above probability guide translate to troubleshooting PLCs as ...


  1. Output Cards
  2. Input Cards
  3. Power Supply
  4. Backplane
  5. CPU (Processor Card)


Environment can change basic order above. Examples: In a foundry, the metal dust causes backplane to fail more frequently. Machining industry, penetrating fluid getting into sensors causes input cards to fail more than output cards because sinks more current than designed to. Wire manufacturing a lot of high voltage insulation testing is going on, CPUs fail more often because control voltage for PLC does not have line filter in series with it.

(More PLC Troubleshooting)


Don (Follow me on Industrial Skills Training Blog and on Twitter @IndTraining .) Be sure to to stay on top!