Explain the operation and selection criteria for automatic transfer switches.

Automatic Transfer Switches (ATS) sense utility power failure and transfer critical loads to generator power automatically. Selection criteria include: voltage and current ratings, number of poles (3-pole or 4-pole with switched neutral), transfer time (standard 10 seconds or fast for critical loads), transition type (open, closed, or soft), bypass capability for maintenance, and withstand/closing ratings for fault conditions. Healthcare facilities require specific configurations per NEC Article 517.