Allen-Bradley PLC Major Fault — Power Loss (Type 01)

CompactLogix or ControlLogix processor detected a power-up or power-cycle event. Controller faulted on startup or after a brown-out condition. Program execution stopped.

Probable Causes (ranked)

• Incoming power supply to PLC rack dropped below threshold — utility fluctuation or loose wiring
• PLC power supply module failing — output voltage sagging under load
• Backplane power distribution issue — loose module seating causing intermittent power to CPU
• UPS feeding the PLC panel failed or battery depleted during outage
• Power supply overloaded — too many modules drawing more current than PSU rating

Fix Steps

1. 1 Check PLC power supply output voltage — 1756-PA72 should read 5.1V and 24V on status LEDs
2. 2 Measure incoming AC voltage at power supply terminals — should be stable 120VAC or 240VAC
3. 3 Reseat the CPU module firmly into the backplane — push until retention clips click
4. 4 Check power budget — sum all module current draw and compare to power supply rating in RSLogix
5. 5 If power loss is intermittent, add a 24VDC UPS to the panel or check utility power quality
6. 6 Clear the major fault in RSLogix 5000/Studio 5000 — Controller Properties > Major Faults tab
7. 7 After clearing, put controller back in RUN mode — verify program executes normally

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