What the 1500W rating means for your circuit
The 1.5KE51CAT/R: That 1500W figure is the maximum power it can absorb in a single pulse without failing; the actual energy handling depends on the pulse duration and duty cycle. The reverse standoff voltage is 43.6V, meaning the part will not conduct appreciably below that level. So a 48V nominal rail gets clamped to under 71V during a surge — well within the typical 80V-100V abs-max of downstream DC-DC converters.
Package and field-service fit
DO-201AA (DO-27) axial through-hole package — the leads are long enough to form through a standard 1.0 mm PCB hole and solder on the opposite side. No hot-air station needed; a soldering iron and a pair of cutters is all it takes to swap one in the field. Polarity is marked by the cathode band, but since this is a bidirectional device, orientation does not matter for the protection function — it clamps both polarities. Supplied in Tape & Reel (TR) — the axial leads are taped between carrier tapes for automated insertion, but the same reel works for hand-kitting if you are populating a small batch.