Voltage ratings: standoff, breakdown, and clamp — the real protection window
The SMCJ28CA-13: The reverse standoff voltage is 28 V — the diode draws negligible leakage below this level, so it sits quietly on a 24 V or 28 V rail. Breakdown starts at 31.1 V minimum, meaning the device begins conducting before the protected circuitry sees a dangerous overvoltage. The clamping voltage is 45.4 V maximum at the rated 33 A pulse current — this is the voltage the downstream components must survive during a transient event.