What the 1500 W rating means in a transient event
The 1.5KE540: That 777 V clamping voltage is the ceiling the diode holds the line to during the surge; downstream components see no more than that.
Breakdown and standoff — the two voltages that define the protection window
The reverse standoff voltage is 486 V — the diode stays off below this, so normal 480 V bus ripple does not trigger leakage. Breakdown starts at a minimum of 486 V, meaning the diode begins conducting right at the standoff threshold, leaving no dead zone between normal operation and protection.
Obsolete — sourcing through independent channels
Littelfuse has marked the 1.5KE540 as obsolete. No pin-compatible direct replacement from Littelfuse is documented — a board-level substitution would require re-evaluating the clamping voltage and package footprint.