Automotive TVS selection criteria
The 1.5SMCJ51CA-AQ: Bidirectional is listed as 1 channel — this is a unidirectional polarity. For a 12 V rail the standoff of 51 V gives roughly a 4× safety margin over nominal, which is typical for single-polarity protection. The 18.2 A peak pulse current corresponds to the 1.5 kW rating at the clamping voltage — above that current the clamping voltage rises and the peak pulse power limit becomes the governing constraint, not the standoff rating.
Diotec does not appear to publish an automotive-grade suffix variant or a formal AEC-Q101 qualification entry for this specific order code in the available record; the listed application field is Automotive, but the compliance documentation scope should be confirmed directly with the distributor or Diotec before signing off on an AEC-qualified BOM position.