The 1.5SMCJ30: The 30 V reverse standoff is the maximum DC working voltage the protected line can sit at continuously without the TVS conducting — the part is off and invisible to the circuit under normal operation. Breakdown triggers at 33.3 V minimum, clamping the transient event. The 53.5 V maximum clamping voltage at 28 A peak pulse is what the downstream load actually sees during a surge — that number determines whether your downstream semiconductors survive.
General-purpose TVS — where it sits in the protection hierarchy
Diotec lists this as General Purpose and explicitly flags Power Line Protection: No — the part is suited for secondary-side protection on signal rails, control lines, and low-current distribution rather than at the AC mains input where higher energy ratings are required. The 30 V standoff fits 24 V industrial control rails comfortably with margin.
Sourcing and lifecycle posture
The 1.5SMCJ30 is listed Active by Diotec Semiconductor.