Standoff, breakdown, and clamping — the protection window
The 1.5SMCJ51: The standoff voltage of 51 V nominal sets the working voltage the TVS sees in normal operation — the diode stays off and invisible to the circuit until a transient arrives. Breakdown begins at 56.7 V minimum, which means the clamping window opens above that threshold and the TVS conducts to ground, clamping the overvoltage before the protected node exceeds its rating. At the peak pulse event, the clamping voltage ceiling is 91.1 V maximum — this is the voltage the downstream circuit actually sees at 16.5 A peak pulse current. For a 51 V rail, the clamping ratio is roughly 1.79×, which is the normal range for a unidirectional Zener TVS in this class.
The DO-214AB (SMC) package is the standard surface-mount package for 1500 W bidirectional and unidirectional TVS diodes — the foot pattern matches every major brand's SMC offering, so a second-source search at RFQ is straightforward.
Listed as Active under Diotec Semiconductor.