Current HP is almost irrelevant to door traps
I see the following behavior with exploding rune, fireballs, and stone block door traps:
Characters maximum health is 1243. Getting hit at full health brings the character to 124 HP. Getting hit when current health is anywhere between 125 and 1242 brings the character to 124 HP. Getting hit when current health is 124 does no damage. Getting hit while below 124 deals a small amount of damage.
EDIT: The HP threshold (124 in the example) changes depending on the character's luck. A doomed character at 1243 HP can be brought to 0 by a door trap.