Littelfuse lists the 1.5KE400-B as Obsolete.
Key ratings for fit
The reverse standoff voltage is 342 V, meaning the protected line can sit at up to 342 V DC without the diode conducting. Breakdown occurs at a minimum of 360 V, and the clamping voltage at peak current is 575.4 V — the voltage the load sees during a surge event. Peak pulse current is 2.8 A under the same 10/1000 µs waveform. The DO-201AA axial-lead package is through-hole — hand-solderable and easy to rework on a breadboard or prototype board.
Where it fits in a circuit
General-purpose transient suppression — clamping overvoltage spikes on AC or DC power rails, signal lines, or industrial control buses. The 342 V standoff suits 240 VAC or 300 VDC rails with margin.