1500 W clamping margin in a DO-201 axial package
The 1.5KE91A B0G is a unidirectional Zener-type TVS from Taiwan Semiconductor's 1.5KE series, rated for 1500 W (1.5 kW) peak pulse power across a 10/1000 µs waveform. It clamps transients at 125 V maximum when hit with a 12.6 A peak pulse current, starting from a 77.8 V reverse standoff level. The breakdown voltage window runs 86.5 V minimum, so for a nominal 78 V rail you have margin above the operating voltage before the diode begins conducting.
What the clamping and standoff numbers mean for your rail
Reverse standoff of 77.8 V means the diode stays effectively off on a 75 V or 78 V nominal bus, leaking only until the transient pushes past the breakdown threshold at 86.5 V minimum. The 125 V clamping voltage is the worst-case peak the protected circuit sees — anything downstream must survive at least 125 V for the pulse duration. At 12.6 A peak pulse current (10/1000 µs), the 1500 W rating is derived as Vclamp × Ipp, so the actual energy the diode absorbs depends on the surge impedance. Power line protection is listed as No — this is a signal-line or DC-bus TVS, not designed for continuous AC mains clamping. For a 48 V telecom bus or a 72 V industrial DC rail it provides a single-direction clamp; the unidirectional configuration (one channel) means you get one diode per package, so bipolar rails require two parts back-to-back.