SV0494 DCS: VRDY ON (Safety Mismatch) fault in Fanuc drive

SV0494 DCS: VRDY ON (Safety Mismatch) fault in Fanuc drive

Description

SV0494 is a critical alarm specific to machines equipped with Dual Check Safety (DCS). DCS is a redundant safety system where the CNC monitors safety signals through two independent channels (Path A and Path B). SV0494 occurs when the redundant check for the "Velocity Ready" (VRDY) signal fails. The CNC commanded the Safe Torque Off (STO) function to engage (shutting down power to the motor), but one of the two safety channels reported that the drive was still active. This is a "Safety Cross-Check" failure.

Cause

• Welded Contactor: The Safety Contactor (MCC) has welded contacts. One channel sees it closed, while the software commanded it open.
• STO Timing: The "Safe Torque Off" wiring to the drive (STO1 and STO2 signals) did not drop out simultaneously. If there is a lag >50ms between STO1 and STO2, the alarm trips.
• Hardware Failure: The redundant safety circuit on the amplifier control board has failed.

Solution

1. Check Wiring: Verify the STO connector (usually CX4 or CX36). Ensure both channels are wired correctly and tight.
2. Cycle Power: DCS alarms often latch to prevent reset. You must completely power down the main breaker for 1 minute to reset the safety relay logic.
3. Check MCC: Manually inspect the main magnetic contactor. If the plunger is stuck down, replace it immediately.