Ah yes - it seems I've mixed up sections 2.1.2.10 and 2.2.1.10 in the GUE standards pdf(
http://www.gue.com/classroom/standar...ds_2003v2a.pdf)
I was looking at the Tech 1 equipment rather than the DIRF requirement.
D'oh.
Oh well now we're here, is the reason that Roel (statistical failure rate of lights) the reason for two backup lights at levels beyond DIRF?