FCOM DSC-32-30-10 P7/14
Automatic braking is activated
-at the command for ground spoilers extension, for LO and MED
-at the command for ground spoilers extension and the aircraft speed is above 40 kt, for MAX mode.
(Bearing in mind that command for spoilers will not activate until above 72 kt anyway)
So to answer your question it's at the command for spoilers, nothing to do with how far they're deflected.
Last edited by HPbleed; 31st October 2012 at 10:30.