What the 1.5 kW rating means for your rail
The 1.5KE300A-E3/73: The 256 V reverse standoff voltage means it sits across a 240 VAC or 250 VDC rail without conducting in normal operation, and the 414 V clamp voltage limits the spike the downstream circuitry sees during a transient. The 3.6 A peak pulse current rating at the 10/1000 µs waveform tells you the maximum surge this part can absorb — useful for sizing the upstream fuse or PTC to coordinate protection.
Through-hole package and board fit
Housed in a DO-201AA (DO-27) axial-lead package, this is a through-hole part that hand-solders easily — no hot-air station needed, just a clean iron and a bit of flux. The 1.5KE supplier device package marking is consistent across the series, so a replacement from the same family drops into the same board holes.