The 1.5SMCJ18AS_R1_00001: The 18 V reverse standoff sets the maximum working voltage the protected rail can run at continuously without the TVS conducting — design the rail to sit below this in normal operation. When a surge arrives, the device breaks down at 20 V minimum and clamps the line at 29.2 V with a 51.4 A (10/1000 µs) surge applied — what the downstream IC actually sees is the clamped level, not the raw transient. This is a unidirectional Zener-type TVS, so it clamps positive transients only — reverse-polarity events are not suppressed. If the protected line sees bidirectional or negative surges, a different polarity configuration is required.
Package, orientation, and sourcing posture
Surface-mount SMC (DO-214AB) package — the cathode bar on the package body is the polarity reference; the marked side is cathode in the unidirectional configuration. Orientation is unambiguous on reflow, but a misplaced part will clamp in the wrong direction on a polarity-sensitive rail.