What the 1500W peak pulse rating means for your board
The 1.5KE440A-E3/51: For a 376 V reverse standoff (the voltage it ignores during normal operation), it breaks down at 418 V minimum and clamps the transient to 602 V maximum at 2.5 A peak pulse current. This is a unidirectional Zener-type TVS, so it protects a single polarity rail; the cathode band goes toward the positive supply.
Through-hole axial package and temperature range
Housed in a DO-201AA (DO-27) axial-lead package, it's a through-hole part — hand-solderable and easy to breadboard or retrofit into an existing PCB.
Obsolete — sourcing through independent channels
Vishay lists the 1.5KE440A-/51 as obsolete. No pin-compatible direct replacement is documented, so a board spin may be needed if you're migrating to a current-generation TVS.