04 ALARM — OPEN PHASE / POWER SUPPLY FAILURE

Measure the 3-phase input voltage (L1, L2, L3 terminals) at the main circuit breaker using a multimeter. If the voltage is significantly unbalanced, below 85% of the rating, or missing a phase, correct the facility power supply and check for blown facility fuses.

Inspect the main magnetic contactor (MCC) and its sequence. If the MCC turns off while the emergency stop is released, or if the contacts are worn, investigate the MCC drive sequence (check connector CX3) and replace the magnetic contactor if it is defective.

Inspect the input power cables connecting the main breaker, AC reactor, MCC, and the Power Supply Module (L1, L2, L3). If any connection is loose or a cable is damaged, securely tighten the terminal screws or replace the damaged power cables.

Check for momentary power failures or severe voltage dips in the factory power grid. If the voltage drops momentarily during operation or heavy machine load, improve the factory power capacity or install a stable power backup system.

For specific models like the alpha-i-D series, inspect the power supply voltage detection cable (connector CX48). If the cable is disconnected, loose, or wired incorrectly (e.g., L1/L2/L3 phase sequence mismatch), correct the wiring and securely attach the connector.

If the input power, MCC, and all cables are perfectly normal but the alarm persists, the internal control printed-circuit board or a power element in the Power Supply Module (PSM/Converter) is defective. Replace the Power Supply Module.

If all previous steps have been completed and the alarm persists, contact an authorized FANUC service representative.