The 1.5SMCJ70: The clamping voltage ceiling of 125 V at 12 A peak pulse sets the let-through voltage the downstream circuit sees during an event — the 125 V figure is the max at rated pulse current, not the typical clamp level, which lands lower on the curve.
Clamping ratio and the standoff-to-breakdown gap
Breaking down the voltage ladder: 70 V standoff is where the diode begins to look resistive; the 77.8 V minimum breakdown is where it snaps into conduction; 125 V at 12 A is the clamping ceiling. The gap between standoff and breakdown gives the protected rail ~8 V of differential before the TVS activates — enough to let normal operation sit clean without the clamp eating rail noise on a 48 V system.