Applicable Systems
SKY-2
Symptoms
The Otis SKY-2 drive controller displays fault code 303 indicating VAC UNDER voltage. The AC line voltage has fallen below the lower limit. The drive will end the current run and then lock out. The elevator will be out of service until power is restored and the fault is cleared.
Possible Causes
- Utility power outage or voltage sag below drive threshold
- Phase loss on three-phase input to the drive
- Building main breaker trip or undersized feed transformer
- Loose input wiring connection creating voltage drop
Troubleshooting Steps
- Step 1: Read fault code 303 on drive controller display
Code 303 displayed indicating AC undervoltage - Step 2: Measure three-phase AC input voltage at drive input terminals
All phases present and above minimum voltage threshold - Step 3: Check building main breaker and upstream power supply
Breaker closed, utility power stable - Step 4: Inspect input wiring connections at drive terminal block
All connections clean and torqued to specification
How to Reset
To reset: 1) Restore AC input voltage to acceptable range. 2) Power cycle the drive controller. 3) Verify fault clears and elevator returns to service. Caution: Ensure power is stable before returning to service.
Frequently Asked Questions
- Q: What is the AC undervoltage threshold for SKY-2?
The exact threshold varies by drive model and input voltage rating. Typically, it triggers when AC voltage drops below approximately 80% of nominal (e.g., below 304V for a 380V system).
- Q: Will the elevator trap passengers during fault 303?
The drive will attempt to complete the current run to the nearest floor before locking out. If the voltage drops too rapidly, the elevator may stop between floors. Emergency rescue procedures should be followed.
Source: Otis SKY-2 Drive Fault Code Manual (1lift.cn), Otis 21VF/MCS321 Fault Code Manual
Category: Elevator