The 1.5KE300CA-E3/51: The breakdown voltage window starts at 285 V minimum, so a 256 V standoff rail sits comfortably below the conduction threshold, meaning the diode stays out of the circuit during normal operation and only conducts when a transient pushes the line above that level.
DO-201AA axial package and through-hole mounting
Housed in a DO-201AA (DO-27) axial-lead package, this part is through-hole mounted — a good fit for point-to-point wiring, terminal blocks, or boards where hand-soldering is preferred over reflow. The axial leads make it easy to retrofit into existing designs or prototype on a breadboard.
Obsolete — sourcing through independent channels
Vishay lists this part as obsolete. Because this is a through-hole TVS with a standard DO-201AA footprint, a functional replacement from another manufacturer (same 1500 W rating, similar standoff and clamp voltages) can be sourced and cross-referenced at quote time if the original code is unavailable.