TVS diode — what the 51V standoff and 1.5kW rating decide
The 1.5SMCJ51AS_R1_00001 is a unidirectional Zener-based transient voltage suppressor in the 1.5SMC series — it clamps the line when a surge exceeds the standoff threshold and absorbs it up to the rated peak pulse power. The 1500W peak pulse rating and 18.2A peak pulse current tell you it handles the first-hit energy of a lightning-induction or switching transient without cascading — the part's job is to limit the voltage at the protected node and survive the event intact.
Clamping chain — breakdown to clamp voltage
The protection window runs from 56.7V minimum breakdown to 82.4V maximum clamping voltage at the rated 18.2A peak pulse current — so the downstream IC or bus sees no more than 82.4V during the event. For a 48V system that means roughly 34V of headroom in the suppression zone, which is the region where the TVS is doing useful work. A lower clamping voltage would reduce stress on the protected load, but at 1.5kW this is the standard trade-off between handling energy and holding down the let-through voltage in this series.
DO-214AB SMC package — SMT rework note
The unidirectional polarity means this is for DC rails only — it does not protect AC lines the way a bidirectional TVS would. If the application is a 48V bus or 24V supply rail, this is the right orientation; reverse-bias it and it clamps as described.