Zero Endurance and Burning Stun

[Tempuswolf]

Can a character's endurance drop below zero when performing stun burning actions? Is the stun loss a substitution in lieu of losing more END or an additional penalty to the endurance loss?

For example, after a character with 5 END performs an action that costs 15 END would his END be 0 or -10?

[Steve Long]

Characters cannot spend themselves into negative END; once they hit 0, they have no more END to spend. All points of “END” “below” 0 must be paid for with STUN, as described on 5E 286. In your example, the character would drop to 0 END and then take 5d6 STUN damage.