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Troubleshooting PLC Analog Output Modules, Part 2

Aug 24, 2010 11:13 AM

Does the module's output correspond correctly to its input? Determine that by forcing the PLC output to 10%, 25%, 50%, 75%, and 90% and measuring on the output terminals at the corresponding range. For example, if it's a 4mA to 20mA module, then at 10% input from the PLC you should measure 5.6mA (10% of 16mA + 4mA).

If you don't get the anticipated results, disconnect the field wiring and use a test load (typically 250Ù to 1,000Ù; your loop simulator instrument should provide this). If the results are correct, the problem is in the field wiring. If you get incorrect results, then replace the module. However, don't replace the module until you've verified the field wiring and devices in the output loop are OK, or you may be replacing that module again.

Simply putting a DMM on the field terminals isn't a good way to check that field wiring. Use an insulation resistance tester to test wiring integrity, point to point (disconnecting as needed). Verify reconnection with point-to-point ohms testing at the terminals (the classic "loop check").

If the wiring and connections check out OK, then you’ll need to find the defective device before putting the loop back in service. Also check equipment bonding, per Art. 250 Part IV. Possibly, that loop is getting a high-voltage flashover due to differences of potential.

 

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